adsorption

Technology development in the solar adsorption

refrigeration systems

K. Sumathy*, K.H. Yeung, Li Yong

Department of Mechanical Engineering, University of Hong Kong, Hong Kong, People’s Republic of China

Received 9 May 2002; accepted 30 April 2003

Abstract

Solar adsorption refrigeration devices are of significance to meet the needs for cooling requirements such as air-conditioning,

ice-making and medical or food preservation in remote areas. They are also noiseless, non-corrosive and environmentally

friendly. Various solar powered cooling systems have been tested extensively; however, these systems are not yet ready to

compete with the well-known vapor compression system. For these reasons, research activities in this sector are still increasing

to solve the technical, economical and environmental problems. The objective of this paper is to provide fundamental

knowledge on the adsorption systems and present a detailed review on the past efforts in the field of solar refrigeration systems.

A number of attempts have been made by researchers to improve the performance of the solar powered adsorption subsystems.

It is seen that, for successful operation of such systems, a careful selection of the adsorbent-adsorbate pair is essential apart from

the collector choice, system design and arrangement of subsystems.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Adsorbate; Adsorbent; Adsorption; Performance; Refrigeration; Solar energy

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

2. Principle of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

2.1. Adsorption equilibria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

2.2. Refrigerants and adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305

2.3. Heat of adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306

3. Thermodynamics of adsorption cycles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

3.1. Basic adsorption cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

3.2. Heat recovery adsorption refrigeration cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308

3.3. Mass recovery adsorption refrigeration cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

3.4. Thermal wave cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309

3.5. Convective thermal wave cycle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

3.6. Multi-stage and cascading cycle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

4. Solar cooling technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

4.1. Intermittent adsorption systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312

4.1.1. Silica-gel/water and silica-gel methanol systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

4.1.2. Zeolite–water systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

4.1.3. Activated carbon–methanol systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

4.1.4. Activated carbon–ammonia systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

0360-1285/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0360-1285(03)00028-5

Progress in Energy and Combustion Science 29 (2003) 301–327

www.elsevier.com/locate/pecs

* Corresponding author. Tel.: þ852-2859-2632; fax: þ852-2858-5415.

E-mail address: [email protected] (K. Sumathy).

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

4.2. Continuous adsorption systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

4.2.1. Multi-stage and cascading systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315

4.2.2. Thermal wave adsorption systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

4.2.3. Convective thermal wave adsorption systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

4.2.4. Hybrid system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

4.3. Solar desiccant cooling and air-conditioning systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324

1. Introduction

The use of solar energy for environmental control is

receiving much attention as a result of the projected world

energy shortage. Refrigeration is particularly attractive as

a solar energy application because of the near coincidence

of peak cooling loads with the available solar power.

Solar refrigeration has the potential to improve the quality

of life of people who live in areas with electricity

insufficient. It is usually used for storage of agricultural

products, food and medicines (e.g. vaccines) in remote

areas. Solar cooling to produce ice accumulates latent

heat, thus leading to smaller volume of ice-makers. The

adsorption system is one of the promising solar thermal

refrigeration methods, and it is environmentally friendly

along with low cost and low maintenance requirements

[1]. Adsorptive processes have been applied extensively

for gas separation and catalysis, but it is only recently that

adsorptive processes have been widely studied for

refrigeration and heat pumps.

Despite a large potential market, existing solar refriger-

ation systems are not competitive with electricity-driven

refrigeration systems because of their high capital costs.

Improvements such as reduced collector area, improved

system performance, and reduced collector cost will lower

the cost of solar components. Several solar refrigeration

systems have been proposed and are under development

such as sorption systems including liquid/vapor, solid/

vapor absorption, adsorption, vapor compression and

photovoltaic-vapor/compression systems. Most of the

above mentioned systems have not been economically

justified.

Solar refrigeration is highly dependent upon environ-

mental factors such as cooling water temperature, air

temperature and solar radiation. The energetic conversion

efficiency is low, and solar cooling and refrigeration are not

yet competitive economically with the conventional

systems. This article details the various research aspects

of adsorption refrigeration, which includes adsorption

mechanism, the criteria to choose an appropriate working

pair, thermodynamic analysis of several refrigeration

cycles, adsorbent properties and various solar powered

adsorption refrigeration systems based on various cooling

technologies.

2. Principle of adsorption

Adsorption occurs at the surface interface of two phases,

in which cohesive forces including electrostatic forces and

hydrogen bonding, act between the molecules of all

substances irrespective of their state of aggregation [2].

Unbalanced surface forces at the phase boundary cause

changes in the concentration of molecules at the solid/fluid

interface. The process of adsorption involves separation of a

substance from one phase accompanied by its accumulation

or concentration at the surface of another. The adsorbing

phase is the adsorbent, and the material concentrated or

adsorbed at the surface of that phase is the adsorbate.

Adsorption processes can be classified as either physical

or chemical, depending on the forces causing the adsorp-

tion process. Physical adsorption (physisorption) occurs

when Van der Waals forces bind the adsorbing molecule to

the solid phase, these intermolecular forces are as same as

ones that bond molecules to the surface of a liquid.

Molecules that are physically adsorbed to a solid can be

released by applying heat; therefore, the process is

reversible. Chemical adsorption (chemisorption) occurs

when covalent or ionic bonds are formed between the

adsorbing molecules and the solid substance. The bonding

forces of chemical adsorption are much greater than that of

physical adsorption. Thus, more heat is liberated. This

bonding leads to change in the chemical form of the

adsorbed compounds and hence, it is irreversible. For this

particular reason, most of the adsorption processes

applicable to the thermal system or cooling machine

mainly involve physical adsorption.

Adsorption is an exothermic process accompanied by

evolution of heat, the quantity of heat release depends upon

the magnitude of the electrostatic forces involved, latent

heat, electrostatic and chemical bond energies. The heat of

adsorption is usually 30–100% higher than the heat of

condensation of the adsorbate. In general adsorption is

stronger than condensation to liquid phase. Hence, if a fresh

adsorbent and adsorbate in liquid form coexist separately in

a closed vessel, transport of adsorbate from the liquid phase

to the adsorbent occurs in the form of vapor. The liquid

temperature becomes lower while the adsorbent temperature

rises. Air-conditioning and refrigeration utilize this

phenomenon to obtain a cooling effect [3].

K. Sumathy et al. / Progress in Energy and Combustion Science 29 (2003) 301–327302

Heat of adsorption is either derived from adsorption

isotherms, generally referred to as either the isosteric heat

(the energy released in the adsorption process), or, as the

differential heat of adsorption determined experimentally

using a calorimetric method. Differential heat of adsorption

for some adsorbent/adsorbate pairs are given in Table 1. The

performance of adsorbents used in physisorption is governed

by surface properties, such as surface area, micro-pores and

macro-pores, size of granules in powders, crystals or in

pellets. Adsorbents having special affinity with polar

substances like water are termed ‘hydrophilic’. These include

silica gel, zeolites and porous or active alumina. Non-polar

adsorbents, termed ‘hydrophobic’, have more affinity for oils

and gases than for water. These substances include activated

carbons, polymer adsorbents and silicalites.

The general term ‘sorption’ is used when both adsorption

and absorption occurs simultaneously. ‘Desiccants’ are a

type of adsorbent having special affinity for water and have

been used extensively for dehumidification or drying in air

processing applications.

Adsorbents are characterized by surface properties such

as surface area and polarity. A large specific surface area is

preferable for providing large adsorption capacity, but the

creation of a large internal surface area in a limited volume

inevitably gives rise to large numbers of small sized pores

between adsorption surfaces. The pore size distribution of

micropores which determines the accessibility of adsorbate

molecules to the internal adsorption surface, is important for

characterizing adsorptivity of adsorbents. Materials such as

zeolite and carbon molecular sieves can be engineered

specifically for precise pore size distributions and hence

‘tuned’ for a particular separation.

2.1. Adsorption equilibria

To understand the adsorption process, the adsorption

equilibria are introduced to describe the adsorption process

and several state equations known as isotherms of adsorp-

tion are proposed [2,4]. Several basic theories have been

proposed and ussed to define the main isotherms of an

adsorption process and are listed below:

Henry’s law. For an adsorption process on a uniform

surface at sufficiently low concentrations (such that all

molecules are isolated from their nearest neighbors), where,

Nomenclature

A adsorption potential (J mol21)

Cm adsorbed concentration (mol kg21)

Cms saturation adsorbed concentration (mol kg21)

E interaction energy between solid and adsorbing

molecule (J mol21)

E0 characteristic energy used in Dubinin equation

(J mol21)

H enthalpy (J mol21)

k characteristic parameter of adsorption pair

K adsorption parameter for D-A equation

l micropore half width (nm)

l0 half width of a slit sharped micropore (nm)

n adsorption parameter of adsorption pair

P pressure (Pa)

PC condensing pressure (Pa)

PE evaporation pressure (Pa)

Ps saturated vapor pressure (Pa)

Rg gas constant (J mol21 K21)

T temperature (8C, K)

TA ambient temperature (8C)

TC condensing temperature (8C)

TE evaporation temperature (8C)

TR regeneration temperature (8C)

Ts saturated refrigerant liquid temperature (8C)

W volume adsorption capacity (l kg21)

W0 maximum volume adsorption capacity (l kg21)

W00 total micropore volume (l kg21)

Greek symbol

b affinity coefficient

d variance

u fractional loading

Table 1

Heat of adsorption of some adsorbent/adsorbate pairs

Adsorbent Adsorbate Heat of

adsorption

(kJ kg21)

Remarks

Activated

alumina

Water 3000 Water is applicable

except for very low

operating pressures

Zeolite

(various

grades)

Water 3300–4200 Natural zeolites have

lower values than

synthetic zeolites

Ammonia 4000–6000

Carbon

dioxide

800–1000

Methanol 2300–2600

Silica gel Methyl

alcohol

1000–1500 Not suitable above

200 8C

Water 2800 Used mostly for

desiccant cooling

Charcoal C2H4 1000–1200 Reacts at approximate

100 8C. Ammonia and

methanol are not

compatible with

copper at high

temperature

Ammonia 2000–2700

Water 2300–2600

Methanol 1800–2000

C2H5OH 1200–1400

K. Sumathy et al. / Progress in Energy and Combustion Science 29 (2003) 301–327 303

the equilibrium relationship between fluid phase and

adsorbed phase concentrations will always be linear [5].

Langmuir’s approach. In order to understand the

monolayer surface adsorption on an ideal surface, Lang-

muir’s approach is used and this approach is based on

kinetic equilibrium; that is, the rate of adsorption of the

molecules is assumed to be equal to the rate of desorption

from the surface [6].

Gibbs’ theory. This is based on the ideal gas law, in

which the adsorbate is treated in microscopic and bi-

dimensional form and provides a general relation between

spreading pressure and adsorbed phase concentration. In this

concept, volume in the bulk phase is replaced by the area

and the pressure is replaced by the so-called ‘spreading

pressure’. By assuming some forms of thermal equation of

state relating the number of moles of adsorbate, the area and

the spreading pressure and using them in the Gibbs equation,

a number of fundamental equations can be derived, such as

the linear isotherm, the Volmer isotherm, etc.

Adsorption potential theory. It is purely a thermodyn-

amic approach, suitable for adsorption in microporous

materials which was based on a model originally proposed

by Polanyi and further developed by Dubinin by the end of

the 1920s.

Recent equations for ideal solids are introduced by Nitta

and his co-workers [4] are based on statistical thermo-

dynamics with features similar to the Langmuir theory and

encompasses the Langmuir equation as a special case.

Due to the complexity in structure of practical solids, the

above fundamental adsorption isotherm cannot completely

describe the process. Hence, the analysis of ideal solids are

initially used as a basis for the development of equilibria

theory for practical solids, and a number of empirical as well

as semi-empirical equations to describe adsorption equili-

bria have been developed.

In order to describe adsorption of gases and vapors below

the capillary condensation region, equations such as

Freundlich, Langmuir–Freundlich (Sips), Toth, Unilan,

and Dubinin–Radushkevich (DR) have been used. Simi-

larly, to describe equilibrium data in the region of multi-

layering adsorption, the classical equation of Brunauer,

Emmett and Teller (BET) is used. To account for the various

features inherent with real solids, BET equation is further

modified and is listed in the reference [4]. The above

discussed empirical relations are applicable to both

subcritical as well as the wider range of supercritical vapors.

To analyse more critically the adsorption process due to

the presence of subcritical vapors in the micropores solids,

the semi-empirical DR equation’ was developed by Dubinin

et al. [7].

W ¼ W0 exp 2A

E

� �2" #

ð1Þ

In the above equation, A is the adsorption potential and is

defined as A ¼ RgT lnðP0=PÞ; E; is the interaction energy

between solid and adsorbing molecule. The DR equation has

been widely used to describe adsorption isotherms of sub-

critical vapors in microporous solids such as activated

carbon and zeolite.

The DR equation describes fairly well many carbon-

aceous solids with low degree of burned-out. For carbon-

aceous solids resulting from a high degree of burn-out

during activation, the degree of heterogeneity increases

because of a wider pore size distribution, and for such cases

the DR equation does not describe well the equilibrium data.

To take into account of this surface heterogeneity,

Dubinin and Astakhov [4] proposed another equation (DA)

of the following form:

W ¼ W0 exp 2A

E

� �n� �ð2Þ

where, the exponent n describes the surface heterogeneity.

Rewriting the above equation in terms of the characteristic

energy of the reference vapor, the expression becomes,

W ¼ W0 exp 2A

bE0

� �n� �ð3Þ

where, the parameter b represents the affinity coefficient,

which is the ratio of the liquid molar volume to that of the

reference vapor. With the additional exponent n in the

adsorption isotherm equation, the DA equation provides

flexibility in the description of adsorption data of many

microporous solids ranging from a narrow to wide

micropore size distribution.

Adsorption isotherms of many microporous solids do not

usually conform to the simple DR equation. Even with the

inclusion of adjustable exponent n in the DA equation, it

cannot reflect experimental data corresponding to different

situations. This inability to fit the data is attributed to the

heterogeneity of the system, in which the characteristic

energy varies with the different regions in the solid.

Since adsorption of many adsorbates in micropores of

carbonaceous solids is due to the dispersion force, the

micropore size is therefore playing a major role in the

attraction of adsorbate molecules. In this sense, distribution

of the micropore size dictates the heterogeneity of the solid

surface than the distribution in characteristic energy. For a

micropore volume, W00 f ðxÞ; (f ðxÞ represents the micropore

size distribution), having micropore size between l and l þ

dl; the volume of micropore occupied by adsorbate at a

given adsorption potential A is:

W ¼ W00

ðlmax

lmin

uðA=bE0; nÞf ðlÞdl ð4Þ

where u is the fraction of the micropore volume occupied by

the adsorbate, which is a function of ðA=bE0; nÞ [4]. l is the

micropore half width, with lmax and lmax are its minimum

and maximum, respectively. However, for mathematical

convenience, those limits are usually replaced by 0 and 1,

respectively, to facilitate the analytical integration of the

above integral.


To evaluate the integral Eq. (4), the parameters of the

local isotherm must be expressed in terms of the micropore

half width, l:The parameter n is usually regarded as a

constant. The parameter E0; is the characteristic energy of

the reference vapor, which follows the following relation-

ship with the micropore half width, as:

E0 ¼K

lð5Þ

The micropore size distribution can then be assumed to take

the following Gaussian distribution:

dW0

dx¼

W00

dffiffiffiffi2p

p exp 2ðl 2 l0Þ

2

2d2

" #¼ W0

0 f ðlÞ ð6Þ

where, W00 ; is the total micropore volume, l0 is the half-

width of a slit shaped micropore which corresponds to the

maximum of the distribution curve, and d is the variance.

Using the relationship between the characteristic energy and

the micropore half width, the DR equation can be written in

terms of this half width l as:

W ¼ W0 exp½2ml2A2� ð7Þ

where m ¼ 1=ðbkÞ2

In order to obtain an equation for a heterogeneous solid,

the above adsorption equation is differentiated to obtain

micropore volume element dW :

dW ¼ dW0 expð2ml2A2Þ ð8Þ

Combining Eqs. (6) and (8), yields the volume occupied by

the adsorbate ðWÞ :

W ¼W0

0

dffiffiffiffi2p

pð1

0exp 2

ðl 2 l0Þ2

2d2

" #expð2ml2A2Þdx ð9Þ

Evaluation of the above integral results:

W ¼W0

0

2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 þ 2md2A2

p exp 2ml20A2

1 þ 2md2A2

!

� 1 þ erfl0

dffiffi2

p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 þ 2md2A2

p !" #

ð10Þ

This equation was first obtained by Dubinin and Stoeckli

[8], and hence referred as DS equation. In this equation, ‘erf’

represents the error function. Using the above equation to fit

experimental data, three parameters can be extracted from

this fitting process, namely W00 ; l0; d: Knowing these

parameters, the micropore size distribution in terms of

volume can then be calculated from Eq. (6).

Thermodynamic analysis suggests that an adsorption

isotherm must exhibit the Henry law behavior when

pressure is very low. Unfortunately, the DA as well as the

DR equations do not follow the perfect Henry law when the

pressure is approaching zero. The above DR, DA, DS

equations, however, suffer from the disadvantage of zero

Henry constant at zero pressure. This difficulty in describing

the adsorption behavior at low pressure has been addressed

by a number of workers [9].

2.2. Refrigerants and adsorbents

There are several working pairs for solid adsorption. For

the successful operation of a solid adsorption system,

careful selection of the working medium is essential. It is

because, the performance of the system varies over a wide

range using different working pairs at different tempera-

tures. The advantages and disadvantages of different

working media and their properties are listed and discussed

in this section. For any refrigerating application, the

adsorbent must have high adsorptive capacity at ambient

temperatures and low pressures but less adsorptive capacity

at high temperatures and high pressures. Thus, adsorbents

are first characterized by surface properties such as surface

area and polarity. A large specific surface area is preferable

for providing large adsorption capacity, and hence an

increase in internal surface area in a limited volume

inevitably gives rise to large number of small sized pores

between adsorption surfaces. The size of the micropores

determines the effectiveness of adsorptivity and therefore

distribution of micropores is yet another important property

for characterizing adsorptivity of adsorbents.

Based on the above discussion, the choice of the

adsorbent will depend mainly on the following factors:

† high adsorption and desorption capacity, to attain high

cooling effect;

† good thermal conductivity, in order to shorten the cycle

time;

† low specific heat capacity;

† chemically compatible with the chosen refrigerant;

† low cost and widely available.

The selected adsorbate (working fluid) must have most

of the following desirable thermodynamics and heat transfer

properties:

† high latent heat per unit volume;

† molecular dimensions should be small enough to allow

easy adsorption;

† high thermal conductivity;

† good thermal stability;

† low viscosity;

† low specific heat;

† non-toxic, non-inflammable, non-corrosive; and

† chemically stable in the working temperature range.

Based on the above criteria, some of the appropriate

working pairs are zeolite–water, zeolite–organic refriger-

ants, silica gel–water, zeolite–water and activated carbon–

methanol in solid adsorption systems. Several refrigeration

applications have been studied using various adsorbent and

adsorbate pairs. Initially, zeolite and activated carbon were


the only adsorbent most popularly used in many systems. In

the early 1980s, Tchernev [10] carried out an investigation

of adsorption refrigeration with zeolite–water pair. Also,

Pons et al. [11] worked on the adsorption pair zeolite and

water, to produce refrigerating effect achieving a coefficient

of performance (COP) of only about 0.1. Later, in 1987,

Pons and Crenier demonstated [12,13] that activated carbon

and methanol can serve as a suitable pair for a solar

powered, solid-adsorption ice-maker. Critoph [14,15] had

studied the performance limitations of adsorption cycles

with different adsorbates, for solar cooling and concluded

that, in general, activated carbon–methanol combination

was preferable for solar cooling, giving the best COP

achievable in a single-stage cycle. In order to increase the

desorption temperatures and have a good cooling effect

during the adsorption period at night, Headley et al. [16]

constructed a charcoal–methanol adsorption refrigerator

powered by CPC concentrating collectors, but the solar COP

achieved was very low, of about 0.02.

The recent new development of activated carbon fibre—

ACF, shows the possibility for applications in adsorption

refrigeration. One good example is the development in

Byelorussia of a refrigerator prototype using ACF–ethanol

and ACF–acetone pairs has been reported [17]. New

experiments have also been shown to use a heat pipe for

heating/cooling ACF adsorbers for the ACF–NH3 pair [18].

In China, several studies had been carried out on solar

powered refrigerators using different adsorption pairs such

as zeolite–water and activated carbon-methanol. In search

of new adsorbent materials, Wang et al. [19] have reported

that, for specially treated ACF, the measured adsorption

capacity of methanol is two to three times greater than that

of normal activated carbon, and the estimated adsorption

time is only about 1/5 to 1/10 of that of normal activated

carbon [20].

Meunier et al. [20,21] have compared synthetic zeolite–

water, synthetic zeolite–methanol, and reported that

charcoal–methanol resulted in a better system COP

provided that the night time temperature were low. When

the night time ambient temperature to evaporating tempera-

ture difference is particularly high, zeolite–water combi-

nation showed better performance. However, this will also

require a higher generating temperature from the solar

collector during the day.

Critoph and Vogel [22] compared zeolites and activated

carbon with refrigerants R11, R12, R22 and R114, and in all

cases found activated carbon a preferable adsorbent for solar

cooling. A theoretical analysis was carried out by Oertel and

Fischer [23] for a prediction of the achievable COP using

methanol/silica gel. Since water was used as refrigerant, the

adsorption chiller was restricted to cooling temperatures

above the freezing point of water at 0 8C. It was shown that

by changing to the working pair silica gel–methanol it

might be possible to reach cold brine temperatures of 25 8C

with a COP ¼ 0.25. A similar study on the dynamic

performance of a closed-cycle, solar-adsorption refrigerator

was carried out by Hajji et al. [24] numerically using zeolite

13X-water and chabazite–methanol working pairs for a

chosen set of weather conditions.

Recently, Tamainot and Critoph [25] investigated the

thermophysical properties of two types of monolithic

activated carbons with an intention to design and fabricate

a high performance generator for sorption refrigeration

systems and heat pumps using ammonia as refrigerant. It

was found that, reduction in volume from granular bed to

monolithic bed was up to 50% which could lead to a

substantial economic gain.

2.3. Heat of adsorption

All adsorption processes are accompanied by heat

release, i.e. they are, exothermic processes. Since in

adsorbed phase, adsorbate molecules are in more ordered

configuration, entropy decreases; and from DG ¼ DH 2 T

DS; exothermic character of the process is obvious. The

three terms which are often used in adsorption process are:

(i) Integral heat of adsorption—which is the total heat

released from initial state to final state of adsorbate loading,

at constant temperature; (ii) Differential heat of adsorp-

tion—is the change in integral heat of adsorption with

change in loading; and (iii) Isosteric heat of adsorption is

defined by using adsorption isosters and Clausius Clapryron

relationship.

In practice, the difference between differential heat of

adsorption and isosteric heat of adsorption is so small that it

can easily be neglected, as well as, these two can be

considered as identical. At constant loading ðdCm ¼ 0Þ;

using the DA Eq. (3) and the following Clapeyron equation,

DH

RgT2¼ 2

› ln P

›T

� �Cm

ð11Þ

an expression for the isosteric heat can be obtained.

2DH ¼ A þ DHvap þðbE0Þ

ndT

nAn21ð12Þ

Thus, the isosteric heat is a summation of three terms. The

first term is due to the adsorption potential, the second is

the heat of vaporization and the third corresponds due to the

change in maximum capacity with temperature.

To express the isosteric heat of adsorption in terms of

adsorption rate, the above expression can be rewritten as:

2DH ¼ DHvap þ bE0 ln1

u

� �þ

ðbE0ÞdT

nln

1

u

� �2ðn21Þ=n

ð13Þ

where the fractional loading u can be expressed as Cm=Cms:

The above expressions (11)–(13) are often used in the

study of adsorption refrigeration system and the detail of

adsorption equilibria, working pair and heat of adsorption is

summarized in Table 2.


Table 2

A review of adsorption equilibrium equations used in different models.

References Equilibrium equation Adsorbent-adsorbate pair Adsorption heat

Hajji, 1995, 1996

Zhang, 1999, 2000 [61,93]

lnðpÞ ¼ aðwÞ þ bðwÞ=T aðwÞ ¼ a0 þ a1w þ a2w2 þ a3w3;

bðwÞ ¼ b0 þ b1w þ b2w2 þ b3w3 ¼ DHðwÞ=R

4A zeolite–water, 13X zeolite–water,

AC35 activated carbon–methanol

Enthalpy of adsorption:

bðwÞ ¼ b0 þ b1w þ b2w2 þ b3w3 ¼ DHðwÞ=R

Cho, 1992 Simplified Freundlich’s equation: simplified Freundlich’s

equation: q ¼ qsatðP=PsatÞ1=n; qsat ¼ 0:522 kg kg21, n ¼ 1:6

Fuji RD-type silica gel–Water

[14] D-A equation: q ¼ W0rðTÞ exp½2ðA=bE0Þn� Adsorbent (charcoal): AC35, DEG,

LH, BPL, NORIT RB, PKST,

207E and C; adsorbate: NO2,

methanol, SO2, amine, methyl,

ammonia, acetonitrle and

formaldehyde.

Clausius–Clapeyron equation:

DH ¼ R lnðp2=p1Þ=ðð1=T1Þ2 ð1=T2ÞÞ

[43] Modified DR equation Carbon–ammonia Constant

Gui, 2001, 2002,

Chahbani, 2002 [38,75]

Modified D-A equation: q ¼ q0exp½2kðT=Tsat 2 1Þn� Active carbon–methanol [17,18,28];

monolithic carbon–ammonia [35]

Constant

Guilleminot, 1987,

Boubakri, 2000, Passos, 1989,

Mhimid, 1998, [41]

q ¼ W0r exp 2D T lnPsatðTÞ

PsatðTcondÞ

� �n� �Activated carbon AC-35–methanol;

activated carbon AS–methanol;

carbon AX21–ammonia; zeolite–water

Clapeyron equation for ideal gas:

DH ¼ RT2ð› lnp=›TÞ

Llobet, 2000 D-A equation Adsorbent (activated carbon):

PX21, KL93, KF1500, TA90, TA60,

BPL; adsorbate: ammonia

QstðT ;PevapÞ ¼ RT2½› lnðpÞ=›T�

¼ DHNH3þ A þ 2:5 £ 1023TbnEn

0A12n=n

Luo, 2000 Modified DR equation: q ¼ q0exp½2kðT=Tsat 2 1Þn� Activated carbon AC-35–methanol,

Activated carbon AC-35–ethanol

DH ¼ RT þ L þ ð2303b=ffiffik

pÞ £ ½ln1=2ð1=uÞ

þðaT=2Þln21=2ð1=uÞ� k;b are coefficients,

a is the thermal expansion coefficient of

liquid, and u ¼ q=q0

Saha, 1995, Chua, 1999 q ¼ AðTadÞ½PsatðTrÞ=PsatðTadÞ�BðTadÞ; AðTadÞ ¼ A0 þ A1Tad þ

A2T2ad þ A3T3

ad; BðTadÞ ¼ B0 þ B1Tad þ B2T2ad þ B3T3

ad ; A:

26.5314, 0.72452 £ 1021, 20.23951 £ 1023, 0.25493 £ 1026; B:

215.587, 0.15915, 20.50612 £ 1023, 0.53290 £ 1026

Silica gel–water Constant

Sakoda, 1984,1986,

Saha, 1995, Alam, 2000

Simplified Freundlich’s equation: q ¼ qsatðP=PsatÞ1=n; qsat ¼

0:346 kg kg21; n ¼ 1:6

Fuji-gel A type silica gel–water Constant

Sami, 1996 lnðp=psatÞ ¼P

i Aici þ ð1=TaÞP

i Bici Carbon–methanol, AC-HCFC123/

HCFC124

Clapeyron equation for ideal gas

Shelton, 1990, Fuller, 1994 q ¼ a1 þ a2ðTz=TsatÞ þ a3ðTz=TsatÞ2 þ a3ðTz=TsatÞ

3 Zeolite–ammonia Activated

carbon–ammonia

Constant

Sward, 2000 [41,52] qv ¼qs1b1p

1 þ b1pþ

qs2b2p

1 þ b2pþ

qs3b3p

1 þ b3p

qs;k and bk ðk ¼ 1; 2; 3Þ are function of temperature

Zeolite 13X–water Zeolite

13X–ammonia

Constant

[40] q ¼ 221:2 exp½21:916 £ 1027ðT lnðPsðTÞ=PÞÞn� Zeolite NaX–ammonia Constant

[74] Q ¼ f ðTad; pÞ Activated carbon–ammonia Constant

K.

Su

ma

thy

eta

l./

Pro

gress

inE

nerg

ya

nd

Co

mb

ustio

nS

cience

29

(20

03

)3

01

–3

27

30

7

3. Thermodynamics of adsorption cycles

Adsorption refrigeration with different cycles have been

studied extensively [26], and the typical adsorption

refrigeration cycles are: basic cycle, continuous heat

recovery cycle, mass recovery cycle, thermal wave cycle,

convective thermal wave cycle, cascade multi effect cycle,

hybrid heating and cooling cycle etc.

3.1. Basic adsorption cycle

A basic adsorption cycle consists of four thermodynamic

steps, which can be well represented with the aid of the

Clapeyron diagram, as shown in Fig. 1. The idealized cycle

begins at a point (point A in Fig. 1) where the adsorbent is at

low temperature TA and at low-pressure PE (evaporating

pressure). A–B represents the heating of adsorbent, along

with adsorbate. The collector is connected with the

condenser and the progressive heating of the adsorbent

from B to D causes some adsorbate to be desorbed and its

vapor to be condensed (point C). When the adsorbent

reaches its maximum temperature TD; desorption ceases.

Then the liquid adsorbate is transferred into the evaporator

from C to E and the collector is closed and cooled. The

decrease in temperature D to F induces the decrease in

pressure from PC to PE: Then the collector is connected to

the evaporator and adsorption and evaporation occur while

the adsorbent is cooled from F to A. During this cooling

period heat is withdrawn to decrease the temperature of the

adsorbent.

3.2. Heat recovery adsorption refrigeration cycle

The semi-continuous heat recovery cycle is usually

operated with two adsorption beds. The adsorber to be

cooled will transfer its heat to the adsorber to be heated,

which includes sensible heat as well as heat of adsorption.

This heat recovery process will lead to a higher system COP.

Multi-beds could be also adopted to get more heat recovery

and thereby to attain higher COP, but the operation of

a practical system will be complicated. A quasi-continuous

adsorption refrigeration system with heat recovery was

investigated by Wang et al. [26,27] and the flow path is

shown in Fig. 2. While adsorber 1 is cooled and connected to

the evaporator to realize adsorption refrigeration in

evaporator, the adsorber 2 connected to the condenser is

heated to obtain heating-desorption-condensation. The

condensed refrigerant liquid flows into evaporator via a

flow control valve. The operation phase can be changed, and

the go-between will be a short time heat recovery process.

Two pumps are used to drive the thermal fluid in the circuit

between two adsorbers (the connection to the heater and

cooler are blocked during this process).

Miles [28] obtained an experimental COP of 0.8 for the

activated carbon/ammonia system with an evaporation

temperature and a condensation temperature of about 5

and 35 8C, respectively. At the same time, Istria et al. [29]

achieved approximately the same theoretical value for these

temperatures with the multi-salt system. Pons et al. [30]

investigated the zeolite þ water adsorption system under

the fixed test temperature and offers the best possible

theoretical maximum COP of 1.5.

Jones [31] suggested an improvement to the process by

installing more than two adsorbers into the system. The

operating principle of the cycle remains the same, relying on

heat transfer flowing between the adsorbers and the

desorbers. As compared to the basic cycle, heat recovery

in this process is only effective if the heat transfer fluid

temperature leaving the adsorbers is sufficiently high.

Simulation results have shown that the maximum value of

the COP depends on the number of adsorbers and desorbers

installed. The analysis was further extended to a system

containing six adsorbers and six desorbers at the same test

temperature conditions (evaporation at 5 8C and conden-

sation at 35 8C) and it was possible to obtain COPs in the

range of 1.16 [32].Fig. 1. Clapeyron diagram (ln P vs.21=T) of ideal adsorption cycle.

Fig. 2. Schematics of heat recovery two-beds adsorption refriger-

ation system.


In order to simplify the management of the cycles and to

achieve continuous operation avoiding any control during

the cycle as well as to obtain good heat regeneration

between adsorption and desorption, systems with several

elementary adsorbers coupled with evaporators/condensers

and rotated about a central axis, have been proposed [33,34].

Maier-Laxhuber [35] proposed a rotary system which, in

principle, can be used for continuous production of cold

based on a single effect adsorption cycle. Erickson [36]

adapted the principle of the rotary system to a double effect

chemical reaction cycle operating at three pressure levels.

Although there is a problem for the practical management of

the air flows, i.e. the heat transfer fluid used in rotary

systems, these processes seem highly attractive as continu-

ous cooling effect as well as high COP, in the range of 1, can

be obtained.

Based on above reasons, Ebbeson [37] proposed a rotary

process which is designed for continuous operation with the

concept of a heat regeneration developed for solid sorption

cold production systems. The theoretical COP was about

0.9, which is comparatively low, due to the consideration of

the thermal masses and thermal pinches necessary for heat

exchange between the air and the elementary modules.

In a recent work, Critoph [38] describes the operation of

a continuous multiple-bed regenerative adsorption system.

The complete system, consisting of 32 modules was

modeled in detail. The design has not yet been optimized

but a parametric study has revealed the way in which key

parameters affect COP and SCP. The effect of five major

parameters such as thermal capacity ratio, number of

modules, temperature of air leaving the heater, generator

heat transfer coefficient and the evaporator air inlet

temperature were reported.

3.3. Mass recovery adsorption refrigeration cycle

Apart from the above discussed heat recovery operation,

it had been proved that mass recovery is also very effective

for heat recovery adsorption heat pump operation. In this

process, at the end of each half cycle, one adsorber is cold

and the other one is hot. Meanwhile, the former one which is

at low pressure ðPCÞ must be pressurized up to the conden-

ser pressure, and similarly, the other one which is at high

pressure must be depressurized down to the evaporator

pressure. With just one tube between the adsorbers and a

vapor valve, part of this pressurization–depressurization

can be achieved by transferring vapor from the latter

adsorber to the former one. This process can also be called

as an ‘internal vapor recovery process’, and is reported to

enhance the cooling power of the unit without reducing the

COP by more than 10%.

The above explained process involves only mass transfer

and hence the process is rapid. To obtain a ‘double effect’,

mass recovery could be initiated followed by heat recovery.

An ideal heat and mass recovery cycle is shown in Fig. 3, in

which the heat recovery state for a two bed system is shown

by the state points e 2 e0:

The mass recovery cycle ða2 2 a3 2 g01 2 g1 2 g2 2

g3 2 a01‘ 2 a1 2 a2Þ is an extended form of a two bed

basic cycle or two bed heat recovery cycle

(a2 2 g1 2 g2 2 a1 2 a2 shown in Fig. 3), and the cycled

mass is increased from Dx to Dx þ dx; which causes the

refrigeration effect to increase. The principle of these cycles

can be described using Fig. 3. The very first part of each half

cycle is the mass recovery process (path g2 2 g3 and

a2 2 a3). Then the heat recovery process proceeds: heat is

transferred from the hot adsorber to the cold one (path

g3 2 e0). As a consequence, the hot adsorber is first

depressurized (path g03 2 a01), it then adsorbs vapor from

the evaporator (path ). Meanwhile, the cold adsorber is first

pressurized (path a3 2 g01), and then vapor that is desorbed

passes into the condenser (path g01 2 e). Theoretically, the

heat recovery process develops until the adsorbers reach the

same temperature. Actually, there still remains a tempera-

ture difference between the adsorbers at the end of this

period. Then, for closing each half cycle, the adsorbers are,

respectively, connected to the heat source and heat sink

(path e 2 g2 and e0 2 a2). The second half-cycle is

performed the same way except that the adsorbers now

exchange their roles. Due to this process, about 35% of the

total energy transmitted to each adsorber can be internally

recovered, including part of the latent heat of sorption.

3.4. Thermal wave cycle

To further improve the heat regenerative ratio, Shelton

[39] had proposed an attractive cycle called ‘thermal wave

cycle’. In this process, it is assumed that a large temperature

gradient exists along an adsorption bed.

Heating and cooling of the adsorbent beds is achieved

via a heat transfer fluid such as high temperature oil. The

system consists of two adsorber beds and two heat

Fig. 3. Diagram of heat and mass recovery cycle.


exchangers connected in series (Fig. 4) to effect semi-

continuous process. The function of the bed and heat

exchanger is to combine a large area of heat transfer surface

with a low oil flow rate.

A typical thermal wave cycle is shown in Fig. 4. The

cycle consists of two phases: In the first phase, the oil

recovers heat from bed 2 (hot), has a further heat addition

from the heat exchanger and then proceeds to heat bed 1

(cold). As the heating of the bed proceeds, bed 1 desorbs

refrigerant which passes to the condenser (giving a useful

heat output in the case of a heat pump) and bed 2 adsorbs gas

from the evaporator which provides cooling. In the

following phase (second phase) of the cycle the pump is

reversed, and hence, bed 1 is cooled (adsorbing) and bed 2 is

heated (desorbing) in a similar fashion until the original

conditions are reached and the pump can again be reversed.

Though the procedure is simple, significant heat recovery

can be achieved. Further, the system would achieve much

better performance due to the combination of the special

nature of the internal bed heat exchangers and the low flow

rate.

Although many researchers have studied the cycle, up to

now, there is no report of a successful prototype adopting

thermal wave cycle. Also, some experimental reports had

shown that the performance of the thermal wave cycle is not

very good. The efficiency of the thermal wave regenerative

system depends on a relatively large number of parameters:

for example, rates of various heat transfer processes, the

flow rate of the circulating fluid, the cycle time, the adsorber

configuration, etc. A numerical analysis of adsorptive heat

pumps with thermal wave heat regeneration had been

presented by Sun et al. [40]. They had derived two time

constants which can be used directly to quantify the relative

importance of the two heat transfer processes. This allows

ready determination of which of the two processes is rate

limiting and needs to be improved. The work has also

confirmed that the performance of an adsorptive heat pump

system using a traditional packed-bed would be too low,

even with a heat regeneration, and therefore a significant

enhancement of heat transfer properties inside the adsorber

is necessary.

Similar to the above numerical study, the effect of

various operating parameters on the performance of an

adsorptive thermal wave regenerative heat pump had been

studied by Ben Amar et al. [41], theoretically. They had

developed a two-dimensional model which simultaneously

considers heat and mass transfer in the bed. The results have

shown that under ideal conditions, the performance of a

thermal wave regenerative heat pump is considerably better

than that of a basic ‘uniform temperature’ heat pump. The

study showed that a COP greater than 1 and a power of cold

production of near 200 W per kg of adsorbent could be

obtained. For air-conditioning applications, these figures are

slightly higher than those obtained with other single-stage

solid-gas systems as chemical heat pumps [42].

3.5. Convective thermal wave cycle

Thermal wave cycles normally suffer from low power

density because of poor heat transfer through the adsorbent

bed. Rather than attempting to heat the bed directly, it is

possible to heat the refrigerant gas outside the bed and to

circulate it through the bed in order to heat the sorbent. The

high surface area of the grains leads to very effective heat

transfer with only low levels of parasitic power needed for

pumping. Hence, Critoph [43,44] has presented a modified

version of a thermal wave cycle, known as ‘convective

thermal wave cycle’. The concept is the same as thermal

wave cycle, however, the thermal fluid for heating and

cooling to the beds is initiated by the refrigerant itself, thus

the heat transfer between thermal fluid and adsorption bed is

a direct contact heat transfer, which is incorporated with

mass transfer in the system.

A practical schematic of the proposed system is shown in

Fig. 5. The two ‘active’ beds are packed with activated

Fig. 4. Thermal wave cycle.


carbon and the two ‘inert’ beds are packed with non-reactive

particles such as steel balls. The diagram shows the first half

of the cycle, during which Active bed 1 is heated and

desorbs ammonia while Active bed 2 is cooled, adsorbing

ammonia.

In the fluid circulation loop shown on the left, a low

power pump circulates ammonia steam through inert bed 1

which is initially hot. The gas stream is heated by the bed

and a ‘cold’ wave passes through the bed from right to left.

Having been preheated by the inert bed, the ammonia stream

is heated to the maximum cycle temperature (150–200 8C)

in a heat exchanger. The ammonia gas then passes to active

bed 1 where it heats the carbon. A ‘hot’ thermal wave passes

from left to right through the active bed. As the temperature

of the active bed rises it desorbs ammonia which first

increases the pressure in the left hand loop and then

condenses in the condenser, rejecting heat to the environ-

ment. The mass flow rate of circulating ammonia is typically

ten times that of the condensing stream of ammonia and it

may take about ten minutes for the two thermal waves to

travel the length of their respective beds. In similar fashion

to the left hand loop the circulating flow might be ten times

the adsorption flow from the evaporator.

The advantages of this system are:

† The four packed beds are in effect heat exchangers of

very high surface areas but at minimal cost. They are not

only cheap but very compact.

† There are only four conventional heat exchangers and

this is the minimum number allowed by thermodyn-

amics. These are the evaporator and condenser, a gas

heater whereby high-grade heat is input and a gas cooler

whereby the low grade heat of adsorption is rejected to

the environment.

† The cycle is highly regenerative since the packed beds

act like large counterflow heat exchangers. This results

in good energy efficiency (i.e. high COP).

3.6. Multi-stage and cascading cycle

The adsorption cycles discussed in previous sections are

applicable only to a single stage cycle. The single stage

cycle systems have certain limitations, that is, they cannot

effectively utilize high temperature heat source, as well as

do not perform well at very low temperatures. Hence, to

improve the system performance under such situations,

adsorptive processes may be adapted for advanced cycles,

such as, multi-stage and cascading cycle.

The basic idea of a multi-stage cycle is to perform the

desorption –condensation processes and evaporation–

adsorption processes at different temperature/pressure levels

by using the ‘same working pair’. The internal re-use of heat

of condensation or adsorption can increase the system

performance significantly. Another practical cycle that can

make good use of high temperature heat source is the

‘cascading cycle’, which operates with ‘different working

pairs’ (either liquid/liquid or solid/liquid), such as zeolite–

water/activated carbon–methanol, or zeolite–water/silica

gel–water, etc. These cascading cycles are applied to

situations especially, when there exists a large temperature

difference between the heat source/ambient and the

temperature in the evaporator/refrigeration space. For such

situations, it may not be practical to use single stage cycle.

Hence, one way of dealing with such situations is to perform

the evaporation/refrigeration process in stages, that is, to

have two or more cycles that operate in series at different

temperature levels (cascading). A high temperature heat

source (e.g. boiler) is used to drive the high temperature

Fig. 5. Adsorption refrigeration system with convective thermal wave cycle.


stage adsorption refrigeration cycle. The low temperature

stage adsorption refrigeration is driven by sensible heat and

heat of adsorption obtained from high temperature stage. To

minimize the contribution of sensible heat, special care has

been attached to the heat management of the adsorbers;

n-adsorber cycles operating with a single evaporator and a

single condenser have been proposed with sequences of heat

recovery between adsorbers. Such cycles offer some

advantages: for example, a single condenser is used and

pressure in the n-adsorber unit is not higher than that in the

unit operating an intermittent cycle; moreover, adsorption

heat at high temperature is used as desorption heat at low

temperature. Counteracting heat transfer fluid circuits

between adsorbers reduces entropy generation in compari-

son with what happens in intermittent cycles. The driving

heat supplied to the cycle is at high temperature level (Fig. 6)

so that the entropy generation—due to the inadaptation

between the temperature levels of the source and of the

adsorber is much less in an n-adsorber cycle than in an

intermittent cycle. The same thing happens for the rejected

heat: the rejection temperature is much closer from the

utility temperature with an n-adsorber cycle than with an

intermittent cycle.

Very similar conclusions to that drawn by Scharfe et al.

[45] have been presented by Meunier [46] in the case of heat

recovery between adsorbers. In a particular case, Meunier

has shown that using an infinite number of adsorbers with

ideal heat recovery between adsorbers, the maximum

achievable—with given conditions of operating tempera-

tures—would be a cooling COP equal to 1.85 corresponding

to 68% of ideal Carnot COP. An other cascading cycle

which includes a triple effect machine operating a cascade

between a water zeolite heat pump and a single stage LiBr–

H2O refrigerator has been tested in Munich [47]. A lot of

theoretical studies exists on the possibilities of cascading

cycles while few experimental data are available.

4. Solar cooling technologies

In order to evaluate the potential of the different solar

cooling systems, a classification has been made by Best

and Ortega [48]. The relevant cooling technologies are:

† intermittent adsorption;

† continuous adsorption;

† diffusion; and

† absorption systems;

Adsorption cycles which were detailed in Section 3.6,

can be used in heat-driven refrigerators, air conditioners or

heat pumps in which the energy source can be solar energy

or waste heat. Examples include the use of waste heat to

provide air-conditioning, heating, industrial refrigeration,

and domestic refrigeration in parts of the developing world.

There is a well-documented need for domestic refrigeration

in areas that do not have access to grid electricity. Spoilage

of many products, particularly fish, can be as high as 50%.

This crisis has led to various international agreements

including the Kyoto ‘Declaration and Plan of Action on the

Sustainable Contribution of Fisheries to Food Security’

(NRI, 2000) [49].

Renewed interest in adsorption refrigeration is based on

the advantages of (i) being CFC-free; (ii) cost effective;

(iii) simple construction; (iv) no need for solution pumps,

and (v) they can be driven by low grade energy heat.

Nineteen different possible technologies have been ident-

ified [48] for solar refrigeration. Photovoltaic/vapor com-

pression and photovoltaic/thermoelectric systems have been

predominant in the application of small refrigerators for

vaccine storage in isolated areas where high cost is justified.

In this section, brief details on various solar refrigeration

systems have been listed and their unique features are also

discussed. Absorption and diffusion systems are not

included in this study as the main focus is on adsorption

systems.

4.1. Intermittent adsorption systems

Because of the intermittent nature of solar energy,

intermittent adsorption refrigeration cycles have long been

considered as logical approaches to solar cooling systems.

Adsorbent–refrigerant combinations which have been

examined for solar application include silica gel–water,

Fig. 6. n-Adsorber cascading cycle.


zeolite–water, activated carbon–methanol and activated

carbon–ammonia. The theoretical principles of basic

intermittent adsorption cycles have been described in detail

in Section 3.

4.1.1. Silica-gel/water and silica-gel methanol systems

Since early 1980s, the work on silica-gel/water systems

have been popular and lot of work was carried out mainly in

Japan. In an effort to utilize solar heat, Sakoda and Suzuki

[50] achieved a solar COP of about 0.2 with a solar collector

500 £ 500 £ 50 mm3 depth, packed with 1 kg of silica-gel

particles and with 1.5 kg of distilled water in the evaporator.

On a clear day with total solar insolation of 19.3 MJ m22

day21, it was estimated that a COP of about 0.4 can be

possible just with 0.4 m2 solar collector.

Hisaki et al. [51], utilizing the benefits of cogeneration,

manufactured a 34.5 kW (10 TR or tonne of refrigeration)

adsorption chiller and ice thermal storage system, compris-

ing two adsorption units, an evaporator and a condenser, as

shown in Fig. 7. Both adsorbers were installed in an

evacuated hermetic enclosure, and each adsorber was

alternatively heated and cooled in a cycle time of 7 min.

The COP of the system was found to be 0.4. They had

reported that a 100 TR system can obtain a COP of about

0.6. Based on the above approach, Saha et al. [52], also had

worked on a 10 kW system using silica gel as the working

media. The silica gel was packed around finned tubes and

the experiments had shown that each adsorption/desorption

cycle takes nearly 3–13 min and heat-exchange cycle is

about 30 s. The system could achieve a COP of 0.48

and was found that the COP tends to rise with cycle time

above 5 min and with the flow rate of chilled water.

A solar powered refrigeration system with a 0.25 m2 flat

plate collector has been developed [53]. This system would

realize an evaporating temperature of 5 8C when the

condensation temperature was around 35 8C and the attained

regeneration temperature was 100 8C. Ortel and Fischer [54]

used methanol with silica gel instead of water, so that the

system could operate at an evaporating temperature below

0 8C. It was found that a two-chamber silica-gel/water

system could be operated by methanol also, but because of

methanol’s inferior thermodynamic properties the COP of

the system was considerably reduced by about 30%.

4.1.2. Zeolite–water systems

Since the late 1970s, significant efforts were made by

Tchernev [55] to develop a solar-powered natural-zeolite/

water system. Natural zeolites, with a heat of adsorption of

2.8 MJ kg21, were found to be more suitable for cooling

systems than synthetic zeolites having a heat of adsorption

of 4.2 MJ kg21. This is due to the fact that with lower heat

of adsorption the adsorbent bed can be cooled more

effectively, which in turn results in better adsorption of

refrigerant. This air-conditioning system with the exposed

collector area of 1 m2 could produce ice at a rate of

6.8 kg day21. Tchernev [55] also designed and tested a solar

adsorption machine with different combinations of working

media. Flat plate collector having an exposed area of

2.44 m2 was used to produce 100 kg of ice per day. An

additional 1.5 m2 flat plate collector was used along with the

system to refrigerate milk. However, its performance

Fig. 7. Adsorption chiller.


was found to be unsatisfactory, which was due to the

presence of inert gas inside zeolite bed chamber.

Dupont et al. [56] developed a 13X-zeolite-water cold-

storage system, which had an ice-making capacity of

10 kg day21. The system could attain a solar COP of 0.15.

Similarly, in France, Grenier et al. [57] had developed a

system using a simple solar collector with an area of 20 m2

containing 360 kg of NaX zeolite for a cold storage plant.

The cold production during evaporation was 1.93 MJ m22

and net production was only about 1.88 MJ m22 when the

incident radiation was 17.8 MJ m22. The operation con-

ditions were set to a condensation temperature of 32 8C,

evaporating temperature at 1 8C and the regeneration

temperature at 118 8C. The system could attain a net solar

COP of 0.105, while its cycle COP was 0.38. Tather and

Erdem-Senatalar [58] also utilized zeolite–water as work-

ing pair and reported that, the system could operate

successfully if the ambient temperature is less than 20 8C.

In the literature [53] it is mentioned that Guilleminot et al.

tried to use zeolite composites to replace zeolite as an

adsorbent and two different composites with different ratios

was used in his study. (i) 65% of zeolite þ 35% of metallic

foam; (ii) 70% of zeolite þ 30% of natural expanded

graphite. It was expected that, the latter combination

resulted in a better performance. A similar performance

was noticed by Poyelle et al. [59] and found that, if the

evaporation temperature is set to 24 8C, the required

condition temperature should not exceed 30 8C.

Meunier [60] had been a pioneer in the field of

adsorption. Initially, in 1979, his group theoretically

analyzed the zeolite and water pair and found that the

COP depends largely on the condensation temperature,

followed by evaporation temperature and then regeneration

temperature. In 1984, they developed an adsorption machine

which was operated at different temperature conditions.

When TC ¼ 30 8C; TE ¼ 5 8C and TR ¼ 70 8C, the system

could attain a COP of 0.14, while when the TC and TE was

set to 48 and 121 8C, respectively, the COP reduced

drastically to 0.04. Further work was continued and in

1986, they had attempted to use various working media like

Z13X and Z4AX-water in order to improve the performance

of the system.

A detailed thermodynamic model had been developed by

Cacciola and Restuccia [61] to obtain all the data needed

during the design step of an adsorption machine. Using this

model, it is possible to calculate the operative conditions,

the energy balance in each component of the machine and

the COP.

From the comparison of performance reported for

different adsorbent/adsorbate pairs, it appears that zeolite/

water is the most suitable pair to realize machines to be used

in domestic applications in southern European countries,

where the market demand for devices that are capable to

produce heating during winter and cooling during summer is

increasing considerably.

4.1.3. Activated carbon–methanol systems

A solar-powered adsorption system using activated

carbon–methanol pair was investigated by Delgado et al.

[62] in France. With an exposed area of 4 m2 solar collector

area, it would produce nearly 25 kg ice in a day. While the

thermodynamic COP was 0.4, the overall system COP was

only 0.15. Li and Sumathy [63] had presented the

description and operation of a solar-powered ice-maker

with the solid adsorption pair of activated carbon and

methanol using basic adsorption cycle. A domestic type of

charcoal was chosen as the adsorbent, and a simple flat-plate

collector with an exposed area of 0.92 m2 was employed to

produce ice about 4–5 kg per day at an evaporator

temperature of about 26 8C. The above system could

achieve solar refrigeration COP of about 0.1–0.12. A

similar experiment was performed by Pons et al. [11] with

activated carbon and methanol as the working pair and the

condensers used were of air-cooled type. The system was

loaded with 130 kg of activated carbon, which produced

around 30–35 kg of ice on sunny days. The system

employed flat-plate collector having an exposed area of

6 m2 and was packed with charcoal at a ratio of 20 kg m22.

It was reported that the system COP was about 0.12 and was

comparable with Delgado’s results.

They further extended their work at Agadic, Morocco

and demonstrated the operation of a mini-ice maker which

could produce about 4 kg of ice a day. The limitation of the

performance was mainly due to the ambient temperature,

and an increase in the ambient temperature beyond 23 8C

reduced the system performance significantly.

Since 1986, at AIT Bangkok, Exell et al. [64] have

worked extensively on the development of a charcoal–

methanol system. With just 1 m2 collector area, the system

could produce 4 kg of ice a day. The COP of the system was

about 0.123 which is marginally better than the systems

reported by other researchers. With a similar collector of

area (0.92 m2), Meunier et al. [60] as well as Cacciola and

Restuccia [61] also carried out many experimental investi-

gations on development of activated carbon–methanol

adsorption machines. Their observations were similar to

that reported by Pons and Guilleminot.

4.1.4. Activated carbon–ammonia systems

Investigations in the use of charcoal–ammonia are

apparently more recent—mainly during the 1990s. Jones

[65] used a novel carbon-moulding technique and incorpor-

ated a thermal wave-regeneration concept employed in the

drying of gas streams; a small unit consuming 0.51 kg of

charcoal produced 293 W cooling with an adsorbent heating

and cooling cycle of 6 minutes, with ammonia as the

adsorbate. With R22 and R134a as adsorbates, cooling rates

of 113 and 99 W were reported. Jones [65] also reported that

larger multi-bed systems were under development which

could have a cooling COP of 1.0. Miles and Shelton [66]

were involved in a similar development work, they claimed

to have achieved a heating COP of 1.21 and a cooling COP


of 0.76, using a two-bed system with half-cycle time of

2.6 min.

In order to improve the heat transfer effectiveness, finned

tubes are used instead of a simple flat-bed [53], which had an

outer diameter of 53 m and relatively length (L ¼ 0:2 m).

The system was tested at various operating conditions, and

was shown to be optimum, when the condensation

temperature of and regeneration temperature were 20 and

25 8C, respectively.

4.2. Continuous adsorption systems

Continuous solar adsorption refrigeration systems are

being reported widely because of their higher system

performance over intermittent alternatives and for their

timely coincidence with the requirement of the cooling and

refrigeration demand. Continuous adsorption can be rea-

lized through various cycles which are detailed in (Sections

3.2–3.6). Hence, various continuous adsorption cycles

depending on their technologies, can be grouped as:

† multi-stage and cascading systems;

† thermal wave adsorption systems;

† convective thermal wave adsorption systems; and

† hybrid systems.

In general, thermal wave and convective thermal wave

adsorption systems are also called as ‘heat regenerative

systems’ owing to its heat recovery properties.

4.2.1. Multi-stage and cascading systems

A lot of theoretical studies exist on the possibilities of

cascading cycles while very few experimental data are

available. Douss and Meunier [67] had reported the

experiments on a cascading adsorptive heat pump. The

cycle comprised of a two-adsorber zeolite–water system at

high temperature stage and an intermittent active carbon–

methanol system at low temperature stage. Driving heat was

supplied by a boiler to zeolite adsorbers while active carbon

adsorber was heated by heat recovered from zeolite adsorber

under adsorption. Evaporators from both basic cycles

operated at the same temperature and contributed to the

evaporating load. Experimental cooling COP was found to

be 1.06, which is much higher than the COP of an

intermittent cycle (<0.5) and that of a two-adsorber zeolite

water cycle (<0.75). An analysis of the results showed that

the components which limit the power of the unit were the

evaporators and basically it was the water evaporator.

The COP of the cascading cycle was very sensitive to the

evaporating temperature lift. Also, it was reported that

the cycle had adapted well to air conditioning as long as the

evaporation temperature lift was less than 45 8C. A similar

study on combined cycle comprising of an adsorption–

absorption cascading multi-effect refrigeration cycle was

carried out by Shu et al. [68]. The system consists of a high

temperature stage of solid adsorption unit working with

zeolite water and a low temperature stage of double effect

absorption unit working with LiBr–water The working

principle of this multi effect cascading cycle can be

explained using Clapeyron diagram shown in Fig. 8. The

single stage cycle with heat source Qh represents the

adsorption cycle using zeolite þ water as working medium.

The double stage cycle obtains the absorption heat of the

single stage cycle as its driving heat ðQadsÞ for its high

temperature stage. The heat obtained from the condenser of

the single stage cycle is used as the heat source for its low

temperature stage. The study has shown that the COP of the

cycle can be greatly improved by efficient utilization and

recovery of energy within the system. Also, they had

reported that there exists no corrosive problem though the

temperature of the working pair is higher than 200 8C in the

adsorption unit.

Saha et al. [69] has proposed a two-stage non-

regenerative adsorption chiller design and experimental

prototype using silica gel–water as the adsorbent refrigerant

pair. The main advantage of the two-stage adsorption chiller

is its ability to utilize low temperature solar/waste heat

(40-75 8C) as the driving heat source in combination with a

coolant at 30 8C. With a 55 8C driving source in combi-

nation with a heat sink at 30 8C, the COP of the two-stage

chiller is 0.36. Wang [26,70] carried a study on a four-bed

adsorption refrigeration system and the multi-stage system

Fig. 8. Adsorption–absorption cascading multi-effect refrigeration

cycle: (1) adsorption cycle; (2) high pressure absorption cycle and

(3) low pressure absorption cycle.


configuration is shown in Fig. 9. The system consisted of

four adsorbers ðA1;A2;B1;B2Þ; one condenser (C), and one

evaporator (E). The working principle can be explained as

follows according to the Clapeyron diagram as shown in

Fig. 10: To begin with, A1 and A2 are in the high temperature

stage, for which generation (temperature about 200 8C) is

initiated by heat input. The desorbed water vapor will go

through the adsorber B1 or B2 at the low temperature stage to

release heat for desorption. The adsorption pressure of the

two stages are the same because only one evaporator is used

in the system. During phase 1, the desorption process in

desorber B1 is furnished by the heat from desorber A1 and

adsorber A2 in the high temperature stage. The coordination

of adsorption and desorption of the two stages is very

important to operate the system properly. By the way, the

desorption pressure in desorber A1 must be higher than

the desorption pressure of desorber B1 in order to facilitate

the vapor desorbed from A1 to pass through desorber B1

effectively. In this multi-stage system, water is the

refrigerant and the desorbed vapor flows through the adsor-

bent bed in desorber B1 to get a triple-effect installation.

Here, a double effect is accomplished with sensible and

adsorption heat recovery to generate low temperature stage

in addition to the heat input to the high temperature stage, a

triple effect is accomplished with the heat recovery of the

desorbed vapor in the high temperature stage in addition to

Fig. 9. The system configuration of four-bed cascading adsorption refrigeration cycle.

Fig. 10. Clapeyron diagram of four-bed cascading adsorption refrigeration cycle.


the heat used in a double effect arrangement to generate low

temperature stage.

The maximum energy recovery from high temperature

stage can be arranged by a triple effect system, in which the

sensible heat of adsorbent bed, heat of adsorption and the

latent heat of refrigerant vapor are fully used to drive the low

temperature stage system. The simulation work has shown

that, the performance of a cascading cycle interve of COP

can be triple (COP ¼ 1.56), compared to two independent

stage system (COP ¼ 0.6). This concept could be used for

multi-effect adsorption systems, as well as extended to

adsorption–absorption cascading systems [70,71].

4.2.2. Thermal wave adsorption systems

Thermal wave adsorption is the most commonly

attempted technology. In a two-bed adsorption system

shown in Fig. 4, substantial part of heat from the adsorbent

bed is rejected at a temperature greater than the temperature

of the bed being heated. Therefore, it was soon recognized

that part of this rejected heat could be used for heating the

desorbing bed, thus providing gains in efficiency. Shelton

et al. [72] and Fuller et al. [73] were among the pioneers in

devising analytical models for this technique, by means of a

single heat transfer fluid loop passing through adsorbent

beds, a fluid cooler, a fluid heater and a reversible

circulation pump. For a two-bed system, high temperature

thermal fluid flows into the adsorber, exchanges heat with

the bed, and the temperature goes down along the bed

rapidly, thus the outlet temperature will be close to ambient.

After being cooled by ambient surroundings, the fluid flows

into another adsorption bed, absorbs heat from the bed, and

the temperature of the fluid goes up. At the exit of this bed,

the thermal fluid temperature will be very close to the

temperature of heat source. In this case, only less heat is

added to the system, and less heat released to the

environment. Thus, heat recovery ratio is high and the

COP is increased significantly.

Analytical models of square and ramp waveforms

predicted that heating COP of up to 1.6 is possible. In a

similar effort, Zheng et al. [74], using a charcoal–ammonia

pair, found that system performance improves significantly

with a reduction of ambient temperature, and is also affected

by cycling speed and regeneration temperature.

Wang et al. [75] had developed two heat regenerative

adsorption systems, with two different types of heat

exchangers: (i) heat regenerative adsorption refrigerator

with a spiral plate heat exchangers adsorbers; (ii) plate fin

heat exchangers or plate fin shell heat exchangers adsorbers.

The activated carbon–methanol adsorption pair was used

for the above two adsorption systems. The two adsorbers

were independently operated for heating and cooling, along

with the intermediate heat recovery process. Fig. 4 shows

the schematic of the unit. A receiver was installed for easy

observation of refrigerant flow in the system. An ice box

was used which was cooled by a salt water cooling heat

exchanger. With a heat source temperature of 100 8C,

the refrigerator achieved a refrigeration power density of

more than 2.6 kg per day of ice per kg of activated carbon

with a COP of 0.13, and the heat pump achieved

150 W kg21 activated carbon for air conditioning with a

COP of about 0.4.

4.2.3. Convective thermal wave adsorption systems

Critoph et al. [43,44] suggested the use of a forced

convection effect by direct use of refrigerant vapors. During

desorption, the refrigerant is heated externally and passes

through the adsorber bed, where it heats the adsorbent

rapidly because of good heat transfer coefficient and the

large surface area of the adsorbent bed. The desorbed vapor

is condensed in a condenser in the usual way. During the

adsorption process, the flow is reversed and heat of

adsorption is removed by the refrigerant, which is cooled

externally. This technique enhances performance by virtue

of the combined effects of forced convection and thermal

wave. Flow of hot gas through the adsorbent gives rise to a

thermal wave traveling through the bed during desorption,

and vice versa during adsorption process. Based on this

theory, a new continuous adsorption refrigeration/heat

pump system using a number of simple tubular adsorption

modules was developed. Each single module comprised of a

generator and a receiver/condenser/evaporator. A complete

module was tested in a simple rig, which was subjected to

alternating hot and cold air streams, desorbing and

adsorbing ammonia.

The performance of a temperature wave regenerative

heat pump has been analyzed using a two-dimensional

model by Ben Amar et al. [41]. The model simultaneously

considers heat and mass transfers in the adsorbent bed. The

mathematical model has been solved numerically by use of

the ADI finite different method. The consideration of both

heat and mass transfer in the model has made it possible to

determine the impact of the convective transport of the

gaseous adsorbate in the adsorber on the global perform-

ance, in addition to that of the heat transfer. It has been

shown that with a traditional packed bed, which generally

has a low thermal conductivity (,0.2 W m21K21), the

performance of the resulting heat pump is very poor. In

contrast, with a consolidated adsorbent material, which can

have a much improved thermal conductivity but relatively

poor mass transfer properties, the heat pump performance

could be poor as well, due to the large mass transfer

resistances encountered by low pressure adsorbates like

water. A proper design of the adsorber should therefore

ensure good properties for both heat and mass transfer, in

particular when the adsorbate used has low working

pressures.

4.2.4. Hybrid system

Although solid adsorption refrigeration has received

much attention from most of the refrigeration companies

and laboratories around the world, some of the main

disadvantages such as long adsorption/desorption time


and small refrigeration capacity per unit mass of adsorbent,

have become obstacles for the real mass production of the

system.

Better design of the adsorbers and improvements in the

thermal conductivity of the adsorption bed are the main

tools to reduce cycle time, while treatment of adsorbent to

increase its adsorption capacity on a refrigerant could

increase the refrigeration capacity of adsorbent per unit

mass and also the COP of the cycle. Besides, several

investigations had attempted to further improve (hybrid) the

performance of the existing solar refrigerators, (i) by

combining the basic adsorption cycle with other refriger-

ation cycles, as well as, (ii) in designing a dual system. The

features of the different hybrid systems are discussed in

Section 4.3.

In an attempt to overcome the intermittence of adsorp-

tion refrigeration, and realize the continuity, a novel

combined cycle of a solar-powered adsorption–ejection

refrigeration system has been studied by Li et al. [76]. As

shown in Fig. 11, it includes two subsystems: the adsorption

sub-system, which refrigerates at night time and the ejection

subsystem, which refrigerates during the day.

During the daytime, with valves v1; v4 kept open while v2

is closed, the vapor at high temperature and high pressure

produced in generator ðgÞ and auxiliary heater ðbÞ enters the

ejector and forms supersonic primary fluid at the exit of the

nozzle. This enables the entrainment of the secondary fluid

from the evaporator. The secondary fluid subsequently

mixes with the primary fluid in the mixing chamber of the

ejector, and when flowing into the condenser, the mixed

fluid condenses into liquid. This liquid is finally divided into

two parts: one part goes into the evaporator as in other

cycles; the other is pumped back to the generator [77]. At

the same time, the adsorber gets heated up by solar energy.

When the pressure in the adsorber rises to a certain value Pb

(correspondent with Tb), the valve v5 opens, the desorbed

vapor then enters the generator as the primary fluid for the

ejection cycle. The valve v5 should be closed and v3 should

be open, as the temperature within the adsorber reaches Tg2.

During night time, valves v1; v3; v4 and v5 are closed, and

when the temperature within the adsorber decreases to Ta2;

valve v2 opens. As a result, the adsorbent begins to adsorb

the refrigerant. In order to recover the sensible heat and the

heat of adsorption, a heat pipe is used. The recovered heat is

applied to heat the refrigerant or thermal storage material in

the generator for the ejection cycle the next day. When

thermal equilibrium is reached, the heat pipe will stop

working, then a heat exchanger ðx2Þ is needed to cool the

adsorber further.

The main advantage of this system is to overcome the

intermittence problem in solar-powered adsorption refriger-

ation, together with increasing the performance of the

ejection system. It is reported that increasing the tempera-

ture or reducing pressure within the adsorbent bed, the COP

of the ejection subsystem improve very slightly, but if much

more adsorbent is used, a better result might be obtained.

Though solar ice making is attractive by an adsorption

system, it needs both good solar collecting (high desorption

temperature) and heat release characteristics for the

adsorber (low adsorption temperature), which seems to be

Fig. 11. Schematic of the ejection refrigeration and adsorption refrigeration systems.


contradictory, specially for a single bed system. Because, in

order to obtain a desorption temperature, the losses from the

bed should be as low as possible. Whereas, during

adsorption period, the same bed should attain a low

adsorption temperature, by releasing heat to the surround-

ing. It should be noted that, heat release means energy

losses; therefore, in order to make the system more efficient

and useful in practice, heat recovery must be considered. A

hybrid system of solar-powered water heater and ice-maker

(Fig. 12) has been developed by Wang et al. [78]. The

system consists of a solar collector, water tank adsorber/

generator, condenser, evaporator, receiver, ice-box, etc. The

working principle is based on the combination of a solar

water heater and an adsorption refrigerator. In the morning,

the solar collector heats the water tank and along with the

increase of water temperature, the temperature in the

adsorbent bed rises. In an ideal process, the adsorbent

temperature could reach a level very close to the water

temperature in the tank.

When the temperature in the adsorbent rises up to a

temperature which causes the vapor pressure of the

desorbed refrigerant up to the condensing pressure,

desorbed vapor would be condensed in the condenser and

collected in the receiver. This liquid refrigerant then flows

to the evaporator via a flow rate regulating valve. On the

other hand the temperature of the water and the adsorbent

bed continues to rise to a maximum temperature of about

80–100 8C. The high temperature water is finally drained

out into a separate tank, be used more flexibly for any

domestic purposes.

The features of the hybrid system include (1) water

heating and refrigeration with one solar collector, which is

suitable for household applications; (2) adsorber/generator

is separated from collector, thus high efficiency vacuum

collector can be used for water heating, thereby heating

the adsorber at same time. The high efficiency heating does

not mean a bad cooling of the adsorber through the night, as

by draining the hot water from the tank, cold water is refilled

to the tank, thus the adsorber is cooled and refrigeration will

take place.

Recently, Dai et al. [79] have investigated the perfor-

mance of a hybrid solar cooling system (Fig. 13), which

combines the technologies of rotary desiccant dehumidi-

fication and solid adsorption refrigeration, for cooling grain.

The key components of the system are a rotary desiccant

wheel and a solar adsorption collector. The former is used

for dehumidification and the later acts as both an adsorption

unit and a solar collector. The heating load from sunshine

can thus be reduced to a greater extent since the solar

adsorption collector is placed on the roof of the grain depot.

Compared with the solid adsorption refrigeration system

alone, the new hybrid system performs better. Under typical

conditions, the COP of the system was round to be 0.4 when

the outlet temperature was ,20 8C. It is believed that the

system can be used widely in the regions with abundant

solar resources due to such advantages as environmental

protection, energy saving and low operation costs.

Yet another application of natural ventilation using the

adsorption refrigeration cycle has been studied by Dai et al.

[80]. The study presents a parameter analytical study of

a solar house (Fig. 14) with enhanced natural ventilation

derived by a solar chimney and a solar adsorption cooling

cavity.

The proposed solar house consists of two subsystems, i.e.

the solar chimney and the solar adsorption cooling cavity.

Both subsystems contribute to inducing outdoor air into the

room space. The components of a solar chimney include

glazing, air channel, selective coating, and a thermal storage

wall. It works on the principle of thermal buoyancy induced

by solar heating. Similarly, the solar adsorption cooling

system is configured by a flat plate adsorber, a condenser,

and an evaporator, etc. The adsorber is regenerated during

daytime when solar irradiation heats the adsorption bed and

desorption process begins; during night time cooling effect

is produced when the adsorber cools down and adsorption

process is initiated. A water tank is provided to store the

chilled water from the evaporator, which is circulated

(thermosyphoning) through a heat exchanger in the cooling

cavity. During this period, the ambient air is captured

through the cavity inlet (0), gets cooled by the chilled water

through the heat exchanger, and further descends down-

wards into the room. The air motion is mainly due to the

density difference of the air at the inlet and outlet of the

system.

Hence, during daytime, both solar chimney and cooling

cavity induce ventilation, while at night, ventilation is

induced mainly by the heat storage in solar chimney, the

heat released during adsorption process, and by a chilled

water circulation in the cooling cavity. In order to enhance

Fig. 12. Schematic of the hybrid system: (1) solar collector; (2)

water tank; (3) adsorber; (4) condenser; (5) receiver; (6) valve; (7)

evaporator; (8) refrigerator.


Fig. 14. Schematic of the solar house with enhanced natural ventilation driven by solar chimney and adsorption cooling cavity [80].

Fig. 13. Schematic of the solar cooling system with rotary desiccant dehumidification and solar adsorption refrigeration [78].


the adsorption process, two dampers that enclose the space

between the glass cover and the adsorption bed are released

apart which facilitates the adsorption bed to be cooled more

efficiently. The chimney effect can thus be realized in the air

channel between the glass cover and the adsorption bed.

4.3. Solar desiccant cooling and air-conditioning systems

Desiccant technology has become a valuable tool in the

industry’s arsenal of space-conditioning options. In certain

cooling applications, desiccant cooling units provide

advantages over the more common vapor-compression and

absorption units. For example, desiccant units do not require

ozone-depleting refrigerants, and they can use natural gas,

solar thermal energy, or waste heat as heat input, thus

lowering peak electric demand.

An open-cycle desiccant cooling system operating in a

recirculation mode is shown in Fig. 15. The room air is

recirculated and ambient air is used only for regeneration.

Room air is dehumidified and heated by the desiccant wheel,

regeneratively cooled, and then evaporatively cooled prior

to re-entering the room. Ambient air is evaporatively

cooled, regeneratively heated, and then heated by an energy

supply. The heated ambient air passes through the

dehumidifier and regenerates the desiccant. For this system,

the sensible heat regenerator and dehumidifier are assumed

to be rotary elements, but direct transfer exchangers and

fixed beds could equally well be used.

A 10.5 kW open-cycle liquid desiccant dehumidification

system was designed and operated by Patnaik et al. [81].

Packed bed units were used to dry the air in the dehumidifier

and to concentrate the desiccant in the regenerator. Liquid

distribution in the regenerator was studied for two systems:

a gravity tray distributor, and spray nozzle system. Higher

capacities (40–50% increase) and lower pressure drop

(30–40% reduction) for the air flow were observed with the

spray system. Cooling capacities of 3.5–14.0 kW were

achieved for both the regenerator and dehumidifier.

Functional relationships correlating the independent vari-

ables to the rate of vaporization in the regenerator and rate

of condensation in the dehumidifier were obtained by

statistical analysis of experimental data. These studies thus

provide data and correlations useful for design guidance and

performance analysis of similar open-cycle liquid desiccant

systems.

Recent research in the development of a solar desiccant

cooling system has focused on the development of advanced

desiccant materials that give improved sorption capacity,

favorable equilibrium isotherms, and better moisture and

heat rates. Improved performance of these systems will

lower their initial costs and make these systems a more

attractive alternative to existing vapor compression systems.

An improved system with Type 1M isotherm shape

(modified Langmuir Type 1) has been proposed by Collier

et al. [82]. Further development of such a system were

continued by Belding et al. [83] and Worek et al. [84]. The

suitability of different technologies for binding silica gel

particles has been investigated [85].

Performance of various types of commercially available

desiccant wheels have been experimentally and analytically

evaluated by Collier et al. [86]. This study included the use

of lithium chloride, molecular sieve, silica gel and activated

carbon in the manufacture of wheels. Observations revealed

that the rate of moisture removal was roughly the same for

core materials of all types, which implies that cooling

system’s performance is similar for desiccant wheels of all

types. The important merits of solid-desiccant cooling

systems include: simplicity in construction and maintenance

Fig. 15. An open solid desiccant cycle.


Table 3

An overview of developments in adsorption technologies

System Heat source Collector

area (m2)

Operating temperature (8C) Performancea Working pairs Investigators Remarks

TC TE TR TA COP SCP

(W kg21)

Ice

(kg)

Intermittent Solar heat 1 – – – 21–29 0.12 – – Activated carbon/methanol Boubakri [94] Collector-

condenser

technology

Experiment

Solar heat 2 – – – – 0.09–0.13 – – Activated carbon/methanol Buchter

et al. [95]

Experiment

Measured solar

irradiation

– 33 20.9 105 20.6–27.3 0.24 – 7.3 Activated carbon/methanol Leite and

Daguenet [96]

Simulation

Solar heat 0.92 – 26 70–78 6–13 0.1–0.12 – 4–5 Activated carbon/methanol Li and

Sumathy [63]

Experiment

Quartz lamp 1 28–35 3 – – 0.11 – 5–6 Activated carbon/methanol Li et al. [97] Experiment

– 1 40 0 – – 0.15 – – Zeolite 13-X/water Tchernev [55]

Heat

recovery

– – 35 – 120 – 0.75–0.85 – – Activated carbon/methanol Cacciola and

Restuccia [61]

Simulation

þZeolite/water Single stage

system

Boiler – – – – – 0.21–0.48 – – Silica gel/water Saha et al. [52] Experiment

– – – – – – 0.3–0.5 – – Silica gel/water Saha et al. [52] Simulation

Heater – 24–29.6 6–9.9 96.8–106 – 0.32–0.4 133–151 – Activated carbon/methanol Wang [27] Experiment

Mass

recovery

– – 30 5 80–120 – 0.6–0.8 – – Activated carbon/methanol Wang [27] Simulation

Mass

recovery

Generator – 38 8.9 – – 0.60 142 – Monolithic active carbon/

ammonia

Critoph [38] Experimental

simulation

Multi-bed

system

Cascading Thermostated

bath

– 35 25 – – 1.06 – – Zeolite/water þ activated

carbon/methanol

Douss and

Meunier [67]

Experiment

Cascading – – – – – – – – – Zeolite/water þ LiBr–water Shu et al. [68] Simulation

of COP

Multi effect

Thermal

wave

– – 35 2 – – – – – Ditto Miles and

Shelton [66]

Heater – 40 5 – – – – – Zeolite NaX/ammonia Sun et al. [40] Simulation

K.

Su

ma

thy

eta

l./

Pro

gress

inE

nerg

ya

nd

Co

mb

ustio

nS

cience

29

(20

03

)3

01

–3

27

32

2

Convective

thermal

wave

Heater – 42 0 – – 0.95 – – Carbon/methanol Critoph [43,44] Experiment

Hybrid – – 20 22.5 – – 0.064–0.067 – 10–10.5 Activated carbon/methanol Wang et al. [78] Experiment

Heating þ

ice making

Assumed solar

irradiation

2 15–35 210 84.9–100 – 0.039–0.046 – 3.05–8.7 Activated carbon/methanol Wang et al. [78] Simulation

Heating þ

ice making

– 0.4 – – 5 – 0.18 87.8 – Activated carbon/methanol Zhang and

Wang [98]

Simulation

Hybrid

collector

– 2 30 5 – – 0.33 5.67 – Zeolite 13-X/water Zhang and

Wang [99]

Simulation

Adsorption-

ejection

technology

Generator – 40 10 120 – 0.4 – – Zeolite 13-X/water Li et al. [76] Experiment

Adsorption-

ejection

technology

Binary

coupling

Electric heater – – – 130 – – – – Zeolite/Ammonia/water Wang and

Zhu [100]

Adsorption þ

absorption

Experiment

a The cooling performance of the system.

K.

Su

ma

thy

eta

l./

Pro

gress

inE

nerg

ya

nd

Co

mb

ustio

nS

cience

29

(20

03

)3

01

–3

27

32

3

(as only air and water are needed); low pressure operation;

improved indoor air quality; potential for high energy

savings; and operation with potential use of diverse energy

sources such as solar energy, natural gas and bio-gas. Their

major limitation is that their performance may be influenced

by variations in weather conditions. Their COP ranges from

0.5 to 1.0, although analytical models for modified systems

have predicted that COP values greater than 2.0 are possible.

An experimental investigation with a lithium-chloride-

impregnated wheel regenerated, by solar energy (using a

commercially available evacuated tube collector) showed

that 0.5 units of cooling can be obtained per unit of solar

energy incident on solar collectors. In summer, an average

cooling of 10 MJ m21 per day could be achieved [87]. The

use of desiccant wheels for humidity control has been tried

successfully in applications such as the prevention of metal

corrosion in storage, and in photolithography, by Harriman

[88,89]; their potential has been evaluated by Banks [90,91]

for use in applications including ice rinks and supermarkets.

Hybrid systems, which integrate desiccant dehumidifiers

with conventional cooling systems are proven to provide

substantial energy savings. The required energy can be

supplied by low temperature heat sources for the regener-

ation if the rotary dehumidifier matrix is properly con-

structed and appropriate selections are made for the type and

amount of the desiccant. The thermal energy can be

provided by a combination of an array of solar flat plate

collectors and natural gas [92].

5. Conclusions

The development of adsorption system for refrigeration

is promising. An overall thermodynamics-based comparison

of sorption systems shows that the performance of

adsorption systems depend highly on both the adsorption

pairs and processes [93]. The technology continues to

develop, and the cost of producing power with solar thermal

adsorption refrigeration is falling. If the costs of fossil fuels,

transportation, energy conversion, electricity transmission

and system maintenance are taken into account, the cost of

energy produced by solar thermal adsorption systems would

be much lower than that for conventional refrigeration

systems. This paper presents an overall review on the

fundamental understanding on the various adsorption

refrigeration cycles and the applicability of solar adsorption

both in air conditioning and refrigeration, with the

improvement of the COP.

Of the several kinds of adsorption systems analyzed in

this paper, the intermittent system has been extensively

studied both theoretically and experimentally, owing to its

simplicity and cost effectiveness. However, the main

disadvantages such as long adsorption/desorption time

have become obstacles for commercial production of the

system. Hence, to compete with conventional absorption

and vapor-compression technologies, more efforts should be

made in enhancing the COP and SCP (A summarization on

the different systems along with their respective COPcooling

and SCPs is listed in Table 3). Heat recovery, mass recovery,

multi-bed and multi-stage technologies are promising

technologies in improving the COP and SCP. Also, recent

developments [94,95] of activated carbon series—activated

carbon fibre (ACF)—has called attention to the possibility

for application in adsorption refrigeration. For instance,

with a specially treated ACF, the measured adsorption

capacity on methanol is 2–3 times that of normal activated

carbon, and the estimated adsorption time of ACF on

methanol is about 1/5 to 1/10 of that of normal activated

carbon. Besides, activated carbon can be made with

properties to suit particular applications by varying the

activation time and temperature. Hence, with such technol-

ogies the specific refrigeration capacity as well as the COP

of the adsorption refrigeration cycle can be improved.

Adsorption technology combined with other tech-

nologies for multi-purpose application seems to be a new

trend in the research. This will widen the area of

applications of adsorption technologies and make the

adsorption refrigeration more cost effective. Any method

that improves the efficiency even marginally would improve

the economic viablilty of operating such devices. Thus,

further studies need to be carried out to validate the potential

for possible application in household refrigerators.

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