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Proposal of an eco-friendly high-performanceair-conditioning system. Part 1. Possibility ofimproving existing air-conditioning system by anevapo-transpiration condenser

Huynh Thi Minh Thu a, Haruki Sato b,*aGraduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku,

Yokohama 223-8522, JapanbDepartment of System Design Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,

Kohoku-ku, Yokohama 223-8522, Japan

a r t i c l e i n f o

Article history:

Received 27 January 2012

Received in revised form

28 March 2013

Accepted 6 April 2013

Available online 15 April 2013

Keywords:

Air-conditioning

Energy saving

Heat island

Condenser

Evapo-transpiration

Exergy

* Corresponding author. Tel.: þ81 45 563 114E-mail address: hsato@sd.keio.ac.jp (H. S

0140-7007/$ e see front matter ª 2013 Elsevhttp://dx.doi.org/10.1016/j.ijrefrig.2013.04.004

a b s t r a c t

Air-conditioning (AC) system consumes high energy and releases waste-heat. In the pre-

sent study, we propose a method to improve its performance and minimize waste-heat by

replacing existing air-cooled condenser by an evaporation and transpiration, evapo-

transpiration, condenser. The improvement is confirmed by performing experiment for a

conventional air-cooled AC system and a water-cooled AC system. Condenser temperature

in the air-cooled system is higher than outdoor-temperature by 5e10 �C, while it is �5 to

5 �C in case of the testing system. From simulation results, saving energy consumption is

expected to reach up to 30% in summer with the testing system. Based on these results, an

evapo-transpiration heat-exchanger was developed as a new condenser. Heat-transfer

coefficient of the testing heat-exchanger is at least 4 times higher than that of air-cooled

condenser. Even hot fluid is used inside copper-tubing, its outlet-air temperature is as

nearly as outdoor temperature.

ª 2013 Elsevier Ltd and IIR. All rights reserved.

Proposition de systeme de conditionnement d’air hautementperformant et ecologique. Partie 1. Possibilite d’ameliorer unsysteme de conditionnement d’air existant a l’aide d’uncondenseur a evapotranspiration

Mots cles : conditionnement d’air ; economies d’energie ; ılot de chaleur ; condenseur ; evapotranspiration ; exergie

1x43045; fax: þ81 45 566 1729.ato).ier Ltd and IIR. All rights reserved.

Nomenclature

Air-conditioning system

To outdoor temperature [K]

Tcond condenser temperature [K]

DT ¼ Tcond � To temperature difference between condenser

and outdoor [K]

Tao outlet-air temperature from outdoor unit [K]

P power consumption rate [W]

_Qcond condenser heat-transfer rate [W]_E exergy [W]

Heat exchanger

Tfi, Tfo temperatures at inlet and outlet of fan [K]

Thwi, Thwo temperatures at inlet and outlet of hot water [K]

Uo overall heat-transfer coefficient [W m�2 K�1]

qhw heat-transfer rate releases from hot water [W]

Ao outside surface area [m2]

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 5 8 9e1 5 9 51590

Fig. 1 e Sketch of conventional AC (left) and testing AC

systems (right).

1. Introduction

Annual global average temperature trend continues increasing.

One-degree Celsius increase in summer has been correlated

with 3.8% increase in peak demand load for air-conditioning

(Peck and Richie, 2009).

Air-cooled condensers have contributed large amount of

small-scale air-conditioning due to its advantages of easy

maintenance with convenient size. However, cooling by sen-

sible heat from air is only expected to get low heat-transfer

performance that makes high condensing temperature, i.e.

15e20 �C above that of the ambient air in some cases (Hosoz

and Kilicarslan, 2004). In studies of Chow et al. (2002) and

Hajidavalloo (2007), they mentioned that the coefficient of

performance (COP) of an air-conditioner decreases about

2e4% by increasing each degree Celsius in condenser tem-

perature. In addition, by releasing waste-heat to the sur-

roundings, it further increases temperatures outside, which

contributes to heat island problem in urban area. Moreover,

hot-air flow of the waste-heat contains exergy, which is

available energy that can transfer to work, generally it is not

re-used.

Other types of condensers that commonly used in air-

conditioning system are water-cooled and evaporative con-

densers (Hosoz and Kilicarslan, 2004). Most of water-cooled

condensers reject heat by connected with cooling tower,

while evaporative condenser is compact by combining func-

tions of an air-cooled condenser with a water-cooled

condenser and a cooling tower. Cooling by water evapora-

tion has much higher performance compared to air-cooled

condenser. In evaporative condenser, fin combined with

packing material have been used (Ettouney et al., 2001). Cel-

lulose is a common material for evaporative packing

(Hajidavalloo, 2007; Hu and Huang, 2005), but it requires large

space for evaporation. However, size of these condensers is

large and recently they are applied for medium- and large-

scale cooling system, in which, additional pump is required

to operate.

We propose a new air-conditioning systemusing an evapo-

transpiration heat-exchanger for higher performance

condenser to reduce its temperature with convenient size and

creating comfortable space at the outdoor unit. The possibility

for developing a new air-conditioning system will be dis-

cussed in this paper.

� By experiments, we examine the relationship of power

consumption of conventional air-conditioning system

and average condense-temperature for every hour. Effect

of condenser-temperature to the system performance is

also demonstrated by simulation. Besides, waste-heat

and its exergy from air-cooled outdoor-unit are also

evaluated.

� Temperatures of condenser and compressor of a water-

cooled system are measured to confirm the possibility

of reducing those temperatures in the new system. Per-

formance of new system is also expected based on this

result.

� An evapo-transpiration heat-exchanger, which is proposed

for new condenser, will be explained by its heat-transfer

coefficient and possibility to minimizing environmental ef-

fect of the new outdoor-unit.

2. Experiment description

2.1. Air-conditioning systems

An existing commercial air-conditioning system using R410A

as refrigerant, nominal cooling capacity of 2.5 kW and cata-

logue COP of 5.68 is used as a baseline system. This conven-

tional system has an air-cooled condenser, which is copper-

tubing of 22.3 m length and 8 mm outside diameter. A

water-cooled air-conditioning system, which was modified

from conventional system by using a water-cooled condenser

that connected with cooling tower, as shown in Fig. 1, is used

as a testing system. Water-cooled condenser is a double

copper-tubing adjacent to each other, with total length of

21 m, refrigerant outside diameter of 6.35 mm and water

outside diameter of 8 mm. The cooling tower used in the

testing system has nominal cooling capacity of 13.6 kWwith a

0.25-kW pump and a 0.05-kW fan being commercially avail-

able as the minimum capacity.

Fig. 2 e Testing heat-exchanger. A: Heat exchanger (copper tube and ceramics), B: water-drop pipe, C: bath-pump, D: cooling

water tank, E: fan, F: duct.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 5 8 9e1 5 9 5 1591

For the present purpose, we are testing the condensing

ability under various conditions for the testing system

compared to conventional system in actual summer weather.

Experiment of the conventional systemwas operated 4 days in

summer 2009 (daytime) and 5 days in summer 2010 (24 h);

while testing system was operated 5 days long (24 h) in sum-

mer 2010.

2.2. Testing heat-exchanger

A prototype of a new heat-exchanger has been set-up and

tested in summer 2009. The experiment is sketched in Fig. 2.

The heat-exchanger consists of copper tubes covered with

porous ceramics. Hot water flows inside copper tube and is

circulated. Tap water drops from top to ceramic surface and is

circulated from bottom tank by a 13-W bath-pump. A fan of

1740 m3 h�1 flow-rate is put in front of the heat-exchanger.

Airflow is ducted by an acrylic duct.

Type-T-thermocouples are used to measure tempera-

tures; data is connected to computer and recorded by data

Fig. 3 e Positions of thermocouples in the air-conditioning

system.

logger CADAC with the uncertainty of 0.02% of reading

þ0.03 �C. For the air-conditioning systems, temperatures

were measured at inlet, middle, outlet of condensers and

evaporators; before and after compressors; before expansion

valves; and inlet and outlet of cooling water, as in Fig. 3. For

heat exchanger, temperatures at inlet and outlet of hot

water; top, middle and bottom of copper-tube and ceramics

surfaces; inlet and outlet of air flow; in the water tank; and of

the ambient are recorded.

3. Calculation

3.1. Air-conditioning performance

3.1.1. Coefficient of performanceA typical ideal vapor compression cycle is sketched in Fig. 4. In

order to estimate COP of the air-conditioning system, prop-

erties of refrigerant are evaluated using REFPROP 8.0.

COPtheo ¼ h1 � h4

h2 � h1(1)

COP ¼ hsys,COPtheo (2)

Fig. 4 e Heat pump cycle on Peh diagram.

Fig. 5 e Middle-condenser temperature (every minute).

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 5 8 9e1 5 9 51592

In which, hsys is system correction factor and is calculated at

nominal condition by:

hsys ¼COPcata

COPtheo n(3)

COPtheo_n is theoretical COP at nominal condition and COPcatais COP in manufacturer’s catalogue.

Nominal condition is assumed that: outdoor temperature of

35 �C, indoor setting temperature of 27 �C, indoor relative hu-

midity is about 43%, dew point temperature is 13.5 �C and evap-

oration temperature is 7 �C lower thandewpoint,which is 6.5 �C.

3.1.2. Power consumptionFor the same outdoor temperature and indoor setting tem-

perature for the same space, cooling load required for the two

systems is assumed to be the same. Hence, we have:

Pconv

Pnew¼ COPnew

COPconv(4)

Then; power consumption of new system is estimated :

Pnew ¼ COPconv

COPnewPconv ð5Þ

Fig. 6 e Integral power consumption vs. condens

3.2. Overallheat-transfer coefficientofnewheat-exchanger

Heat released by hot water is evaluated by :

_qhw ¼ _mhwcpwðThwi � ThwoÞ(6)

Heat� transfer rate is also calculated by : _qhw ¼ UoAoDTlm

(7)

where DTlm ¼ DTo � DTi=ðlnðDTo=DTiÞÞ: logarithmic mean

temperature difference with DTo ¼ Thwo � Tfi and

DTi ¼ Thwi � Tfo.

4. Results and discussions

4.1. Effects of condenser temperatures to theperformance of a conventional air-conditioning system

For the present purposes, condenser and compressor

temperatures are concerned. Thermocouple at the middle of

the condenser copper-tubing, which is supposed to be very

close to condenser temperature, is used as condenser

er (left) and compressor (right) temperatures.

Fig. 9 e Temperatures of condenser vs. outdoor.

Fig. 7 e Estimated COP vs. condenser temperature.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 5 8 9e1 5 9 5 1593

temperature. Thermocouple right after the compressor,

which is close to compressor temperature, is used as

compressor temperature.

In the experiment, data is recorded every minute. At low

outdoor-temperature, temperature at condenser is divided

into two regions: high temperature (ON or operating mode)

and low temperature (OFF or stop mode), as in Fig. 5. In order

to compare the performance among air-conditioning systems,

integral power consumption for an hour is considered. In this

paper, we consider integral power consumption and average

temperatures of outdoor, condenser, compressor for every

hour. From now on, hourly average temperature of outdoor,

middle-condenser and right after compressor are briefly

called as outdoor temperature, condenser temperature and

compressor temperature.

From experimental results of the conventional AC system,

higher condenser temperature, which leads to higher

compressor temperature, makes integral power consumption

higher, as shown in Fig. 6. Hence, higher performance is ex-

pected to achieve at lower condenser temperature, as simu-

lation result in Fig. 7.

Fig. 8 e Waste-heat and its exergy of air-cooled outdoor-

unit.

4.2. Exergy of waste-heat from air-cooled outdoor unit

For air-cooled outdoor unit, hot-air flow of waste-heat con-

tains exergy, but it is not utilized for other purposes. In

consequence, it transfers directly to the ambient air and per-

forms some change to the surrounding air. Amount of this

exergy is calculated by:

_E ¼ _Qcond

�ðTao � ToÞ � Toln

To

Tao

�(8)

Waste-heat and its exergy of hot-air flow from air-cooled

condenser are described in Fig. 8. Because condenser-

temperature increases drastically as outdoor-temperature

increases, exergy also increases considerably. For example,

in this experiment, at 34.5 �C outdoor-temperature, condenser

temperature is nearly 45 �C, waste-heat is about 0.9 kW and its

exergy is estimated to be approximately 0.15 kW.

4.3. Condenser and compressor temperatures in air andwater-cooled systems

Temperatures of compressor and condenser are sketched in

Figs. 9 and 10, respectively, for conventional and testing air-

conditioning systems at outdoor temperatures from 27 �C to

35 �C. In Fig. 9, condenser temperature increases sharply with

increasing of outdoor temperature. Temperature at condenser

surface is approximately from5 to 10 �Chigher than that of the

water condenser, which is nearly the same as outdoor

Fig. 10 e Temperatures of compressor vs. outdoor.

Fig. 13 e Estimated COP vs. outdoor temperature.Fig. 11 e Temperature difference between condenser and

outdoor vs. outdoor temperature.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 5 8 9e1 5 9 51594

temperature. Likewise, temperature right after the compressor

of the conventional system is higher about 10e20 �C than that

of the water system, which is just higher than outdoor tem-

perature about 5 �C Fig. 10.

4.4. Expected performance of the new air-conditioningsystem

Temperature difference between condenser and outdoor, DT,

is getting higher as outdoor temperature increases, as shown

in Fig. 11.When outdoor temperature is higher, integral power

consumption increases not only due to higher cooling load but

also caused by higher pressure difference between condenser

and evaporator. For that reason, higher DT is, higher energy

requires. Fig. 12 shows the change of integral power con-

sumption with respect to temperature difference between

condenser and outdoor temperatures.

The integral power consumption of the testing system is

not included in the results since cooling tower has high

power consumption. Based on experimental results, COP of

the testing system is expected to be higher than that of the

Fig. 12 e Integral power consumption vs. temperature

difference between condenser and outdoor.

conventional system up tomore than 30%, as shown in Fig. 13.

Therefore, with the same cooling space, if any new

air-conditioning system can have low condenser-temperature

as the testing system without using cooling tower, its integral

power consumption is expected to save 30%, as shown

in Fig. 14.

4.5. Testing heat-exchanger

Transpiration and evaporation are main principles in devel-

oping the new heat-exchanger. Transpiration helps to mini-

mize energy used for pump using porous ceramics, while

heat-exchange rate between inside and outside is enhanced

by evaporation of water. Even ceramics’ performance should

be carefully investigated, its property to spread water auto-

matically and continuously along copper-tube surface is

satisfied. This experiment was carried out in summerweather

to confirm the actual performance.

Heat-transfer coefficient is evaluated in order to compare

with that of the air-cooled condenser. The heat-transfer co-

efficient of the proposed heat-exchanger, which is calculated

from Eq. (7), is demonstrated in Fig. 15. The overall heat-

Fig. 14 e Integral power consumption vs. outdoor

temperature.

Fig. 16 e Outlet-air temperature of the new heat exchanger.

Fig. 15 e Heat-transfer coefficients of the new heat-

exchanger.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 6 ( 2 0 1 3 ) 1 5 8 9e1 5 9 5 1595

transfer coefficient is about 400e900 W m�2 K�1, which is

much higher than the air-cooled condenser whose range of

heat-transfer coefficient is from 50 to 90 W m�2 K�1 with air

velocity of 0.5e3 m s�1 (Seshimo and Fujii, 1992). By having

higher performance, size of condenser of the new system is

expected to be smaller than the existing one.

By using water evaporation to cool hot water inside copper

tube, outlet-air temperature is nearly or even lower than

ambient temperature as shown in Fig. 16. In this prototype,

the distance between two rows of copper-tubing is long

enough that humidity of air at the outlet is not so higher than

that of the ambient due to mixing with the air that does not

contact with wet ceramics surface.

5. Conclusions

High temperatures are measured in condenser and

compressor of conventional air-cooled air-conditioning

system. Higher condenser temperature makes higher energy

consumption. By using ambient air to cool condenser, con-

ventional outdoor unit releases heat including the exergy to

the surroundings, which causes heat island problem in cities.

In this experiment, at about 35 �C outdoor-temperature,

waste-heat is about 0.9 kW and exergy of outlet hot-air flow

is estimated to be about 0.15 kW.

It is confirmed by using testing air-conditioning system

that condenser temperature is as low as outdoor-temperature,

which makes temperature right after compressor lower, too.

From simulation result, COP of the new testing system is

estimated to be 30% higher than that of the conventional

system.

In order to achieve energy saving estimation above, a new

heat-exchanger using evapo-transpiration has been proposed.

The heat-transfer coefficient of this new heat-exchanger is

evaluated at least 4 times higher than that of the air-cooled

heat-exchanger used in the conventional system. Even hot

fluid is used in this testing heat-exchanger, outlet-air tem-

perature is as near as ambient temperature.

Acknowledgments

Authors are grateful to Toshiba Carrier for supporting

to develop the new air-conditioning system. The study is

supported by Global COE Program “Center for Education

and Research of Symbiotic, Safe and Secure Design”, MEXT,

Japan.

The authors would like to thank to AUN/SEED-Net JICA

(Japan International Corporation Agency) for giving scholar-

ship to student.

r e f e r e n c e s

Chow, T.T., Lin, Z., Yang, X.Y., 2002. Placement of condensingunits of split-type air-conditioners at low-rise residences.Appl. Therm. Eng. 22, 1431e1444.

Ettouney, H.M., El-Dessouky, H.T., Bouhamra, W., Al-Azmi, B.,2001. Performance of evaporative condensers. Heat TransferEng. 22, 41e55.

Hajidavalloo, E., 2007. Application of evaporative cooling on thecondenser of window-air-conditioner. Appl. Therm. Eng. 27,1937e1943.

Hosoz, M., Kilicarslan, A., 2004. Performanceevaluations of refrigeration systems with air-cooled,water-cooled and evaporative condensers. Int. J. EnergyRes. 28, 683e696.

Hu, S.S., Huang, B.J., 2005. Study of a high efficiency residentialsplit water-cooled air conditioner. Appl. Therm. Eng. 25,1599e1613.

Peck, S., Richie, J., 2009. Green Roofs and the Urban Heat IslandEffect. Stamats Commercial Buildings Group, Cedar Rapids,The United States. Web site URL: http://www.buildings.com(quoted in 2012).

Seshimo, Y., Fujii, M., 1992. Compact Heat Exchanger. NikkanKogyo Press Co., Ltd, Tokyo (in Japanese).