SCHOOL OF DESIGN
MSc in Design and Engineering
Alternative solutions in automotive HVAC systems.
Comfort, efficiency and sustainability in car cabin
temperature control
Master of Science Thesis
Thesis supervisor: prof. Silvia FERRARIS
Adjunct supervisor: prof. Emmanuele VILLANI
Author: Manol DIMOVSKI (10522815)
Academic year 2016/2017
Manol DIMOVSKI | Alternative solutions in automotive HVAC systems. Comfort, efficiency and sustainability in car cabin temperature control | MSc thesis
POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 1
Abstract
The vehicles cabin temperature control is essential to car passengers
and thermal comfort, providing appropriate conditions all seasons. Taking into consideration
that in a hot weather sunny day, due to the higher solar radiation, it is normal every car cabin
to suffer from high rise of the indoor heat, which leads to peak in the surface and interior air
temperatures. Controlling the thermal comfort with adjustment in temperature, however, is
acknowledged to be a difficult task especially when the cabin is a rapidly changing
environment, non-uniform with respect to parameters such as air temperature, air velocity and
solar load. The current existing Heating, Ventilation and Air Conditioning (HVAC) mobile system
have a main disadvantage it is a big consumer of energy. Since it needs to achieve the desired
thermal comfort temperature as fast as possible, it directly reflects in fuel consumption in the
combustion engine cars, but also affects electric vehicles as it could substantially reduce the
. In both cases this requires more frequent charges of petrol or electric
power. Additionally the excessive usage of private vehicles nowadays directly leads to an
increase in over-consumption of natural resources, producing more CO2 and NOX greenhouse
gas emissions. This redirects in increasing the natural imbalance and over-pollution. It could be
easily seen that the problem, based on the passengers microclimate comfort adjustment,
reflects directly into economic and ecological problem, which need to be managed.
This thesis paper investigates the possibilities of the existing alternative solutions for
assisting or improvement of the mobile air-conditioning system performance in hot weather.
The focus of the research is directed to private vehicles, which are more affected from the
temperature misbalance in the summer since they are left parked up to several times and hours
per day under hot solar radiation. Thus the simultaneous prevention of overheating inside the
cabin, necessary in moment of entering of passengers, is much harder to be controlled and
managed. In the process is researched variety of design solutions for active and passive
influence over interior air temperature and heat reduction of the cabin. Based on multiple
comparisons of experimental works and literature surveys, the research is exposing effectively
ways of preventing the rise in cabin temperature. The installation of such products in mass
production cars as a standard or optional feature will significantly help reducing the overall
vehicle compartment needs of energy for acclimatization and therefore extend a combustion
engine travel mileage or hybrid/ electric vehicle travel distance per charge, reducing the impact
of the heat over the HVAC work. High lightening the opportunities for optimization of vehicle
cabin physical characteristics and properties from the further research results will be made
conclusions and further suggestions for possible directions for future design of heat-preventing
solutions and product improvements in reduction of the usage of the HVAC system and
increasing its efficiency/performance.
Manol DIMOVSKI | Alternative solutions in automotive HVAC systems. Comfort, efficiency and sustainability in car cabin temperature control | MSc thesis
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Abstract (Italian)
Il controllo della temperatura della cabina dei veicoli è essenziale per la salute, le prestazioni
e il comfort termico dei passeggeri, condizioni appropriate per tutte le
stagioni. In una giornata di sole calda, a causa della maggiore radiazione solare, è normale che
ogni cabina dell'auto soffra di un aumento elevato del calore interno, che porta a un picco
nella superficie e nelle temperature . Il controllo del comfort termico con la
regolazione della temperature in un autoveicolo, tuttavia, è riconosciuto come un compito
difficile, perché è un ambiente in continua variazione, non uniforme rispetto a parametri come
la temperatura dell'aria, la velocità dell'aria e il carico solare. l sistemi mobili esistenti di
riscaldamento, ventilazione e condizionamento dell'aria (RVCD) presentano un grande
svantaggio sono grandi consumatori di energia. Dal momento é necessario raggiungere la
temperatura di comfort termica desiderata il più velocemente possibile, questosi riflette
direttamente sul consumo di carburante nelle auto dei motori a combustione, ma colpisce
anche i veicoli elettrici in quanto potrebbe ridurre sostanzialmente l autonomia. In entrambi i
casi ciò richiede più frequente rufornichento di benzina o energia elettrica. Inoltre, l'uso
eccessivo di veicoli privati oggigiorno conduce direttamente a un aumento del consumo di
risorse naturali, producendo più emissioni di gas serra CO2 e NOX. Si puó facilmente vedere
come il problema, basato sulla regolazione del comfort microclimatico dei passeggeri, si riflette
direttamente nel problema economico ed ecologico, che puó e deve essere gestito.
Questa tesi di laurea indaga sulle possibili soluzioni alternative esistenti per l'assistenza o il
miglioramento delle prestazioni del sistema di climatizzazione mobile nella stagione calda. Il
focus della ricerca è rivolto ai veicoli privati, che sono più colpiti dallo sbilanciamento della
temperatura in estate poiché sono lasciati parcheggiati fino a diverse volte e per piú ore al
giorno sotto la radiazione solare. Pertanto, la prevenzione del surriscaldamento all'interno
della cabina, che si riscontra al momento dell'ingresso dei passeggeri, è molto più difficile da
controllare e gestire. Nella ricerca sono state studiate una varietà di soluzioni progettuali per
l'influenza attiva e passiva sulla temperatura dell'aria interna e sulla riduzione del calore della
cabina. Sulla base di confronti multipli di lavori sperimentali e indagini sulla letteratura, la
ricerca sta espone in modo efficace per prevenire l'aumento della temperatura della cabina.
L'installazione di tali prodotti in auto di serie contribuirà in modo significativo a ridurre le
esigenze complessive di energia per l'acclimatazione del veicolo econtribuendo a prolungare
del motore a combustione o la distanza percorsa dai veicoli ibridi / elettrici,
riducendo l'impatto di il calore sul lavoro RVCD. L'ottimizzazione delle caratteristiche fisiche e
delle proprietà della cabina del veicolo sonoi ulteriori risultati della ricerca da cui saranno tratte
conclusioni e ulteriori suggerimenti per la progettazione futura di soluzioni di prevenzione del
e miglioramenti del prodotto.
Manol DIMOVSKI | Alternative solutions in automotive HVAC systems. Comfort, efficiency and sustainability in car cabin temperature control | MSc thesis
POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 3
List of figures
Figure 1 http://www.hybridcars.com/wp-content/uploads/2015/09/vauxhall-astra1.jpg
Figure 2 http://gtspeed.us/wp-content/uploads/2015/12/danger-to-children-sun-heat-750x400.jpg
Figure 3 http://www.adnradio.cl/images/3639675_n_vir3.jpg?u=161155
Figure 4 https://fthmb.tqn.com/fxoGknFuFhT-5TQjT_c0eTrzc3s=/768x0/filters:no_upscale()/
tesla_model3_roof-57044e5f3df78c7d9e800918.jpg
Figure 5 Redrawn from Atkinson and Hill (2015)
Figure 6 http://atermalnaya.ru/wp-content/uploads/2013/05/tonirovochnaya-plenka.jpg
Figure 7 Redrawn from Farrington, et al.(1999a)
Figure 8 https://dantricdn.com/2017/o-to-do-duoi-nang-1498634227012.jpg
Figure 9 Redrawn from Aljubury, et al. (2015)
Figure 10 Al-Kayiem, et al., (2010)
Figure 11 http://www.theseus-fe.com/ths_content/images/thematic/thermal/manikin-with-seat-and-
steering-wheel_transparent.png
Figure 12 Thermal comfort factors
Figure 13 Redrawn from https://static.wixstatic.com/media/
5dfbab_441b9188859e4100b8bac948df406524~mv2_d_3004_2272_s_2.jpg
Figure 14 Redrawn from Fanger (1972)
Figure 15 http://technox1.cafe24.com/wp-content/uploads/2016/11/newton-thermal-manikin1-1-1-
1024x613.jpg
Figure 16 Ivanescu, et al. (2010)
Figure 17 http://www.kramautos.nl/wp-content/uploads/2017/09/ac-730x350.jpg
Figure 18 http://www.linzing.de/wp-content/uploads/2012/01/rl11Klima02_01_1024.jpg
Figure 19 http://www.audiocoustics.co.za/car%20air%20conditoning%20diagram.jpg
Figure 20 Redrawn from Farrington & Rugh (2000)
Figure 21 Redrawn from Farrington & Rugh (2000)
Figure 22 Redrawn from Barrault, et al. (2003)
Figure 23 Redrawn from Farrington & Rugh (2000)
Figure 24 https://qph.ec.quoracdn.net/main-qimg-68b8b6d8a46a7f7cd46c30981e04de7a
Figure 25 Redrawn from Clodic, et al. (2005)
Figure 26 Redrawn from Clodic, et al. (2005)
Manol DIMOVSKI | Alternative solutions in automotive HVAC systems. Comfort, efficiency and sustainability in car cabin temperature control | MSc thesis
POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 4
Figure 27 Ivanescu, et al. (2010)
Figure 28 Ivanescu, et al. (2010)
Figure 29 https://www.liebherr.com/shared/media/aerospace-and-transportation/transportation/images/
products-and-solutions/air-conditioning-systems/liebherr-air-cycle-air-conditioning-system-
architecture-zoom.jpg
Figure 30 https://thermaxprofetherm.files.wordpress.com/2016/01/img_4273.jpg?w=2000&h=1500&crop
=1
Figure 31 http://www.apexinst.com/cms/wp-content/uploads/2015/06/Stirling-Cooler1.jpg
Figure 32 https://daisanalytic.com/applications/nanoair/
Figure 33 http://www.gentherm.com/sites/default/files/Zonal_Hvac_product-shot.jpg
Figure 34 https://www.researchgate.net/profile/Gevork_Karapetan/publication/258436493/figure/fig1/AS:
297375907237888@1447911349632/Figure-1-The-semiconductor-thermoelectric-cooler-of-old-
type-with-hot-and-cold-sides.png
Figure 35 https://blogs.scientificamerican.comblogsassetsFileBeCoolSchematic.png
Figure 36 https://www.cibsejournal.com/technical/the-appeal-of-magnetic-refrigeration/
Figure 37 http://snowders.com/curtains-for-cars-2/
Figure 38 http://www.theseus-fe.com/ths_content/images/thematic/applications/climatization/
applications_climatization_climate-chamber-results.png
Figure 39 http://bronxmrc.com/wp-content/uploads/2015/12/techpage_applications3.jpg
Figure 40 http://www.nissan-global.com/JP/TECHNOLOGY/FILES/2013/05/f51a4547ab11fd.gif
Figure 41 https://www.virtualmarket.innotrans.de/en/Thermobreak-RT,p1509489
Figure 42 http://www.classicgarageblog.com/wp-content/uploads/2013/07/1968-camaro-headliner-b-
1024x768.jpg
Figure 43 https://www.heatshieldproducts.com/hp-stealth-shield
Figure 44 Redrawn from Purusothaman et al.( 2017)
Figure 45 http://www.conservationsolutions.com/assets/images/insulation%20classes.jpg;
Figure 46 https://i.pinimg.com/564x/1a/1d/ca/1a1dcab5e7aa67333d47d659121ee659.jpg
Figure 47 Redrawn from Levinson, et al. (2011)
Figure 48 https://www.dispersions-pigments.basf.com/portal/streamer?fid=814255
Figure 49 Redrawn from BASF( 2017b)
Figure 50 http://www.aerogel.org/wp-content/uploads/2009/03/fenanofoamsem-lanl.jpg
Figure 51 https://upload.wikimedia.org/wikipedia/commons/6/69/Aerogelflower_filtered.jpg
Figure 52 https://cnet4.cbsistatic.com/img/1FFvwaDU-SleB33nAPen_W6M3eM=/770x433/2010/02/02/
ec51ee01-f4d5-11e2-8c7c-d4ae52e62bcc/Aerogel.JPG
Figure 53 https://upload.wikimedia.org/wikipedia/commons/2/2c/Aerogel_hand.jpg
Figure 54 http://www.johnsonwindowfilms.com/dealer/articleView.php?ARTICLE_ID=244
Figure 55 Redrawn from Atkinson and Hill (2015)
Manol DIMOVSKI | Alternative solutions in automotive HVAC systems. Comfort, efficiency and sustainability in car cabin temperature control | MSc thesis
POLITECNICO DI MILANO | SCHOOL OF DESIGN | MSc Design and Engineering Page | 5
Figure 56 http://www.thecrashdoctor.com/Images/PGW-sungate-weathermaster-glass-windshields.gif
Figure 57 http://midwestglasstinters.com/wp-content/uploads/2017/08/Window-Films.jpg
Figure 58 Isa, et al. (2015)
Figure 59 https://www.raynofilm.com/wp-content/uploads/2014/09/nano-carbon2-01.png
Figure 60 Redrawn from Rugh, et al.(2006)
Figure 61 https://www.6speedonline.com/forums/attachment.php?attachmentid=507903&d=1501791993
Figure 62 Redrawn from Aljubury, et al. (2015)
Figure 63 https://asklegal.my/p/what-does-malaysian-law-say-about-car tinting?fb_comment_id=
1419052528202099_1421105507996801
Figure 64 http://veteranlending.info/retractable-window-blinds/retractable-window-blinds-solar-screens-
windows-motorized-back-awning/
Figure 65 http://www.audizine.com/gallery/data/500/273IMG_0245.jpg
Figure 66 https://ssli.ebayimg.com/images/g/CLMAAOSwnHZYV~Cm/s-l640.jpg
Figure 67 Redrawn from Jasni & Nasir (2012)
Figure 68 https://sc02.alicdn.com/kf/HTB1ASy1JVXXXXcUXVXXq6xXFXXXC/outdoor-portable-car-covers-
garage-automatic-universal.jpg
Figure 69 https://sc02.alicdn.com/kf/UT824x.XMhaXXagOFbXh.jpg
Figure 70 Redrawn from Aljubury, et al. (2015)
Figure 71 http://assets.weathertech.com/assets/1/22/713x535/82556_Odyssey_2011.jpg
Figure 72 Redrawn from Manning & Ewing (2009)
Figure 73 http://www.automobilesreview.com/gallery/toyota-prius-solar-pack/toyota-prius-solar-pack-
03.jpg
Figure 74 https://blogmedia.dealerfire.com/wp-content/uploads/sites/190/2015/07/How-Does-the-
Toyota-Prius-Solar-Roof-Feature-Work-1024x400.jpg
Figure 75 http://3.bp.blogspot.com/-eW4J4j8tZHo/VUDrIS9RqiI/AAAAAAAA5-
4/7g3_GphPXEM/s1600/1kulcar.jpg
Figure 76 Redrawn from Rugh, et al.(2006)
Figure 77 https://www.audiworld.com/forums/attachments/q5-sq5-mki-8r-discussion-
129/8778d1271910380-ventilated-seats-availability-074__scaled_600.jpg
Figure 78 Theseus-Fe (2008)
Figure 79 https://marketinginsidergroup.com/wp-content/uploads/bfi_thumb/lead-scoring-criteria3-
n3nbow2brrcie38ru4xvse4dj6ifog9zd59rcok3o4.jpg
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Table of contents
INTRODUCTION .......................................................................................................................................... 9 1.
PROBLEM DEFINITION ............................................................................................................................. 10 1.1
RESEARCH OBJECTIVES ............................................................................................................................ 11 1.2
METHODOLOGY ...................................................................................................................................... 13 1.3
ANALYSIS OF THE HEAT FACTOR ............................................................................................................... 14 2.
“GREENHOUSE EFFECT” .......................................................................................................................... 16 2.1
CAR CABIN HEAT ACCUMULATION AND THERMAL DISTRIBUTION ......................................................... 17 2.2
THERMAL COMFORT RESEARCH ............................................................................................................... 21 3.
AIR TEMPERATURE .................................................................................................................................. 23 3.1
MEAN RADIANT TEMPERATURE .............................................................................................................. 24 3.2
AIR VELOCITY ........................................................................................................................................... 24 3.3
RELATIVE HUMIDITY ................................................................................................................................ 24 3.4
HUMAN ACTIVITY LEVEL (METABOLIC RATE) .......................................................................................... 25 3.5
CLOTHING INSULATION ........................................................................................................................... 26 3.6
ADDITIONAL FACTORS FOR THERMAL COMFORT ................................................................................... 26 3.7
THERMAL COMFORT RESEARCH MODELS ............................................................................................... 26 3.8
3.8.1 FANGER’S MODEL ........................................................................................................................... 27
3.8.2 THERMAL MANIKIN ........................................................................................................................ 28
3.8.3 COMPUTATIONAL FLUID DYNAMICS (CFD) .................................................................................... 29
RESEARCH OF ACTIVE HVAC SOLUTIONS ................................................................................................... 31 4.
CONVENTIONAL AUTOMOTIVE HVAC SOLUTIONS .................................................................................. 33 4.1
4.1.1 MECHANICAL VAPOR-COMPRESSION HVAC SYSTEM .................................................................... 33
4.1.2 ALTERNATING CURRENT (AC) VAPOR-COMPRESSION HVAC SYSTEM ............................................ 33
4.1.3 IMPACT OF HVAC SYSTEM USAGE .................................................................................................. 34
4.1.3.1 Fuel/energy consumption and equivalent CO2 emissions ..................................................... 34
4.1.3.2 Tailpipe emissions .................................................................................................................. 37
4.1.3.3 Greenhouse gases and footprint impact on the environment .............................................. 37
4.1.3.4 Thermal comfort and air-flow distribution issues of HVAC ................................................... 40
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4.1.4 POSSIBILITIES FOR IMPROVEMENT OF CONVENTIONAL HVAC SYSTEM ........................................ 42
4.1.4.1 Electrical instead of mechanical driven compressor .............................................................. 42
4.1.4.2 Alternative refrigerants .......................................................................................................... 42
EXPERIMENTAL ALTERNATIVES OF AUTOMOTIVE HVAC SYSTEMS ......................................................... 43 4.2
4.2.1.1 Air cycle .................................................................................................................................. 43
4.2.1.2 Absorption systems ................................................................................................................ 44
4.2.1.3 Metal hydrides/ Chemical heat pump ................................................................................... 45
4.2.1.4 Pulse/Thermoacoustics .......................................................................................................... 45
4.2.1.5 Thermoelastic......................................................................................................................... 45
4.2.1.6 Duplex Stirling cycle heat pump ............................................................................................. 46
4.2.1.7 Membrane heat pump ........................................................................................................... 46
4.2.1.8 Thermoelectric devices .......................................................................................................... 47
4.2.1.9 Evaporating/desiccant system ............................................................................................... 48
4.2.1.10 Magnetocaloric system .......................................................................................................... 49
PASSIVE SOLUTIONS FOR HEAT TREATMENT ............................................................................................ 50 5.
VEHICLE THERMAL CONSERVATION (INSULATION) ................................................................................. 51 5.1
5.1.1 BODY PANEL THERMAL INSULATION (HEAT BARRIERS) ................................................................. 52
5.1.1.1 Aluminium foil ........................................................................................................................ 53
5.1.1.2 Polyethylene/polyester insulators with additional aluminium foil layer ............................... 53
5.1.1.3 Carbon-fiber based insulations .............................................................................................. 54
5.1.1.4 Phase Change Materials (PCM) insulation ............................................................................. 55
5.1.1.5 Color significance ................................................................................................................... 57
5.1.1.6 Solar reflective paint .............................................................................................................. 58
5.1.1.7 Aerogel blanket ...................................................................................................................... 60
5.1.2 WINDOWS THERMAL FILTERING: GLAZING AND TINTING ............................................................. 61
5.1.3 SHADES ........................................................................................................................................... 65
5.1.3.1 Windshield and windows sunshade covers ........................................................................... 66
5.1.3.2 Side/rear curtains and blinds ................................................................................................. 67
5.1.3.3 Car tents and external shields ................................................................................................ 69
LEAVING THE WINDOWS DOWN (CRACK) ............................................................................................... 71 5.2
SOLAR POWERED VENTILATION .............................................................................................................. 72 5.3
VENTILATED SEATS .................................................................................................................................. 74 5.4
ANALYZING DATA PROCESSES ................................................................................................................... 77 6.
RESULTS AND RESEARCH OUTCOMES ..................................................................................................... 78 6.1
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CONCLUSIONS ......................................................................................................................................... 80 6.2
DISCUSSION AND SUGGESTIONS FOR FURTHER DESIGN SOLUTIONS ..................................................... 82 6.3
REFERENCES .............................................................................................................................................. 83 7.
BIBLIOGRAPHY ........................................................................................................................................ 83 7.1
WEBPAGES .............................................................................................................................................. 85 7.2
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Introduction 1.
1
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PROBLEM DEFINITION 1.1
Nowadays the automobile is one of the most important transportation and spread
vehicles for a lot of people compared to public transport. Since its debut in the end of 18th
century, the gasoline cars have being improved and modified, enlarging their role for the
human life, becoming an asset that every modern family or individual needs to own.
Meanwhile the extensive usage of these vehicles has brought an enormous change to the
The high demand of the private
transportation causes multiple circumstances in multiple terms of traffic organization and
available spaces for parking not only in the downtowns of the big cities. This leads to
alternative ways for parking, one of the most popular of which is leaving the car at opened
parking space outside, in a lot of cases without any sufficient shadow nearby, which is
critical in the summer time.
In hot weather, when the ambient air hits over 40°C, such exposure to the sun radiation
causes multiple further consequences for the car and its passengers. The amount of heat,
received from the intensive sun radiation activity leads to highly uncomfortable and
disturbing warmth, which is incompatible with the sense of comfort and even dangerous
for the driver and the passengers once they enter inside the vehicle. The reached heat is
harmful and even fatal for any left belongings, including pets as well.
Hitting more than 72°C in a summer day, the interior of a car becomes as a real oven.
year in the United States (GTSpeed, 2015).
From another point of view, the comfort inside the cabin of vehicle is expected by
default from the user. In the high-end class of vehicles, where the buyer is ready to pay
more than 60-100 000 for a luxury car, is inadmissible to have such disturbing factors as
Fig.1 Vehicle
exposure outside
a hot and clear
sunny day
Figure 2 (GTSpeed, 2015)
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the unpleasantly high temperatures. Of course, the luxurious and higher class vehicles
already have multiple installed features as an option. But this does
all so effective in terms of sustainability. An example of this could be the remote accessed
Heat, Ventilation and Air-Conditioning (HVAC) system of most high-class vehicle
manufacturers, which start working several minutes before the passenger enters the
vehicle in order to reach the desired internal air temperature without suffering from the
over heat.
This significant amount of heat, which the vehicle gains from the sun activity in the hot
weather, affects directly on the fuel consumption of every gasoline, hybrid or electric
vehicle, because further usage of the HVAC system consumes a lot power. In general, a
reliable car nowadays is expected to be driven for at least 500 km before another easy and
quick refueling. In addition the vehicle is made to be capable of keeping up with the
overall flow of traffic, which additionally increases the fuel consumption. Thus the tailpipe
smoke and burn emissions from gasoline vehicles have lead in greatly pollution of the
environment and is believed to be one of the leading source to recent dramatic change in
climate.
RESEARCH OBJECTIVES 1.2
Since the first implementation into the vehicle, throughout the evolution steps which it
has made up to nowadays, the development of the interior thermal comfort maintenance
with different solutions is object of unstoppable evolution. The main purpose of this thesis
work is to examine the potential and the opportunities of the alternative systems and
solutions reliable for this thermal comfort of the occupants. The directions of the research
topic are:
giving a guideline in improving and developing the overall thermal comfort of
the occupants in terms of better temperature and heat distribution inside the
vehicle compartment;
limiting or reducing the influence of factors leading to excessive usage of the
HVAC system and thus minimizing the effect of its circumstances;
exploring possibilities of reduction the heat load in a vehicle in warm weather
through investigation, comparing and highlighting the most effective existing or
conceptual solutions above them;
analysis of their advantages and disadvantages as a general impact of improving
their further development.
Highlighting the reasons and main related factors for extensive rising of cabin
temperature, when the vehicle is closed and parked and under direct sunlight, is critical for
further analysis and proposal of proper and effective solutions for ensuring safety and
normal thermal conditions in vehicle compartment. The existing risk of damage and harm
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children or pets left by chance in the vehicle or property and belongings, is enough
powerful motivation for the manufacturers to qualify the problem as a not only thermal
discomfort. Inside of the vehicle cabin, the users get in touch with the circumstances of
thermal misbalance, led by the exposition of the vehicle compartment to the intensive
solar energy (heat) high temperature of inside breath soak air, extensively hot surfaces as
dashboard, steering wheel, consoles, burning seats (especially leather ones) and other
cabin objects, lack of airflow and fresh air.
elopment, as seen from the evolution of the
HVAC design, is led by the user needs in such conditions, which are related to their first
instinctive reactions:
eliminating the subtractive factors for the heat;
applying temperature reducing methods;
avoiding immediately entering inside the car if possible.
Taking action in this proper order is a key point for a fastest, safest and less disturbing
way to remove the heat from the internal compartment and to use the car without further
thermal comfort disorders.
of view in two main groups:
instantaneous;
preventive
The heat reduction devices and solutions are being classified in terms of their action to
the given temperature of the vehicle interior. The active solutions are designed to manage
directly with decreasing of the environment temperature and act simultaneously, while
the passive ones are preventive or just limiting the heat increase, so their action is indirect
and generally not related to the present moment.
Appropriate design improvements and practices will be essential to help generalizing
more objective and fair overall model pattern, which creates future opportunities of
improving and revolutionizing of temperature management in the cabin comfort via
alternative and environmentally friendly solutions for cabin HVAC systems. The final
conclusions will consider the diverse priorities for future development and improvements
in this area in order to give a reliable direction for new design concepts and solutions.The
right oriented proposals for such of improvements are essential for revolutionizing the
comfort inside the vehicle cabin and the overall work of the HVAC system, making it much
more efficient. Reducing the indirect pollution made from its usage will decrease its
footprint index to the environment with closer to zero values and thus helping to the
efforts of mankind for a balanced nature.
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METHODOLOGY 1.3
In this research will be analyzed several main aspects related to the continuous
development and evolution of the interior thermal comfort technology in vehicles.
Starting from the main factor the solar load and its influence over air-temperature,
continuing to the consequences related to them, which are directly affecting the physical
and psychological perception of the human body for comfortable conditions. The
definition of thermal comfort is going to be presented, and issues in terms of its
application, related to the drivers and passengers of motor vehicles, will be examined.
Thus will be revised already made models, testing methodologies, and applying current
standards. The leading purpose in the development of this research work is the sustainable
and ecological aspect of assuring comfort temperatures in vehicle cabin. After deep
analysis it will be assumed that even simple design steps and improvements such as good
cabin insulation, window sunlight tinting/glazing, solar reflective paint or conserving
temperature devices/solutions could be essential key points in reducing the amount of
internal heat, further less usage of the HVAC system and assuring the comfort traveling of
the passengers, without negative affecting of the environment. This will directly cut the
overall vehicle consumption coming from smaller amplitude in cabin temperature
between operating/ non-operating HVAC system, which will increase and assure the better
comfort in the cabin as well it is essential for the driver, passengers, pets and goods.
In the active and passive products research sections will be investigated the solutions
and technologies for thermal comfort coverage. It will be examined multiple literature
surveys and research papers, based on study cases, scientific publications and reports,
including multiple scale model experiments, temperature distribution investigations,
passengers and cabin heat experiments, existing HVAC efficiency tests and calculations as
source of a deeper research on the topic for better understanding the main challenges in
providing effective cooling in vehicle cabins nowadays. Experimental reports from
developing concepts will be also analyzed. It will be considered the user perceptions and
existing habits for solving problems related to the thermal comfort area. Thus will be
examined different scenarios of overheat prevention, but also different possibilities for
maintaining cooler temperatures inside of the car cabin in hot days, leading to evaluating
the opportunities and to find possible alternatives and assistance to the existing HVAC
solutions. The variety of researched possible solutions will be examined in terms of overall
temperature/ heat reduction of the vehicle cabin, coefficient of performance (COP),
feasibility and user interaction point of view and cost for implementation.
In the final section, based on the realized findings from the research process, will be
made conclusions and further suggestions. There will be proposed alternative ways in
solving the problem that could have the potential to change the further development of
the automotive HVAC systems, thus making vehicles much more effective, eco-friendly,
but also much more comfortable for their passengers.
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Analysis of the heat 2.factor
3
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There are several factors that cause a parked vehicle interior to become that hot almost
like an oven place. Based hese include its exterior and interior
surface colors, their materials, the type, shape, angle and size of the windows, and also the
size of the passenger compartment.
According to numerous experiments, including a detailed one of Atkinson et al. (2015),
the sun radiation (Figure 3) is considered as a main factor on elevating the vehicle interior
temperature even on mild temperature days (Atkinson & Hill, 2015).
Due to the necessity of reduction of the environmental poluion made of the usege of
fosil fuels (directly from combustion engine vehicles or indirectly by producing energy for
charging electric vehicles), the car manufacturers have been taken in consideration the
aerodynamic aspect of the vehicles deeper in development of every new model for less
airflow resistance and thus reduction of the overall vehicle fuel consumption.
Since the modern cars are designed to have better air-dynamic performance, but also
increased perception for the surrounding environment and overall visual comfort, their
windows use an extensive amount of glass. It is coincidence of the lower-mounted angle
of the windshield and windows, which shape have become more oval, thus including more
space, which directly leads to a bigger impact of sun energy entering the interior of the
vehicle. The implementation of the panorama windshield and roofs nowadays (Figure 1,
Figure 4) is a common design solution, which affects all interior surfaces that can be
subjected to direct sunlight.
Of course, the mounted glass in a lot of the modern cars, specially those on the
rooftops and bigger windshileds, has been tempered with particular technologies in order
to reduce their impact of gained solar light and energy from the solar load. Therefore,
multiple experiments confirm the influence of the window size and angle over the
soak temperature in vehicles. An experiment of Atkinson (Atkinson & Hill, 2015) gives a
Figure 4 Modern vehicle windows
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good idea of the effect of window properties (Figure 5) over 7 vehicles, produced in 1985.
It could be seen the huge difference in the average breath temperatures in the cabin of
several different model types of cars, especially the gap between Mercedes 190 and
Chevrolet Camaro of more than 10°C. The key point of understanding and managing the
GREENHOUSE 2.1
The solar load is consisted of ultraviolet (UV), visible and near-infrared (NIR) radiation.
can be found in conditions where the short wavelengths of
Figure 6 Transmittance of conventional cabin windshield
Figure 5 Windows type affecting the breath soak temperature
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visible light pass through transparent environment and being absorbed after that (Figure
6). However the longer wavelengths (750-1000nm) (Figure 7) of the infrared waves, re-
radiated from sun-warmed objects are unable to pass through the environment since it
cannot transmit them effectively.
The trapping of the long wavelength radiation in vehicle compartment is the main
factor in increasing the heat of the interior objects and air (Figure 6), causing drastic higher
resultant interior temperature since the car is left completely closed as an empty taped
glass jar
compartment.
The short infrared waves are slightly transmitted through the clear glass which is
proved by the warmer surface of the glass when it is exposed to solar light. The long
infrared radiation waves (mainly in the far infra-red band above 5µm) are being 100%
blocked. Even ordinary float glass is practically opaque to radiation with a wavelength
higher than 5µm. Short wavelengths of visible light are readily transmitted through the
transparent
of ultraviolet light are largely blocked by glass since they have greater quantum
energies which have absorption mechanisms in the glass, which could be proved even if
a person is uncomfortably warm with bright sunlight streaming through, he will not
be sunburned (Nave, 2015).
CAR CABIN HEAT ACCUMULATION AND THERMAL DISTRIBUTION 2.2
The temperature of car cabin can rise drastically if the vehicle is left parked outside
without shade. This is due to the materials which the automobiles exterior is made from. In
Figure 7 Spectrum wavelength transmittance of conventional glass
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order to be enough strong, rigid, cheap, easy to maintain and reliable, almost every car
chase, body parts, panels are made from metal, mainly steel sheet panels. The windows are
commonly produced from glass and in some rare cases from transparent thermoplastics.
Since the steel is one of the best thermal- and electro conductors, this reflects directly to
the temperature distribution from the warmed body panels by the solar load into the
other parts of the vehicle compartment, which are in contact with the metal chase. Since
the engine is not working, it cannot provide effective ventilation in order to decrease the
rising amount of heat from the interior. The level of heat from the solar energy is so big
that it can damage some internal parts, left property and harm children or pets left in the
car, even in a short time of 10 min. As described in Section 2.1, the direct sunlight over
vehicle is converted by its windows from solar radiation into long wave thermal radiation,
. The already entered sunlight is absorbed and re-radiated
from the warmed interior air, objects and surfaces, and thus being trapped inside with no
way to escape (Figure 8).
The process of warming the cabin interior is consequence of 3 processes, attributed to
conduction (air volume inside), convection (presence of metals and heat absorbing
materials inside) and radiation (from the glass and body of the car by the sun). Thus the
internal soak temperature amplitude of the vehicle is directly related to the amount of
thermal radiation exchanged between the ody and the environment, but also to
the radiation absorbed and eradiated by the cabin compartment.
Multiple experimental, numerical and simulation tests and investigations of different
studies and researchers show the exact process of increase and diffusion of accumulated
heat from the cabin, its further consequences to the exposed surfaces and the regularity in
the temperature distribution. The optimal and hottest possible scenario could be seen in
the research report of a team from Baghndad, Iraq (Aljubury, et al., 2015) as a refference.
Figure 8 Heat load from the solar activity
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Their experiment investigates the change in the breath soak temperature of a vehicle
cabin, which is left under hot for couple of hours. It gives a lot and
precise information (Figure 9) about the effects of solar radiation on car cabin components
(steering wheel, dashboard, seats, inside air temperature) in an intensive case, where the
ambient temperature reaches 43°C. Orienting the vehicle to face south ensures tracking
the sun most of the day with maximum (thermal) sun load from the front windshield the
biggest glass surface in most of nowadays . Taking in consideration the average values
of the actual , the proposed exploitation of energy loads and heat
transfer into the cabin can be examined accurate and dependable.
The results from measurements (Figure 9) show the progress of change and maximum
temperature values of front dashboard, inside air, front seats and internal air in the parked
car. The temperature inside the car rises in the morning more rapidly than the ambient.
The rising in the first one hour is the highest compared to the other periods, which shows
the impact of - the visible radiation is transferred into thermal
infrared re-radiation. It is also observed that the temperature of the measuring points cool
in the afternoon faster than the ambient. Reasonably, around 12:00h the values of the
temperature reach its maximums due to the maximum value of solar load of 954 W/m2
available around 13:00h. The dashboard has the maximum temperature of 99°C between
components due to the largest projected area of glass facing the solar rays, the
dark color and the big exposed surface.
The results of another experimental research of a university team (Al-Kayiem, et al.,
2010) could be used as confirmation of the overall result from the numerical simulation
they are publishing. It gives much better illustrative information of how the temperature is
Figure 9 Temperature variations into unshaded parked car
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distributed inside the common cabin interior. It is noticed that the highest temperature
spots are close to the front and rear windows (Figure 10). The warmest air regions are near
the top roof and near the windshield glass at the front and the rear, confirming almost the
same temperature distribution pattern from the experiment of Aljubury et al.(2015).
It can be assumed that the experimental results indicate the dashboard as a
functioning thermal sink of solar radiation and source of convection heat, which is
transfered to the adjacent air particles and directly affects the cabin air temperature (Al-
Kayiem, et al., 2010). Elimination of this factor will improve the overall effect of less usage
of HVAC system and further fuel consumption.
Figure 10 Temperature simulation of a 3D vehicle cabin
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Thermal comfort 3.research
11
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Taking in consideration that thermal comfort is that state of mind the individual
expresses satisfaction in relation to thermal environment and is assessed by subjective
evaluation (ASHRAE Standard 55, 1992), achieving and maintaining it inside the cabin is a
fundamental ergonomic aspect for customer satisfaction in any kind of vehicle. Through
the years it has become more important due to the increasing mobility of people, which
results into more time spent and distances overtaken by people inside the vehicles.
The thermal comfort is subjective perception for the surrounding environment, because
it is different for every individual. It is maintained when the heat generated by the human
metabolism is able to distribute at a rate that maintains thermal equilibrium in the body,
produced from the body stays in balance with the heat loss. However, as consequence of
changing factors, the balance is able to incline the body temperature as well. When
temperature changes above the scope of a human body automatic temperature
regulation, the body produces more heat than it dissipates, which leads to substantial
discomfort. If it is available a big difference between gain and lost heat from the human
body, there are even severe risks for human health and specially babies and children, but
also pets, deliberately left or inadvertently trapped inside a closed vehicle. If the body
temperature of an adult person exceeds 40°C, the chance of getting heat stroke or
hyperthermia is absolute real. The situation with the children is even worse
since they can reach and maintain dangerous body temperatures much faster.
Despite the human perceptions of receiving feedback from the surrounding
environment are very sensitive, it is common issue that a thermally comfortable
environment generally is accepted unconsciously, but an uncomfortable and disturbing
one is recognized simultaneously, leading to immediate response of impatience and
further necessity of taking sudden actions in order to change the conditions.
According to ASHRAE Standard 55 (1992) and Fanger (1972) the heat balance of a
human body is affected by six environmental and personal key factors (Figure 12) air
temperature, air motion, relative humidity, mean radiation, metabolic rate and clothing
insulation. These factors are always kept in mind in developing effective and precise air-
conditioning solutions. The t
to surrounding thermal environment, which makes ambience quality an important
criterion inside the cabin. It influences not only the thermal comfort inside the vehicle, but
it also decreases the risk of possible , led from
damaging and disturbing factors of environment perception. Every one of the six factors of
thermal comfort plays significant role in the overall perception, so analyzing them is
obligatory in providing a precise thermal comfort model and building a real perception for
the environment.
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AIR TEMPERATURE 3.1
The air temperature is an average temperature of air, which surrounds
the body, with the respect of location and time. The spatial average takes into account the
ankle, waist and head levels, which vary for seated or standing occupants (ASHRAE
Standard 55, 1992). Since the interior has a relatively small air volume, it is much easier to
be influenced by heat exchange or HVAC system airflow. It is also an inhomogeneous zone
with significant differences between soak air temperature and surface temperatures due to
the unequal exposure to the sun, respectively the area of the exposed surfaces of the
different components. T air temperature is taken as the average of several
positions measured if not specified.
Since humans create different temperatures at the different parts of the body, ASHRAE
Standard 55 prescribes 3ºC for the vertical air temperature difference between head and
ankle level. (ASHRAE Standard 55, 1992)
The air temperature alone is one of the leading and most sensible factors for thermal
comfort. Its change is able to easily manipulate the levels of perceptions for it in humans.
Thermal comfort and the related air temperature effect driver alertness. For example, in an
experimental study, examined by ASHRAE Standard 55 (1992), drivers of a moving vehicle
missed 50% of test signals at 27 with reaction times 22% slower than those at 21°C.
Figure 12 Thermal comfort factors
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MEAN RADIANT TEMPERATURE 3.2
The Mean Radiant Temperature of any environment is defined as that uniform
temperature of an imaginary black enclosure which would result in the same heat loss by
radiation from the person as the actual enclosure (ASHRAE Standard 55, 1992)
It is related to the amount of radiant heat which is absorbed or emitted from a surface,
depending on the of soaking, but from the emissivity of the surrounding
The precise measurement of mean radiant temperature is very complex and time
consuming in the case of vehicle interior, because it is consisted of the temperatures of all
surrounding surfaces and additional index factor for any of them. So it is introduced the
operative temperature, combining the effect of air and mean radiant temperature in a
given place, which could be easily and directly measured. In the mean temperature the
influence of the air velocity is neglected. It should be taken in mind that the operating
HVAC produces high local air velocities, which cooling effect will be neglected, and this
could result to false conclusions. (Zhou, 2013)
AIR VELOCITY 3.3
Air velocity is the average speed of the air to which the body is exposed, with respect
to location and time, without regard to direction. (ASHRAE Standard 55, 1992)
Since the HVAC cools or heats the interior space with concentrated air-flow, which
increases the velocity of it has an interconnection with many other factors such as heat
exchange and human physical activity. Controlling the relevant air-flow and speed is
essential for the perception, because the presence of large or irrelevant air motion is
leading to thermal discomfort to the most sensitive parts of the body as human neck and
head. According to ASHRAE Standard 55 (1992) the acceptable value for the air velocity is
between 0.1m/s~0.4 m/s. Due to the existing nature of temperature distribution in the air
and the nearest areas with higher temperature (bigger exposure to the sun) as roof and
windows, the higher levels of the interior have bigger temperature, thus the cool air
should be directed to the upper body parts. In the article published by Rugh & Bharathan
(2005) they introduce the equivalent temperature to express the combined effect of air
velocity, air temperature and mean radiant. Thus Madsen et al. (1986) explains that the
equivalent temperature is the preferred parameter for the evaluation of thermal comfort, if
there is presence of high air velocities. (Madsen, et al., 1986)
RELATIVE HUMIDITY 3.4
It is defined by ASHRAE Standard 55 (1992) that the ratio of the
amount of water vapor in the air to the amount of water vapor that the air could hold at
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the specific temperature and pressure Humidity inside the vehicle is influenced by the
evaporation of sweat from the human body in a warm temperature. Sweating is an
effective heat loss mechanism that relies on evaporation from the skin. Keeping the
relative humidity in the cabin within a proper range is essential in the hot weather.
However at RH>70%, the air has close to the maximum water vapor that it can hold,
which limits the evaporation and therefore the heat loss. The recommended level for RH is
between 30%~70% for automotive interiors. If the RH<30%, the dry air is going to lead to
uncomfortable dry sensation and discomfort. (Zhou, 2013)
Relative air humidity is directly connected with interior air temperature. In the related
research of Zhou (2013) the increase of temperature leads to decrease of humidity and
contrary. Additionally, when the difference between the inside temperature and outside
temperature increases, the maximum relative humidity decreases. There is a particular
zone of comfort, which is a ratio between air temperature and humidity (Figure 13).
HUMAN ACTIVITY LEVEL (METABOLIC RATE) 3.5
As the human generates internal heat, the amount of the generated heat generated is
assumed as a quantity called Human Activity Level. It is an index of the work intensity or
action performed by the subject, valued as Metabolic Equivalent of Task (MET). It is a
human personal parameter (1 MET=58.2W/m2). which is equal to the energy produced per
unit surface area of an average person seated at rest. According to ASHRAE Standard 55
(1992), which provides numerical rates for variety of activities, typically the value for the
driver is 1.2 MET and 1.0 MET for other passengers. As a refference, the resting condition is
normed between 0.7 MET for sleeping and 1.0 MET for seated position. For heavy physical
work and normal sports activities, values between 8 10 MET are reached.
Figure 13 Psycrometric diagram of relation air temperature and humidity
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CLOTHING INSULATION 3.6
Clothing insulation reduces the heat loss from the body and thus effecting on the heat
balance, which directly means that it could either keep the body warm or lead to
overheating. Normally, the thicker clothes have higher insulating ability (Innova AirTech
Instrument, 1997). Of course, this depends on the type of material the clothing is made
from. The insulation value of the cloth depends on the Clo unit value 1 clo = 0.155
m2*K/W. (ASHRAE Standard 55, 1992)
The skin temperature differs in different parts of the body. Clothing insulation increases
will prevent less heat loss, which means the differences between air temperature and skin
temperature of each part decreases as shown. (Zhou, 2013)
ADDITIONAL FACTORS FOR THERMAL COMFORT 3.7
The establishing of thermal comfort in vehicle cabin is consisted not only by personal
and environmental factors. The thermal sensation of the occupants is affected also by the
physiological and psychological aspect of their perception for level of comfort. These,
according to Zhou, include shades, glazing, coating, light intensity, sun load (see Section
2.1), acceleration, internal and external colors and size of vehicle. (Zhou, 2013) (These
factors and their influence over the vehicle and passengers are going to be analyzed
further in Section 5 as a part of product research).
Due to non-uniform conditions in interior compartment of vehicle and the variable
differences in thermal perceptions of every individual occupant, it is hard to be given exact
criteria of what is an optimal thermal comfort in numeric numbers. For this reason the
definite for optimal environment must satisfy the majority of the occupants, which
according to ASHRAE should be at least over 80% (ASHRAE Standard 55, 1992).
As Musat and Helerea (2009) have described,
ensuring temperatures of 20°C ÷ 22°C, as a result of air temperature, delimitation areas,
humidity and air velocity in accordance with the activity level and clothing insulation of
the occupants, (ii) by avoiding situations such as the occupants coming into contact with
very cold or very hot surfaces, (iii) by avoiding air currents. These requirements must be
(Musat & Helerea, 2009).
ASHRAE suggests temperature between 20°C~23°C for winter and 22°C~26°C for the
summer as a standard, which matches with the Psychrometric chart (Figure 13) (ASHRAE
Standard 55, 1992).
THERMAL COMFORT RESEARCH MODELS 3.8
The convective, conductive and radiative heat exchange inside a vehicle interior effects
the thermal environment, characterizing it as a non-uniform environment. It is mainly due
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to the action of solar load and HVAC system, which at the different part of the cabin are felt
different. In extreme weather conditions like hot summer the heat balance is influenced by
evaporation as well. The researchers have developed effective
methods for evaluating and collecting the necessary properties of the environment. So far
they are two a thermal comfort test and numerical simulation.
3.8.1 MODEL
The thermal comfort model, introduced by Fanger (1972) uses a time-dependent heat
balance equation between thermal heat developed by metabolism of a human body and
the heat transferred in environment (through convection, conduction, radiation and
evaporation). According to Fanger, the thermal comfort is able to be predicted if the six
with two
coefficients: PMV (Predicted Mean Vote) and thermal discomfort, analyzed by PPD
(Predicted Percentage Dissatisfied). They are based on the physiological processes that
underlie human heat balance and establish the thermal comfort levels according to
exposed to different thermal conditions and circumstances.
(Fanger, 1972).
The PMV index has a range between -3 to +3 (-3: cold, -2: cool, -1: slightly cool, 0 neutral,
1: slightly warm, 2: warm, 3: hot), seen on Figure 14, relevant to the human sensations for
cold and warm conditions. The point of thermal neutrality is pointed with PMV=0, where
the temperature of a human body (or rather that on its surface) remains constant over
time. The PMV model is used as a basis for most current standards prescribing methods for
evaluating thermal comfort in vehicles (ASHRAE Standard 55, 1992), (Fanger, 1972).
Figure 14 Fanger's model PMV scale
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The PPD index (Predicted Percentage of Dissatisfied) is associated with the parameter,
indicating the percentage of occupants under thermal discomfort. A PPD=10% index
corresponds to the interval between -0.5 a
PMV=0, about 5% of occupants are in discomfort. This means that in any case there will be
dissatisfied users. (Zhou, 2013)
Unfortunately, the PMV model was developed based on data from uniform thermal
environments. So it has limitations related to: (i) dynamic state, (ii) distinction between
local and whole-body thermal comfort, (iii) environmental particularities of the vehicle. it
represents the entire body as one object since the clothing is assumed to cover the entire
body uniformly. It does not distinguish between different parts of the body, which has
different sensation and local temperature and thus is unable to predict local discomfort. If
one side is warm and the other cold, the PMV model will calculate a zero thermal load
PMV=0, defined as a neutral condition. It is noted that the optimum value for thermal
comfort (PPD is 5% and PMV is 0) can be obtained only with automatic HVAC systems,
because of the influence of outside temperature. (Musat & Helerea, 2009)
3.8.2 THERMAL MANIKIN
Nowadays several tools were developed to apply on the research of thermal comfort
which makes the thermal comfort test much easier and more accurate. These tools include
thermal comfort manikin, physiological model and human comfort empirical model. Each
model has its function and provides accurate feedback while measuring. For instance the
Figure 15 Thermal automotive manikin
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advanced automotive manikin (ADAM) on Figure 15 is composed of 120 individually
controlled surface zones, which are connected with sensors and collect multiple type of
data. The physiological model is used to simulate the human body internal physiology
system and provide physiological responses as a real human. The comfort empirical model
predicts local and global thermal comfort based on the collected data from manikin and
physiological computes. Unfortunately, the cost of a thermal manikin and test are
relatively high for researchers (Zhou, 2013).
3.8.3 COMPUTATIONAL FLUID DYNAMICS (CFD)
With this method, the passenger compartment of a 3D model car (Figure 16) is set up as
figure shows, which adds the human body, a computational grid model, considering the
thermal radiation and adding the solar ray tracing, calculates by using CFD software. The
thermal comfort of passenger compartment is evaluated by equivalent temperature.
Dividing the human body into 15 segments, it calculates the heat exchange between each
segment and surrounding environment. This method provides a precise guidance for the
design and development of automobile HVAC and overall interior design. (Zhou, 2013)
(Theseus-Fe, 2008)
The rapid development of computer technologies and their cheaper to build virtual
models has overtaken the traditional thermal comfort test method and made this method
a primary for researchers. The factors in examining the thermal comfort are properly
analyzed in order to be implemented into realistic simulation human (manikin) and
environmental models (3D cabin), which are essential in predicting the possible change in
the cabin environment and passengers, thus improving the existing solutions for thermal
comfort. The aim is reduction of the negative effect of extreme environmental conditions.
Examining the impact on the thermal conditions within a passenger cabin by different
Figure 16 CFD analysis in the vehicles
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manikin simulation drastically reduces the costs and time compared to physical
experiments and thus providing reliable information for further product developments.
(Zhou, 2013)
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Research of active 4.HVAC solutions
17
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The main instantaneous method for climate control in the vehicle is its Heat, Ventilation
and Air-Conditioning (HVAC) system. Nowadays HVAC is a basic standard equipment
installed in almost every automobile (Figure 17). The demand for more comfortable and
luxury vehicular thermal environment has led to a promotion in vehicles thermal control.
For less than 60 years since its first implementation in vehicles (Swenson, 1995) HVAC
develops from high-end extra feature for luxury vehicles into a standard system for almost
every class vehicle sold nowadays. HVAC system is a technology for a precise climate
control in vehicule compartment. It is designed to provide constant fresh air (through
filtering system) and at the same time controlling the interior temperature by cooling or
heating, in order to meet the comfort demand of the passengers. It also plays additional
significant role inside interior safety considerations such as clearing the fog, mist and
moisture from the windshield and windows and thus providing adequate thermal comfort
in any condition.
HVAC includes three main functions heating, ventilation and air conditioning. They
are interrelated and work together in order to provide and ensure the occupants a safe
and comfortable temperature, good air quality in summer or in winter.
The fresh air enters into the vehicle through vents near the base of the windshield. The
air is drawn into the HVAC module by a blower motor and later directted to the heater core
in order to be warmed or through the air evaporator to be cooled. It continues then
directed by air flow controls to the area selected by the user toward the windshield, in
defroster mode, to the floor, in heater mode or through dash vents in HVAC or vent mode.
Figure 18 Air-flow distribution in a multi-zonal HVAC system
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The modern HVAC systems can have several individually ventilated thermal zones, with
up to 2 blowing motors, situated in the front and back compartment of the vehicle. In this
thermal zones, where the temperatures can vary between each other in order to satisfy any
Figure 18).
CONVENTIONAL AUTOMOTIVE HVAC SOLUTIONS 4.1
4.1.1 MECHANICAL VAPOR-COMPRESSION HVAC SYSTEM
Briefly, the basic working principle behind the most popular type of HVAC system is
exchange between conduction and convection of low/high compressed vaporized
refrigerant. Absorbed heat is transferred from a low-temperature area to a higher-
temperature area in the vehicle, which is caused by the pressure difference, called
refrigeration. The system is based on use of five major components: Evaporator,
Compressor, Condenser, Receiver/Drier, Expansion device (Figure 19). They are divided
into two pressure regions: high-pressure side, including a condenser and a receiver/drier
unit, and low-pressure side, with air conditioning evaporator. The separation between low
and high pressure is the compressor from one side and the expansion valve from the
other, situated as it is shown on Figure 19. (Gupta, et al., 2012)
4.1.2 ALTERNATING CURRENT (AC) VAPOR-COMPRESSION HVAC SYSTEM
The hybrid and electric vehicles have a little, but significant difference in the HVAC
anatomy. Since an electric motor powers the car when it is driven as an electric/hybrid
Figure 19 Components of mechanical HVAC system
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vehicle, the belt-driven compressor from the HVAC system is unable to run with non-
working combustion engine. Most of the hybrid and electric vehicles have a 200V AC
(alternating current) electric motor implemented into the compressor assembly. Thus it
takes the place and function of the belt-driven pulley found in the typical combustion
engine vehicles. There is also another difference since the
heated coolant from the engine. In this case the heat for the cabin comes from an
additional electric heater. All of this energy power used for the HVAC system is delivered
-voltage battery.
4.1.3 IMPACT OF HVAC SYSTEM USAGE
Since the HVAC system has become an inseparable part of ensuring thermal comfort for
automobile cabins, it needs to be very efficient and reliable system. The high-efficient
overall performance of the present system is capable to satisfy the occupants even with
drastic amplitude difference between the ambient temperature, internal breath air
temperature and user desired temperature in significantly short time (depending on the
condition). Of course, the high COP of 2.5 in vapor-compression HVAC (Clodic, et al., 2005)
has its coincidences, which means that there is an influence over other aspects of the
system like fuel/energy consumption, environmental impact, economic situation, thermal
and other factors.
4.1.3.1 Fuel/energy consumption and equivalent CO2 emissions
The vehicle HVAC system is sized to provide adequate cool down time for a high
cooling load even for extreme places like Arizona, United States or Baghdad, Iraq, where
the solar load can reach up to 1000 W/m2 and ambient temperature up to 44°C. According
to several experiments, including that of Aljubury et al (2015), such extraordinary weather
conditions can lead to surface temperatures of more than 100°C and cabin air
temperatures higher than 70°C. Thus the requirement for necessary energy power of the
cooling mode is significant.The combustion engine vehicles use a belt-
driven mechanical compressor to produce the necessary power, directly connected to the
engine. In this way it puts additional load to the motor when is working, which could rise
up to 5 kW (6hp).
A vehicle simulation of Farrington and Rugh (2000) has shown that although the impact
of the additional load is significant for combustion engine (conventional) vehicles, it is
much more critical for high fuel economy vehicles (HEV). From the experiment is visible,
that moment fuel economy of a nominal 37km/L vehicle could drop to about 21km/L if the
additive loads increase from 400W to 4kW (Figure 20). (Farrington & Rugh, 2000)
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The big additional energy consumption due to the HVAC usage in the HEV is
unacceptable. From the table is seen as well that even a minimal load of 400W on a
conventional engine can decrease the fuel economy by cutting 0.4 km/L (adding
0.01L/100km to fuel economy). (Farrington & Rugh, 2000)
According to the federal US tests FUDS (an urban driving cycle) and HWFET (a highway
driving cycle), the direct impact of the hybrid regime of a vehicle in motion has a big
influence over the fuel consumption. Combined with a higher energy load by the HVAC
unit, the overall distance range of HEV is significantly affected (Figure 21).
Though the HVAC compressor in hybrid/electric vehicle is controlled by an electronic
control module (ECM) (minimizing the electrical power draw), the air-conditioning usage
(Figure 22) has a significant negative effect over the fuel economy, reducing the mile range
Figure 21 HEV range simulation
Figure 20 Auxiliary load impacts on fuel economy
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of the vehicle with 9-22% at high-speed (129km/h), respectively with 16-38% in urban
driving with low speed and frequent start/stops. (Farrington & Rugh, 2000).
The additional fuel consumption due to HVAC operation mainly depends on climate
conditions Figure 22 shows calculated mechanical power required
by the mechanical compressor in conventional gasoline vehicle. It varies between
0.9 3.5kW, depending on the ambient temperature and the engine speed (Barrault, et al.,
2003). It is visible the significant increase in the energy consumption related to higher air
temperatures, when the compressor works on maximum rotation speed of 3500 rpm. The
biggest load is at maximum power, which happens when the occupants enter inside a left
under the sun vehicle and turn on the HVAC on maximum speed for cooling down the
temperature as fast as possible in order to not struggle from the excessive warm saloon.
The huge difference between real air temperature and desired comfortable
temperature lead to excessive amount of power loads necessary to compensate the hot
interior, mainly in the first 20-30min, when the control knob is positioned on Maximum
cool airflow.
Normally, the HVAC systems are not used the whole period of the journey, but in
warmer weather conditions its usage respectively increases. For instance in northern
Europe, where the temperate climate is moderate and the summer temperatures are not
passing over 30°C, the HVAC operate for about 24% of the vehicle running time (reference
with Frankfurt, Germany). Going on the south, where the climate of some regions is
subtropical, will increase the usage of the HVAC up to 60% in southern Spain and up to
70% in Phoenix, Arizona. For European diesel engines the additional consumption ranges
from 21,5 L/year (Frankfurt) to about 80 L/year (Sevilla). If climatic conditions, engine type
(diesel or gasoline) and user profile for thermal sensation are taken into consideration, the
annual additional fuel consumption is between 2.5 and 7.5% (Clodic, et al., 2005).
Figure 22 Internal temperature vs. compressor power consumption
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Based on Fischer (1995) research, the annual fuel required to run the additional load of
HVAC system is about 12.7 liters per vehicle. Given the above assumptions, the estimated
total fuel used for air-conditioning is about 40 billion liters of gasoline annually, and this is
only for the United States. (Fischer, 1995)
4.1.3.2 Tailpipe emissions
From a published investigation model of Farrington & Rugh (2000), (Figure 23) is visible
the increase in tailpipe emissions depending on the engine modeled for a conventional
vehicle in the Federal SC03 test drive cycle more than doubling the CO and NOx, when
the baseline load without HVAC is sat as an auxiliary load of 500 W. Despite the significant
model variation in the test of HVAC use, where the net coefficient of performance (COP) is
sat as a product of the HVAC COP and the compressor efficiency, the results confirm the
fact that smaller and high economy engines suffer from bigger consumption/ tailpipe
emissions when they are overloaded.
Using the Federal SC03 test, Farrington measures the effect of the air-conditioning
system on fuel economy and tailpipe emissions for a variety of vehicles. The average
increase (Figure 23) of CO2 with 0.42g/km and NOx with 0.053g/km in HVAC operating
mode take a big impact to the 19,300km driven annually, with HVAC operating 45% of the
time. In this case it is assumed that vehicle air-conditioning usege increases CO2 by 655
ktonnes and NOx by 82 ktonnes only in United States (in case 80% of the light duty vehicles
has HVAC system installed) (Farrington & Rugh, 2000).
4.1.3.3 Greenhouse gases and footprint impact on the environment
Figure 23 Predicted Increase in tailpipe emissions and fuel consumption
SC03 test
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Another sufficient disadvantage, related to the HVAC usage in vehicles, is its direct
effect over the atmosphere. It is already well known and proved that during the last two
decades the O3 layer of the atmosphere is slowly, but effectively destroyed because of the
wide usage of refrigerant (Freon) for the refrigeration and air-conditioning purposes,
which are fundamental part of working principle of these systems. The refrigerants used in
the automotive HVAC are Chlorofluorocarbons (CFC), Hydrochlorofluoro-carbons (HCFC)
and Hydrofluoro Carbons (HFC) (Figure 24).
According to report of Clodic et al.(2005), the properties of three of them have similar
distructive and/or harmful effects to the atmosphere. The CFC refrigerant (also known as
CFC-12) has the highest ozone depleting rating among the three and is a greenhouse gas
as well. That leads to total ban nowadays from use or production of this gas in all countries
covered by the Montreal Protocol. Unfortunately there are still a lot of old produced
systems in operation with it. The HCFC (CFC-22) refrigerant has a potential to damage
ozone (rating 0.05) and is also a greenhouse gas. There are still many systems utilizing
these refrigerants, as well, nevertheless it is also banned for use and production since 2015.
The HFC gases (also known as HFC-134a) are used extensively in every day RAC systems.
Actually, there is no current ban upon these gases but it is subscribed mandatory presence
of responsible usage and regular equipment inspections under the "F gas" regulations.
(Clodic, et al., 2005)
The HFC refrigerants (the latest category) do not have ozone depletion potential, but
unfortunately, they act as a greenhouse gases as well. Implemented for a very first time at
1990, by 1994 almost all vehicles, including cars, light commercial vehicles, and truck
cabins sold and manufactured in developed countries use this refrigerant. This is a
circumstance of a big environmental conserving step the implementation of the
Montreal Protocol from 1989, which represents a global decision of switch from CFC-12
Figure 24 Depleting O3 and greenhouse potential of different types of refrigerant
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refrigerant to HFC-134a one. The decision is taken by all car manufacturers in order to
decrease the harmful progress of depletion of the O3 layer of the atmosphere.
The further development of the regulations is effecting globally the overall emissions
from vehicle HVAC systems range from approximately 105 ktonnes in 1993 to 180 ktonnes
(Figure 25), where the most harmful CFC-12 gas emissions decrease drastically from 102
ktonnes to less than 4ktonnes at 2015. The increase of the HFC-134a and the alternative
HFC-152c (similar to CO2) is directly connected to the substitution of CFC-12 and the
further expansion of the HVAC devices as a standard equipment of the modern vehicles.
From the same study (Clodic, et al., 2005)(Figure 26) it can be seen the result of the
Montreal protocol involvement in the CO2-equivalent emissions of the Freons. It leads to a
Figure 26 MAC CO2 eq. refrigerant emissions in kt from 1990 to
2015
Figure 25 MAC refrigerant emissions in kt from 1990 to 2015
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significant decrease in CO2-eq. emissions from about 850 Mtonnes in 1990 down to 270
Mtonnes at 2015. The reason is not the increased HVAC production fleet of the
effect of the much-bigger CO2 equivalent of the CFC Freon respectively to the replacement
HFC-134a gas. With the Fluorinated Gas regulation project of European Commission it is
proposed the phase-out of HFC-134a, thereby creating a totally new context for refrigerant
choices (Clodic, et al., 2005).
4.1.3.4 Thermal comfort and air-flow distribution issues of HVAC
Since the microclimate in the interior of a vehicle is very inhomogeneous environment,
due to varying radiation from the sun and breath air temperature, but also from the air
velocity from the HVAC ventilation, this creates a climate that may vary considerably in
space and time. (Ivanescu, et al., 2010)
The vehicle HVAC system has the aim to provide a precise control of the cabin air
temperature and humidity in order to provide effective thermal comfort for the occupants.
In Section 3 are examined and analyzed all of the factors related to this parameter,
including the sun position.
According to an analysis of Ivanescu et al (2010), which investigate the effect of solar
radiation over the further surfaces and air temperature, but also air-distribution flow
generated by the HVAC system and their influence over the thermal comfort of
passengers, in both highway and city drive mode. (Ivanescu, et al., 2010)
In the results, shown on Figure 27, is visible the effect of the sun radiation into the
surface temperatures after 1hour of driving without HVAC working. The cool-down
working time of HVAC system is shown for 10, 20 and 30 min after its start (respectively 70,
Figure 27 Distribution of the cabin temperature at 60, 70, 80 and 90 min
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80 and 90 min of the experiment). The most affected from the heat surfaces are the
dashboard and the parts of the seats, where the sun light is lighting directly, reaching
more than 85°C.
Since the equivalent temperature (ET) is considered as an accurate predictor for thermal
comfort in cabin compartments (Hintea, et al., 2013), in the experiment is simulated the
distribution of ET on the different body parts as well. From Figure 28, which shows the ET
distribution of the driver on the left and the passenger on the ride side, could be seen, that
the considerably high up to the 60th minute of the experiment, when
the HVAC is turned on.
about sensation for the environment, regarding PMV/PPD index database, Ivanescu et al.
(2010) conclude that the back passenger reaches a better thermal comfort state earlier
than the driver. After 20 minutes the PMV index of 1.06 for passenger and the value
obtained of the driver is after 25 minutes. (Ivanescu, et al., 2010)
There are another results from similar article, studied by Lee (2015), according to whom
achieving the comfort parameters in the shortest time, the best place for installing
additional ventilation diffusers is behind the rear seat passengers (Lee, 2015).Thus it is
obvious the need of different way for providing adequate temperatures in the cabin
compartment. In most of the modern nowadays this issue is taken into consideration,
where the vehicle HVAC system provide air distribution from several positions situated
around each passenger, creating zonal airflow division (described in Section 4.1). Thus the
parameters temperature and humidity becomes more flexible, because every occupant
can select the optimal combination for himself without affecting directly the other
passengers. Thanks to estimating models of the level of passenger comfort perception
allows generating the exact amount of energy needed, rather than wasting additional
energy by warming-up or cooling-down the whole cabin to a certain set-point
temperature. This feature is implemented into the automatic air-conditioners with
multiple sensors. Future development of the precise position for the vents is going to help
to improve the existing airflow distribution HVAC systems.
Figure 28 Distribution of the local equivalent temperature on each segment of manikin body
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4.1.4 POSSIBILITIES FOR IMPROVEMENT OF CONVENTIONAL HVAC SYSTEM
4.1.4.1 Electrical instead of mechanical driven compressor
As already mentioned in Section 4.1.1, the mechanical compressor of the HVAC system
in the conventional vehicle with combustion engine is directly driven by the engine and
thus making the speed of the compressor equal to the speed of the crank shaft, which
cannot be controlled. An alternative solution already exists in big vehicles like buses and
trucks, which use an individual internal-
compressor. Something similar, but much more effective and environmentally friendly is
implemented into the hybrid/electric vehicles. An AC compressor is driven with electric
current directly from the alternator, connected to the battery of the vehicle (as explained
in Section 4.1.2). This difference makes the AC compressor independently working device
from the combustion engine (in hybrids) and the electric engine (electric vehicles), which
means that it can work much smoother and with no relation to the engine loads
accelerating, decelerating, start, top, low or high motion speed of the vehicle.
In experiment of Dahlan et al. (2014) he replaces the conventional belt-driven
mechanical compressor of a gasoline car with AC compressor, connected directly to the
12V battery of the vehicle (compared to the 48V battery of the most electric vehicles). The
further comparison between the two types of compressors for HVAC is illustrated in Figure.
However, as seen from the figure, there is a significant limitation for the AC compressor,
which is
the AC compressor in the perfect conditions can provide less than 1/3 of the conventional
cold impacts. Nevertheless, it shows much better practical COP, which is related to the
least losses in the electric engine compared to the mechanical driven process. The
comparison is related more to the possible solution in future for implementing an
adequate AC compressor instead of mechanical into the gasoline cars Thus the work of the
HVAC will be much more efficient, with lower fuel consumption, better temperature
distribution as the comparison shows.
4.1.4.2 Alternative refrigerants
Before implementing any alternative systems into mass production, it is much more
efficient, cheaper and fast to redesign or optimize the work of the already existing and
developed technology of vapor-compression systems, which generally uses HFC-134a
refrigerant. The optimization of improved HFC-134a can decrease considerably the CO2
and refrigerant emissions released in the atmosphere.
According to Clodic et al. (2005), part of the possible modifications could be eliminating
leakage of the harmful refrigerant, improved COP of the entire cycle, higher efficiency and
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better service in recovering, recycling and end-of-life recovery. Switching to an alternative
and non-harmful refrigerant, such as HFC152a or CO2, is also reasonable method,
nevertheless that CO2 HVAC need around 5 times higher pressure than the existing HFC-
134a freon. A provisional increase of the cost for improved HVAC system is around 40-150
higher for a CO2 HVAC (mainly because of the more powerful compressor) and around 40
for the HFC152a operated, since it uses the same parts as HFC134a (Clodic, et al., 2005).
EXPERIMENTAL ALTERNATIVES OF AUTOMOTIVE HVAC SYSTEMS 4.2
There are a lot of different alternative technologies to the conventional vapor-
compression one, used in mobile HVAC system. The main concern is that most of the
alternatives are used in another fields like buildings, aerospace, ships, heavy trucks, trains,
etc. since they are big, heavy, some of them with need of constant water supply. Since
some of them are only prototypes on experimental and development process level of
implementation, limited production rate or impractical in use with automotive light
vehicles, they will be just specified as possible alternatives to the HVAC system. There is no
coincidence of using mainly the vapor-compression principle it is reliable, high-
performance and suitable in both cooling/heating providing modes and it can fit into the
vehicle compartment. It is need a lot of development and research work to make any of
these technologies more efficient and practical for mass-production vehicles. There is a list
of the most promising of them, mentioned briefly.
4.2.1.1 Air cycle
As one of the first implemented methods for air-conditioning cycle, the air cycle
machine acts as its own working fluid, eliminating any ozone damaging chemicals. The
COP of the system, however, is rather low at approximately 1.5. (Moran & Shapiro, 1988)
Figure 29 Air cycle HVAC system for trains and airplanes
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The air enters into the system (Figure 29), where is directed to a compressor, rising the
temperature of the air and the enthalpy of the air increases by the amount of work put into
the compressor. The heated, pressurized air is directed to a heat exchanger and cooled to
ambient temperature. The cooled air is then expanded through a turbine, further cooling
the air both by expansion and by removing some of its enthalpy as work to help drive the
compressor. The system is consisted of relatively few and cheap parts, excluding the
expansive compressor and turbine. The need of large air-to-air heat exchanger makes it
difficult to locate inside a vehicle, so the system is used in airplanes and trains.
In an analysis by Bhatti (1988) he concludes that the amount of CO2 emitted per year is
in the same range (303 kg CO2 for the air cycle compared with 282 kg CO2 for current HFC-
134a systems). (Bhatti, 1998)
4.2.1.2 Absorption systems
The absorption systems use ammonia water or LiBr-water as working fluid. It is well
known in the building absorption chillers (Figure 30), commonly used in Japan up to
nowadays because of their relative high COP=2. Thanks to the fact that ammonia is natural
chemical, it is non-harmful for the environment, but is toxic if exposed to humans. These
systems use heat for the process of evaporation of the fluid, but they need electrical
energy for fans and pump. In the case with vehicles, the absorbing parts are installed over
the exhausting pipes, which are wormed-up by waste heat of the combustion engine
working. The main disadvantage of the system for vehicle HVAC usage is the heating-up
time (around 30mins) necessary for reaching the temperature for effective process of
evaporation. (Clodic, et al., 2005)
Another serious issue is the amount of heat, required to drive the system, which is
bigger than the waste heat produced by a normal automotive engine, especially from
Figure 30 Absorbtion system for building appliances
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high-fuel economic vehicle. Thus it requires a separate heat source to be added to fulfill
the gap (Charters & Megler, 1974), which makes it non-effective. It is large and complex
and it needs many individual components that should be assembled and maintained.
4.2.1.3 Metal hydrides/ Chemical heat pump
The metal-hydride or chemical heat pump uses the heat of a chemical reaction to
provide the cooling effect without compressing a working fluid. The used metalhydrydes
(lanthanide and transition metal) absorb and desorb large amount of H2, the main
principle in the system. (Sheft, et al., 1979)
The system uses available heat from the exhaust gas and no refrigerants, which makes it
relatively ecological. It is consisted of low number and simple, no moving parts, but with
big dimensions and weight. The COP is significantly lower, between . The fitting of
the metal hydride/chemical heat pump inside a limited car compartment will be an issue.
With its relatively high COP, the system could be competitive using strong energy
conservation measures. (Multerer & Burton, 1991)
4.2.1.4 Pulse/Thermoacoustics
Sound waves create small temperature oscillations, which release or absorb heat. If the
are used as a regenerator, a significant temperature difference can be realized between
the hot and the cold ends. This heat pump works with helium-based fluid using high-
amplitude sound waves to compress and expand the gas, thus generating a temperature
gradient. A resonant cavity is used to enhance the efficiency of thermo-acoustics systems.
A COP up to 2 has been achieved in a laboratory test. However, the integration of the
system into cars could be difficult due to the possible size of system needed for a capacity
of about 3kW. Researchers have developed several prototypes for refrigeration
applications, but it cannot be found proofs for working model. (Clodic, et al., 2005)
4.2.1.5 Thermoelastic
This system stresses and releases an shaped memory alloy (SMA) or special polymers to
absorb heat from the supply air and expels heat to exhaust air. By altering the timing
sequence or circulation, the thermoelastic system could supply space heating as well. The
energy saving potential due to the high COP, significant non-energy benefits and cheap
simple design is making them potential successors of the traditional systems, but still a lot
research and development work need to be done in order to be made working mass-
produced products. This technology is in early stages of research and development level,
with a working prototype development for HVAC applications. (Goetzler, et al., 2014)
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Researchers have demonstrated thermoelastic systems with temperature differentials of
17°C and potentially leading to COP up to 3 or greater with further improvements. (Cui, et
al., 2012)
4.2.1.6 Duplex Stirling cycle heat pump
Stirling cycle heat pump involves four internally reversible processes in series, powered
by a mechanical piston/cylinder arrangement, driven by an electric motor or belt-driven
from the combustion engine. This system uses the mechanical energy from a gas-fired
Stirling engine to compress and expand helium as a refrigerant. It is transferred between
two exchangers, a hot one for carrying out the heat from the engine and a cold one that
removes heat from the inside air compartment delivering it to the engine.
This equipment consists of a hot heat exchanger which transports rejected heat from
the Stirling engine to the ambient air and a cold heat exchanger that removes heat from
the air inside the vehicle and transports it to the Stirling engine. Since the use helium gas is
safe for the environment and the system works with a high COP of 2.5 this is one of the
most promising alternative systems (Figure 31). However it is a complex and expensive to
produce, containing a large number of precision machined parts which present a potential
maintenance problems with sealings and mechanical properties of the helium used into
the system. These particular concerns are both for repair and helium replacement.
(Multerer & Burton, 1991)
4.2.1.7 Membrane heat pump
This relatively new and revolutionary air treatment method known as membrane heat
pump uses nanotechnology to eliminate the need for refrigerants use. It provides cooling
Figure 31 One side of a Stirling cooler/heater HVAC
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and dehumidification by transferring moisture across a number of membranes using a
vacuum pump and harnessing energy using nanoparticles.
According to Dias (the manufacturer of the prototype named NanoAir®) the system
shown on Figure 32 produces 30BTU/h of cooling energy compared to 13BTU/h per W to
current HVAC unit. This will decrease more than twice the electrical consumption,
producing 57% less CO2 emissions, excluding the CO2-equivalent emissions of not using
the harmful HFC-134a refrigerant. (Nanogloss, 2011) (Dais, 2017)
The used parts and operation processes are simple, which means that the whole unit
will be installed at almost the same or lower cost than the current systems because it uses
regular HVAC parts. The main concern is the necessary supply of water, which makes it
impractical in implementing inside vehicles.
4.2.1.8 Thermoelectric devices
In thermoelectric materials, electrical energy can be directly converted into thermal
energy and thermal energy into electrical energy. These materials provide air-conditioning
with conversion between electrical into thermal energy (Figure 34) thanks to two
important thermoelectric effects: the Seebeck effect, referring to the existence of an
electric potential across a thermoelectric material subject to a temperature gradient and
the Peltier effect, which refers to the absorption of heat into one end of a thermoelectric
material and the release of heat from the opposite end due to a current flow through the
material. The thermoelectric modules (TED) are commercially available in low-capacity
and low-lift applications because of their low COP up to 0.6. The high-efficiency TED tech-
Figure 32
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nology, which is suitable for HVAC applications, is still under development. There are
solutions for zonal air-conditioning, using the TED technology (Figure 33).
4.2.1.9 Evaporating/desiccant system
Evaporative coolers use water or special solid material to absorb sensible heat from
airstreams, which evaporate the water and cool the air. These systems have been
commercially available for decades for use in hot-dry climates, but mainly for domestic and
building appliances (Figure 35). Unfortunately, they have achieved very low market
penetration because of their inability to meet moisture removal requirements at all times
(even in hot-dry climates). They suffer from a huge water consumption (water based
solution) and they dehumidify only (solid solutions, which require a supplementary system
to remove sensible heat), complex for installation and supply of water, maintenance
concerns, which makes them inapplicable for automotive solutions. (Goetzler, et al., 2014)
Figure 35 Desiccant-enhanced evaporative cooler
Figure 34 Peltier element working principal in
Figure 33 TED zonal HVAC module
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4.2.1.10 Magnetocaloric system
Since its implementation in 1933, the magnetic refrigeration system is used mostly in
the past for conducted adiabatic demagnetization experiments, producing cooling at
source temperatures from 269.65°C to 272.65°C. (Gauger, 1993)
It is a promising alternative to traditional refrigeration systems based on the gas
compression. The technology uses magnetocaloric effect (MCE) which consists in the
variation of the internal energy of paramagnetic materials, as the result of the magnetic
field variation. This system s main advantage is the lack of usage at all of greenhouse gases
and others pollutants, showing higher energy efficiency than the vapor-compression
system. (Wikipedia, 2017a).
The main disadvantage of the system is the weak supply of rare-earth magnets and
their high price, which are potentially significant barriers to market adoption. The need for
an electrical system will be a major penalty for mobile magnetic refrigeration applications
in terms of system complexity, size and weight, maintenance, and useful life. (Gauger,
1993)
In 2015, Cooltech Applications said it had produced a 150-700W product as part of a
refrigeration system (Figure 36
such as supermarkets, in 2015. (Yebiyo & Maidment, 2016)
Figure 36 Prototype of magnetocaloric unit from Cooltech
Applications
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Passive solutions for 5.heat treatment
37
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One of the possible directions for reducing the high amount of heat in vehicle
Thus
limiting the entering solar energy, which increases the internal temperature, can be made
in numerous ways, including solar reflective glazings or tinting of windows, solar reflective
paints, ventilation, thermal insulation and window shading. Thus the further generated
heat loads by exposed car-objects will be significantly reduced as well, which directly
reflects to the internal air temperature and further time for cooling the passenger
compartment. This directly will empower saving potential of the overall HVAC system since
it will achieve desired level of thermal comfort within a shorter period of time, which
depends directly on the absorbed heat from the vehicle compartment and the cool-down
time of HVAC operation.
VEHICLE THERMAL CONSERVATION (INSULATION) 5.1
According to the principles of heat transfer between objects in thermal contact, the
exposed parts of the vehicle shell windows, roof, body panels will absorb most of the
energy from the solar load, in different proportions, depending on the thermal
conductivity of the materials which they are made from (Figure 38). This will directly
increase the internal soak temperature of the interior since there is no other barrier
between the body parts and the interior air. As circumstance this corresponds with
increase of the temperature of all interior surfaces and objects as well.
If a thermal insulation product is putted between the interior and the exterior of the
vehicle or exterior and surrounding environment, this is going to reduce the heat transfer
with decrease in thermal conduction between them. Regarding the different properties
and fields of affection caused by the solar load, there are multiple solutions for thermal
insulation of any of the different kind of body parts or all together with one product.
Figure 38
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5.1.1 BODY PANEL THERMAL INSULATION (HEAT BARRIERS)
As the roof is the biggest and directly horizontal positioned part of almost every vehicle,
it absorbs the most loads
steel metal which is heated from the sun with solar radiation of up to 1000W/m2 in a clear
summer day. According to an experiment of Purusothaman et al. (2017), the amount of
absorbed heat energy from the roof varies between 20 and 95% (Figure 38), according to
the conditions and materials. Air temperature measurements, found in experimental and
investigation research works, revealed that the highest air temperature are present near
the roof (ceiling), which is direct consequence of the absorbed energy by the rooftop.
Most of the efficient thermal insulation products like polyurethane, polystyrene and
polystyrol are used in the insulation of refrigerators, warehouses, buildings, campers,
yachts, mobile homes, etc. Their high efficiency and low cost is the main factor of making
them relatively common and competitive. As any other material, the level of insulation of
these materials is directly related to their thickness as thicker they are better level of
insulation they offer. Unfortunately, they are not suitable for the automotive interiors,
because they require thick layer of several cm in order to be effective; they are not flexible
as well, which cannot fit to the requirements of the curve-shaped car body panels (Figure
39) and limited space inside the cabin components as roofs, door panels and floors.
Generally, there are only few known thermo-insulation types of products, which are
used for thermal (and some cases sound) insulation into the automotive vehicles. They
work throughout the whole year reduce the interior temperature in the summer, but
keep it warmer in the winter. The most suitable and effective thermal insulations for the
automobile interiors nowadays among the variety which are offered, are aluminium foil
layers, polyethylene/polyester based foams with textile fiber and aluminium foil
integrated, carbon-fiber based materials, aerogel and Phase-Change Materials (PCM).
Figure 39 Insulation layers implemented in luxury class vehicles
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5.1.1.1 Aluminium foil
Available as a product in a lot of fields, the aluminium foil is used as a thermal barrier in
cooking, computer hardware, electric parts, etc. The strange fact is that the aluminium by
its nature is a metal, with relative high conductive heat and electric parameters. The
aluminium reflects only visible light, so it absorbs over 90% of the thermal heat (Innovative
Insulation, Inc., 2017). The key process is actually happening into the air space. Since the air
is a bad conductor, the absorbed heat from the already hot aluminium is distracted into
the air and thus decreasing its temperature.
The technology, shown on Figure 40, is already used by some of the car manufacturers
like Nissan, because it works effectively with significant temperature conservation inside
the interior of the vehicle, and it is a cheap solution. A thin layer of aluminium is attached
to the roof trim of the car and a hollow layer of air space (25mm) is set between the roof
In this case losing/gaining cold/warm loads from the outside
environment becomes harder, which reflects in less frequent and more effective usage of
the HVAC, respectively decreased fuel consumption. (Nissan Motor Corporation, 2017)
5.1.1.2 Polyethylene/polyester insulators with additional aluminium foil layer
The insulation has closed cell foam core and layer of thin pure aluminium foil cover on
one or both of sides, reinforced with textile (Figure 41). It makes s interior not only
much more thermally comfortable, but quiet and with better performance of the
audiosystems, because they are used for sound deadeners from the environment. In order
to face the human needs inside the cabin, they are made from non-toxic sources, prevents
condensation, thus not causing rust and absorption of moisture, which prevents the
Figure 40 Aluminium foil roof insulation in Nissan Leaf
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creation of mold and mildew. The flexibility of the polyethylene/ polyester insulators
makes them easy to handle, cut, shape, flex and install into vehicle interiors (Figure 42).
According to the producers, they are able to block up to 97% of radiant heat transfer
due to its high thermal resistance R=1.3 and high nominal temperature rating of around
250°C, they are used in professional racing cars as well, where the conditions are even
harder than in conventional vehicles. Relatively cheap (starting from 9 /m2), they are one
of the most widely spread and preferred methods for thermal and sound insulation.
5.1.1.3 Carbon-fiber based insulations
This kind of insulation seems similar to textile wool, but it has much higher functional
parameters than the polyethylene/polyester insulation, regardless its thin (3.2mm) and
light (0.54kg/m2) structure. This is mainly because it is based on carbon-fiber material,
Figure 41 Polyethylene/polyester insulator with
aluminium foil Figure 42 Insulated roof interior
Figure 43 Carbon-fiber insulation material and its temperature resistance
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reinforced with wool. Thus it could be installed almost everywhere. According to the
manufacturer (Heatshield Products Inc., 2017), the carbon fiber shield could withstand
continuous up to 982°C, which is much more than needed for solar load insulation. There is
almost no information about experimental tests, except one only. It shows extremely
efficiency (based on the low conductivity of 0.03 W/mK), where the direct measurement of
the temperature from the side of a heated metal roof is compared to the side with the
shield installed, detecting decrease between 41% and 67% (Figure 43). According to the 2 (150-
not so affordable, but instead of that with high efficiency characteristics (Heatshield
Products Inc., 2017)
5.1.1.4 Phase Change Materials (PCM) insulation
The Phase Change Materials (PCM) have the ability to absorb the heat and store it
within the material itself, changing their phase from solid to liquid when they absorb heat
and convert back to a solid state when they release heat during the process of melting and
freezing.
They are becoming more and more attractive nowadays because of their high
effectiveness of collecting big amounts of thermal energy in variety of temperatures range
from - 40°C to more than 250°C. They are able to store between 5 and 14 times more heat
per unit volume than traditional materials like water, masonry or rock. Among various heat
storage options developed for different thermal requirements and implementation to
applications, PCMs are particularly attractive because they offer high-density energy
storage and store heat within a narrow temperature range.
The PCM is initially at solid state. As the environmental temperature increases, the
material absorbs the energy in mean of heat in large amount at an almost constant
temperature. When the ambient temperature reaches the melting point (particular for
every type of PCM), this leads to higher rate of heat absorption, leading from solid phase to
liquefaction, which continues until all the materials is melted and converted into liquid
phase. In this way the heat is stored in PCM and the temperature is maintained in a level
that is optimum for the material. When the temperature around the material starts to get
low and reaches the melting point again, the material starts to solidify as it releases the
heat back into the atmosphere.
Based on different temperature and heat capacity requirements, nowadays there are
several known and developed kinds of PCM materials water-based (ice, gel-packs), salt
hydrates (inorganic), petroleum based (paraffins) and biomassed PCMs(biodegradable).
(Entropy Solutions LLC, 2017)
An experiment of Purusothaman (2017) with an integrated layer of PCM (1-dodecanol)
into a normal roof structure of a vehicle (inserted between the insulator layer and the
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upper metal sheet of the roof) shows the potential of the material as a thermal barrier
compared to a normal non-insolated metal roof. The PCM arrests the heat of the roof
within itself without allowing the external heat to enter into the interior compartment and
prevents further internal air and objects temperature increase, which effect could be used
with a great success into the vehicle industry.
The PCM material (1-dodecanol) used in the experiment has maximum effectiveness up
to 259°C. With a phase change between 24°C and 27°C, it starts to melt when the air
temperature passes through this interval. The results from the experiment of
Purusothaman et al. (2017) are shown on Figure 44. The measured values of both interior
temperatures are result of a considerably permanent ambient temperature (between 22-
27°C) and clear sky. It could be seen a big difference between the conventional non-
insulated roof interior and that one insulated with PCM. The material maintains smooth
and light line of with regard of time in the actual temperature thanks to its heat absorbing
nature. The difference between the temperatures is the biggest around 14:00h, when the
intensity of the sun is highest and the ambient temperature is around 25°C. The reduction,
due to the better thermal distribution into the insolated roof, corresponds in 13°C of
difference in favour of the PCM roof.
According Phasechange Energy Solutions (one of the producers of such products) the
PCM material is able to reduce the HVAC consumption between 25-35% (for buildings).
(Phasechange Energy Solutions, 2017)
The PCM materials have different properties, performance, environmental impact and
specially prices starting from 9 /kg for unencapsulated row material up to 1000 /kg for
encapsulated layers, depending on the material for the PCM cells. The high price of some
Figure 44 Temperature variation comparison with time
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of them is one the only limit for wide usage, but throughout the spread of the material as
an alternative solution, the prices are going to fall down.
From the experiment of Purusothaman et al. (2017) could be summarized that an
alternative design solution with integrated PCM layer into the roof could be implemented,
which will significantly decrease its temperature. The better thermal performance will
correspond up to 30% energy savings for the HVAC system or even more in windy and
motion car conditions.
5.1.1.5 Color significance
It is well known that the different painted color surfaces have different thermal
absorption of the solar energy due to properties like solar reflectance and thermal
radiation acceptance for every color. This is the reason why dark colours of instrument and
control panels, seats, steering wheels, but mainly exterior color grow unbearably hot
Figure 45
Figure 46 Solar waves absorption by exterior paint
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under the scorching solar radiation. All dark surfaces that are exposed to sunlight heat up
strongly, while light surfaces remain distinctly cooler.
Black surfaces usually absorb up to 90% (Figure 46) of incident sunlight and convert it
into heat, therefore getting much hotter in terms of minutes than light surfaces like white,
which absorbs only up to 25 % and tend to stay much cooler. (BASF, 2017a)
In an experiment between black and silver painted car (Figure 47) Levinson and
assocciates compare the roof, ceiling, dashboard, windshield, seat, door, vent air and cabin
air temperatures after exposing the vehicles for a whole day under the sun. The autors
confirm that darker coloured cars are warmer than lighter coloured. As expected, the
biggest temperature difference is described to be observed at the roof, where the black
painted car has a roof temperature of up to 24°C warmer than the silver car. It is based on
the difference of their solar reflectance and thermal radiation, respectively 0,05 and 0,83
for black vs. 0,58 and 0,79 for the silver color. The biggest difference of the interior surfaces
has the ceiling temperature with a peak decrease of 11°C comparing black and silver
interior. This automatically leads to 6°C difference in the average cabin breath
temperature. (Levinson, et al., 2011)
A direct relation between the lower internal temperature with the fuel consumption of
the HVAC system, in this case leads to an overall assumed reduction for the HVAC
consumption of around 24% and to further decrease in the fuel consumption as well.
5.1.1.6 Solar reflective paint
Although the physical principles at first appear irrelevant, there are already existing
solutions of innovative IR reflective pigments and preparations (Figure 48), which make it
Figure 47 Comparison between black and silver paint maximum
temperatures
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possible to create surface coatings with significantly reduction of the heating effect in
sunlight, despite their color.
Carbon black, which is the most commonly used black pigment for paints, strongly
absorbs the invisible near infrared (NIR) radiation that accounts for more than 50 % of total
incident solar energy. In contrast, the solar-reflective black pigment (Figure 48) reflects or
transmits the same radiation with very low levels of absorption. They soak the visible light
completely, like any conventional black pigment, which does not change the optical
perception of blackness
percent, whereas the advanced pigment paints achieve TSR values up to 30% even for
black or very dark colors. (BASF, 2017b)
Solar-reflective pigments can be added to any conventional color (for internal or
external use), adding heat-reduction benefits. Since the carbon black pigment is contained
Figure 49 Comparison between conventional and NIR reflective colour
pigments
Figure 48 pigments
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in different percentage also in most of other color shades, adding such pigment will
reduce significantly the thermal loads from solar radiation.
To achieve the desired solar reflectance, from BASF use an organic transparent or
inorganic reflective substrate as a base and add a layer of TiO2 to overcoat the organic and
empower the inorganic substrate. As a result (Figure 49), for the lighter color shades are
achieved even higher TSR values, reducing the overall paint absorption significantly. In a
direct comparison, the improved TSR performance results in a temperature reduction
between 8 and 20°C difference between standard color and modified paint. The white
overcoat with TiO2 has lowest temperature from all the samples with up to 14°C less than
the other overcoat colours and up to 30°C less than the carbon black. The benefits of using
NIR reflective pigments, which main purpose is reduction of the NIR waves absorption, are
flexibility of the colors, lower cooling power from the HVAC, enlarged lifetime of the
coatings through the reduced temperatures and overall improved sustainability from the
above factors. (BASF, 2017a)
5.1.1.7 Aerogel blanket
The material Aerogel is an ultralight nanomaterial, developed by NASA, generally made
from nanoparticles of silica gel, where the liquid nano-drops are extracted and replaced
with nano-sized filled pores of gas (Figure 51). This results in a solid material with
extremely low density of 1200g/m3 (the same as the air) and extreme thermal conductivity
of up to 0.012 W/mK due to the amount of enclosed gas, which acts as an insulator (Figure
50 , but making
the material very brittle (Figure 53) and vulnerable to impacts. (Wikipedia, 2017b)
Reinforcing with fibers (ceramic or carbon) aerogel becomes much more resistant to
impacts, flexible and durable, with a little impact on its conductivity increased to 0.013-
0.015W/mK. It is available to the market as a sheet insulation layer blanket (Figure 52) with
Figure 51 Microscopic image of Aerogel Figure 50 Aerogel's ultimate insulation
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different thickness and additive compounds for different final results. The implementation
of the aerogel from aerospace material into commercial product automatically makes it
the material with lowest thermal conductivity of any known insulation alternatives for
mass production nowadays.
Aerogel based material has a big potential in almost every field, because its excellent
thermal resistance, enhanced acoustic insulation, light weight, non-toxic contents,
reduced thickness, fire and water resistance are suitable for a lot of applications and
improvements. Implementing it as a competitive insulation material in some fields is still
limited because of the high price, based on the complicated and expensive production of
the material.
In the sector of automotive thermal insulation an aerogel blanket with thickness of 3-
6mm could be used as a barrier installed between the roof and internal ceiling, thus
protecting the interior from the excessive heat of the roof. There were no found official test
results for usage in automotive interior, but generally the aerogel reduces the direct
temperature loads around 10 times. With a convenient price of 18 /m2 and low weight of
200kg/m3, the material is a competitive alternative for effective and ultimate protection
from solar loads.
5.1.2 WINDOWS THERMAL FILTERING: GLAZING AND TINTING
Since the implementation of laminated glass technology into automotive industry in
multiple aims in car cabin comfort and security fields. There are multiple study cases and
existing literature on confirming the role of the acrylic glazing and tinting upon glasses,
which affect not just the visual comfort, privacy, sound insulation and overall security (in
case of braking of windshield), but has much bigger impact over the thermal comfort of
Figure 53 Aerogel in pure form Figure 52 Aerogel carbon reinforced blanket
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the passengers, filtering mainly infrared, partly visual and ultraviolet light (Figure 54). Films
that block infrared and visual light keep the interior cool on sunny days. Blocking some of
the visual light provides privacy, while blocking UV light protects the skin and cabin
surfaces from the adverse effects of solar exposure.
The infrared-reflective (IRR) automotive glazing is an example of advanced automotive
technology, which prevents infrared radiation from entering the vehicle cabin by reflecting
it back into the environment. Results of several experiments show that IRR windshields
consistently reduce cabin and interior surface temperatures. This effect is increased when
IRR glazing is also applied to the side and rear windows. The use of IRR glazing has been
shown to reduce HVAC workload, thus assisting to reduce A/C compressor usage and/or
size. Based on the researches, IRR glazing has been shown to be more efficient than
Figure 54 Transmittance of solar-
Figure 55 Transmittance and absorption of window glasses
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infrared-absorbing glazing (a widely used solar-control glazing). (Devonshire & Sayer,
2002).
Since it is an acrylic layer, integrated between the glass layers (Figure 56), the glazing is
integrated permanent solution. The glazing of windows is used in a lot of car models with
different level of light transitivity.
On the aftermarket exists another similar solution as well, named window tint. It is a
plastic layer thick a part of mm (Figure 57), which is made to be installed by sticking over or
under the factory glass. It is characterized with Its Visible Light Transmission (VLT)
coefficient, defined as the percentage of solar visible light (daylight) transmitted through a
glazing system in which the lower the VLT percentage, the darker the tint (valid also for
glazing). The addition of solar control glazing to the glass or installing tinting layer, almost
always reduces the transmission of light in the visible range, even if only marginally, which
Most types of window glazing or additional tinting
film layers (Figure 57) have lower VLT (darker tinting) coefficient due to the metallic/
dielectric nature of their coatings. Like the tinted spectral-absorbing glass used on many of
metallic nano-particles used in most low-
range price IRR coatings can affect the transmission of visible light (Mori & Koursova, 2000).
Figure 58 Image of cyclist as seen through un-tinted windows (left) and tinted (right)
Figure 56 Sungate glazing structure Figure 57 Tinting film layer
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Thus reducing the VLT level may severely affect the visual performance of the drivers,
especially the elder ones (75+) and mainly in the twilight/night period. The possibility of
missing an obstacle, pedestrian, bike-, motorcyclist in the darker part of the day at higher
VLT coefficient is relatively bigger (Figure 58). A confirmation of the effect of VLT increase
(Freedman, et al., 1993) is an important step in limiting the windshield VLT transmittance
up to 70% regulated in the law requirements of the United States and respectively 75% in
European Union requirements for vehicles.
According to another research report (Shi, et al., 2008) the optimal VLT level which do
not affect the visual acuity and glare response, is 75%. Since the visible transmission is a
crucial consideration for advanced glazing technology most of the nowadays available
advanced quality glazed windshields (Sungate®) and tinting films (3M Crystalline®) filters
the solar light almost without affecting the visibility (even at 90% VLT film).
These filtering solutions allow only 3% of the infrared (IR) and 33% of the total solar
energy to be transmitted through the glass, most of it in the visible spectrum, thus
rejecting more than 50% of the total solar energy (Rugh, et al., 2006).
Since the window tints are consisted of several types, based on the materials they are
made from, they could be: dyed polyester films (standard); two-layer (high performance)
films combining dyed and metallic films; carbon and ceramic window tint, made from
nonmetallic nanoparticles, relatively small, which are even hard to distinguish under a light
microscope, to not affect the visual performance of the user.
The carbon-ceramic tints of 3M® Figure 59) are close to the highest performance
possible within the constraints of the visibility requirements (without darkening). The glare
reduction of 22% is an additional advantage of the additive tinting. (3M, 2016).
Figure 59 Carbon-ceramic window tint's principal
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An issue of using low-transmittance (VLT<70%) tints from the lower class (dyed and
metallic films) is the possible interference problem in using cell phone, smartphone, GPS
navigation or satellite radio inside the car since the increasing concentration of
metallic/dielectric nanoparticles in the layers rises with the solar filtering. This
circumstance is eliminated in high-performance ceramic and carbon tints without metal
particles, which cannot interfere with the reception of electronic signals.
An experiment of Isa et al. (2015) with different percentage VLT tinted windows reports
that the VLT level has a minimal effect in reducing the soak cabin temperature (only about
0.5°C) as infrared rejection. Instead of darker tinting with lower VLT (i.e. visible light
rejection), infrared rejection plays a more significant role in reducing the heat penetration
into the vehicle. (Isa, et al., 2015).
In direct comparison, both solutions succeed to reduce the temperature inside the
cabin (Figure 60), mainly because of the insulation effect which is made to the windscreen,
side the interior.
According to Farrington et al. (1999a), the realized decrease in the temperature could
result to the overall fuel consumption between 3,4 and 4,5%. (Farrington, et al., 1999a)
Adding the price for an OEM glazed Sungate® windshield of around 200 and the price for
high-quality tinting (between 200 and 60 per complete tinting of a vehicle), for the price
of the glazed window it could be installed full tinting with different percentage VLT
shading. Both solutions are proving that they resist against the solar heat and more
specifically to the dashboard, respectively steering wheel.
5.1.3 SHADES
Shades provide real insulation from direct solar load through the windows. They could
be several different types, depending on the solution they are realized and mounted
Figure 60 Reduction in temperatures with Sungate windscreen and tinting
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externally or inside, integrated or additionally putted to the car. A lot of tests and
investigations, including that of Al-Kayiem et al. (2010), support the statement that those
interior spots, which are exposed directly to transmitted solar radiation, accumulate much
more thermal energy than the spots subjected to convective heat transfer. (Al-Kayiem, et
al., 2010)
The shades are represented by different design solutions, varying from type to type and
from internal to external mode of use, but their easy, simple to use and relatively low price
makes them very attractive for a first in mind thermal isolating solution.
5.1.3.1 Windshield and windows sunshade covers
It is proved from multiple tests and research approaches over the heat distribution
inside vehicles, the dashboard is considered as a functioning sink of solar radiation and
source of convection heat (Figure 10), which is transferred into the cabin compartment by
the air particles. Isolating the windshield, which is the main reason for excessive
dashboard, steering wheel and front seats surface temperature and overheated breathe
air, will cut the direct solar radiation and further heat air distribution around the
windshield. This as well is one of the easiest and simple solutions to reduce vehicle
compartment temperature. Covering the windshield with a reflective shade, mostly
common attached to aluminium foil coating, reflecting the visible light and absorbing up
to 90% the radiant heat.
The sun shade installation underneath the front windshield reduces the thermal
accumulation considerably with about 27% inside the car cabin (Figure 61), according to
Al-Kayiem et al. (2010). It decreases the maximum temperature of the dashboard surface
Figure 61 Front internal windshield cover
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from 75°C down to 50°C. Consequently, all the measured temperatures in the front part of
the interior have shown lower values compared with unshaded windshield as well, but
with less cooling effect on the rear windshield and rear ambient air.
The experiment of Aljubury et al. (2015) confirms the simulation of the work of Al-
Kayiem with even higher measured parameters (Figure 62), due to the higher solar
intensity in his experiment. According to him, the windshield shade does not decrease
reasonably the ambient temperature (just 1-2°C), but affects in drastically reduction of
dashboard 40% and steering wheel temperature with 28%. (Aljubury,
et al., 2015)
An advanced insulation to the windows can be achieved with overall shade installation
to all of the windows and particularly to the back windshield, which is similar as angle and
area to the front windshield. However the process of building the shade, from
of view, will take much more time and effort if the shades are internal, since every of them
need to be fixed to a window. Additional (even flat) space should be provided into the
trunk to be stored and kept always on board. The windshield shades starting price of 5
makes them one of the cheapest way to protect a dashboard and steering wheel from
excessive high temperatures and gain additional degrees on seats around 4°C.
5.1.3.2 Side/rear curtains and blinds
The shading of the windshield proves that the direct insulation against solar light has
significant effect to the front part of the interior and a smaller decrease into the overall
breath temperature. Since the sun enters not only through the front windshield inside the
vehicle compartment, optimizing the solar reflection could be achieved with covering the
rest of the windows as well. In this case covering the back window, which dimensions and
angle are closer to the windshield (thus allowing bigger amount of solar load to penetrate
Figure 62 Windshield shade mounted under the window
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into the cabin) will affect considerably the ambient air temperature into the back
compartment.
It is common for the high-end and luxurious class vehicles to have implemented
blinds/curtains for the side and rear windows as standard (Figure 37, Figure 63, Figure 65),
which makes them an expansive (250-350 for a rear roll) additional extra to other class
vehicles, but reasonable investment against the solar heat. The rollers are made of vinyl
polymers, but the curtains are from textile and sometimes from special fabric, including
wool or cashmere for finest ones.
There is a big choice of aftermarket rolling blinds and shades, which are considerably
cheaper, but not always fitting aesthetically with the interior or the size of the particular
window (Figure 64). A lot of the models are even electrically operated. Most of them are
universal, but those, which are made especially for a particular model vehicle, has better
window fitting and coverage, which reflects to better looking and increased overall
Figure 63 Luxury built-in side curtains Figure 64 Entry-level universal blind
Figure 66 Aftermarket side roller blinds Figure 65 Integrated rolling blinds
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thermal protection. The use of curtains/blinds isolates the direct propagation of solar loads
into the vehicle cabin and decreases the temperature of the interior compartment and
increases the overall comfort of the passengers.
Taking into consideration the opportunities of this type of insulation, there are couple
of researches over their or similar products efficiency. Close to the work of Al-Kayiem et al.
(2010) is an experiment (Figure 67) of Jasni and Nasir (2012), showing that using a
sunshade (placed on all car windows, not just on the windshield) could cool the dashboard
by an average of 22°C (with a maximum reduction of 30°C). It has no cooling effect on the
car ambient air. In the experiment the interior air is measured at front and back, showing
that actually it is with °C higher than without the shades. The used nylon material for
shading is similar to the vinyl of most entry-level blinds (Jasni & Nasir, 2012)
The experiment makes evident the fact, that the shades are useful to reduce the peak
temperatures of the surfaces, which is a big advantage, since the dashboard and
overheated saloon are acting as a heat sink to the rest of the vehicle elements and internal
air. Experiment surveys with higher class materials like textile or fabric are not found, but it
is assumed that their decrease in the temperature will be similar or slightly better in term
of temperature decrease and more smooth air distribution because they have bigger
accumulative surface. For this reason, the usage of these solutions will lead to direct HVAC
economy, because the time needed to cool down the air into the cabin will be much less
compared without the direct solar barrier.
5.1.3.3 Car tents and external shields
The external alternative to the blinds and curtains is the vehicle plastic cover, which
(Figure 69) for the vehicle exterior. Isolating the roof and the
windows with solar reflective material actually eliminate the chance of direct sunlight
radiation to access the interior of the vehicle.
Figure 67 Temperature decrease by shades to all windows
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The closest reference for their overall performance is an experiment of Aljubury et
al.(2015), where he puts sunshades over the cabin to simulate exterior shield like the
shown on Figure 70. The total insulation from the direct sunlight which provide such kind
of design solutions has its effect over the thermal parameters to all of the measured zones
and surfaces. Most visible (Figure 68) is the huge decrease of the dashboard temperature
and steering wheel, respectively 48 and 44°C reduction. The other referant temperatures
are also cooled with more than 17°C. (Aljubury, et al., 2015)
As it is seen from the experiment the general effect of the external shields/tents is
reduction in the overall temperature of the interior compartment. The effectiveness and
reliability of the different solutions depend from their design properties like type of used
material, technology for spreading, ensuring. In some cases they could have more issues
rather than advantages from the internal solutions, but according to experimental tests
with similar products, they seem to be one of the most efficient solutions for conservation
Designed to stay as a shield above the car they require preparations and montage/
demontage activities when exiting/entering the vehicle longer than the other solutions.
The tents cover shields start at 30 for half size (only cabin insulation) and 60 for a full
Figure 68 Temperature decrease in use of external shield
Figure 70 Solar shield as an external cover Figure 69 Electric solar "tents"
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90 and reach up to 200
LEAVING THE WINDOWS DOWN (CRACK) 5.2
Before HVAC system to become widely used and spread around the modern vehicles,
most of the people probably have tried to reduce the internal temperature of vehicle in
motion by lowering the front or rear window with couple cm. The method works in motion
thanks to the speed of the vehicle, which makes the static environmental air to enter as a
wind air-flow into the cabin, putting fresh ambient temperature air and reducing the hot
air inside the vehicle compartment.
Using the lowering of the windows in steady vehicle (Figure 71), however, is not
working the same way, nevertheless that it provide ambient air to the cabin. In the hotter
climate countries is not a rare situation for left parked vehicles outside. It is used widely
from a lot of people all around the world since it is assumed that this method helps to
decrease the internal hot air, but without any specific regardless information.
Measurements from a simulation test of Manning & Ewing (2009) provide concretic
results in the decrease of the interior temperature by leaving front and back lowered
windows with 4,5cm (Figure 72). According to their test this lowering has the role to
decrease the speed of heat distribution into the vehicle compartment around 2,6 times
rather than significant reduction into the internal air temperature. The maximum
reduction of the internal air temperature reaches 12,1°C from 65,8°C to 53,7°C, at 32,3°C
average ambient temperature. (Manning & Ewing, 2009)
Figure 71 "Cracked" window left for natural air-distribution
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The openings have caused a slow airflow circulation between the hottest air inside the
cabin, situated under the roof and the ambient air, thus achieving a reduction of the
internal air.
As already mentioned, the method is wide accepted and used by a lot of people, but it
has severe disadvantages: safety risk for the vehicle since the windows are not closed and
entering inside is easier; the wind airflow can put dust and other particles inside the cabin
through the opened space; in case of rain, the compartment will become wet from
entering of water inside. Based on these circumstances, this method is not proposed as a
competitive solution but just as a reference for other technologies of ventilation.
SOLAR POWERED VENTILATION 5.3
Figure 72 Temperature decrease with lowered windows
Figure 73 Toyota Prius and its solar ventilation system
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Taking in consideration the heat distribution of a vehicle and the opportunities which
the ambient air gives for natural recirculation from the previous Section 5.2 these could be
used to remove the internal entrapped hot air with fresh ambient tempered one. It has
become an interesting design innovation the implementation of a solar ventilating system
for parked and non-driven vehicle.
There are multiple experiments of enthusiasts, university institutions researches and a
lot of prototypes of big companies working mass production solution could be found in
several vehicle models nowadays (2009- Toyota Prius (Figure 73), 2008- Audi A8, etc.) The
idea in this design solution is to be used the blower of the HVAC system in a ventilation
mode without load of cooling power. The electric energy for running it comes from a solar
panel on the rooftop Figure 74
need of further recharge.
According to Ponce (2009), the innovative design feature delivers an average reduction
of 5,5°C in the interior air compared to non-working condition (from 35.5°C down to 30°C).
However, the high extra price for this ecological and innovative technology of
a big limit for entering of the system in the rest of the vehicles.
Figure 74 The solar panel is integrated into a glass rooftop
Figure 75 Alternative cheap solutions for solar ventilation
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Because of the implementation of the idea with solar powered ventilation, there have
been made multiple small alternatives to the solar ventilation system (Figure 75). However
these small devices cannot be a real solution for air ventilation because of their limited and
weak characteristics compared to the power of a sunroof FV panel.
In a comparison test of Rugh et al. (2006), where he prepare almost the same features of
the solar ventilation of Toyota, he installs into a car 12V-17W solar panel wtih 6x1,7W fans
to provide reasonable airflow. Depending on the design requirements, the blowers can
either vent hot air or pull in cool air through the roof while the car is parked. The details in
both cases are shown on Figure 76. It is clearly visible the big difference between the two
modes of operation of the ventilation the better performance with bigger temperature
reductions are in air blown out mode. The breath air temperature is reduced with 8,3ºC,
and the same is for the dashboard temperature. The seat and windshield are also with
lower temperatures, respectively 6 ºC and 2,3 ºC. The compensating air is being pulled in
through HVAC heater/defroster ducts and other body leakage areas. (Rugh, et al., 2006)
VENTILATED SEATS 5.4
Optimizing the use of HVAC system could probably seems inapplicable in hot weather
since the needs of cooling power in days is excessive. Based on the factors of thermal
comfort applied into the interior of a vehicle and specially the dependence of clothing
insulation of the passengers (already explained in Section 3.6), the human body is
insulated from the surrounding environment at a different level. This is mainly because of
the enclosed thin layers of air between the body and the clothing. Since the air is
considered as a bad conductor of heat, it is known as a really effective isolative material. In
the vehicle therefore, the thin layer of encased air in the clothing is squeezed from the
body In the points of contact with the seat when a passenger sits on it. This leads to
decrease into the cloth insulation in those areas and increase of the dependence from the
Figure 76 Solar ventilation system and its effect over interior temperatures
temperature
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seat which represents another insulator. Such assumption is very illustrative in extreme
cases, when the insulation of the contact zones are so effective insulated from one side
inevitable.
Such optimizing solutions are the ventilated seats (Figure 77), which are considerably
new technology, offered mainly in the high-class and luxury vehicles because of their
higher price. Thanks to them reaching of thermal neutrality prevents sweating and
uncomfortable sensation from the heated air and surfaces of the vehicle compartment
such as the warmed seat. Improving the overall cooling process of the body with the
implemented vents into the seats reduces significantly the uncomfortable time lag
between achieving conditioned air and further overall decrease of the surfaces during the
HVAC operation. This solution enhances the comfort level much faster due to their large
contact areas with the body of passenger. Thus they eliminate the insulation barrier with
the body and increase the evaporative cooling, preventing hot feeling and further sweat.
The two types of ventilated seats, which are offered nowadays: ventilated only and with
thermoelectric cooling/heating module implemented. The second one is more expensive,
but it provides a thermo regulated control for optimal temperature adjustment and fast
response.
An experiment of Rugh et al. (2006) confirms that state with collecting information from
ADAM . The experiment shows a cooler sensation for the
vehicle cabin environment due to the operation of ventilated seat. Based on the results of
decreased seat temperature with 4,7°C, which improves the overall thermal sensation with
Figure 77 Ventilated seat with 4 vents Figure 78 Main zones of body
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0.28, leads
to assumed reduction of 7,5% on the fuel use (equal to 1.97million tons of fuel only for the
United States) (Rugh, et al., 2006).
The ventilated seats have low energy requirements (in case they provide just an
ventilated ambient air from the cabin). They are effective in reducing the temperature, but
mostly successful in terms of user-related point of view (Figure 78)
to the thermal comfort needs of the passenger reflects to its overall competitiveness with
the other technologies, directly leading to higher thermal comfort level in less time and
less usage of the HVAC system. The relatively high price (between 1500~5000 per set of 4)
makes them available mainly in the high class luxurious and expansive car models.
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Analyzing data 6.processes
79
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RESULTS AND RESEARCH OUTCOMES 6.1
The main objective of the present work was to investigate the optimal and alternative
design
technologies for proceeding of such solutions in vehicles during hot environment weather.
Examining the different parameters and particular influences of the analyzed factors and
circumstances, which lead to extensive temperatures into the vehicle compartment, are
helping for better understanding s and concerns, and forming more
adequate ways of providing thermal comfort inside a vehicle. This is essential for future
developing of competitive alternative design solutions and products.
The results from the research show some already known information, but reveal as well
new technological states of art, based on new materials and conceptual ideas from
another fields of application, which can be derived into the automotive HVAC systems
evolution. They concern not only solutions and products, but factors and properties of the
process of heat distribution (from the sun into the vehicle) with biggest impact for
efficiency, which consideration has a key role in further development of successful and
competitive products
The two main categories, in which the product analysis investigation took place, are
active and passive alternative solutions for HVAC systems and assistance. On Figure 80 are
presented the active methods for air-conditioning in terms of complexity for
implementation into the mobile HVAC system as alternative and the percentage of
improved efficiency, compared to the existing vapo-compression HVAC system. Placed
with some assumptions in the order, because the information is based on analysis from
Goetzler et al. (2014), Gauger (1993), Multerer & Burton (1991) as this particular filed of
Figure 80 Active solutions for HVAC systems
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mechanical engineering analysis is not coherent with the design orientation of the
proposals of this thesis report.
Analyzing the possibilities for new systems that are able to fit with for automotive HVAV
systems, the main requirements should always follow environmental safety, energy
efficiency (COP), small size and simplicity from design point of view. This automatically
eliminates the pretty heavy, big and too complex systems for further implementation into
. It should be considered, that the most of the alternative
solutions are in research and development state, or they are used in another fields, which
modification for the automotive HVAC system is going to complicate them even much
more and making them unprofitable. The most practical and feasible of them are not with
highest COP, nevertheless that it is another key element of the development of
competitive future solutions.
In terms of passive alternative solutions of assisting the present HVAC systems, they
could be seen on Figure 81. They are diverse in demonstrating overall decrease of the
interior temperature, because all of them work in a particular way. The heat loads which
every of the solutions prevent as thermo-potential reducer cannot be compared directly
between them, because of the different parameters characterizing any of them and the
given information from experiments in terms of efficiency, fuel consumption, temperature
decrease or general COP. ey are presented in the following chart in terms of
investment and overall heat reflection efficiency in %.
Their pros and cons are examined throughout the research part not only regarded to
performance and economic investment, but also expectation and satisfaction
Figure 81 Passive solutions for HVAC systems assistance
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point of view, mode of use and characteristics, in order to
be competitive and working alternatives.
CONCLUSIONS 6.2
From the research work it is clear that significant changes need to be done in the sphere
of thermal comfort providing in vehicles. The aims for development and upgrade of
existing and future alternative solution in the field of HVAC assistance are multiple
providing safer, quitter, more comfortable, compact, fuel efficient and environmentally-
friendly vehicles. Otherwise developing of systems that are relatively large, very complex
or adding circumstances to the already given ones from the present one is not going to
work in the demanding automotive environment.
After a deep analyze and research over the proposed alternatives for active mobile air-
conditioning, there are several promising systems, which could be further spread into the
global mobile HVAC market. The Stirling cycle and the chemical heat pump systems could
be feasible alternatives to the existing vapor-compression system. A third system with
competitive COP is the absorption cycle, but with a big notice its multiple dimensions
and complexity should be modified in order to be fitted inside the vehicle compartment
and to increase the maintenance/reliability factor.
The Stirling cycle has high efficiency and it is compact, so the heat exchangers could be
placed throughout the car compartment. The main limit therefore is its complexity, which
will result in maintenance and reliability issues. A further research in improving its noon-
problematic work will make it a practical mobile HVAC solution. (Multerer & Burton, 1991)
The chemical heat pump from other side has also many advantages, including its high
overall efficiency and simplicity, due to the lack of moving parts. This waste-heat driven
system would also be reliable in operation because of its simplicity. Its size is a distinct
drawback for automotive use, and the system would therefore benefit strongly from
energy conservation.
imperfections. But before alternative systems are brought to the market, modifications of
the existing one can be made to reduce the greenhouse gases concentrations. The present
HVAC systems reduce range mileage in average of 7% when they are used. This
penalty could be cut to 3% (a 4% savings) by improved window and thermal design, which
corresponds with conserving feature of the mentioned in the research. (Multerer & Burton,
1991)
Improving the delivery methods for conditioned air in an automobile is one of the
effective ways to increase thermal comfort with little energy cost. This reduces HVAC
needs and thus fuel use or electric power. The others are conservational methods of
possible. Research results show that some of the solutions have a potential to vastly assist
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to the HVAC not only in reducing the heat into the cabin in a hot day, but also to prevent
the vehicle cabin to lose the warmth in cold weather.
The conservation of the vehicle has the biggest effect in solving the overheating
of air and surfaces temperatures inside the interior, relative to a COP with 2-3, because it is
directly related to the fuel consumption of the vehicle. The different solutions for thermal
barrier between the cabin and the environment reduce with different amounts and
specific areas of higher temperature decrease effect. The interesting point is that the high
efficiency of the solutions is not related to their price. The best insulating solutions like
cover shields, ceilings/blades and low-price thermal insulation and tinting films are
actually relevantly cheap and simple for implementation, which is making them much
more affordable. Of course, there are some factors, like the color of the paint, which are
just a matter of personal choice, but also play a relevant role in car interior temperature
and overall heat load.
of several different solutions. The implementing of a lot of them at once to act against the
heat as theoretical maximum heat decrease will not be practically and economically
feasible. Choosing one basic solution, with additional one or two, is the optimum.
For example, the cover shield (Section 5.1.3.3) for external use has a relatively medium
to high potential in isolating the vehicle, decreasing the internal air temperature levels
with around 18°C (corresponding to 40% decrease) in really low price. The solar
ventilating system (Section 5.3)(which represents more or less the principal of the lowered
window) has even less potential for temperature decrease of 8,3°C for the ambient air.
However, in
significant overall decrease, with more than 90% heat decrese. (Aljubury, et al., 2015).
Another possibility could be advanced glazings/tinting and cabin ventilation, which are
going to work better together to reduce the peak cabin temperature. The combination of
solar-reflective windows, solar-powered ventilation and solar-reflective paint resulted in
significant temperature reductions as well (decrease of 12°C of the internal breath
temperature).
From Figure 80 and the considerations for any of the solutions given, it is visible that all
of the possible operational principles for HVAC systems (experimental or existing) will
strongly benefit from energy conservation of the cabin interior, because their COP is not
significantly higher than the existing vapor-compression system. The conservation
technologies are the easiest to implement, effective and promising design solutions. They
are able to protect not only the interior part from degradation, but also will decrease the
difference in temperatures, which is critical for the work of the HVAC system and thermal
comfort. They could be mass produced due to its simple structure and reasonable price.
A significant step in reduction the work of the HVAC in vehicles is already made using
a built estimating model of human for the examination and prediction of thermal comfort
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in real-time. Such a model is already used in development of automatic HVACs, also known
as Climatronic Automatic Air-Conditionig system. The further development of this
technology, implementing better prediction, more ergonomically adequate positions for
the vents, particular airflow speed for the different parts of the body and overall control
are able to reduce significantly the overworking of the HVAC system and in parallel to
increase the thermal comfort of the occupants with better temperature distribution
around the interior with less discomfort.
DISCUSSION AND SUGGESTIONS FOR FURTHER DESIGN SOLUTIONS 6.3
Some of technologies reviewed into the research of alternative HVAC principles are
struggling from low COP and thus making them inappropriate methods for using into the
air-conditioning unit. But implementing them into smaller products for local action, with
less consumption/performance requirements, could be productive for the overall thermal
comfort. An example of already implemented device are the ventilated seats with thermo-
regulated thermoelectric device (TED), which produces both cold and warm temperatures,
and thus provides the seat not only with ventilation, but with a cold or warm airflow as an
air-conditioner.
A further similar use of TEDs into the blower of the HVAC system, connected with a solar
panel, similar to the used from Toyota principal, can be efficient for cooling down the
interior air temperature when the car is left outside under the sun. The low COP (0,6)
means that the device will require significant electricity in order to provide satisfying
result, which is the only concern.
Another possible direction is changing the power source for the vapor-compression
HVAC system from mechanical belt-driven compressor to electric 200V compressor will
significantly benefit in reducing the fuel consumption by HVAC up to 20% compared to
the existing mechanical compressor and also increases the performance of the engine.
In any case, the present performance of the vapor-compression system is related to
ecological issues, including the leakage of greenhouse gases from them. Its further
development and replacing the harmful HFC-134a refrigerant with ecological one, like the
CO2, will significantly reduce the footprint of these systems over the global environment.
In terms of technology, used for producing an competitive and satisfying alternative
HVAC system, the multiple alternatives possess some more significant disadvantages over
advantages, which results in their overall performance and further development. It is still
necessary further deepen research and analytical work of the alternative systems before
releasing a competitive solution for the existing vapor-compression HVAC. Up to then, the
alternative passive ways of conserving the ambient temperature in the vehicle could work
to satisfy the global problem with overheating and increase in the fuel consumption.
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