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Proposal of an eco-friendly high-performance
air-conditioning system. Part 1. Possibility of
improving existing air-conditioning system by an
evapo-transpiration condenser
Huynh Thi Minh Thu a, Haruki Sato b,*
a
Graduate School of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku,
Yokohama 223-8522, Japan
b
Department of System Design Engineering, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi,
Kohoku-ku, Yokohama 223-8522, Japan
article info
abstract
Article history:
Air-conditioning (AC) system consumes high energy and releases waste-heat. In the pre-
Received 27 January 2012
sent study, we propose a method to improve its performance and minimize waste-heat by
Received in revised form
replacing existing air-cooled condenser by an evaporation and transpiration, evapo-
28 March 2013
transpiration, condenser. The improvement is confirmed by performing experiment for a
Accepted 6 April 2013
conventional air-cooled AC system and a water-cooled AC system. Condenser temperature
Available online 15 April 2013
in the air-cooled system is higher than outdoor-temperature by 5e10 C, while it is 5 to
Keywords:
expected to reach up to 30% in summer with the testing system. Based on these results, an
Air-conditioning
evapo-transpiration heat-exchanger was developed as a new condenser. Heat-transfer
Energy saving
coefficient of the testing heat-exchanger is at least 4 times higher than that of air-cooled
Heat island
condenser. Even hot fluid is used inside copper-tubing, its outlet-air temperature is as
Condenser
nearly as outdoor temperature.
5 C in case of the testing system. From simulation results, saving energy consumption is
Evapo-transpiration
ª 2013 Elsevier Ltd and IIR. All rights reserved.
Exergy
Proposition de système de conditionnement d’air hautement
performant et écologique. Partie 1. Possibilité d’améliorer un
système de conditionnement d’air existant à l’aide d’un
condenseur à évapotranspiration
Mots clés : conditionnement d’air ; économies d’énergie ; ı̂lot de chaleur ; condenseur ; évapotranspiration ; exergie
* Corresponding author. Tel.: þ81 45 563 1141x43045; fax: þ81 45 566 1729.
E-mail address: hsato@sd.keio.ac.jp (H. Sato).
0140-7007/$ e see front matter ª 2013 Elsevier Ltd and IIR. All rights reserved.
http://dx.doi.org/10.1016/j.ijrefrig.2013.04.004
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1.
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Nomenclature
Q_ cond
E_
Air-conditioning system
outdoor temperature [K]
To
condenser temperature [K]
Tcond
DT ¼ Tcond To temperature difference between condenser
and outdoor [K]
outlet-air temperature from outdoor unit [K]
Tao
P
power consumption rate [W]
Heat exchanger
temperatures at inlet and outlet of fan [K]
Tfi, Tfo
Thwi, Thwo temperatures at inlet and outlet of hot water [K]
overall heat-transfer coefficient [W m2 K1]
Uo
heat-transfer rate releases from hot water [W]
qhw
outside surface area [m2]
Ao
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 sensible 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 temperature. In addition, by releasing waste-heat to the surroundings, 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 airconditioning system are water-cooled and evaporative condensers (Hosoz and Kilicarslan, 2004). Most of water-cooled
condensers reject heat by connected with cooling tower,
while evaporative condenser is compact by combining functions of an air-cooled condenser with a water-cooled
condenser and a cooling tower. Cooling by water evaporation has much higher performance compared to air-cooled
condenser. In evaporative condenser, fin combined with
packing material have been used (Ettouney et al., 2001). Cellulose 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 largescale cooling system, in which, additional pump is required
to operate.
We propose a new air-conditioning system using an evapotranspiration 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 discussed in this paper.
By experiments, we examine the relationship of power
consumption of conventional air-conditioning system
condenser heat-transfer rate [W]
exergy [W]
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 watercooled system are measured to confirm the possibility
of reducing those temperatures in the new system. Performance 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 effect 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 catalogue COP of 5.68 is used as a baseline system. This conventional system has an air-cooled condenser, which is coppertubing 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 kW with a
0.25-kW pump and a 0.05-kW fan being commercially available as the minimum capacity.
Fig. 1 e Sketch of conventional AC (left) and testing AC
systems (right).
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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.
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 system was 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 summer 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 h1 flow-rate is put in front of the heat-exchanger.
Airflow is ducted by an acrylic duct.
Type-T-thermocouples are used to measure temperatures; data is connected to computer and recorded by data
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 performance
A typical ideal vapor compression cycle is sketched in Fig. 4. In
order to estimate COP of the air-conditioning system, properties of refrigerant are evaluated using REFPROP 8.0.
COPtheo ¼
h1 h4
h2 h1
COP ¼ hsys ,COPtheo
Fig. 3 e Positions of thermocouples in the air-conditioning
system.
Fig. 4 e Heat pump cycle on Peh diagram.
(1)
(2)
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Fig. 5 e Middle-condenser temperature (every minute).
In which, hsys is system correction factor and is calculated at
nominal condition by:
hsys ¼
COPcata
COPtheo n
(3)
COPtheo_n is theoretical COP at nominal condition and COPcata
is COP in manufacturer’s catalogue.
Nominal condition is assumed that: outdoor temperature of
35 C, indoor setting temperature of 27 C, indoor relative humidity is about 43%, dew point temperature is 13.5 C and evaporation temperature is 7 C lower than dew point, which is 6.5 C.
3.1.2.
3.2.
Overall heat-transfer coefficient of new heat-exchanger
Heat released by hot water is evaluated by :
_ hw cpw ðThwi Thwo Þ
q_ hw ¼ m
Heat transfer rate is also calculated by :
(6)
q_ hw ¼ Uo Ao DTlm
(7)
where DTlm ¼ DTo DTi =ðlnðDTo =DTi ÞÞ: logarithmic mean
temperature difference with DTo ¼ Thwo Tfi and
DTi ¼ Thwi Tfo.
Power consumption
For the same outdoor temperature and indoor setting temperature for the same space, cooling load required for the two
systems is assumed to be the same. Hence, we have:
Pconv COPnew
¼
Pnew COPconv
(4)
Then; power consumption of new system is estimated :
COPconv
Pnew ¼
Pconv
COPnew
ð5Þ
4.
Results and discussions
4.1.
Effects of condenser temperatures to the
performance 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
Fig. 6 e Integral power consumption vs. condenser (left) and compressor (right) temperatures.
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Fig. 9 e Temperatures of condenser vs. outdoor.
Fig. 7 e Estimated COP vs. condenser temperature.
4.2.
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 expected to achieve at lower condenser temperature, as simulation result in Fig. 7.
Fig. 8 e Waste-heat and its exergy of air-cooled outdoorunit.
Exergy of waste-heat from air-cooled outdoor unit
For air-cooled outdoor unit, hot-air flow of waste-heat contains exergy, but it is not utilized for other purposes. In
consequence, it transfers directly to the ambient air and performs some change to the surrounding air. Amount of this
exergy is calculated by:
To
(8)
E_ ¼ Q_ cond ðTao To Þ To ln
Tao
Waste-heat and its exergy of hot-air flow from air-cooled
condenser are described in Fig. 8. Because condensertemperature 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 and
water-cooled systems
Temperatures of compressor and condenser are sketched in
Figs. 9 and 10, respectively, for conventional and testing airconditioning 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 from 5 to 10 C higher than that of the
water condenser, which is nearly the same as outdoor
Fig. 10 e Temperatures of compressor vs. outdoor.
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Fig. 11 e Temperature difference between condenser and
outdoor vs. outdoor temperature.
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 temperature about 5 C Fig. 10.
Fig. 13 e Estimated COP vs. outdoor temperature.
conventional system up to more 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.
4.4.
Expected performance of the new air-conditioning
system
4.5.
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 consumption 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
Transpiration and evaporation are main principles in developing the new heat-exchanger. Transpiration helps to minimize 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 automatically and continuously along copper-tube surface is
satisfied. This experiment was carried out in summer weather
to confirm the actual performance.
Heat-transfer coefficient is evaluated in order to compare
with that of the air-cooled condenser. The heat-transfer coefficient of the proposed heat-exchanger, which is calculated
from Eq. (7), is demonstrated in Fig. 15. The overall heat-
Fig. 12 e Integral power consumption vs. temperature
difference between condenser and outdoor.
Fig. 14 e Integral power consumption vs. outdoor
temperature.
Testing heat-exchanger
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Fig. 15 e Heat-transfer coefficients of the new heatexchanger.
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system. Higher condenser temperature makes higher energy
consumption. By using ambient air to cool condenser, conventional 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 temperature 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 scholarship to student.
Fig. 16 e Outlet-air temperature of the new heat exchanger.
references
transfer coefficient is about 400e900 W m2 K1, which is
much higher than the air-cooled condenser whose range of
heat-transfer coefficient is from 50 to 90 W m2 K1 with air
velocity of 0.5e3 m s1 (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.
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units of split-type air-conditioners at low-rise residences.
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5.
Conclusions
High temperatures are measured in condenser and
compressor of conventional air-cooled air-conditioning