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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

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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.

mailto:[email protected]

http://www.iifiir.org

http://www.elsevier.com/locate/ijrefrig

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


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).


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).


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


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).


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).


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).


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).


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).


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).


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.


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.


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).


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).


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.


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).


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.


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.


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.


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.


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.


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.



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.


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.


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).


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).


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.

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