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Review
Solar sorption cooling systems for residential applications:Options and guidelines
R.Z. Wang*,1, T.S. Ge, C.J. Chen, Q. Ma, Z.Q. Xiong
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
Article history:
Received 27 November 2008
Received in revised form
20 January 2009
Accepted 3 February 2009
Published online 13 February 2009
Keywords:
Air conditioning
Chiller
Adsorption system
Water
Silica gel
Ammonia–water
Review
Solar energy
Residential building
* Corresponding author. Tel./fax: þ86 21 3420E-mail address: [email protected] (R.Z.
1 IIR-B2 vice president and member of IIR0140-7007/$ – see front matter ª 2009 Elsevidoi:10.1016/j.ijrefrig.2009.02.005
a b s t r a c t
Solar powered sorption cooling systems have been researched and demonstrated in recent
years, which contain adsorption cooling, absorption cooling and desiccant cooling. The
various typical systems with small scale for potential residential applications are discussed
and analyzed, in which the working principals, system suitability for solar cooling,
performance, maintenance and economic viability have been discussed in this paper. With
such analyses and the available real operation systems, the detailed options and guidelines
of solar cooling for residential applications are shown.
ª 2009 Elsevier Ltd and IIR. All rights reserved.
Systemes de refroidissement a sorption solaire pour lesapplications residentielles : options et recommandations
Mots cles : Conditionnement d’air ; Refroidisseur ; Systeme a adsorption ; Eau ; Gel de silice ; Ammoniac–eau ; Enquete ; Energie solaire ;
Immeuble d’habitation
6548.Wang).
strategic committee.er Ltd and IIR. All rights reserved.
Fig. 1 – Conventional adsorption refrigeration systems. (a)
Basic refrigeration system. (b) Continuous refrigeration
system.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 639
1. Introduction
Energy consumption in buildings has been increased in recent
years with the development of the economy worldwide, the
energy consumed by buildings accounts for 30% of the total
energy used. Thus solar heating integrated with buildings has
been thought to be an efficient way to reduce building energy
consumption. Solar energy can provide heating, cooling, hot
water and even electricity and lighting for buildings.
There are four steps of solar thermal applications world-
wide: (1) hot water supply usually 2–4 m2 with 150–300 L per
family, for example in China there are about 90,000,000 m2
cumulative installed aperture solar collectors in use in 2008;
(2) solar energy for hot water supply in the whole year and
heating in winter, a family may need 20–50 m2 solar collector
for winter floor radiation heating, such modules have been
becoming common in Germany and other EU countries,
demonstrations in China have been well accepted now; (3)
solar cooling in summer, which is based on the solar collectors
integrated on building roof, in which a 5–10 kW cooling is
welcome for residential applications. Such demonstrations
have been well accepted in Europe (Henning and Wiemken,
2007; Balaras et al., 2007), a lot of newly start-up companies
have joined the marketing processes; (4) solar integrated
energy systems for a building, in which solar heating in
winter, cooling in summer, hot water supply in the whole year
and also solar enhanced natural ventilation in spring and
autumn have been well considered (Zhai et al., 2007).
A real success of solar integrated energy system requires
a good solar cooling system, which is in good match with the
solar collector integrated on the roof or wall. There are already
a lot of demonstrations of solar air conditioning systems in
public buildings such as office buildings or workshops (Wang
and Dai, 2006), LiBr–water absorption systems were usually
adopted (Henning, 2007), silica gel–water adsorption chillers
have been also welcomed for such applications (Wang and
Oliveira, 2006). Solar cooling systems with solid desiccant or
liquid desiccant cooling have been considered reasonable in
humid areas, in which dehumidification is considered
important, or dehumidification is integrated with water
evaporative cooling (Daou et al., 2006). The above solar sorp-
tion systems have been also incorporated with a normal vapor
compression cooling system to ensure the full availability of
heating or cooling for buildings, or a backup heating might be
used to drive solar sorption cooling system. The problems for
all available solar sorption air conditioning systems are
mainly high cost, not easy to maintain, not as reliable as that
of electric driven systems, big size and also need backup
energy systems. The available products of sorption chillers are
usually with cooling power above 100 kW.
A lot of efforts have been taken regarding solar sorption
cooling for residential applications. There are absorption,
adsorption and desiccant cooling systems with various oper-
ation cycles powered by 60–90 �C hot water, or 150 �C water
vapor. The required cooling capacity should be less than
10 kW for residential buildings, better with a range of 3–10 kW.
Regarding solar sorption systems, the solar COP (the ratio of
cooling power to the total heat extraction by total solar input)
is usually in the range of 0.15–0.6 depending on the solar
heating efficiency and the thermal COP (ratio of cooling power
to the thermal energy consumed to heat regeneration air) of
sorption cooling systems.
In this paper the feasibilities, economics, technical merits,
maintenances, etc. of the above solar sorption cooling
systems are discussed and analyzed. Detailed suggestions and
guidelines are proposed for the proper means of solar air
conditioning.
2. Adsorption chiller
Adsorption chiller using silica gel–water working pair has
been well accepted for solar cooling, the reason is that such
cooling system can be powered with 60–80 �C hot water
directly. Adsorption chiller can match solar collector water
heating for the whole day due to its possibility driven with
60 �C hot water (Wang and Oliveira, 2006).
The working principle of an adsorption chiller can be
described schematically as in Fig. 1. Two valves are necessary
to fulfill the adsorption and desorption processes of a basic
adsorption refrigeration system (shown in Fig. 1(a)). During
the desorption process, the valve between adsorber and
condenser is opened while the other one is closed, so the
refrigerant desorbed from adsorber is condensed in the
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0640
condenser. After the desorption process, the adsorber is con-
nected to the evaporator via the valve between adsorber and
evaporator. For a basic adsorption refrigeration system, the
cooling power output is intermittent. In order to get a contin-
uous cooling power output, an adsorption refrigeration
system including two adsorbers, one condenser and one
evaporator should be adopted (shown in Fig. 1(b)). Thus at
least four valves should be adopted in the system. Since
a mass recovery processes could greatly improve the perfor-
mance of adsorption refrigeration system (Wang, 2001),
a valve to connect the two adsorbers for mass recovery could
be installed which will improve the COP significantly. In
addition, heat recovery between the two beds should be
considered to improve the COP. Since density of water vapor is
typically low, big vacuum valves are necessary in order to
have a large water vapor passage, which results in a high cost
of vacuum valves. In a real system, vacuum valves installed
between the adsorber and the evaporator/condenser can be
canceled by integrating the adsorber, condenser and evapo-
rator into one vacuum chamber, hence the cost of the vacuum
valves can be reduced.
2.1. Heat source needed related to solar heating/hotwater
Since the silica gel–water adsorption chiller can be driven by
low-grade heat source with a temperature lower than 90 �C, it is
attractive to be used in a solar cooling system. For a solar
cooling system, which adopts silica gel–water adsorption
chiller, the most commonly used solar collectors are flat plate
collector and evacuated tube collector. When the heat source
temperature ranges from 45 to 95 �C, flat plate solar collectors
can be used (Varga et al., 2005). Zhai et al. (2007) have concluded
that a silica gel–water adsorption chiller can be operated more
than 8 h daily for continuous air conditioning when powered
with solar water heating, the thermal COP could be as high as
0.3 when 60 �C hot water is used for generation.
Fig. 2 – Two-stage silica gel–water ads
2.2. Prototypes or products
Adsorption chillers which were produced by the Nishiyodo
Kuchouki, Co. Ltd., appeared in the market in 1986. Chillers
produced by this company are sold in the American market by
the HIJC USA Inc (HIJC USA Inc, 2005). The chillers can be driven
by heat source temperature from 50 to 90 �C, and the temper-
ature of the chilled water can be as low as 3 �C. When the chiller
is driven by the hot water at 90 �C, a COP of 0.7 can be reached.
Mycom is another company producing silica gel–water
adsorption chiller in Japan (Mycom-AdRef, 2005). The chillers
produced can be powered by hot water at 75 �C and yield chilled
waterat 9 �C witha reported COP of 0.6. And the nominal cooling
capacity of the chillers ranges from 70 to 350 kW.
Saha et al. (2001) designed a four-bed and two-stage silica
gel–water adsorption chiller (shown in Fig. 2) in order to utilize
solar energy or waste heat source of temperature from 40 to
75 �C. The cooling power and COP of the chiller were 3.2 kW
and 0.36 when the hot water and cooling water temperature
were about 55 and 30 �C, respectively. Based on this adsorp-
tion chiller, a six-bed silica gel–water adsorption chiller was
developed by Saha et al. (2006). The six-bed adsorption chiller
was operated under two modes: the single-stage mode and
the three-stage mode, which aimed to use efficiently different
heat source temperature ranges from 60 to 90 �C and from 40
to 60 �C respectively.
The available silica gel–water adsorption chiller with small
scale (less than 10 kW) is quite limited. SorTech in Germany
developed a silica gel–water adsorption chiller with a dimen-
sion of 0.795 m (length)� 1.10 m (depth)� 1.19 m (height), as
shown in Fig. 3 (Jakob, 2008; Jakob et al., 2007). The cooling
capacity is 5.5 kW at the hot water temperature of 75/67 �C
and the chilled water of 18/15 �C. The experimental results of
the chiller, which is installed in the solar cooling system at
CritrinSolar office building, showed the cooling power
increased from 1.2 to 5.5 kW when the heat source
orption chiller (Saha et al., 2001).
Fig. 4 – Silica gel–water adsorption chiller developed in
SJTU (a) Type A; (b) Type B.
Fig. 3 – Silica gel–water absorption chiller chillii� STC6
(source: SorTech, Jakob et al., 2007).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 641
temperature was from 57 to 80 �C. The specifications rated are
actually a sensible cooling power connected to a dry fan coil.
Fig. 4 shows two different types of silica gel–water
adsorption chillers developed in SJTU in recent years: Type A
(nominal cooling power: 5 kW) and Type B (nominal cooling
power: 10 kW) (Liu et al., 2005; Wang et al., 2005).
There are two vacuum chambers in Type A adsorption
chiller, each with one evaporator, one condenser and one
adsorber. Vacuum valves are not adopted in this chiller due to
the integration of evaporator, condenser and adsorber into
one chamber. A mass recovery-like process, which is actually
a heat recovery process between the two evaporators, is
carried out. The experimental results showed that the cooling
power and COP are about 4.84 kW and 0.33 respectively when
the cooling water temperature was about 28 �C and the
evaporating temperature was 7 �C (Liu et al., 2005).
Type B adsorption chiller consists of three vacuum cham-
bers: two water chamber and one methanol chamber. Each
water chamber comprising of one evaporator, one condenser
and one adsorber. And only one vacuum valve is installed
between the two water chambers to fulfill the mass recovery
process.
Capillary-assisted evaporation is adopted to enhance the
heat transfer performance of the water evaporators and
methanol evaporator (Xia et al., 2008). Heat pipe technique is
used to output the cooling power in this chiller. The experi-
mental results showed that the cooling power and COP were
about 7.15 kW and 0.38 respectively when the hot water
temperature, cooling water temperature and chilled water out
temperature were 84.8 �C, 30.6 �C and 11.7 �C, respectively
(Wang et al., 2005).
A 1 kW adsorption cooling system was also demonstrated
by SJTU (Yang et al., 2006). The prototype has a size as 500 mm
width, 300 mm thickness, and 950 mm height. Fig. 5 shows the
outview and schematics of the prototype. At the work condi-
tion of 85 �C of heating water inlet and 28 �C of cooling water
inlet, a cooling capacity of 995 W and a COP of 0.477 can be got.
If the work condition of 85 �C of heating water inlet and 30 �C
of cooling water inlet is considered, a cooling capacity of
907 W and COP of 0.446 can be reached.
2.3. Problems of adsorption chiller
There are two main problems for silica gel–water adsorption
chiller. One is the low thermal conductivity of silica gel, which
results in large volume of the chiller. The other is the low cycle
mass which means the difference between the maximum and
the minimum water uptakes of silica gel during the whole
adsorption refrigeration cycle. In a real system, it could be
related to the refrigerant flow rate. The operation at vacuum
may have another problem for maintenance.
Due to the low COP of the silica gel–water chiller (w0.4–0.5),
the solar COP of the solar driven adsorption air conditioning
system is typically low (w0.16–0.2). Thus large solar collector
Fig. 5 – A 1 kW silica gel–water adsorption air conditioning unit developed in SJTU (Yang et al., 2006).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0642
area might be needed. Besides, a cooling water tower and a hot
water tank should also be adopted in this system. As a result,
the initial cost of the solar cooling system employing
adsorption chillers is high. According to Jakob, the initial cost
of solar cooling system in Europe so far ranges from 5 to
8 kV kW�1, but the price is expected to be reduced in the
future (Jakob et al., 2007). The future solar cooling system may
have a price 2–3 kV kW�1 when the market is extended based
upon our studies.
Thus the future main research points of solar driven
adsorption refrigeration systems concentrate on the
improvement of silica gel–water adsorption chillers and the
control strategy of adsorption chillers under a variable heat
source temperature. Performance of the chiller should be
improved in order to get a high COP. Firstly, composite
adsorbent (SWS) (Aristov et al., 2002), which is made from
silica gel and metal salt (LiCl or CaCl2), should be developed to
obtained a high cycle mass. As well known, the metal salt of
composite adsorbent will become a solution during the
adsorption process, which will lead to the corrosion of
adsorber metal. As a result, the composite adsorbent should
be able to avoid the corrosion problem. Secondly, heat transfer
between the adsorbent particles and at the interface of the
adsorbent and heat exchanger should be improved in order to
reduce the volume of the adsorption chiller. Since the silica
gel–water adsorption chiller operates under a low pressure,
the mass transfer should be taken into consideration while
enhancing the heat transfer. Finally, the compact adsorption
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 643
chiller integrated with cooling water tower, chiller water tank
and other components, which aims to reduce the electricity
consumption and promotes the installation’s convenience, is
also one of the main research points in the future.
Due to the variation of solar energy, the adsorption chiller
is operated under a variable heat source temperature. As
a result, the control strategy of the chillers should be opti-
mized in order to obtain a high solar COP.
3. Absorption chiller using water–ammonia
Most of the thermally driven cooling systems today, including
solar air conditioning systems, are based on absorption
chillers. Actually the absorption chillers have been produced
commercially for many years. Fig. 6 shows a schematic of the
working principle of the single effect ammonia/water
absorption chillers, without GAX cycle (a) and with GAX cycle
(b) (Gomez et al., 2008).
In a single effect ammonia–water absorption chiller, the
absorbent is the solution of water and ammonia, and
the refrigerant is ammonia. It includes four main parts, the
generator (including the rectifier), the condenser, the evapo-
rator and the absorber. The rich solution (ammonia as solute)
in the generator is heated and separated into the weak solu-
tion and the ammonia vapor, and the ammonia vapor is
condensed in the condenser, and then the condensed
Fig. 6 – Schematics of the working principle of the single
effect ammonia/water absorption chillers. (a) Normal
absorption cooling cycle, (b) GAX absorption cooling cycle
(Gomez et al., 2008).
ammonia goes through the throttle valve and is evaporated in
the evaporator, where the cooling is produced. The evapo-
rated ammonia is absorbed by the weak solution, which is
from the generator, to become the rich solution. The rich
solution is sent into the generator by a solution pump.
Compared to the single effect ammonia absorption cycle,
the GAX (generator–absorber exchanger) one needs higher
heat source temperatures. In the GAX absorption cycle, the
concentration difference between the rich solution and the
weak solution is large. It is possible that the temperatures of
part of the absorber are even higher than that of the generator,
so an extra internal heat recovery subcycle may be included,
in which part of the absorption heat is recuperated by the
generator. By doing this, the thermal energy needed is greatly
decreased and the COP can be increased by 30% (Jakob and
Pink, 2007).
3.1. Heat source needed related to solar heating/hotwater
Flat plate collectors, evacuated tubes, and concentrating
collectors can be used to supply the heat to the absorption
chillers. For the solar collectors which are capable of working
at 80–120 �C, a single effect ammonia–water absorption
machine can be considered, whose COP is 0.3–0.7. For the GAX
cycles, the minimum driving temperature of 160 �C is
required, and the COP can reach 0.75. The COP may increases
to 1.0 when the driving temperature reaches nearly 200 �C
(Sabatelli et al., 2007).
3.2. Prototypes or products
The company SolarNext in Germany distributes a 10 kW single
effect ammonia–water absorption chiller which uses a newly
developed membrane pump, as shown in Fig. 7 (Jakob and
Pink, 2007). The chiller is developed for residential and
commercial heating/cooling applications. For air condi-
tioning, the driving temperatures are 75–68 �C, when the
cooling water temperatures are 24–29 �C and the chilled water
temperatures are 19–16 �C for cooled ceilings. At this oper-
ating condition, the COP reaches 0.64. When the chilled water
temperatures of 12–6 �C are requested for fan coils’ units, the
driving temperatures of 85–78 �C are required.
The chiller chillii� PSC10 has been used in the new training
centre and the office building of Bachler Austria, where 40 m2
flat plate collectors and three hot water storage with 1.5 m3
each are installed to support the solar heat needed. The chiller
supplied 16–19 �C chilled water for ceil radiation cooling with
a cooling capacity 9 kW, meanwhile a 26 kW wet recooling
tower is used.
The company Robur in Italy produces a kind of directly air-
cooled ammonia/water absorption chillers as shown in Fig. 8,
whose cooling capacity is 17 kW. The chiller is originally
designed to use the direct fired gas. However, it can be
modified to be driven by pressurized water of a Fresnel
collector (Haberle et al., 2007), and the driving temperatures
required are 180–200 �C.
A prototype of an ammonia–water absorption heat pump
system operated with solar energy was built in Gazi
University in Turkey (Sozen et al., 2002), which is shown in
Fig. 7 – Ammonia–water absorption chiller chillii� PSC10 and its demonstration in the Training centre and office building
Bachler Austria (from SolarNext, Germany, Jakob and Pink, 2007).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0644
Fig. 9. The prototype used a parabolic slot type collector with
an electric control system through north to south axes
following the solar beam to supply the high temperature
water up to 100 �C in order to drive the generator. In the
experiments, the generator temperature at 90 �C was
obtained, which is the optimum generator temperature to
get the best COP according to the theoretical computations.
The evaporator temperature could reach to as low as 3 �C,
thus it is possible to be used for air conditioning and pres-
ervation of food.
A 2 kW prototype of a low-power ammonia–water
absorption driven by solar energy was constructed by
University of Madrid in Spain (De Francisco et al., 2002), which
is shown in Fig. 10. The condenser and the absorber were air-
Fig. 8 – Ammonia–water absorption chiller ACF60
cooled by natural convection, so no cooling tower was
involved. The prototype used a transfer tank instead of the
solution pump. The transfer tank is controlled by valves to
alternatively connect to the zones of high and low pressure,
and the solution can be discharged by the opening and closing
of the valves. In this prototype, a parabolic cylindrical
collector was used, which reached temperatures exceeding
150 �C. The heat was transmitted to the generator by a thermal
oil circuit. Unfortunately, this newly design (the transfer tank
instead of the solution pump) was not operating well, and the
experimental COP was lower than 0.05.
The University of Applied Sciences, Stuttgart in Germany
developed a solar power ammonia–water diffusion absorption
machine, whose cooling capacity is 2.5 kW at temperatures
-00 (from ROBUR, Italy, Haberle et al., 2007).
Fig. 9 – Schematic of the ammonia–water absorption heat pump operated with solar energy by Gazi University in Turkey
(Sozen et al., 2002).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 645
between �10 and 5 �C, heating by CPC (compound parabolic
concentrators) vacuum tubes’ collectors (Jakob and Eicker,
2002), as shown in Fig. 11. Helium was used to keep the
pressure equilibration between the high and the low pressure
sides. The generator heating inlet temperatures were from 150
to 170 �C. In this case, the best ever reached cooling capacity
was 1.5 kW, and the COP values were between 0.2 and 0.3.
The Technical University Graz in Austria and the company
Heliotherm/Helioplus developed a prototype of an ammonia/
water absorption chiller with 5 kW cooling capacity (Moser
and Rieberer, 2007). An ammonia/water absorption chiller
prototype with 10 kW cooling capacity was also developed at
the ITW Stuttgart in Germany (Zetzsche et al., 2007). There are
also other prototypes or products which are developed by ABB,
Colibri, Mattes, etc. (Sabatelli et al., 2007).
More recently, researchers in Shanghai Jiao Tong Univer-
sity tested the feasibility of single effect ammonia–water
absorption system for heat or cold transportation over long
distance (Ma et al., 2008).
Both the mathematic model and experimental set-up of
a long-distance thermal energy transportation system are
built and analyzed, and satisfactory and attractive results are
obtained. When a steam heat source at 120 �C is available, the
user site can get hot water output at about 55 �C with the
thermal COP of about 0.6 and the electric COP (ratio of cooling
power to the electrical power consumed in the system) of
about 100 in winter, and cold water output at about 10 �C with
the thermal COP of about 0.5 and the electric COP of 50 in
summer. The work shows that the ammonia–water absorp-
tion system is suitable to transport thermal energy over long
distances.
3.3. Problems of ammonia–water absorption chiller
In general ammonia–water absorption chiller is cheap to
produce and with easy maintenance. It could be advantageous
for residential solar applications. The main problems of the
ammonia–water absorption chiller in solar powered air
conditioning systems are as follows:
1) At low driving temperature, the efficiency of the single
effect cycle is lower in comparison with the single effect
LiBr–water absorption chiller. If the normal solar collector
is used to power the system, it can only supply high
temperature chilled water (15 �C or higher), thus without
capability of dehumidification.
2) In order to drive an ammonia–water absorption chiller with
GAX cycle, higher heat source temperature is needed, more
expensive solar collector types are required, such as CPCs.
3) In a small scale ammonia–water absorption chiller, the
power consumption of the solution pump cannot be
neglected, and the price of the pump is high.
4) Ammonia is toxic and harmful to people, so the location of
the chillers should be carefully considered. Normally they
are located outdoors.
For residential solar systems, the air-cooled ammonia–
water absorption chillers are suggested, and the air-cooled
condenser and absorber could be integrated inner the chiller
to make it compact. The solution pump could be removed if
a diffusion ammonia–water absorption cycle is introduced.
However, the efficiencies of a diffusion absorption cycle are
usually lower than the traditional one, so more solar collectors
are required, and consequently the investment cost would be
increased. Above all, the ammonia–water absorption chillers
have been proved successfully operated, but the costs have to
be further reduced.
4. Liquid desiccant (LiCl–water) cooling
Liquid desiccant cooling system, as a potential alternative to
conventional vapor compression air conditioning system, is
operated based on liquid desiccant’s strong affinity to water.
Fig. 10 – The prototype and operation principle of the ammonia–water absorption system by University of Madrid in Spain
(De Francisco et al., 2002).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0646
Liquid desiccant cooling system can independently control air
humidity and temperature, and provide high quality air.
Compared with absorption system, it works under atmo-
sphere pressure without the capital-intensive pressure-sealed
units. Efficiently utilizing low-grade heat source is another
merit of liquid desiccant cooling system. Theoretical simula-
tions and preliminary experiments on core components
(dehumidifiers and regenerators) have been well explored
(Alizadeh, 2008).
The working principle of a basic liquid desiccant cooling
system is shown in Fig. 12. Process air is dehumidified by
concentrated liquid desiccant solution in a dehumidifier
(DEH), and then further cooled by the cooling water
provided by direct evaporator (HE5), or vapor compression
system, etc. After dehumidification, the liquid desiccant
solution needs to be regenerated back to its original
concentration. Before entering regenerator, the liquid
desiccant solution is heated by hot regenerated desiccant
Fig. 11 – Principle and prototype of the solar driven
diffusion absorption cooling by University of Applied
Science, Stuttgart in Germany (Jakob and Eicker, 2002).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 647
solution (HE2 for heat recovery) and hot water (HE3) in
series. To reduce the heat loss in regeneration air, air to air
heat exchanger is adopted in regeneration. Solar hot water
or solar heated air could thus be used for regeneration of
liquid desiccant.
4.1. Heat source needed related to solar heating/hotwater
Liquid desiccant incorporated with solar collector is one of the
popular research fields. Liquid desiccant cooling system
DEH
Regeneration airWaterLiCl
REG
Process air
DEH - dehumidifier;REG - regenerator;HE1 - liquid desiccant pre-cooler;HE2 -heat recuperator;HE3 - liquid desiccant pre-heater;HE4 - air-air heat exchanger;HE5 - water-air heat exchanger
Fig. 12 – Basic liquid desiccant cooling systems.
shows great potential to be powered by solar energy due to the
following two reasons. Firstly, liquid desiccant cooling system
can be powered by 50–80 �C heat sources, even as low as 40 �C
(Jain and Bansal, 2007), which could be provided by flat plate
solar collector. Secondly, Liquid desiccant could have energy
storage capacity which can overcome the problem of the
unavailability of solar heat source at night. Three kinds of
regenerators powered by solar energy exist, i.e. open regen-
erator/collector, closed regenerator/collector, and regenerator
plus collector (Mei and Dai, 2008). Even though the former two
kinds of regenerators are proven to have higher regeneration
efficiency with compact structure, regenerator plus collector
is commonly adopted due to the more stable regeneration
performance.
4.2. Prototypes or products
One of the most famous prototype of solar driven liquid
desiccant system for cooling, dehumidification and air
conditioning is the system built by Gommed and Grossman in
Haifa, Israel (Gommed and Grossman, 2007), which is shown
in Fig. 13. The system has been operated since April 2003. Its
average dehumidification capacity reaches 16 kW powered by
20 m2 solar collector area. Energy is stored by 120 L LiCl solu-
tion and 1000 L hot water, aiming to store energy enough for
4 h continuous operation without solar insolation. The
thermal COP based on the heat gained from solar collector, is
about 0.8. Air absolute humidity ratio is reduced from
16 g kg�1 dry air to 8 g kg�1 dry air in a typical August day.
In China, Tsinghua University demonstrated a series of
liquid desiccant cooling systems. In 2003, first fresh air liquid
desiccant cooling system driven by waste heat of heat pump is
installed in a hospital in Beijing, shown in Fig. 14 (Li et al.,
2005). Liquid desiccant cooling system is incorporated with
total heat recovery device and heat pump. The maximal
design fresh air flow rate is 4000 m3 h�1, supplied to an
emergency ward with an area of 300 m2. It is reported that the
EER (energy efficiency ratio, defined as the ratio of the cooling
capacity (or heating capacity in winter operation) gained by
the fresh air to the power consumption) of the system is in the
range of 6.3–7.3 in summer and 4.7–5.0 in winter. Then
a hybrid liquid desiccant cooling system driven by waste heat
from BCHP system is developed in a 10-story office building
(Liu et al., 2006), as shown in Fig. 15. The operating hours of the
cogeneration system are lengthened by adding the liquid
desiccant cooling system. The average COP of the desiccant
system is about 1.0. A hybrid system combined by a waste
heat powered liquid desiccant cooling system and a solar
powered adsorption system, shown in Fig. 16, was built by
Tsinghua University and Shanghai Jiao Tong University (SJTU)
(Ma et al., 2006).
A two-stage solar liquid desiccant (LiCl) cooling system
assisted by CaCl2 solution has been designed by SJTU as
shown in Fig. 17. Its design dehumidification capacity is 3 kW.
Test has been carried out on two-stage liquid desiccant cool-
ing unit driven by 65–70 �C hot water. The thermal COP is
about 0.7. To better explore the liquid desiccants’ energy
storage capacity, liquid desiccant concentration variance is
increased to 5%.
Fig. 13 – Photograph of the liquid desiccant system (Gommed and Grossman, 2007).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0648
4.3. Problems of liquid desiccant cooling
The major problems concerned with liquid desiccant cooling
system are corrosive caused by inorganic salts and carryover
of liquid desiccant in air. Thus polymer based materials are
selected for the components to solve corrosive problem.
Demisters or filters are adopted to prevent the carryover of
liquid desiccant.
The following guidelines are suggested in constructing
solar powered liquid desiccant cooling system for residential
uses:
1) Low cost solar collector, such as flat plate solar collector or
collector/regenerator, is suggested to power the system.
2) Appropriate control strategy shall be developed to coordi-
nate liquid desiccant concentration and energy storage.
3) Sensible cooling unit needs to be carefully chosen accord-
ing to the climate and initial cost.
4) A hybrid energy system to use heat pump and solar energy
is recommended. Liquid desiccant cooling can be thereby
efficiently used.
5. Solid desiccant cooling
Solid desiccant cooling system is a good alternative to
conventional vapor compression (VC) system due to its
energy saving and CFC-free characteristics. Also, compared
with the liquid desiccant system in which the liquid and air
directly interact, the solid one is compact and less subject to
corrosion.
The working principle of a solid desiccant cooling system is
shown in Fig. 18. In process air side, ambient air flows through
the desiccant wheel in which the latent load is removed by the
adsorption of desiccant material. Then a sensible heat
exchanger is adopted in the system to remove the releasing
Fig. 15 – Liquid desiccant cooling systems driven by waste
heat from BCHP (Liu et al., 2006).
Fig. 14 – Fresh air liquid desiccant cooling system driven by
waste heat of heat pump by Tsinghua University (Li et al.,
2005).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 649
adsorption heat and preheat regeneration air. Usually, an
evaporative cooler is installed before process air is supplied to
the conditioned room to adjust the temperature and humidity
ratio of supply air.
Simultaneously, in regeneration air side, return air from
the conditioned space is cooled in an evaporative cooler and
then flows through the heat exchanger to cool process air.
Afterwards, regeneration air is heated up in the air heater to
required temperature and used to regenerate the desiccant
material. The psychrometric chart representation for desic-
cant cooling system is also shown in Fig. 18.
5.1. Heat source needed related to solar heating/hotwater
The most commonly used adsorbents in desiccant wheels are
LiCl, silica gel and molecular sieve. LiCl is the first adsorbent
which is adopted as desiccant material in desiccant wheel.
Compared with silica gel and molecular sieve, LiCl has better
moisture removal capacity and its required regeneration
temperature is among 60 �C and 120 �C. Silica gel is another
widely used adsorbent in desiccant wheel. It has better
stability compared with LiCl and the required regeneration
temperature is between 80 �C and 150 �C. Besides, lots of
researches are focused on using molecular sieve as the
desiccant material due to its good dehumidification capacity
under the condition of lower humidity ratio. On the other
hand, its regeneration temperature is normally higher than
160 �C which hinders the application of low-grade thermal
energy such as solar energy. However, desiccant cooling
systems which adopt LiCl or silica gel as the desiccant mate-
rial still can be driven by solar energy. Flat plate collector can
be used in solid desiccant cooling system when the required
regeneration temperature is not very high. And for the higher
regeneration temperature, solar collector like vacuum tube
collector is recommended in this system.
In order to reduce the regeneration temperature and make
good use of solar energy, researchers in SJTU have conducted
many investigations during the past years. They proposed
a novel compound silica gel–haloid desiccant wheel, which
can work well under lower regeneration temperature and
achieve higher dehumidification capacity due to the contri-
bution of new composite desiccant material. It is indicated
that the moisture removal capacity improves about 20–30%
compared with regular silica gel one (Jia, 2006; Jia et al., 2007).
Recently, Mitsubishi Chemical developed a new adsorbent
called FAM, which can utilize heat source below 100 �C (Mit-
subishi Plastics, AQSOA web site). Recently, a desiccant rotor
using FAM (now it is called AQSOA) is developed as
a commercial product by Mitsubishi Plastics, Inc (Shimooka
et al., 2007). The regeneration temperature of AQSOA
Fig. 16 – Scheme of a hybrid system by solar adsorption
cooling and desiccant dehumidification (Ma et al., 2006).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0650
honeycomb rotor can be as low as 40 �C, according to their
brochure. Mitsubishi Plastics and MYCOM have also devel-
oped an adsorption chiller using AQSOA, which can be driven
by 60–80 �C heat source. Besides, an desiccant rotor made of
polymer desiccant is developed by Japan Exlan Co., Ltd (Japan
Exlan web site). The "Eks-Rotor" can be regenerated under
80 �C (Inaba et al., 2002).
Isothermal dehumidification is thought as one of ideal air
conditioning process with the smallest irreversibility. When
the air flows alternately over infinite desiccant wheels and
intercoolers, its thermodynamic process would be close to
isothermal. With other working conditions being unchanged,
the regeneration temperature of an ideal infinite multistage
desiccant cooling system is the minimum. Based on this
principle and by the use of the newly developed compound
desiccant material, a novel two-stage desiccant cooling
system (TSRDC) has been proposed in SJTU to reduce the heat
source temperature (shown in Fig. 19, Ge et al., 2007). Under
ARI (American Air conditioning and Refrigeration Institute)
summer condition, results indicate that TSRDC can provide
satisfied supply air when regeneration temperature is not
lower than 60 �C and at 60 �C the corresponding thermal COP
is as high as 1.16. Also thermal COP of the system keeps over 1
if regeneration temperature is not higher than 80 �C. TSRDC
has the merits of lower regeneration temperature and rela-
tively high thermal COP.
Therefore, plate collector can be adopted in this system.
More recently, two-stage dehumidification process is
realized in one desiccant wheel by the researchers in SJTU
(shown in Fig. 20). Experimental results showed that this
system not only inherits the merits of TSRDC but also greatly
reduced the size of two-stage system. Such concept has now
been utilized for our real demonstration solar desiccant
cooling system. With the application of the novel compound
silica gel–haloid desiccant wheel, and also double stage, the
required regeneration temperature could be from 50 to 80 �C.
SJTU is now using solar air collector for the regeneration of
desiccant wheel, thus the air–water heat exchanger can be
removed in air handling unit (AHU) and its size can be
reduced.
5.2. Prototypes or products
A typical solar desiccant wheel cooling system (principle
similar to Fig. 18) was installed at the building of the chamber
of trade and commerce in Freiburg, Germany (Fig. 21) (Hen-
ning, 2007). Solar air collector was used as the only heat source
with an area of 100 m2. The adopted silica gel rotor is with air
flux of 10,200 m3 h�1. The main purpose of the system is to
provide supply air to the seminar room. It was showed that
this system is a promising concept for building with a high
similarity of cooling load and solar gains.
There is one example of solar desiccant air conditioning in
Himin Solar Co., China, in which an air solar collector inte-
grated on the roof of building is used to drive desiccant wheel
evaporative cooling system. The building has 300 m2
construction area, the typical run shows a dry bulb tempera-
ture of 24.2 �C and relative humidity 54% RH were reached for
the conditioned rooms, while the outdoor temperature was
29.3 �C and relative humidity 36.2% RH. The desiccant evap-
orative cooling unit outputs air 20.3 �C and 76.2% RH.
A desiccant-basedevaporative cooling system has been built
in the test house at Canadian Center for Housing Technology
(CCHT) test house in Ottawa to improve the energy efficiency of
the houses (National Research Council Canada, 2008).
Recently a demonstration unit of 10 kW solar desiccant
cooling system developed by SJTU is installed in Jiangyin
Wanlongyuan Solar Co., China, shown in Fig. 22. The system
uses plate type solar collector of about 70 m2, a two-stage
desiccant dehumidification wheel with evaporative cooler,
the air condition area is about 200 m2. A normal water chiller
is incorporated with the desiccant system in the air handling
unit to ensure the availability of cooling and heating all the
year when needed. The experimental results show that: 1) for
the two-stage desiccant cooling system, when ambient air is
of 35 �C and 23.2 g kg�1. The system can provide the supply air
with temperature of 25 �C and humidity ratio of 17.1 g kg�1.
The cooling power, thermal COP and electrical COP of the
system are 11 kW, 1.1 and 8.3 respectively. 2) For the heat
pump air conditioning system, if ambient air is of 29.1 �C and
processair
solarcollector
strongsolutiontank 1
strongsolutiontank 2
dilutesolutiontank 1
dilutesolutiontank 2
REG2 REG1
pump 1 pump 2pump 3pump 4
pump 5
air
water
CaCl2
LiCl
DEH1
coolingwater
coolingwater
DEH2
Fig. 17 – Scheme of the solar powered two-stage liquid desiccant cooling system.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 651
14 g kg�1. Supply air is with temperature of 21.6 �C and
humidity ratio of 14 g kg�1. The cooling power and electrical
COP are 17.7 kW and 2.96 respectively. 3) For the integrated air
conditioning system, its electrical COP is about 3.92 which is
higher than the separated heat pump system. Also, the power
consumption of the integrated system is 24.5% less than the
conventional air conditioner system.
The more recent research work on solar desiccant cooling
in SJTU focuses on hybrid energy system as shown in Fig. 23,
in which solar air collector and gas boiler are both used to
obtain solid desiccant cooling, heating and hot water supply.
The hybrid energy system may contain 10 m2 solar collectors,
to reach 5 kW cooling for residential uses, the expected
thermal COP is about 1, solar COP about 0.5.
5.3. Problems of solid desiccant cooling
When solid desiccant cooling system is applied to residential
building, it is likely that the operating cost can be significantly
reduced due to the utilization of thermal energy. The initial
Sensible heat exchanger Evaporative cooler Supply airFresh air
Air heater
Desiccant wheel
Pump
AuxiliaryHeater
SolarCollector
Water Tank
Hot water
Process airRegeneration air
1 2 3 4
5Return air
6789
0 10 20 30 40 50 60 70 80 90 1000.00
0.01
0.02
0.03
0
10
20
30
40
50
60
70
80
90
6
Hum
idity ratio (kg/kg)
Dry bulb temperature (ºC)
9
7
18
54
3 2
Ent
halp
y (k
J/kg
)
100%
RH
80%
RH
60%
RH
40%
RH
20%
RH
10%
RH
5%R
H
2%RH
1%RH
Fig. 18 – Operating principle of solid desiccant wheel cooling and its psychrometric chart.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0652
cost for desiccant cooling system can be also reduced in
comparison with other sorption systems. Such system is very
reliable and easy for maintenance. If a backup heating is used,
a thermal COP of about 1.0 can still be reached (solar COP
above 0.4–0.6). The main problem could be the big size of the
AHU. In order to make solid desiccant cooling system more
suited for residential application in the future market,
following guidelines are suggested:
1) Develop novel desiccant material to obtain better dehu-
midification capacity at lower driven temperature. In this
case, the use of compound adsorbent (silica gel and LiCl/
CaCl2, like SWS) is quite reasonable, however the desiccant
321
Exhaustair
HeatExchanger 1
1111114 13 15 12
Ambient air
Exhaustair
Exhaust air
DesiccantWheel 1
DesiccaWheel
Fig. 19 – Schematic dr
materials should be stable all the running year, the formed
salt liquid should be contained inside silica gel matrix.
2) Use low cost solar collector, such as solar air collector, this
makes not only cost reduction of solar collector, but also
the size reduction of the AHU as no water–air heat
exchanger is needed in the AHU. A 5 kW solar system
capable of heating, cooling and hot water supply could be
with a cost of 5 kV, which means the rated kW cooling cost
is 1 kV kW�1.
3) Investigate different regeneration mode such as staged
regeneration to make good use of thermal energy. Propose
new circulation mode in solid desiccant cooling system to
lower regeneration temperature and reduce system size.
54
EvaporativeCooler 1
EvaporativeCooler 2
HeatExchanger 2
Ambient air
Return air
Supply air 6
101111
9Exhaust
air
nt2
8
7
awing of TSRDC.
Desiccant Wheel
10
Heater13
1
9 8
14
2
3
12- 1
8- 1
4
Heat Exchanger
5
6
Evaporative Cooler
Fan
11
7
Valve
Process airin the 1st stage
Regeneration airin the 1st stage
Process airin the 2nd stage
Regeneration airin the2nd stage
Regenerationair 1 & 2
Process air
12
Fig. 20 – Schematics of the one-rotor two-stage rotary desiccant cooling system in SJTU.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 653
4) Determine optimal operating strategies at various condi-
tions and means of implementing the strategies.
6. Absorption chiller using LiBr–water
The principle for solar absorption cooling could be well
understood if we consider the generation–condensation
process and absorption–evaporation process; the first process
needs heat dissipation to the environment for condensation,
while the second process needs heat dissipation for absorption.
As is shown in Fig. 24, the weak solution of LiBr–water is
pumped up to the generator in which the solution is heated by
solar heated hot water, the generated solution turns into
strong solution which returns back to the absorber via an
inner heat recovery heat exchanger. The desorbed water
vapor is condensed into liquid in the condenser, which is then
throttled and flow into an evaporator. The evaporator is con-
nected to the absorber, the strong solution absorbes water
vapor continuously and thus causes evaporation in evapo-
rator, the evaporation cooling effect in the evaporator is
transferred to a chilled water circuit heated via a heat
exchanger. A cooling water tower is needed to serve the
cooling effect to the condenser and absorber. Solar air condi-
tioning is simple as the hot water could be supplied by solar
water heater, however enough high temperature is necessary
and the solar power should match the cooling capacity of
absorption chiller properly.
6.1. Heat source needed related to solar heating/hotwater
A single effect LiBr–water absorption chiller may need a heat
source of about 88 �C or higher (with a COP over 0.6 for the
cooling water temperature about 32 �C and chilled water
about 7 �C), thus plate type solar collector is normally not
matched with its application unless the evaporation temper-
ature is increased or condensing temperature is decreased.
Solar evacuated tube collector is usually considered to drive
LiBr–water absorption chiller. When plate type solar collector
is used, a two-stage LiBr–water absorption chiller could be
required, in which 70–80 �C hot water could be used to power
the absorption cooling system (with a COP about 0.4). Solar
absorption cooling system may always meet the problem if it
can be operated for 8 h daily cooling.
A double effect LiBr–water absorption chiller may need
150 �C heat source, which means that normal building inte-
grated solar collectors cannot meet its request, a parabolic
trough solar collector is thereby needed. Double effect system
could have a solar COP of about 1.2.
Fig. 21 – Solar air collector and the chamber of desiccant
cooling system installed in Freiburg/Germany (Henning,
2007).
Fig. 22 – A solar desiccant cooling unit with 10 kW capacity
integrated with an electric cooling system.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0654
6.2. Prototypes or products
There are various solar absorption cooling systems demon-
strated, mostly with cooling capacity over 100 kW. Single
effect absorption chiller is used which is powered by 88 �C or
higher hot water if the cooling water has a temperature of
above 30 �C, this may cause a problem as the solar heating
could not guarantee 88 �C hot water for 8 h or longer, the
mostly reported results are limited to 3–4 h cooling operation
during the middle noon. Such systems need evacuated tube
solar collector. There are also efforts to use double stage LiBr–
water absorption chiller, in which 70–75 �C hot water could be
used to get cooling thermal COP of about 0.4, thus plate type
solar collector could be used for this task, the daily solar
cooling time could then be extended to 6–8 h.
One good example of solar driven cooling system was
developed in 1987 in China by Guangzhou Institute of Energy
Conversion, its cooling power is 14 kW, in which the single
effect small scale absorption chiller made in Japan, Yazaki, was
adopted. This absorption system needs a heat source
temperature of 88 �C. The similar work was then demonstrated
in the University of Hong Kong (Li and Sumathy, 2001) (shown
in Fig. 25), in which a flat plate collector array with a surface
area of 38 m2 is used to drive a LiBr–water absorption chiller
(Yasaki) of 4.7 kW cooling capacity. The system is provided
with a storage tank (2.75 m3) which is partitioned into two
parts. The study showed a total solar cooling COP¼ 0.07.
Bruno (2007) reviewed the solar driven sorption chiller
which developed nowadays in the market recently. The resi-
dential ones are summarized as following: a compact 4.5 kW
water–LiBr single effect absorption chiller is manufactured by
Rotartica S.A. When the hot water is 90 �C, system COP is
about 0.7. Besides, Phoenix Sonnen Waerme AG developed
a 10 kW water–LiBr single effect absorption chiller for low
driving temperatures, low electricity consumption and high
COP. It was reported that the chiller supplies chilled water at
15–18 �C for cold ceilings, using a hot water driving tempera-
ture of 75–65 �C and cooling water of 27–35 �C, or chilled water
at 6–12 �C in which case the required temperature for hot
water is 85–95 �C. The cooling capacity can be modulated
between 40 and 120% of the nominal capacity by changing the
driving temperature.
In order to combine the commercialized solar technology
with absorption chiller, Ma and Deng (1996) and Li et al. (1999)
had developed a two-stage LiBr–water absorption chiller,
which is driven by the heat source of 60–85 �C. The technical
Fig. 23 – The hybrid energy system.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 655
parameters and performances of the solar air conditioning
system are
1) Solar heating system: 500 m2 collector area, which supplies
30 m3 55–60 �C warm water for living, 65–75 �C hot water for
refrigeration purposes.
2) Refrigeration system: two-stage LiBr–water absorption
chiller with a cooling power of 100 kW, which needs 75 �C
hot water heat source and provides 9 �C chilled water. The
system is designed to satisfy 600 m2 air conditioning.
Such two-stage absorption system is reasonable to couple
LiBr–water absorption chiller with building integrated solar
collector. The daily operation time could be extended up to
6–8 h, the averaged thermal COP could be between 0.3 and 0.4.
But the commercial residential two-stage system is still not
available now, the reason is mainly related to the high cost of
absorption chiller and also the low interests from absorption
chiller manufacturers.
There are several demonstrations of solar powered LiBr–
water absorption cooling system, in which Yazaki-10 RT
(35 kW) absorption chillers were used, both plate type and
evacuated tube type solar collectors were used. The most
recent examples are (1) 35 kW LiBr–water single effect
absorption cooling system in Thailand as shown in Fig. 26
(Pongtornkulpanich et al., 2008), which consisted 72 m2 evac-
uated tube solar collector with LPG as backup to ensure 70 �C
hot water to get absorption cooling, the running experiences
had shown an averaged annual solar fraction of 81%. The
operation of the system is quite satisfactory in Thailand as
solar cooling is needed all the year, its main problem is the
high initial cost and the over cooling capacity of the
Generator
Condenser
Absorber
Evaporator
Chilled water
Cold water
Water vapor
Cold water
Heat exchanger
Solution pump
Water vapor
tosolar waterheater tank
Fig. 24 – Schematics of a solar driven single effect
absorption cooling system.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0656
absorption chiller. (2) 35 kW LiBr–water single effect absorp-
tion cooling system in Madrid (Syed et al., 2005), which con-
sisted 49.9 m2 plate type solar collector to drive the 35 kW
absorption chiller (requested for 5–10 kW, but market not
available), however with 5–7.5 kW cooling output, the solar
cooling COP is about 11%, the daily averaged cooling COP is
about 0.34–0.42.
The Broad Air Conditioning Co. has successfully developed
the direct fired double effect small scale LiBr–water absorption
chiller (BCT type, in which a cooling water tower is integrated
together with the chiller) with the cooling power between 16
and 500 kW and COP about 1.1, this market system might be
incorporated with high temperature solar heating system and
find the solar air conditioning market.
As is shown in Fig. 27, there are several examples of this
solar air conditioning system. The parabolic trough is efficient
Fig. 25 – Schematic diagram of a solar absorption air conditioni
account the collector, generator, chilled water and the cooling w
to get 150 �C hot vapor with heat gain efficiency of about 45–
50%. However the big systems are not easy to install, reliability
could be the main issues to solve.
Surely the double effect system driven by solar energy and
fossil energy is a reasonable way for solar air conditioning
application. The hybrid energy system might be a good
opportunity for building air conditioning in the near future.
Broad has committed to use a 20 m2 parabolic trough solar
collector, with the addition of gas/oil burner to yield 16 kW
cooling for residential buildings.
But the above system is not related to building integrated
solar energy system, an idea shown in Fig. 28, was developed
by the author to use solar water heating system to generate
the low pressure generator of the BCT chiller (Liu and Wang,
2004), while the high pressure generator is generated by direct
fired heating, such system may need good flow rate distribu-
tion control for the weak solution. In principal this concept
could be applied to the BCT absorption chiller, in which its low
pressure generator may need to be modified.
6.3. Problems of solar LiBr–water absorption cooling
The main problem for LiBr–water absorption chiller with small
scale is its cost and reliability. The small system still needs
a pump to circulate weak solution and a pump to circulate
water for falling film evaporation in the evaporator. Single
effect absorption chiller may need 88 �C or high hot water to
generate the system, which is not in good match with building
solar integrated heating system. Double stage is a good way to
match with the building integrated solar collector, 70–80 �C
hot water is acceptable, but the COP is somehow reduced to
0.4 in this case.
For the commercial single effect LiBr–water absorption
chiller, if it is used for solar cooling with heat source of
70–80 �C hot water, the option could be to have a ground
source heat sink, in which the cooling water temperature
ng system comprising four main flow circuits, taking into
ater (Li and Sumathy, 2001).
Fig. 26 – The main testing building with a 72 m2 roof-mounted solar collector array, which forms the main energy source for
the building’s 35 kW LiBr–water absorption cooling system (Pongtornkulpanich et al., 2008).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 657
could be reduced to about 20 �C, which may yield 8–10 h solar
cooling daily operation with COP of about 0.5–0.6. For the new
development of solar cooling for residential application, one
should concern the following issues as
1) Using rising film evaporation to replace falling film evapo-
ration, thus the circulating water refrigerant pump can be
waived, its initial cost and operation cost can be reduced
and also its reliability can be improved. Such work has been
demonstrated well in our adsorption chiller (Wang et al.,
2005; Xia et al., 2008; Wang, 2008).
2) Try to use the bubble pump to replace the solution pump,
there are already a lot of such work, bubble pump is suitable
Fig. 27 – A small scale double effect solar absorption air
conditioning system.
for small flow rate, thus possible to be implemented in
residential solar LiBr–water absorption chiller.
3) Double stage absorption chiller is worth to be developed in
order to couple with 70–80 �C hot water for solar cooling.
7. Recommendations and guidelines of solarsorption cooling
It is shown that solar sorption air conditioning has been
extensively researched, a lot of demonstration has been
down, specially for LiBr–water absorption chillers. But there
are limited successful demonstrations if the initial cost and
payback time are considered, most of the demonstrations
may need more than 10 years payback time. Regarding solar
residential applications, there are a research and develop-
ment wave for adsorption chillers, absorption chillers and
desiccant systems. A lot of demonstrations have been proven
successful, but the cost is still a big problem, so is the main-
tenance needed.
Fig. 28 – Schematic diagram of the solar assisted
absorption system (Liu and Wang, 2004).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0658
Solar sorption cooling for residential uses is closely
related to solar integrated heating systems for buildings in
which 40–45 �C hot water in winter could be directly used
for heating (especially floor radiation heating), 60–85 �C hot
water in summer could be used to power a sorption cooling
system. The hot water can be all year available for sanitary
hot water uses. There are several kinds of sorption cooling
systems which might be feasible for residential
applications.
1) Adsorption chiller using silica gel–water adsorption pair,
which could be powered by 60–85 �C hot water, the cooling
COP is around 0.3–0.5, the cooling capacity is usually 5–
10 kW. The cooling water tower should be integrated with
the chiller. The expected price with market acceptability of
solar cooling system could be 2–3 kV kW�1, the expected
adsorption chiller price could 1 kV kW�1 when the market
is well extended.
2) Absorption chiller using ammonia–water, could be gener-
ated by 80 �C hot water if the chilled water temperature is
as high as 15 �C, the cooling COP is about 0.4, such system
can be air-cooled (no need for cooling water tower), the big
advantages of such system could be really no need of
maintenance as it works at above ambient pressure. It is
specially suited for those areas lack of water. Generally the
price of ammonia absorption chiller should be cheaper
than adsorption chiller and LiBr–water absorption chiller.
But if 7 �C chilled water is required, the required generation
temperature could be as high as 100 �C, for which normal
solar collector cannot be applied.
3) Liquid desiccant (LiCl–water) cooling system driven with
50–80 �C hot water may have a cooling COP of about 0.6.
Various means to prevent corrosive liquid flow into air
ventilation piping and the prevention of the leak and
corrosion of the liquid desiccant in AHU itself should be
well taken. Such system is well for dehumidification and
bacteria sterilization. It might be of big sizes due to the need
of several heat and mass transfer units. The combination
with electric heat pump has proved that such system is
effective, with total electric COP of above 5.
4) Solid desiccant cooling system driven by hot air (50–80 �C)
may reach a thermal COP of above 1, in which desiccant
wheel with high uptake capability of moisture water from
air is used, an evaporative cooler is incorporated together.
This system could be more compact if solar air collector is
used to replace solar water heating system. This solar
system is actually a solar heating system plus an AHU, the
centralized air conditioning AHU send treated air to the end
user rooms.
5) Absorption chiller using LiBr–water, with two-stage set-up
which could be driven by 70–80 �C hot water (COP w 0.4), or
one stage driven by 90 �C hot water (COP w 0.6). Double
effect LiBr–water absorption chiller powered by direct firing
incorporated with solar parabolic troughs (to yield 150 �C) is
another choice, such system could have a thermal COP of
about 1.1.
It seems that desiccant cooling is with high COP and easy
maintenance, solid desiccant cooling incorporated with solar
air collector might be a good choice for future residential
uses, the total system price could be possibly controlled for
less than 1 kV kW�1. But this system is somehow an air
handling unit to supply cold or hot air to the conditioned
space.
For the sorption chillers, silica gel–water adsorption chiller
might be a good choice for small cooling power, in which no
moving parts or pumps are needed, the chiller could be
incorporated to ceil cooling using earth tubes or normal fan
coil units, the daily operation time could be 8 h longer, the
system price could be controlled at 1–2 kV kW�1 in the future.
For other sorption systems, we need more research and
development for small cooling power systems with low cost
and good reliability.
Above all are not all for the work of solar sorption cooling
for residential uses. Thermal storage is another important
issue as residential use may need cooling over the night. The
thermal storage requested is to have PCM materials with
transition temperature of about 75 �C. But if liquid desiccant
cooling is used, the weak solution could be generated during
the sunshine day, the generated strong solution could be then
used for cooling over the night, and in this case one may need
a solution tank for ‘‘cooling’’ storage.
Acknowledgments
This work was supported by National Key Technologies R&D
Program under the contract no. 2006BAA04B03, and also by
Hi-Tech Research and Development Program of China (863)
under the contract no. 2006AA05Z413.
The authors thank Elsevier for the kind permission to use
Figs. 2, 3, 5–11, 13–16, 21, 25, 26 and 28 from the references.
r e f e r e n c e s
Alizadeh, S., 2008. Performance of a solar liquid desiccant airconditioner – an experimental and theoretical approach. SolarEnergy 82, 563–572.
Aristov, Yu.I., Restuccia, G., Cacciola, G., Parmon, V.N., 2002. Afamily of new working materials for solid sorption airconditioning systems. Appl. Therm. Eng. 22, 191–204.
Balaras, C.A., Grossman, G., Henning, H.M., Ferreira, C.A.I.,Podesser, E., Wang, L., Wiemken, E., 2007. Solar airconditioning in Europe – an overview. Renew. Sust. EnergyRev. 11, 299–314.
Bruno, J.C., 2007. The Spanish solar air conditioning market. TheSpanish solar air conditioning market. In: 2nd InternationalConference Solar Air-Conditioning, Tarragona, Spain, 18–19October 2007.
Daou, K., Wang, R.Z., Xia, Z.Z., 2006. Desiccant cooling airconditioning: a review. Renew. Sust. Energy Rev. 10, 55–77.
De Francisco, A., Illanes, R., Torres, J.L., Castillo, M., De Blas, M.,Prieto, E., Garcia, A., 2002. Development and testing ofa prototype of low-power water–ammonia absorptionequipment for solar energy applications. Renew. Energy 25,537–544.
Ge, T.S., Li, Y., Wang, R.Z., Dai, Y.J., 2007. Experimental study ona novel two-stage desiccant cooling system. In: The 22nd IIRInternational Congress of Refrigeration, Beijing, China, 21–26August 2007, ICR07-E1-826(2007).
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0 659
Gomez, V.H., Vidal, A., Best, R., Garcıa-Valladares, O., Velazquez, N., 2008. Theoretical and experimental evaluation of an indirect-fired GAX cycle cooling system. Appl. Therm. Eng. 28, 975–987.
Gommed, K., Grossman, G., 2007. Experimental investigation ofa liquid desiccant system for solar cooling anddehumidification. Solar Energy 81, 131–138.
Haberle, A., Luginsland, F., Zahler, C., Berger, M., Rommel, M.,Henning, H.M., Guerra, M., De Paoli, F., Motta, M., Aprile, M.,2007. A linear concentrating Fresnel collector drivinga NH3–H2O absorption chiller. In: Proceedings of the 2ndInternational Conference Solar Air-Conditioning, Tarragona,Spain, 18–19 October 2007, pp. 662–667.
Henning, H.M., 2007. Solar assisted air conditioning of buildings –an overview. Appl. Therm. Eng. 27, 1734–1749.
Henning, H.M., Wiemken, E., 2007. Solar cooling. In: Proceedingsof ISES Solar World Congress, Beijing, China, 18–21 September2007, pp. 60–67.
HIJC USA Inc, 2005. Waste heat adsorption chiller substituteabsorption. http://www.adsorptionchiller.com/ (accessed 15.01.09).
Inaba, H., Kida, T., Horibe, A., Kaneda, M., 2002. Sorptioncharacteristics of honeycomb-type sorption elementcomposed of organic sorbent. JSME Int. J. Ser. B 45, 183–191.
Jain, S., Bansal, P.K., 2007. Performance analysis of liquiddesiccant dehumidification systems. Int. J. Refrig. 30,861–872.
Jakob, U., 2008. Recent developments of solar air-conditioning inEurope. In: Ruzhu, Wang, Peng, Zhang (Eds.), Cryogenics andRefrigeration – Proceedings of ICCR’2008, 5–9 April 2008,Shanghai, China, pp. 659–667.
Jakob, U., Eicker, U., 2002. Solar cooling with diffusion absorptionprinciple. In: World Renewable Energy Congress VII, Cologne,German, 29 June–5 July 2002.
Jakob, U., Huber, M., Dubbelfeld, D., Aubele, R., 2007. Experimentalinvestigation of a novel solar cooling system based on a small-scale water/silica gel adsorption heat pump. In: InternationalSymposium on Innovative Materials for Processes in EnergySystems, Kyoto, Japan, 28–31 October 2007.
Jakob, U., Pink, W., 2007. Development and investigation of anammonia/water absorption chiller – chillii� PSC – for a solarcooling system. In: Proceedings of the 2nd InternationalConference Solar Air-Conditioning, Tarragona, Spain, 18–19October 2007, pp. 440–445.
Japan Exlan web site. http://www.exlan.co.jp/en/products/eksrotor.html (accessed 15.01.09).
Jia, C.X., 2006. Study on reinforcement dehumidificationmechanism of composite desiccant based on silica gel andapplication. PhD thesis, Department of MechanicalEngineering, Shanghai Jiao Tong University, China, pp. 80–96(in Chinese).
Jia, C.X., Dai, Y.J., Wu, J.Y., Wang, R.Z., 2007. Use of compounddesiccant to develop high performance desiccant coolingsystem. Int. J. Refrig. 30, 345–353.
Li, J.H., Ma, W.B., Jiang, Q., Huang, Z.C., Xia, W.H., 1999. A 100 kWsolar air-conditioning system. Acta Energ. Solar. Sin. 20,239–243 (in Chinese).
Li, Z., Liu, X., Jiang, Y., Chen, X., 2005. New type of fresh airprocessor with liquid desiccant total heat recovery. Energ.Build. 37, 587–593.
Li, Z.F., Sumathy, K., 2001. Experimental studies on a solarpowered air conditioning system with partitioned hot waterstorage tank. Solar Energy 71, 285–297.
Liu, X., Li, Z., Jiang, Y., Lin, B., 2006. Annual performance of liquiddesiccant based independent humidity control HVAC system.Appl. Therm. Eng. 26, 1198–1207.
Liu, Y.L., Wang, R.Z., 2004. Performance prediction of a solar/gasdriving double effect LiBr–H2O absorption system. Renew.Energy 29, 1677–1695.
Liu, Y.L., Wang, R.Z., Xia, Z.Z., 2005. Experimental performance ofa silica gel–water adsorption chiller. Appl. Therm. Eng. 25,359–375.
Mei, L., Dai, Y.J., 2008. A technical review on use of liquid-desiccant dehumidification for air-conditioning application.Renew. Sust. Energy Rev. 12, 662–689.
Ma, Q., Wang, R.Z., Dai, Y.J., Zhai, X.Q., 2006. Performanceanalysis on a hybrid air-conditioning system of a greenbuilding. Energy Build. 38, 447–453.
Ma, Q., Wang, R.Z., Luo, L.A., Xia, Z.Z., Lin, P., 2008. Transportationof low-grade thermal energy over long distance by ammonia–water absorption. Chin. Sci. Bull. 53, 3026–3029.
Ma, W.B., Deng, S.M., 1996. Theoretical analysis of low-temperature heat source driven two-stage LiBr/H2Oabsorption refrigeration system. Int. J. Refrig. 19, 141–146.
Mitsubishi Plastics, AQSOA web site. http://www.yes-mpi.com/other/aqsoa.html (accessed 15.01.09).
Moser, H., Rieberer, R., 2007. Small-capacity ammonia/waterabsorption heat pump for heating and cooling – used for solarcooling applications. In: Proceedings of the 2nd InternationalConference Solar Air-Conditioning, Tarragona, Spain, 18–19October 2007, pp. 51–61.
Mycom-AdRef, 2005. http://www.mayekawa.co.jp/en/special.html (accessed 15.01.09).
National Research Council Canada, 2008. http://irc.nrc-cnrc.gc.ca/ie/iaq/desiccant_e.html (accessed 12.06.08).
Pongtornkulpanich, A., Thepa, S., Amornkitbamrung, M.,Butcher, C., 2008. Experience with fully operational solar-driven 10-ton LiBr/H2O single-effect absorption cooling systemin Thailand. Renew. Energy 33, 943–949.
Sabatelli, V., Fiorenza, G., Marano, D., 2007. Technical statusreport on solar desalination and solar cooling. A technicalreport of the EU-project ‘‘NEGST (New Generation of ThermalSolar Systems)’’ WP5.D1. http://www.swt-technologie.de/html/publicdeliverables3.html (accessed 15.01.09).
Saha, B.B., Akisawa, A., Kashiwagi, T., 2001. Solar waste heatdriven two-stage adsorption chiller: the prototype. Renew.Energy 23, 93–101.
Saha, B.B., Koyama, S., Ng, K.C., Hamamoto, Y., Akisawa, A.,Kashiwagi, T., 2006. Study on a dual-mode, multi-stage,multi-bed regenerative adsorption chiller. Renew. Energy 31,2076–2090.
Shimooka, S., Oshima, K., Hidaka, H., Takewaki, T., Kakiuchi, H.,Kodama, A., Kubota, M., Matsuda, H., 2007. The evaluation ofdirect cooling and heating desiccant device coated with FAM.J.Chem. Eng. Jpn. 40, 1330–1334. http://www.jstage.jst.go.jp/browse/jcej (accessed 15.01.09).
Sozen, A., Altiparmak, D., Usta, H., 2002. Development and testingof a prototype of absorption heat pump operated by solarenergy. Appl. Therm. Eng. 22, 1847–1859.
Syed, A., Izquierdo, M., Rodrıguez, P., Maidment, G., Missenden, J.,Lecuona, A., Tozer, R., 2005. A novel experimentalinvestigation of a solar cooling system in Madrid. Int. J. Refrig.28, 859–871.
Varga, M., Bangens, L., Cavelius, R., Davison, J.M., Garcia, F.A.,Isaksson, C., Laia, C., LeutgOb, K., Lopes, C., Nicol, J.F.,Pagliano, L., Perednis, E., Read, G.E.F., 2005. Service buildingskeep cool: promotion of sustainable cooling in the servicebuilding sector, Produced by: OesterreichischeEnergieagentur-Austrian Energy Agency Otto-Bauer-Gasse 6,A-1060 Vienna. http://www.energyagency.at(accessed 22.12.05).
Wang, D.C., Wu, J.Y., Xia, Z.Z., Zhai, H., Wang, R.Z., Dou, W.D.,2005. Study of a novel silica gel–water adsorption chiller, PartII. Experimental study. Int. J. Refrig. 28, 1084–1091.
Wang, R.Z., 2001. Performance improvement of adsorptioncooling by heat and mass recovery operation. Int. J. Refrig. 24,602–611.
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 2 ( 2 0 0 9 ) 6 3 8 – 6 6 0660
Wang, R.Z., 2008. Efficient adsorption refrigerators integratedwith heat pipes. Appl. Therm. Eng. 28, 317–326.
Wang, R.Z., Dai, Y.J., 2006. Solar Refrigeration. China ChemicalIndustry Press, Beijing.
Wang, R.Z., Oliveira, R.G., 2006. Adsorption refrigeration – anefficient way to make good use of waste heat and solar energy.Prog. Energ. Combust. 32, 424–458.
Xia, Z.Z., Yang, G.Z., Wang, R.Z., 2008. Experimental investigationof capillary-assisted evaporation on the outside surface ofhorizontal tubes. Int. J. Heat Mass Trans. (available online20 February 2008).
Yang, G.Z., Xia, Z.Z., Wang, R.Z., Keletigui, D., Wang, D.C.,Dong, Z.H., Yang, X., 2006. Research on a compact adsorptionroom air conditioner. Energ. Convers. Manage. 47, 2167–2177.
Zetzsche, M., Koller, T., Brendel, T., Muller-Steinhagen, H., 2007.Solar cooling with an ammonia/water absorption chiller. In:Proceedings of the 2nd International Conference Solar Air-Conditioning, Tarragona, Spain, 18–19 October 2007,pp. 536–541.
Zhai, X.Q., Wang, R.Z., Dai, Y.J., Wu, J.Y., Xu, Y.X., Ma, Q., 2007.Solar integrated energy system for a green building. EnergyBuild. 39, 985–993.