CLIMAMED 2007 ENERGY, CLIMATE AND INDOOR COMFORT … · Refrigerazione), in cooperation with AICVF...

Genova, Italy September 5th through 7th 2007Magazzini del Cotone

Associazione Italiana Condizionamento dell’AriaRiscaldamento e Refrigerazione

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CLIMAMED 2007 ENERGY, CLIMATE

AND INDOOR COMFORTIN MEDITERRANEAN

COUNTRIES

AICARR - Associazione Italiana Condizionamento dell’Aria Riscaldamento e RefrigerazioneVia Melchiorre Gioia 168 - 20125 Milano - Italy - Phone +39 02 67479270 - Fax +39 02 67479262 - www.aicarr.it

AICARR (Associazione Italiana Condizionamento dell'Aria, Riscaldamento,Refrigerazione), in cooperation with AICVF (Association des IngénieursClimatique, Ventilation et Froid), APIRAC (Associação Portuguesa da Industria deRefrigeração e Ar Condicionado), ATECYR (Asociación Técnica Española deClimatización y Refrigeración), has organized in Genova (Italy), at the beginningof September 2007, the 4th Mediterranean Conference of HVAC&R EngineeringCLIMAMED 2007. The Technical Programme for CLIMAMED 2007 focuses on sustainable energydevelopment and practical solutions for an updated design, construction,operation and maintenance of buildings and clima services. Main topics of thehigh quality technical papers collected in the Proceedings volume are:- solar and renewable energy in Mediterranean Countries;- climate adaptation, thermal comfort requirements and indoor environment;- load reduction and building envelope optimization in Mediterranean Countries;- energy efficiency and certification in new and existing buildings;- emerging and best available HVAC&R technologies in Southern Europe and

Mediterranean climates;- market, construction, and O&M issues.

CLIMAMED 2007 is organized under the patronage of REHVA (Federation ofEuropean Heating and Air Conditioning Associations) and is sponsored byASHRAE (American Society of Heating, Refrigerating and Air ConditioningEngineers).

PROCEEDINGS

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cop climamed bis 28-07-2007 11:56 Pagina 1

ABSTRACT

Energy performance of a new glass double skin façade equipped with an integratedmovable shading system is presented. A spectrophotometric campaign on transparentand opaque materials has been conducted to evaluate transmittance, reflection andabsorption of each element constituting the double skin façade. The solar radiation pathwith its multiple reflections at the different interfaces has been taken into account,employing a ray tracing method, integrated in the computational fluid dynamic (CFD)code.

The simulation shows that the winter configuration of the proposed façadeenhances the solar heat gain thanks to the heating of the air inside the gap; in the hotseason the system is able to block significantly solar radiation, thus reducing summercooling demand.

An energy analysis of a residential building equipped with the proposed façade isreported, showing good performance both in winter and in summertime, especially ifcompared to the thermal behaviour of common transparent façades.

RIASSUNTO

Nel presente studio sono presentati i risultati dello studio del comportamentotermofluidodinamico ed energetico di una facciata vetrata innovativa equipaggiata consistemi di ombreggiamento mobili, progettati per ottimizzare le prestazioni energetichedell'involucro sia nella stagione invernale che durante quella estiva. La radiazione solareincidente sulla parete è simulata attraverso un modello di solar ray tracing, già integratonel codice di calcolo impiegato (Fluent). Il modello è stato sviluppato con dati climaticirelativi a tre località italiane ed è stato integrato con misure spettrofotometriche per ladefinizione delle proprietà spettrali dei materiali trasparenti ed opachi che costituisconola facciata. L'analisi energetica, condotta in un edificio residenziale, mostra che ilsistema proposto garantisce buone prestazioni in termini di risparmio energetico nellastazione invernale ed al tempo stesso consente di superare il problema tipico dellafacciate a doppia pelle, ossia il surriscaldamento dell'intercapedine nella stagione estiva.

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A new double skin façade withintegrated movable shading systems:numerical analysis and evaluationof energy performanceFRANCESCO ASDRUBALI, GIORGIO BALDINELLI

University of Perugia, Italy

P21 asdrubali baldinelli 25-07-2007 17:10 Pagina 259

A new double skin façade with integrated movable shading systems:numerical analysis and evaluation of energy performance

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1. INTRODUCTION

The need to reduce energy consumption in buildings - according to the goals ofKyoto Protocol - is causing a new interest towards passive solar systems (withoutauxiliary mechanical means for fluid transport). The spatial interaction between incidentradiation and energy supplied to the internal environment is developed in different ways:in direct gain passive systems, the prevailing heat contribution is given by the solarradiation that is transmitted through transparent surfaces; the indirect gain modalitytakes place mainly by convection, being the radiative part supplied by storage masses(such as in Trombe walls or water walls). In the insulated gain systems, a mass storageaccumulates heat by radiation and convection, but there is no passage of air to the innerspace, such as in greenhouses, air collectors, solar chimneys, Barra-Costantini systemsand double skin façades.

Used since ancient times, passive solar systems can nowadays take advantage ofmodern technologies, selective materials and sophisticated regulating systems; amongthem, double skin façades seem extremely interesting. These façades are, in fact,becoming an important and widespread architectural element in office buildings, as theycan provide numerous advantages such as energy saving, sound, wind and pollutantprotection with open windows, solar preheating of ventilation air, night cooling and, lastbut not least, aesthetics.

A double skin façade (Figure 1) consists of an external glass surface, a shadingsystem, a gap filled with air and an insulating double internal glazing system, sometimesintegrated with opaque walls (Wigginton and Harris 2002). The gap is ventilated throughthe air flux driven by the buoyancy effect (natural convection) or by mechanical devices(forced ventilation). The heat carried to the inner rooms (Figure 2) is the sum of theenergy directly transmitted through the transparent surfaces plus the secondary emissionof the inner skin (Fux 2006); the latter depends strictly on the radiation absorbed by thewhole system (Poirazis 2004).

A new double skin façade - called SUNSHADE® - with integrated movableshading systems and a different configuration in summer and wintertime is presented. Adetailed analysis of the new façade is carried out, through an approach that mixes experi-mental spectrophotometric campaigns to characterize the materials with three-dimensional computational fluid dynamics codes, to be used as a support for buildingenergy simulation models.



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Figure 1 - Example of a double skin façade

Figure 2 - Thermal exchange process through double skin façades

2. LITERATURE REVIEW

Double-skin façades represent the result of an architectural movement which paysa great deal of attention to aesthetic matters; there are also many other advantagesassociated with double skin façades which are summarised as follows (Poirazis 2004):• reductions in heating consumption thanks to outdoor air preheating;• reductions in cooling consumption when a shading system is installed;• night cooling of the building by opening the inner windows;• a good sound insulation level of the façade;• high level of daylight;• low risk of condensation on transparent surfaces because of continuous air movement;

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• improved protection against burglary thanks to two glazed skins;• higher protection against air pollution.

On the other hand, some problems could arise with the installation of glazeddouble skin façades instead of typical external walls:• poor cross ventilation and insufficient removal of heat from office rooms during

windless periods; • higher investment costs;• reduced floor building space;• additional cleaning costs;• fire protection can be more difficult depending on the type of façade.

The main topic of scientific research on double skin façades consists on theassessment of the thermal field within the air gap. Different models have been proposed,starting from a non-dimensional analysis that introduces 14 non-dimensional numbers todescribe the total heat transfer process in a naturally ventilated façade, withoutmodelling fluid flows and heat transfer and using easy to get thermal and physical data(Balocco 2004). Other authors (Corgnati, Perino, Serra and Filippi 2003) suggested thatin wintertime the air extracted by the gap could be sent to an air treatment unit as apreheated ventilation flow, thus saving considerable amounts of energy.

Deeper investigations on flow regimes between the two skins began to employcomputational fluid dynamic techniques and compared the results with correlationsbased mainly on experimental results. In (Manz 2003) heat transfer by naturalconvection is studied, with the assumptions of closed, rectangular and vertical cavities inbuilding elements; a good agreement between the CFD simulations and the data obtainedby empirical correlations was found. This study enlarged the possibility of the codeapplications to more complex cases of facade elements, where less or even no experi-mental data was available in literature. What's more, it showed that, using a three-dimensional modelling instead of restricting the simulations to any vertical section of thefaçade, errors were negligible, hypothesizing that, except for the two vertical skins, allwalls are adiabatic.

A successive work of the same author (Manz 2004) underlined that the thermalbehaviour of a double skin façade can be reliably described using a spectral opticalmodel in conjunction with a CFD model including convection, conduction and radiation.This evidence was reinforced in (Manz and Frank 2005): for the complete description ofthe double skin façade, three different modelling levels have to be taken into account:optics of materials, thermodynamics and fluid dynamics of double-skin façade and thebalance of building energy.

Particular attention has also been paid to shading systems: different solutions wereproposed from louvers and venetian blinds to the “natural solution” described in (Stec,van Paassen and Maziarz 2005) where the gap of the double skin façade is equipped withplants.

Dealing with fire hazard assessment (Chow and Hung 2006), some experimentalinvestigations indicated that the gap depth is the key factor: wider gaps are safer, as wellas a horizontal partition of the façade and small tempered glass panels perform betterthan larger sheets because of their higher resistance to thermal stress.



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3. THE PROPOSED FAÇADE

The innovative part of the proposed façade (SUNSHADE®) is the external layer,made of an integrated glass-shading device, permitting the exploitation of the doubleskin façade benefits in winter conditions (figure 3) as well as the cooling load reductiondue to the shading system in summertime (figure 4).

Figure 3 - Investigated double skin façade (SUNSHADE®), winter configuration

Figure 4 - Investigated double skin façade (SUNSHADE®), summer configuration

The system is effective if the façade is exposed towards south. During wintertime(figure 3) air inlet is in the lower part of the outer skin; air heats up in the gap and is inputinside the internal environment at the upper part of the inner skin; shading systems arehorizontal, so that a large amount of solar radiation can pass trough the façade. In the hotseason, to avoid overheating and discomfort, the external layer remains open (figure 4)while the shading devices are configured with a high tilt angle, giving the air thepossibility of escaping from the gap and blocking at the same time the solar radiation,thus avoiding an undesirable overheating effect, typical of double skin façades with fixedconfiguration (Gratia and De Herde 2004).



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The external skin is bounded by a stratified glass (two layers of 5 mm float glassdivided by a 0.37 mm film of polyvinyl butyral); the shading device is made of anodizedaluminium, an alloy that combines good mechanical resistance properties with arelatively low density and an excellent performance against atmospheric agents.

The inner skin is assembled with the coupling of the same laminated glass used forthe external layer with a 4 mm float glass; the two panes are divided by a 10 mm air gap.

4. OPTICAL PROPERTIES

The spectral properties of the materials making up the façade were measuredthanks to a precision spectrophotometer Varian - Mod. Cary 2300; results for normalincidence as indicated in (CEI EN 410 1998) are reported in Table 1 (direct solartransmittance factor τe and direct solar reflectance factor ρe).

Table 1. Single number optical properties of materials used for the proposed double skinfaçade

In figures 5, 6 and 7 the global spectral data are sketched, showing the high levelsof the transparency of both glazing systems (internal and external), as well as the goodreflective properties of the aluminium in the shading system.

Glazing optical properties depend on the incident angle between the surface andthe ray direction: as this angle deviates from the normal direction (0°), transmittancedecreases, and increases in reflectance and absorbance.

The variation of optical properties with the incidence angle depends on glass typeand thickness; in particular it results more pronounced for coated glass and multiple-pane glazing systems; in the case under investigation, the spectral transmittance at anyincident angle is calculated from the relations suggested by the CFD code (Fluent, 1995).

Finally, to properly simulate multiple reflections occurring wherever there is apassage between two different means (e.g. air-glass), a ray tracing algorithm wasemployed.

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Figure 5 - Optical properties of the external skin (laminated glass)

Figure 6 - Optical properties of the shading device (anodized aluminium)

Figure 7 - Optical properties of the internal skin (laminated glass, air gap, float glass)

5. NUMERICAL ANALYSIS

The fluid dynamic models representing air motion inside the gap result rathercomplex, involving turbulent and laminar flows, as well as recirculation phenomena andthe intrinsic difficulty on defining the type of flow (laminar or turbulent). With the



266

hypothesis of free convection, the flux is driven by buoyancy forces; this phenomenon isdescribed by equations of mass, momentum and energy conservation, together with thedefinition of turbulent flow variables.

The most commonly used model to simulate the air motion in double skin façadegaps is the renormalisation group k-_ model (Gan 2001); for this reason, we have chosento use it in this paper. In the segregated solver used for the calculation, the radiation heatexchange has been taken into account by the P1 model (Siegel and Howell 1992). Theheat exchange is permitted only in the external and internal skin, all the other surfacesare considered adiabatic; the lower and upper openings were given respectively an inletand an outlet pressure boundary conditions of 101,325 Pa.

The instauration of natural convection inside the gap is shown in figure 8.

Figure 8 - Velocity vectors inside the gap in winter configuration

The overheating effect that could rise in summer conditions inside double skinfaçades with a closed configuration is avoided in this case by the communicationbetween the gap and the external air. As shown in figure 9 (Cotana and Baldinelli 2007),air recirculation occurs through the outer skin, guaranteeing a ventilation flux that keepsthe air temperature in the gap at levels close to external conditions.

Figure 9 - Path lines coloured by velocity magnitude (m/s) in summer conditions

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6. ENERGY SAVINGS IN BUILDINGS

The actual performance of the proposed facade was tested using a residentialbuilding, equipped with large windows, and therefore suitable for energy analysis (figure10). In the simulations, the building was located in three different Italian cities (Cuneo,Rome, Palermo), representative of the different Italian climates.

The energy performance of the building was evaluated by crossing data comingfrom the thermal and fluid dynamics code - such as air temperature in the gap of thedouble skin façade, in the various conditions - with data coming from a commonly usedsoftware to evaluate energy demand of buildings, according to the Italian Decree 192/05.

Figure 10 - Building investigated for the energetic analysis

The part of the building chosen for the simulations is a 70 m2 flat, located at anintermediate floor, with two external surfaces, one facing north, without openings apartfrom the entrance door, and one facing south; transparent surfaces have been inserted inthe latter. The investigated configurations result as follows:- south façade partly glazed (50%);- south façade partly glazed (50%) + SUNSHADE®;- south façade completely glazed;- south façade completely glazed + SUNSHADE®.

Both transparent and opaque walls comply with the minimum requirements as faras transmittance of Italian Decree 192/05.

Simulations were carried out in the three cities during all the months of the winterseason, according to technical standard UNI 10349; each month was represented by the15th day, and each day was described through three different simulations: one at 8 a.m.,one at 12 a.m. and one at 4 p.m. For the sake of security, the contribution of electricallighting to thermal loads was not considered, as well as the one that the air in the gap ofthe double façade could give to the air conditioning plant, if used as fresh, pre-heated air.Under such conditions, the contribution of the system SUNSHADE® to energy savingsin wintertime is due to the increase of air temperature in the gap, in contact with thesouth façade of the building. The simulation provides the solar heat flux transmitted tothe inner rooms, together with the flux transmitted if no shading were added; the gaptemperature is also evaluated.

In Table 2, winter data for the city of Palermo is reported; it is evident that in



268

central hours of the day, the shading devices become more influential because of theelevation of the sun above the horizon. Similar results were found for Rome and Cuneo.

Table 2. CFD winter results for the city of Palermo

The increase of air temperature in the gap compared to the external air temperatureis the clearest demonstration of the system's efficiency; in Table 3 a comparison betweenthese data for the city of Rome in the winter season is shown. The increase is higher at12 a.m., when the solar radiation reaches its peak.

Table 3. Comparison between external and gap air temperatures for the city of Rome inwinter

The procedure was repeated for the hot season and the simulations showed similarresults in terms of solar incident radiation reduction (see Table 4 relative to the city ofPalermo); the gap temperature is not reported because the opening of the external skinbrings this value close to external conditions, thus overcoming the problem of traditionaldouble skin façades, characterized by significant overheating of air in the gap.

The tendency of stopping solar rays, mainly in the middle hours of the day, isconfirmed; in this case the shadowing effect is desired, since it reduces the building'scooling demand.

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Table 4. CFD Summer Results for the City of Palermo

In Tables 5 and 6 (partially glazed inner façade and completely glazed inner façade,respectively) the values of Primary Energy Demand for winter heating (FEP in the Italianlegislation, given in kWh/m2 year) are shown for the three investigated cities, along withthe reduction due to the application of SUNSHADE®, which reaches about 28% inPalermo.

Table 5. Partially glazed façade: energy demand without SUNSHADE® and reductiondue to SUNSHADE®

Table 6. Completely glazed façade: energy demand without SUNSHADE® andreduction due to SUNSHADE®

The same configurations as far as the south façade of the flat were studied insummertime. The efficacy of the system SUNSHADE® was evaluated considering nonshaded glazing, glazing shaded with an internal curtain (shading factor equal to 0.5) andfaçade with SUNSHADE®. Also in the summertime case, the contribution of electricallighting to the thermal load was omitted; summer results are not reported in terms of FEPsuch as for winter, since up to now Italian legislation has not defined a standard indicatorto describe summer energy demands of buildings. Table 7 reports the reduction of the

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summer thermal load due to a shading with an internal curtain and due to SUNSHADE®,for a partly transparent south façade. SUNSHADE® is more effective than an internalshield and can reduce the thermal load up to 7% in Cuneo and in Rome. Table 8 reportsthe same reduction for a completely transparent south façade; also in this caseSUNSHADE® is more effective than an internal shield and can reduce the thermal loadup to about 14% in Cuneo and in Rome.

Table 7. Summer energy savings for traditional shading system and proposed solution(partially glazed façade)

Table 8. Summer energy savings for traditional shading system and proposed solution(completely glazed façade)

7. CONCLUSIONS

A new double skin façade with a movable integrated shading system wasinvestigated, defining its energy performance starting by measuring the opticalproperties of materials and a computational fluid dynamic analysis. The façadeperformance was tested in a residential building equipped with large windows,evaluating the energy saving both in winter and summer seasons; the system proved itseffectiveness, especially in winter, where savings up to 28% could be reached. Insummer, because of the particular shading design, the proposed solution is more efficientthan a traditional device with a shading factor of 0.5, allowing a cooling load reductionof up to 14%.

It is worthwhile underlining that, apart from the effects on the energy savings, afurther benefit of the glazed façades lies on natural lighting aspects, giving a significantimprovement of internal visual comfort.

Due to the shading surfaces being exposed towards the sun, it is possible tosubstitute the aluminium with photovoltaic modules; in doing so the energy performanceof the façade is increased (Infeld, Mei and Eicker 2004).

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8. ACKNOWLEGDEMENTS

The authors wish to thank Prof. Paolo Belardi, Prof. Fabio Bianconi, Dr. MarcoArmeni and Dr. Luca Martini for contributing to the work with their graphics and Dr.Manuele Battisti for his essential help in the simulations, as well as the Company “LilliSerramenti srl” for their support in developing the project.

REFERENCES

Wigginton, M. Harris, J. 2002. Intelligent skins, p. 72, Elsevier, Oxford, UK.Poirazis, H. 2004. Double Skin Façades for Office Buildings Literature Review.

Dept. of Construction and Architecture Lund University, Sweden, pp. 61-66.Fux, V. 2006. Thermal simulation of ventilated Double Skin Façades. Doctoral

thesis, Loughborough University, United Kingdom.Manz, H, and Frank, Th.2005. Thermal simulation of buildings with double-skin

façades. Energy and Buildings, Vol. 37, pp. 1114-1121.Heimrath, R. Hengsberger, H. Mach, T. Streicher, W. Waldner, R. Flamant, G.

Loncour, X. Guarracino, G. Erhorn, H. Erhorn-Kluttig, H. Santamouris, M. Farou, I.Duarte, R. Blomsterberg, A. Sjöberg, L. Blomquist, C. 2005. Bestfaçade - Best Practicefor Double Skin Façades - WP 1 Report “State of the Art”.

Balocco, C. 2004. A non-dimensional analysis of a ventilated double façade energyperformance. Energy and Buildings, Vol. 36, pp. 35-40.

Corgnati, S. P. Perino, M. Serra, V. Filippi, m. 2003. Analisi del comportamentotermofluidodinamico di un involucro trasparente innovativo: risultati di una campagna dimonitoraggio. Proceedings of 58° ATI Congress, pp. 1457-1468, San Martino diCastrozza, Italy.

Manz, H. 2003. Numerical simulation of heat transfer by natural convection incavities of façade elements. Energy and Buildings, Vol. 33, pp. 305-311.

Manz, H. 2004. Total solar energy transmittance of glass double façades with freeconvection. Energy and Buildings, Vol. 36, pp. 127-136.

Stec, W..J. van Paassen, A.H.C. Maziarz, A. 2005. Modelling the double skinfaçade with plants. Energy and Buildings, Vol. 37, pp. 419-427.

Chow, W.K. Hung, W.Y. 2006. Effect of cavity depth on smoke spreading ofdouble-skin façade. Building and Environment, Vol. 41 pp. 970-979.

Belardi, P. Bianconi, F. Asdrubali, F. 2007. SUN-SHADE - Sistema innovativo difacciata a doppio involucro con frangisole integrato, Libria.

Gratia, E. De Herde, A. 2004. Natural cooling strategies efficiency in an officebuilding with a double-skin façade, Energy and Buildings, Vol. 36, pp. 1139-1152.

CEI EN 410. 1998. Glass in building - Determination of luminous and solarcharacteristics of glazings.

Fluent Inc. 1995. Fluent User's Guide, USA.Gan, G. 2001. Thermal transmittance of multiple glazing: computational fluid

dynamics prediction, Applied thermal engineering, Vol. 21,pp. 1583-1592.Siegel, R. Howell, J. R. 1992. Thermal Radiation Heat Transfer, Hemisphere

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Cotana, F. Baldinelli, G. 2007. Numerical analysis of a double skin façade withintegrated movable shading systems. Proceedings of 5th International Conference onHeat Transfer, Fluid Mechanics and Thermodynamics, Sun City, South Africa.

D. L. 192. 2005. Attuazione della direttiva 2002/91/CE relativa al rendimentoenergetico nell'edilizia.

Infield, D. Mei, L. Eicker, U. 2004. Thermal performance estimation for ventilatedPV façades, Solar energy, Vol. 76, pp. 93-98.

UNI 10349. 1994. Heating and cooling of buildings. Climatic data. ItalianOrganization for Standardization.


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