A Review Of Chemical Heat Pump Technology And Applications

Review
A review of chemical heat pump technology and applications
W. Wongsuwan a
, S. Kumar a,*, P. Neveu b
, F. Meunier b
a
Energy Program, School of Environment Resources and Development (SERD), Asian Institute of Technology,
P.O. Box 4, Klong Luang, Pathumthani 12120, Thailand
b
Institut Franc
ßais du Froid Industriel et du G
enie Climatique, Conservatoire National des Arts et M
etiers, Paris,
75141, France
Received 26 July 2000; accepted 5 February 2001
Abstract
Chemical heat pumps (CHPs) provide high storage capacity and high heat of reaction as compared to
sensible heat generated by absorption. Investigation of material properties and their operation have led to
their heat pumps exploitation for commercial and industrial applications. Integration of solar thermal
system to the CHPs would assist in expanding the utilization of CHPs and also for many applications in the
tropical region. The research done in CHP regarding to status of technology, current applications and their
future prospect has been reviewed, with special reference to their utilization with solar thermal energy for
cold production and upgrading/storage of heat. Ó 2001 Elsevier Science Ltd. All rights reserved.
Keywords: Chemical heat pump; Working pairs; Performance; Metal-hydride chemical heat pump; Chemical reaction
chemical heat pump; Solid adsorption; Solar assisted chemical heat pump
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1490
1.1. Principle of operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493
2. Technology status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495
2.1. Working pairs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495
2.2. Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496
2.2.1. General theoretical studies on CHP performance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1498
2.2.1.1. Solid±gas CHP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1500
2.2.1.2. Liquid±gas system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1501
2.2.1.3. Solid-adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1503
Applied Thermal Engineering 21 (2001) 1489±1519
www.elsevier.com/locate/apthermeng
*
Corresponding author. Tel.: +66-2-524-5410; fax: +66-2-524-5439.
E-mail address: kumar@ait.ac.th (S. Kumar).
1359-4311/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved.
PII: S1359-4311(01)00022-9

2.2.2. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505
3. Areas of further research. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1512
Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1515
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516
1. Introduction
Low temperature sources coupled to a suitable heat pump upgrade heat to a higher tempera-
ture. This is achieved by the consumption of electricity (vapor compression heat pumps) or by
thermal means (vapor absorption and solid±gas sorption heat pumps). Vapor absorption heat
pumps commonly use lithium bromide/water and water/ammonia as the working ¯uids. Though
vapor compression and vapor absorption heat pumps are common, research in the other category
of heat pumps, namely chemical heat pumps (CHPs) has gained momentum in recent years [1±7].
CHP are those systems that utilize the reversible chemical reaction to change the temperature level
of the thermal energy, which is stored by chemical substances [8]. These chemical substances play
an important role in absorbing and releasing heat [9]. The advantages of thermochemical energy
storage [10], such as high storage capacity, long term storage of both reactants and products,
lower of heat loss, etc., suggests that CHP could be an option for energy upgrading of low
temperature heat as well as storage. Sources of low temperature heat could be from waste heat in
industries and/or solar thermal collectors.
Fig. 1 shows the general classi®cation of CHP. Systems involving chemical reaction and re-
quiring only one state variable (e.g. pressure) to be speci®ed (e.g. metal hydrides reacting with
chlorine or ammonia±metal chloride systems) are mono variant systems, and these induce volume
changes, while those that require both the temperature and pressure to be speci®ed are di variant
systems (e.g. solid adsorption).
The general reaction taking place in the CHP reactor is of the form
A ‡ B $ C 1†
where, the forward and backward reactions occur at two di€erent temperatures, thus allowing the
upgrading of heat from low to higher temperature. For example, the hydrogenation/dehydroge-
nation of a 2-propanol±acetone system (liquid±gas) is given by,
CH3†2CO‰gŠ ‡ H2‰gŠ $ CH3†2CHOH 2†
During the backward reaction, 2-propanol is decomposed into acetone and hydrogen by dehy-
drogenation reaction (endothermic). For this reaction to occur (at lower temperature), heat could
be supplied from a low temperature source e.g. solar collector. The hydrogenation reaction is
exothermic and heat is liberated at a higher temperature, thus causing upgrading of heat [11] for
any suitable application.
Various chemical substances could be used in CHP involving chemical reaction. For example,
as shown in Fig. 1, water system (hydroxide/oxide, salt hydrate/salt or salt hydrate), ammonia
system (ammoniate/ammoniate or salt, amine complex with salt), sulfur dioxide system (sulphite/
1490 W. Wongsuwan et al. / Applied Thermal Engineering 21 (2001) 1489±1519

oxide, pyrosulphate/sulfate) [12], carbon dioxide system (carbonate/oxide, barium oxide/barium
carbonate) [13], hydrogen system (hydride/hydride or metal, hydrogenation/dehydrogenation)
[14], etc. have been proposed as the working media.
The solid constituents used in the CHP can be categorized in many ways (Fig. 1). This could be
in terms of adsorbents (e.g. zeolite and activated carbon (AC) with water and methanol), metal
hydrides (e.g. porous metallic foam of Ni and Cu, Al-foam metrics) and compounds with chemi-
cal reaction (i.e. reaction with ammonia derivatives such as monomethylamine or dimethylamine
Nomenclature
COA coecient of ampli®cation (heat
pump e€ect)
COP coecient of performance (refrigera-
tion e€ect)
DEGDME dimethyl ether of diethylene
glycol
E heat of formation of hydride per unit
volume, J/m3
(Eq. (9))
h heat transfer coecient, W/m2
K
DH change of enthalpy, J/mol
DH0 standard reaction enthalpy, kJ/mol
(Eq. (12))
K e€ective thermal conductivity in me-
tal hydride, W/m K (Eq. (9))
L latent heat of vaporization, kJ/kg
m mass of working ¯uid per kg of sor-
bent, kg
M molecular weight, kg/mol
P pressure, Pa
Q quantity of heat exchange, J
rf current radius at hydration front (Eq.
(9))
R reactor bed, perfect gas constant, J/
mol K
R1 internal radius of reaction bed, m
R2 external radius of reaction bed, m
DS entropy, J/mol K
SCP cooling rate, W/kg of pure adsorbent
t cycle time, m
T temperature, °C or K
Greeks
r permeability
q density, kg/m3
g eciency
h Carnot eciency
k thermal conductivity
Subscripts
abs absorption
ads adsorption
amb ambient
b value at bed boundary
chem chemical
cond condensation
des desorption
eq equilibrium
ev evaporation
ex exergetic, e€ective
ext external
f hydration front
h high, high temperature process
H2 hydrogen
i element of
l low
m medium, moderate, metal hydride
max maximum value
o initial value, ambient, sink
reg regeneration
s source temperature
w wall, heat transfer medium ¯owing
through the inner tube of a reactor
module
W. Wongsuwan et al. / Applied Thermal Engineering 21 (2001) 1489±1519 1491

by alkaline, alkaline-earth or metallic halides, double or mixed halides, sulphates, nitrates and
phosphates). Another classi®cation could be based on organic and inorganic substances. For
example, an inorganic reaction system could be calcium chloride/methylamine, calcium oxide/
water/calcium hydroxide, sodium carbonate dehydration/hydration and magnesium oxide/water,
Fig. 1. Classi®cation of CHP.

which normally operate with batch process involving solid±gas or solid±liquid reaction [8]. The
organic reaction system includes hydrocarbon and hydrocarbon derivatives, for example, 2-
propanol/acetone, isobutene/water/tert-butanol, cyclohexane/benzene and paraldehyde/acetalde-
hyde (Pa/A) systems. CHP employing organic reaction have advantages of high energy density
and possibility of continuous cycle because of the liquid±liquid or gas±liquid reaction. Generally,
the gas is condensed and stored as a liquid (e.g. ammoniated salts), condensed and adsorbed (e.g.
zeolite), or reabsorbed (e.g. metal hydrides). In the two later cases, the gas could be stored in
zeolite or in metal hydrides [5].
Depending on the phase of working substance, CHP could be categorized into two types, solid±
gas and liquid±gas. Solid±gas CHPs basically consist of reactor(s) (or adsorber), condenser and
evaporator (Fig. 2) [15]. Liquid±gas systems consist of at least two reactors: endothermic and
exothermic reactors. Besides, other components such as condensers, separators and heat ex-
changer are also usually required.
This paper presents a review of the work done in the recent years on the various types of CHPs
and discusses their applications. For this review, the classi®cation of CHP considered are the
solid±gas system, solid-adsorption and liquid±gas system with focus on the status of technology,
current applications and the areas and activities for further research.
1.1. Principle of operation
The general working of a CHP occurs in two stages: adsorption/synthesis/production and
desorption/ regeneration/decomposition. The synthesis stage is the cold production stage, and this
is followed by the regeneration stage, where decomposition takes place. This can take place in the
same or di€erent reactors depending on the system design (Fig. 2). Basic solid±gas CHPs are not
suitable for continuous production as they are based on two successive phases: synthesis (cold
production stage), which is followed by regeneration (decomposition) [16]. This can be explained
with reference to ammonia±chloride salt system.
During the production phase, the liquid±gas transformation of ammonia produces cold at low
temperature in the evaporator. At the same time, chemical reaction between the gaseous ammonia
and solid would release heat of reaction at higher temperature. In the regeneration phase, the
system is regenerated by heating the reactor and leads to an increase of temperature of solid. This
causes the decomposition of the solid and allows the refrigerant to condense in the evaporator.
When complete decomposition is achieved, the device is ready for a new production phase.
On the other hand, the liquid±gas system is more amenable to be run as a continuous process.
The reactants and products could be fed or removed continuously. Reaction could also take place
Fig. 2. A simple CHP.

by passing the reactants through the catalyst bed. Production and regeneration could also be done
in di€erent reactors [14,17].
Fig. 3 shows the cycle represented in the ClapeyronÕs diagram [18]:
Ln Peq† ˆ
DH
RT
‡
DS
R
3†
For an analysis of the cycle and the design of systems, the working conditions of the CHP cycle
could be plotted in a Clapeyron diagram, from which the various characteristics could be ob-
tained. Chemical reaction will take place when the salt is in a state outside the equilibrium line.
The di€erence of the state in equilibrium and outside of equilibrium is known as ``equilibrium
drop''. Eq. (3) also relates the equilibrium pressure and the temperature. By plotting the systemÕs
cycle along the equilibrium lines of the Clapeyron diagram, the operating pressure, the range of
temperature upgrade, mass of the working pairs required, amount of power consumed and heat
released, etc. could be predicted. Goetz et al. [19] have given a graphical representation of the
temperature levels required to ensure cold production and refrigeration for di€erent chlorides
reacting with ammonia, the variation of coecient of performance (COP) and exergetic eciency
as function of cold production temperature.
The working of an intermittent solid adsorption cycle can be also represented in a Clapeyron
diagram by knowing the relation between vapor pressure of working ¯uid (e.g. water) and the
adsorbent (e.g. NaX zeolite) temperature in equilibrium [20].
CHP could operate in two modes depending on the required output: ``heat pump'' (cold pro-
duction at low temperature and heat generation at medium temperature) and ``heat transformer''
(heat supplied at the medium temperature and heat utilization at higher temperature) [21]. In the
heat pump mode (Fig. 3) [19], in the ®rst stage, heat is supplied to the reactor at high temperature
(Th) to regenerate ammonia which will then be condensed in the condenser at medium temperature
(Tm). The heat required at evaporator at low temperature (Tl) is supplied to vaporize ammo-
nia, which reacts with the chloride salt and releases heat at medium temperature (Tm). In heat
Fig. 3. ClapeyronÕs diagram showing the heat pump mode of operation.

transformer mode, the consumption of heat is at medium temperature, while the heat rejection is at
high temperature (Th) and also at low temperature.
2. Technology status
The technology status of the CHP is described in terms of the working pairs used, the per-
formance of the system and its application, and recent developments promoting energy upgra-
ding.
2.1. Working pairs
Working pairs for absorption/adsorption systems are substances or chemicals used, which
without changing their properties (mechanical or chemical) induce consumption or production of
heat. For example, chloride salts±ammonia [18], zeolite±water [20], AC±methanol [22], and car-
bon±ammonia [23] are some of the common working pairs used, and a discussion on the common
adsorbents and adsorbates used in solid±vapor adsorption heat pump systems has been presented
in Ref. [24]. The amount of heat consumed or released due to the thermochemical process is
utilized for production of heating and cooling e€ect.
The major requirements of working pairsÕ properties are their high storage capacity, thermal
stability within the working conditions (temperature and pressure range), high thermal conduc-
tivity, high external heat transfer coecient, high speci®c power output, transportability (easy to
handle), small speci®c volume, non-corrosive and non-toxic (environmental friendly), low vis-
cosity and surface tension, etc. [25]. Lebrun and Neveu [21] suggest the following criteria be
considered in the selection of an appropriate working pair: economic criteria (cost of working pair
itself or cost of equipment), performance criteria (temperature rise, speci®c power production,
mass of working pair), and cost and performance criteria (speci®c power per unit mass and unit
area of exchange surface or temperature rise by unit mass or unit area of exchange surface). Neveu
and Castaing [18] note that the selection of working pairs needs the understanding of the ma-
chineÕs characteristics as a function of target temperature required. For instance, the ®nal selec-
tion of the ideal working pair requires information of the machineÕs characteristics as a function
of refrigeration temperature for all the possible combinations of the considered working pairs.
A detailed experimental procedure for the estimation of thermal conductivity, permeability,
porosity and speci®c heat of two types of monolithic carbon as adsorbents has been presented by
Tamainot-Telto and Critoph [23]. The heat transfer coecient for the carbon block of coarse
powder has been measured to be 350 W/m2
K, while for ®ne powder, it is about 800 W/m2
K. The
permeability of the coarse powder in both the arial and radial direction is signi®cantly higher than
that of ®ne powder. The speci®c heat of both samples was found to be very close.
The varying properties of working pairs lead to di€erent designs and development of processes,
covering a large temperature range. Table 1 shows the possible systems and their application
(depending on the temperature) to cooling. The chemical reaction system could be utilized in a
larger cold temperature range ( 40°C to 10°C). However, in a narrower range, adsorption sys-
tems could be used ( 20°C to 10°C), while metal hydrides seem to be suitable in the range 30°C
to 0°C.

Heat and mass transfer characteristics of di€erent types of reaction beds are also important
considerations in¯uencing the performance of systems and are given in Table 2. To improve heat
transfer characteristics, limitations due to mass transfer needed to be considered by modi®cation
type and properties of material used (e.g. graphite or binder), applying new assembling or mixing
techniques, varying combination of material (percent in use), etc. For example, using consolidated
zeolite and activated graphite adsorbents as compared with granular beds could improve the
e€ective thermal conductances of zeolite±water system [2,5].
2.2. Performance
CHP performance can be characterized by di€erent parameters, such as, actual upgrading
temperature, speci®c power production (either cooling or heating), COP, coecient of ampli®-
cation (COA) and exergetic eciency.
Actual temperature upgrade gives the temperature gain obtained from the lower temperature to
the higher level, while the speci®c power production gives the amount of heat generated or ex-
tracted by the CHP to the amount of working substance used (e.g. salts, adsorbents). COP is
de®ned as the eciency in cold production (enthalpy of evaporation divided by heat supplied for
regeneration), while COA represents the ratio of hot production to the quantity supplied for
regeneration. Exergetic eciency is de®ned as the ratio of output exergy and the input. These
eciencies could be mathematically represented [18] as:
COP ˆ Qev=Qreg 4†
For systems operating between three temperature levels (and considering the ®rst laws and
second laws of thermodynamics) the COP could be expressed [26] as:
COP ˆ
Qev
Qreg
ˆ
1
Tm
1
Th
1
Tl
1
Tm
1
0
@ DiS
Qreg
1
Tm
1
Th

1
A 5†
COA ˆ Qcond ‡ Qabs†=Qreg 6†
Goetz et al. [19] expressed the equation for exergetic eciency for thermo-transformer system
as:
Table 1
Possible solid±gas systems possible for cold production applications [3]
Applications/temperature level System possible
Air-conditioning/chilled water ‡2°C to ‡10°C† Adsorption
Chemical reaction
Refrigeration ( 20°C to 0°C) Chemical reaction
Adsorption
Metal hydrides
Freezing ( 40°C to 30°C) Chemical reaction
Metal hydrides

gex ˆ COA
1 T0
Th
1 T0
Ts
#
7†
where 1 T0=Ti† is the Carnot temperature.
The speci®c cooling power (SCP), which is another important indicator of the CHP perfor-
mance, is given by [4] as:
SCP ˆ Lm=t 8†
The performance of systems could be obtained from experimental results and/or theoretical
studies. The results of the experimental observations are expressed graphically, for example, in
terms of COP or exergetic eciency as function of cold production temperature, or by developing
nomograms for the speci®cations of the reactor by providing the system characteristics. For ex-
ample, the relation between the theoretical COP as function of refrigeration temperature for
Table 2
Heat and mass transfer characteristics of reaction beds of CHPs [2,5]
Reaction beds Speci®c power output E€ective thermal con-
ductances (keff in W/
m K) and wall heat
transfer coecient (hw
in W/m2
K)
1 Anisotropic heat conduction porous graphite matrices,
thermal contact resistance upto 1000 W/m2
K
Upto 1 kW/kg 5±30 W/m K (radial) 1
W/m K (axial)
2 Consolidated AC and zeolite (NaX, 4A) beds forming
porous block, including highly porous metallic foams
(Ni, Cu) as heat transfer matrices
2±9 W/m K
3 Metallic matrices (highly porous metallic foams, inter-
nal ®n-like structures): carbon methanol
23 W/m K for 31 wt.%
4 Generation of compacts made by mixing metal (hy-
dride) power with aluminum powder and cold com-
pression to pellets, with or without subsequent sintering
5 Lanthanum-rich mischmetal alloy Lm±Ni(5) in a 94%
porous Al-foam matrix
7.5 W/m K
6 Thermal energy storage with 10±20 wt.% graphite and
80±90 wt.% MnCl2, reaction time 5±12 h, R ˆ 3±15 cm,
DT ˆ 20±50 K
200 kW/m3
, 360
Wh/kg of salts
5±15 W/m K (radial) 1
W/m K (axial) 400±600
W/m2
K
7 High power system TES, 25±50 wt.% graphite, 75±50
wt.% MnCl2, reaction time for each phase 0.5±1.0 h
D ˆ 2±8 cm, DT ˆ 43 K†
800 kW/m3
15±30 W/m K (radial)
1 W/m K (axial)
600±1000 W/m2
K
8 Consolidated zeolite or activated graphite adsorbents Cold output of 0.5±1.0
kW/kg adsorbent
5±10 W/m K 500±100
W/m2
K
9 PMH-compacts and equivalent high-conductance
structures
Continuous speci®c
power output 1 kW/
kg of heat pump 0.5
kW/kg for refrigerator
10 Chlorides-type reactants (alkaline, alkaline-earth or
metallic)
0.1±0.4 W/m K 40 W/
m2
K

MnCl2/2NH3 salt has been given [18]. Parametric studies such as the COP-speci®c power pro-
duction function for cycle time, etc. are other methods of representing experimental data [20].
These relations could be used to ®nd the optimum operating condition considering COP and
speci®c power, which are inversely related.
2.2.1. General theoretical studies on CHP performance
A CHP consists of reactors (where adsorption or chemical reaction takes place) and an
evaporator and condenser to supply or reject heat respectively. The condenser and evaporator
is characterized by temperature and pressure of the vapor and liquid used and their speci®ca-
tions (e.g. power, dimensions, and material properties) are usually provided by manufacturers.
Therefore, the performance of evaporator and condenser are not discussed here except where they
are also integrated to the adsorbers [27,28]. Details of the performance of the reactor are discussed
below.
The most critical component of CHPs is the reactor, where heat and mass transfer, chemical
reaction, adsorption and absorption occur. Many researchers have developed models to simulate
the dynamic behavior of the reactor [7,29±33]. The reactor performance predicted by these theo-
retical models generally provides information on the progress of the reaction (% conver-
sion), instantaneous power and mean power per volume of reactive salts, heat exchange area per
volume of salt, progressive of moles of gas adsorbed in salt, temperature and pressure pro®les,
etc.
Stitou and Crozat [34] classify models into three categories: local, global and analytical models.
Local models consider mass and heat transfer, and kinetics of small volume that result in partial
derivatives equations, which are numerically solved. Global models consider variables and ave-
rage values of reactor features such as permeability, thermal conductivity, heat capacity, etc. for
simulation. Numerical solutions for these global models give sets of di€erential equations. Ana-
lytical models consider average values of the variables during reaction time and these di€erential
equations are related to the space variable only.
The local and global models could be solved by the grain±pellet model presented by Goetz [35]
and Goetz and Marty [29], and by shrinking-core models (coupled heat and mass transfer with
chemical reaction) of Lu et al. [36]. This helps to de®ne the parameters; e.g. hydraulic radius of
pores, internal porosity of the grain, kinetic absorption/desorption parameters and radius of
grain. These models calculate the reaction progress in relation to three variables (P; T; X).
Therefore, this helps to simulate:
· the instantaneous rate of desorption and absorption as a function of pressure and temperature,
· temperature pro®les in desorption as a function of radial position,
· degree of advancement (% conversion) pro®les for desorption as a function of radial position
and isokinetic lines, and
· global and local advancement under di€erent operation pressures, permeability, etc.
Lu et al. [36] have given the chemical kinetic equations at grain level. The solution to the
coupled heat and mass equations has been done at the pellet (macroscopic) level. The unreacted-
core model (shrinking core model) has been formulated for the reversible solid±gas reaction and

has been used to interpret the experimental results. This general model was later simpli®ed by
considering the progressive reactive fronts as sharp reactive fronts [37]. Mauran et al. [38] adopted
a simpli®ed approach to demonstrate the reactive fronts, and showed that two sharp fronts of
reaction, mass front and heat front exists. They noted that there are similarities between the re-
sults of these simpli®ed models and the general model on the global advancement of the reactive
front and local temperature and pressure pro®les in the reactor.
Mass transfer in¯uences the heat transfer rate due to low permeability of the reaction bed at
very low pressure. Moreover, the overall heat and mass transfer process is controlled by the
chemical kinetics, especially at the grain (microscopic) level. Experimental studies have shown
that heat transfer also strongly in¯uence the performance of reactor, as compared to di€usion or
mass transfer and chemical reaction [39]. To solve the general coupling heat and mass model
numerically, source-based method (®xed grid enthalpy approach) has been developed, where each
parameter is determined independently. A control volume approach has been used for each
special node to solve the partial di€erential equation. The global conversion was then calculated
by integrating the local conversion overall material volume at each time step. This algorithm helps
to simulate with various geometries in one-, two- and three-dimensional systems and was found to
give good agreement with the experimental results [39].
In the case of temperature (or power) production at a constant level, dimensioning studies give
the characteristics of the installation to enable performance prediction. The analytical heat
transfer model developed by Stitou and Crozat [34] leads to the single di€erential equation (neg-
lecting mass transfer) and is useful for dimensioning purposes. The dimensioning nomograms
developed by Stitou et al. [33] consider the average functional characteristics of a thermochemical
reactor over a given time step. Nomograms give the relationship between the average thermal
power of the reactor and its geometry, thermal characteristics of the reactive medium and working
conditions for various types of heat exchanger structures. They have also demonstrated how these
nomograms could be used to dimension/size a system and illustrated this by examples.
The simpli®ed dynamic numerical models have been used to simulate thermal comfort (heating
and air-conditioning) in electrical vehicles and storage by solid±gas system for persons wearing
insulated garment [15,40]. The model results indicate that the high ¯exibility of thermochemical
transformers at di€erent working constraints (cold and heat production) make it suitable for such
applications.
Recently, Meunier et al. [41] have carried out a study on the second-law analysis of adsorp-
tive refrigeration cycles considering the role of thermal coupling and entropy production. For n-
adsorber uniform temperature cycles, internal thermal entropy production (due to heat recovery
between adsorbers) may reach 50% of the external thermal entropy production. The COP and the
thermal coupling entropy production can be calculated if the thermodynamic properties of the
given pair and the isostere equilibrium chart, are known.
Critoph [42] has described a convection thermal wave adsorption cycle where higher heat
transfer rates between the adsorbent grains and the refrigerant are obtained by forced convection
and higher external surface area of the grains. Theoretical simulation studies for ammonia±carbon
working pair have been done for various evaporating and condensing temperatures, and the
maximum operating temperature would be about 200°C. A simpli®ed diagram of dQ=dT versus T
(e€ective speci®c heat along an isotere) has been shown, which helps also to visualize the heat
rejection and heat input.

2.2.1.1. Solid±gas CHP. There have been signi®cant developments in solid±gas CHPs in recent
years and several are presently in stages of commercialization [2]. For example, thermochemical
system ELF (STELF) includes industrial refrigeration and air-conditioning, hot and cold storage
in transported container, refrigerators, etc. E€orts are also underway to improve their operational
reliability and scale up from laboratory to commercial scale.
Metal hydrides system: Exothermic reaction takes place when hydrogen is absorbed by a hy-
dride and heat is liberated, while cooling is provided due to desorption of hydrogen gas from
hydride. Metal hydride absorbs large amounts of hydrogen gas and could also be used for hy-
drogen storage.
Kim et al. [43] have developed a compressor-driven hydrogen metal-hydride heat pump system
utilizing hydride reactors. This achieves higher eciency and appears to have competitive life-
cycle costs as compared to conventional refrigeration system. Testing this system with 500 full
cycle operation shows that the performance does not degrade and the speci®c cooling power
obtained could reach 1.5 kW/kg of LaNi5. Enhanced internal heat transfer in the hydride reactor
both within the hydride bed and between the hydride bed and the external heat exchanger is
required. To improve the heat transfer in reactor, the hydride-forming metal particles are coated
with a thin layer of copper (around 1 lm thickness) and then compressed and sintered into a
porous powder of metal hydride (PMH) compact. The hydride reactors containing PMH compact
are sintered into lightweight ®nned tubes, and air is forced over the exterior to transfer heat to
surroundings.
Kang and Yabe [44] report a continuous metal-hydride heat transformer, which contains two
pairs of reactors of metal hydrides (LaNi5 at lower temperature and LaNi4:5Al0:5 at higher tem-
perature). The reactors are made of bundles of tubes and are fabricated as cylindrical tubes. Heat
transfer medium ¯ows through the inner tube to supply and extract heat to and from the reactors.
The reactors are packed by hydride material, of inner diameter R1 and outer diameter R2. This
hydride material is surrounded by a ®ne net to allow hydrogen to pass through. The performance
of this heat transformer system is predicted based on heat and mass balances and thermodynamic
properties of hydride material and hydrogen. To determine the reaction progress during the
adsorption phase, the current radius is expressed as a function of temperature front for hydration
(Tf ) as:
rf ˆ
K
E
h Tf Tw†
rf hln rf
R1

‡ K
R1
9†
Therefore, the temperature front and reaction progress is calculated when the thermodynamic
properties of material (e.g. conductivity and heat transfer coecient) and reactor geometries are
known.
The performance analysis shows that the COP is in¯uenced strongly by the cycle period at
higher ratio of internal to external radius R1=R2 ˆ 0:5†, but not much at lower ratios
R1=R2 ˆ 0:25†. The optimum cycle period becomes longer at smaller radius ratios.
Compounds with chemical reaction (ammonia-based): For ammonia-based system utilizing
chloride salts (alkaline, alkaline-earth or metallic), the heat of reaction are of the same order of
magnitude, typically DHR ˆ 50 15 kJ/mol. However, the enthalpy of evaporation or con-
densation of the ammonia gas is only about half of this. The reaction in chloride salts bed of

ammoniated system takes place within the operating pressure range up to 50 bar, temperature
range of 50°C to 300°C, and depends on the number of moles of gas reacting per mole of salt
compound [1]. Table 3 illustrates the upgraded temperature in heat transformer system of various
working pairs of two connected reactors system, in the pressure range between 0.1 and 50 bar and
temperature lifts (temperature di€erence between the heat transfer ¯uid of two reactors) of at least
40°C. When the heat source temperature is lower than 100°C the possible reactive salts are CaCl2±
MnCl2, BaCl2±CaCl2 and CaCl2±MnCl2. Reactor(s) containing chloride salts of Mn and Ni give
the highest output temperature (maximum of 235°C), but the temperature source must be greater
than 100°C.
The temperature lifts are classi®ed in two ranges: 40±55°C and 66±80°C. Eight chloride couples
reacting with ammonia include six alkaline, earth alkaline or metal cations, i.e. Ca, Ba, Sr and Fe,
Mn, Zn, that could be used for refrigeration (with COP of 0.4±0.7, exergetic eciency of 0.02±
0.3). Five chlorides salts of Ca, Mn, Mg, Fe and Ni could be used to upgrade heat from 105°C to
200°C [19].
Lebrun and Spinner [45] developed a dimensioning method using a dynamic numerical model
and an optimization algorithm to calculate the instantaneous or average performances over a
®xed time period for solid±gas CHP operation. The power to be extracted from the reactor was
between 5 and 20 kW in a time period of 1 h. The salt quantity required in the system was about
500 mol corresponding to 30 kW h of energy storage. Lebrun and Neveu [21] fabricated a 20±50
kW (68,000±170,000 Btu/h) prototype of ammonia based CHP using CaCl2/CH3NH2 pair. This
system used a reactive having 10 m2
(100 ft2
) exchanger area and 8.5 mm (0.33 in.) thickness for
reactive layers. The experimental results by testing this system with several di€erent cycles, i.e.
phase sequences showed good agreement with the simulated performances (power output, COP
and COA).
2.2.1.2. Liquid±gas system. The useful heat is obtained from the exothermic reaction, while the
heat supplied (at lower temperature) is used for endothermic reaction. The reactant with the
catalyst is normally in the liquid phase. The vapor thus released from endothermic reaction in-
duces the backward reaction in another reactor, where the exothermic reaction takes place and
temperature upgrading is thus achieved.
Table 3
Upgraded temperature of di€erent working pairs used in ammonia-based reacting system of CHPs [18]
Couple of chloride saltsa
Upgraded temperature (low temperature (Tl) to high
temperature (Th))
CaCl2(4/2) and MnCl2(6/2) 90±125°C
BaCl2(8/0) and CaCl2(4/2) 95±120°C
CaCl2(8/4) and MnCl2(6/2) 100±135°C
CaCl2(8/4) and FeCl2(6/2) 120±185°C
MnCl2(6/2) and NiCl2(6/2) 130±195°C
CaCl2(4/2) and MgCl2(6/4) 145±215°C
MnCl2(6/2) and NiCl2(6/2) 155±220°C
MnCl2(6/2) and NiCl2(6/2) 155±235°C
a
The low temperature heat sink is 15°C except for the last couple, which is 35°C.

Heat pumps based on acetone/hydrogen/2-propanol have been studied by many researchers
[14,46±50]. Mathematical models to simulate the performance of this working pair have been
developed [49]. Studies on a suitable catalyst have been investigated for both endothermic and
exothermic reactions. Kim et al. [50] has studied the use of Raney nickel as a catalyst for both the
reactions, while Doi et al. [47] have studied the dehydrogenation reaction, namely the e€ect of
heat supplied, re¯ux ratio and speed of stirrer on the distillate ¯ow when using carbon supported
ruthenium palladium as catalyst. The use of Ni particles as a catalyst for the dehydrogenation
reaction and the in¯uence of reaction temperature and catalyst concentration have been studied
[48]. Taneda et al. [46] have presented preliminary data on a pilot plant, which shows that the
acetone±2-propanol system could be used to produce steam and note that the system operation
was stable during the 18 h of test.
Magnesium oxide/water CHP has been developed by Kato et al. [9] where the endothermic
reaction takes place at 100±150°C and 12.3±47.4 kPa. The reaction involved is:
MgO s† ‡ H2O g† (
) Mg OH†2 s†; DH ˆ 81:02 kJ=mol 10†
The CHP enhances the thermal energy to temperatures higher than 297°C. For this system,
exhaust heat from industrial processes or other heat sources could be utilized.
For cooling purpose, a CHP with Pa/A has been developed [8]. This has high energy density
and the possibility of the continuous cycle due to the liquid±liquid or gas±liquid reaction. This
organic CHP consists of four parts: an endothermic reactor, an exothermic reactor, a compressor
and an expansion valve. The reaction is as follows:
Paraldehyde 2; 4; 6-trimethyl-trioxane† (
) 3 acetaldehyde l† (
) 3 acetaldehyde g†
DH0
298 ˆ 110:3 kJ mol-Pa†
1
and 26:4 kJ mol-A†
1
: 11†
The working ¯uid of Pa/A system shows the possibilities of energy storage for cooling. Para-
dehyde (2,4,6-trimethyl-trioxane) is depolymerized to acetaldehyde with an acid catalyst. The
depolymerization is endothermic and reversible. Hence, the depolymerization is promoted by
the vaporization of acetaldehyde (endothermic reaction). A comparison of Pa/A system with the
conventional compression heat pump at 13°C heat source and 40°C heat release temperature
shows that although the COP obtained is 6.9 which is similar to that of HFC-134a, the output of
the Pa/A system is larger than that of ¯uorocarbons and require less driving energy.
For high-temperatures, a calcium oxide/lead oxide and PbO/CO2 reaction system has been
recently investigated [6]. The reaction involved is:
CaO s† ‡ CO2 g† (
) CaCO3 s†; DH0
1 ˆ 178:32 kJ=mol
PbO s† ‡ CO2 g† (
) PbCO3 s†; DH0
2 ˆ 88:27 kJ=mol
12†
The system consists of CaO and PbO reactors. Two modes are encountered: heat storage and
heat supply mode. In the storage mode, CaO reactor receives heat from heat source to form CaO
and CO2. This is followed by decarbonation that produces CaCO3. Carbon dioxide also reacts
with PbO in PbO reactor yielding PbCO3 and exothermic heat of carbonation is released. During
the heat supply mode, decarbonation of PbCO3 takes place in the PbO reactor. The carbon di-
oxide produced is led to the CaO reactor. Carbonation of CaO then proceeds and heat is gene-
rated in the reactor (exothermic reaction).

In the case of continuous type liquid±gas CHP, the exothermic reaction heat is produced at
high temperature, whereas the endothermic reaction heat and the evaporation heat are supplied at
low temperature for the decomposition of metal hydride [14]. The maximum COP obtained is
0.36, at a condition of high temperature production of about 200°C, rate of reaction of 0.98 and
ratio of hydrogen to acetone is 5. However, when compact storage of hydrogen is not possible to
accommodate the large variation of heat duty or long timelag between heat supply and demand,
the storage shown in Fig. 4 is more useful as compared to the continuous type.
2.2.1.3. Solid-adsorption. An adsorption cycle consists of one or several adsorbers with a con-
denser and an evaporator, which are connected to either a heat source or heat sink at high, in-
termediate or low temperature. An experimental prototype of AC/methanol pair is one of the
most ecient solar icemakers with a COP of 0.14. The adsorptive solar powered icemakers tested
in Agidir gave a net solar COP of 0.08±0.12. The nominal production of 5.2 kg of ice a day was
obtained for 40% of operation days and the temperature of produced ice was between 15°C and
5°C [27,51].
A comparison of the working pair for absorption and adsorption cycles given in Table 4 shows
that the zeolite/water pairs give the highest performance (COP of up to 0.75). The applications are
for ice making, cold storage, refrigeration and air-conditioning. Applications in ice making and
cold store have been studied by coupling the solid-adsorption system to the solar collectors
[22,27,51]. However, the maximum COP that could be obtained from the coupled of zeolite/water
and AC/methanol system [52] is 1.06.
A summary of the performance of di€erent working pairs utilized in adsorption systems given
in Table 4 shows that solar/solid adsorption system for cold production using water and calcium
oxide gives COP between 0.10 and 0.40 at speci®ed conditions [53]. Experiments done by Tather
and Erdem-Senatalar [54] shows that the average value of 65 and 61 kJ/mol are the heat of
adsorption of water in zeolite 4A and 13X. In comparison, the use of zeolite 13X results in less
optimum mass of zeolite required in an adsorption heat pump than that of zeolite 4A. However,
the eciency of solar adsorption heat pump is higher when zeolite 13X in used. The adsorber
characteristic is improved by directly coating the zeolite±water pair onto the stainless steel wire
gauzes, which are placed vertically in the collector. As a result, the presence of thermal gradient
Fig. 4. Flow diagram of the catalyst-assisted chemical heat pump system of the storage type [14].

Table 4
Performance of di€erent working pairs used in solid-adsorption and absorption systems
Working pair Application Temperature range (low/high
temperature source)
COP/exergetic e-
ciency
Remark References
AC/methanol Icemaker 25°C/130°C 10°C/110°C 0.15 (net COP) Collector equipped with
boosters
[51]
AC/methanol Icemaker 6°C/28°C 24.5°C/87°Ca
0.08±0.12 (solar COP) Ice production of 5.2 kg in
isolation between 18 and 28
MJ/m2
[27]
AC/methanol Icemaker 5°C/25°C 25°C/110°Ca
0.12 (net solar COP) 6 kg of ice/m2
of collector [51]
AC/methanol Air-conditioning 5°C/20°C 30°C/80°Ca
0.35 Two adsorbers, mass recov-
ery (isothermal), SPC
89 W/kg ads., cooling out-
put 3.5 kW
[82]
Zeolite NaX/water
and AC AC35/metha-
nol
Air-conditioning 25°C/35°C 105°C/220°C for
NaX/water 25°C/35°C 35°C/
100°C for AC35/methanola
1.06 Three adsorbers, cascading
cycle with two di€erent
heat recovery phases
[77]
Zeolite (NaX)/water Icemaker 5°C/25°C 25°C/110°Ca
0.30 [22]
Zeolite (NaX)/water Icemaker 10°C/170°C ‡10°C/170°C 0.44/0.21 0.48/0.11 Absorption machine [76]
Zeolite (NaX)/water Cold storage 10°C/170°C ‡10°C/170°C 0.10 (solar COP) 12 m3
of cold stored, aver-
age daily gross production
of ice of 7 kg/m2
[22]
Zeolite (NaX)/water Air-conditioning 22°C/42°C 60°C/200°Ca
0.75 Two adsorbers, SPC 19.6
kg ads., cooling output 1
kW
[52]
CaCl2 ‡ NH3 (Wor-
soe-Schmidt)
Icemaker 0.10 (solar COP) No rectifying column, ini-
tial temperature of water is
0°C
[81]
a
Tev/Tcd and Tads/Tdes.
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becomes more e€ective, and the eciency of the system decreases as the amount of zeolite em-
ployed increases.
Experiments were conducted by Poyelle et al. [20] by introducing heat and mass recovery cycle
using a zeolite±water pair to a new consolidated adsorbent composite. The adsorptive material is
made up of NaX zeolite (5.0 kg), binder (1.3 kg) and expanded natural graphite (ENG) (1.8 kg).
Each adsorber consists of four rows of four tubes comprising of adsorbent composites distributed
in the form of rod around tubes. A heat and mass recovery cycle of zeolite±water pair gave an
experimental SCP of 97 W/kg at an evaporation temperature of 4°C. However, the authorÕs note
that at higher evaporating temperature (25±30°C), a COP of 0.68 and a SCP of 135 W/kg could be
obtained.
For moderate low temperature source ( 10±10°C) and high temperature source (170°C),
zeolite±water system gives relatively high COP values (0.40), as well as the higher exergetic
eciency (up to 0.21). However, with higher temperature of low temperature source and lower
temperature of high temperature source, ammonia±water system shows highest performance in
term of COP and exergetic eciency.
Meunier et al. [41] have proposed an entropy production concept to analyze the quality of heat
management inside the adsorption cycle as well as between the cycle and the heat sources. Internal
entropy production (due to heat recovery) and external entropy production (due to coupling
between heat reservoirs and adsorbers) for n-adsorber uniform temperature cycles and thermal
wave cycles have been determined. They have suggested a design of Ônovel regenerative processÕ
that leads to reduction of the entropy production.
Critoph [55] has evaluated the performance of new refrigerant±adsorbent pairs, namely, bu-
tane, R32 and ammonia with monolithic carbon. The cycle COPs were calculated for one bed
cycle and a two bed cycle. Of the studied pairs, ammonia has about three times the COP of R32,
while those of butane were found signi®cantly lower.
Another study by Critoph [56] using ammonia±carbon in a forced convection adsorption cycle
using inert bed for air-conditioning applications was found to give a COP of 0.56 and a cooling
power of about 300 W/kg. The in¯uence of COP due to design parameters such as bed length, ¯ow
rate, gas exit temperature and initial isothermal temperature indicates that increasing bed length
or decreasing ¯ow rate improves COP, but decreases the cooling power. However, signi®cant
improvements were observed if the heat transfer is increased. A 10 kW demonstration cooling
system is expected to give a COP of 0.95 and a cooling density of about 400 W/kg.
2.2.2. Applications
Among industrial processes, certain unit operations such as drying, distillation, evaporation
and condensation deal with large amount of enthalpy changes where CHP can be e€ectively
utilized. However, the COP of the basic cycle of CHP might be limited because of the sensible heat
consumed during the temperature swings. Using thermochemical substances, designing new re-
actors or new systems could possibly lead to improving the heat rate and thus reduce the size of
units.
The current applications of CHP and related systems can be broadly categorized into areas
such as, refrigeration, heat production (heat pump), combined heat and cold production, lifting
temperature of heat source (thermotransformer), thermal energy storage and the integration of
solar thermal collectors for applications to heating and cooling.

Table 5 summarizes the information on working pairs, their applications, and range of opera-
tion or working conditions of adsorption system, solid±gas system, liquid±gas system, metal-
hydrides, absorption system and solar assisted CHP system.
In zeolite/water systems the heat of adsorption could reach 61 kJ/mol of water adsorbed [54].
Poyelle et al.Õs [20] experimental work obtained a speci®c cooling power of 97 W/kg of adsorbent.
In solar assisted solid-adsorption system, the solar powered adsorption cold store utilizing NaX±
water pair gave a COP of about 0.10. By enhancing the conductivity of the zeolite bed, COP could
be as high as 0.15 [28]. A solar adsorption heat pump [54] utilizing the zeolite±water pair using a
continuous coating of the zeolite on stainless steel wire gauzes has been developed. They are
placed vertically in the collector to ensure a ®rm contact between the absorber plate and the wire
gauzes. The solar collector consists of two separate regions, namely, the glass cover and the
adsorbent metal integrated system. The variation of solar COP with respect to the thermal gra-
dient possible to be found in a conventional solar collector (0±30°C) was between 0.06 and 0.13
for the system using 23.3 kg of zeolite 4A.
Zeolite/methanol pair cannot be used at around 100°C because zeolite is a catalyst of the
reaction methanol ) water ‡ dimethylether†. As AC has a signi®cant volume of micropores, AC±
methanol pair is suitable for solar adsorption icemaker with ¯at-plate collectors [51]. A com-
parison of two collector±condenser units (solar collector merged with condenser) of adsorptive
solar powered icemakers has been done by Boubakri et al. [57], equipped with a radiation shield.
This shield leads to 40% reduction of heat transfer coecient inside condenser. Wang et al. [58]
have presented simulation results for AC±methanol working pair for producing hot water and ice.
This was done by coupling a solar water heater and an adsorber, which is connected to the cold
storage. The results indicate that solar refrigeration COP of about 0.15±0.23 and a heating e-
ciency of about 0.35±0.38 could be obtained. Preliminary tests using an electric heater to simulate
a vacuum solar collector was found to produce about 10.5 kg of ice at 2°C.
In solid±gas CHP system, di€erent chloride salts reacting with ammonia have been studied, and
Lebrun and Neveu [21] have tested 20±50 kW solid±gas (plate reactor) prototype.
In liquid±gas system, both organic and inorganic reactants are in use. Many studies have been
carried out for di€erent catalyst enhancing the endothermic/exothermic reaction of 2-propanol/
acetone with hydrogen [14,17,59±61]. Low-grade heat from solar thermal collectors could supply
heat to an endothermic reactor of a liquid±gas system. Taneda et al. [17] noted that heat at 80°C
could be upgraded by liquid±gas CHP to 150±200°C. The endothermic liquid-phase dehydroge-
nation of 2-propanol release acetone and hydrogen at 80°C with appropriate catalysts (e.g. Ru/C,
Ru±Pt/C and Ru±Pd/C) [59]. In the following phase, exothermic gas-phase hydrogenation of
acetone into 2-propanol takes place at high temperature (200°C) with another catalyst (e.g.
Ni±Cu/alumina [16]). Experiments on 2-propanol dehydrogenation conducted by Mooksuwan
[62] shows that the 10 wt.% and 1.3 g l 1
of Ru±Pt/AC catalyst used could achieve the maximum
heat utilization of 4.5% when oil bath temperature is about 100o
C. For hydrogenation of acetone
using 10 wt.% Ni±Cu/c-Al2O3 catalyst, at the catalyst bed temperature about 110±150°C, the
maximum acetone conversion of 25% was obtained at reactorÕs inlet temperature of 120°C.
In calcium oxide/lead oxide reaction system, the operating decarbonation/carbonation tem-
perature ranges from 300°C to 880°C.
In refrigeration process, new applications for CHP has been discussed by Spinner [2] in-
cluding air-conditioning for automobiles (electrical vehicles), individual air-conditioning

Table 5
Selected characteristics of some CHP systems
Category Working pairs Range of operation or working condition
and application
Heat of reaction or adsorption
and system performance
Author(s)
Adsorption
system
Zeolite±methanol Flat plate selective collectors, desorption
temperature (100°C)
Latent heat of evaporation 19.8
MJ, mass of ice produced 14.7
kJ, incident radiation 120 MJ (6
m2
), cycle COP 0.43, net solar
COP 0.12
Pons and Guilliminot
[51]
Calcium oxide ®xed bed Adsorption of 0.012 g/g s of water vapor
in 1 kg of ®xed bed calcium oxide, store
temperature of 180°C
Heat storage capacity 1±3 GJ/
m3
, heat released 2257 kJ/kg of
water absorbed (9.73 kcal/mol)
Darkwa and OÕCalla-
ghan [53]
Zeolite±water 15 kg/cm2
of zeolite Cold production during evapo-
ration 1.93 MJ/m2
, net cold pro-
duction 1.88 MJ/m2
, incident
radiation 17.8 MJ/m2
, cycle COP
0.38, net solar COP 0.105
Grenier et al. [28]
Zeolite±water (4A and
13X)
minimum Tamb ˆ 20°C,
Tcd Tev ˆ 18°C Tcd ˆ 20°C, Tev ˆ 2°C†
or Tcd Tev ˆ 33°C Tcd ˆ 35°C,
Tev ˆ 2°C†
Heat of adsorption of water in
zeolite 4A: ±65 kJ/mol 13X: ±61
kJ/mol
Tather and Erdem-
Senatalar, [54]
Zeolite±water with
HMR (with new con-
solidated adsorbent
composite)
Evaporating temperature of ±4°C, Con-
densing temperature 25±30°C
SCP 97 W/kg SCP 135 W/kg Poyelle et al. [20]
Activated carbon±
methanol
Maximum temperature of
adsorption ˆ 110°C
COP ˆ 0:14 Net solar
COP ˆ 0:08 0:12, 5.2 kg/day
of ice production ( 15 to 5°C)
Pons and Guilleminot,
[51]; Boubakri et al. [27]
and Grenier et al., [28]
Solid±gas
system
Ammonia based system
in general
Operating pressure 50 bar, temperature
range ±50°C to 300°C;
Heat of reaction about 50 15
kJ/mol
Spinner [1]
Alkaline and earth
alkaline coupled with
ammonia (Ca, Ba, Sr,
Fe, Mn, Zn)
For example; ®ve chlo-
ride salts (Ca, Mn, Mb,
Fe and Ni)
Heat sources from 105°C to 200°C and
temperature lift from 105±120°C (low
pressure steam network) to 155±170°C
(medium pressure network)
For refrigeration, COP ranges
from 0.4 to 0.7, exergetic e-
ciency from 0.02 to 0.3
Spinner [1]
Reactive area 10 m2
, thickness of reactive
layers 8.5 mm, Tc ˆ 65°C, Tev ˆ 18°C,
COP ˆ 0:53
20±50 kW prototype 68,000±
170,000 Btu/h
Lebrun and Neveu [21]
(continued on next page)
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1507

Table 5 (continued)
and application
Author(s)
Thermal comfort (heating and air-condi-
tioning) in electrical vehicles and insula-
tion of garment
Speci®c heat and cold production
is 90 Wh/kg and 45 Wh/kg of salt
used
Goetz et al. [40], Gardie
and Goetz [15]
Liquid±gas
system
Magnesium oxide±
water
Reaction temperature 373±423 K, vapor
pressure 12.3±47.4 kPa; upgrade temper-
ature from 370±440 K to 373±570 K
Heat of hydration 79.9 kJ/mol
water, heat of adsorption 49 kJ/
mol water, heat of condensation
37.4 kJ/mol water
Kato et al. [9]
Pa/A Possible for cooling; 286 K and heat
release of 313 K
COP is 6.9 Kawasaki et al. [8]
Calcium oxide/lead
oxide and PbO/CO2
reaction system
PbO ‡ CO2 ) PbCO3 (2); decarbona-
tion: Pdc 1.0 atm, optimum
Tdc ˆ 440±450°C; carbonation: Pcb ˆ 0:01
atm, Tcb ˆ 300°C CaO ‡ CO2 ) CaCO3
(1); decarbonation: Pdc ˆ 0:4 atm,
optimum Tdc ˆ 860°C; carbonation:
Pcb ˆ 1:0 atm, Tcb ˆ 880°C
Kato et al. [6]
Continuous type liquid±
gas CHP
Exothermic reaction heat is produced at
high temperature; endothermic reaction
heat and evaporation heat supplied at low
temperature; rate of reaction of 0.98 and
ratio of hydrogen to acetone is 5
Maximum COP is 0.36, 200°C Saito et al. [14]
Liquid±gas system
(2-propanol/acetone/
hydrogen)
Upgrading solar thermal energy by endo-
thermic/exothermic reaction (dehydro-
genation/hydrogenation); using
appropriate catalyst: Ru/C, Ru±Pt/C, Ru±
Pd/C, Ni±Cu/c±Al2O3(endothermic); Ni±
Cu/alumina, Ru±Pt/AC (exothermic; take
place at high temperature of 200°C)
Achieve maximum heat utiliza-
tion of 4.5% (oil bath tempera-
ture of 100°C); maximum
acetone conversion of 25% at
inlet temperature of 120°C
Chung et al. [60]; Mears
and Boudart [59]; Tan-
eda et al. [17]; Mooksu-
wan and Kumar [61]
Metal
hydrides
High temperature metal
(LaNi4:5Al0:5) and low
temperature metal
(LaNi5)
Useful heat temperature 423 K, waste
heat temperature 383 K, cooling water
temperature 303 K, cycle period 600 s,
R1/R2 0.325, optimum cycle period is
longer when radius ratio (internal to
external radius) is smaller
Heating output is 40±45 W/kg
COP 0.45±0.5
Kang and Yabe [44]
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Compressor-driven
hydrogen metal-hydride
With fast reactors, 500 full cycle testing Speci®c cooling power is 1.5 kW/
kg
Kim et al. [43]
Solar/
absorption
system or
energy
storage
Solar driven NH3±
LiNO3 and NH3±
NaSCN
Used as coolers or heat pumps; maximum
theoretical heat gain factor is 210%
Maximum theoretical
COP ˆ 0:9, cooling power of 355
W/m2
, useful thermal power 344
W/m2
, COP decrease with in-
creasing ambient temperature
(88% at 20°C to 75% at 40°C),
linear reduction of cooling ca-
pacity with increasing ambient
temperature (1196 kJ/kg NH3 at
20°C to 1144 kJ/kg NH3 at 40°C)
Antonopoulos and
Rogdakis [67]
LiBr±H2O and
NH3±H2O
Two stage vapor absorption system (heat
transformer and refrigerating machine)
lower hot source temperature (thermal
e‚uents or ¯at plate collectors)
Flat plate collector 13 m2
, aver-
age refrigerating e€ect 420 W
Ahachad et al. [74]
Solar-aided R22-
DEGDE
Minimum allowable levels of requirement
is 20°C in winter and over 80°C in
summer; 400 m2
of high eciency collec-
tor s(4 m2
/kW cooling)
Cooling capacity 100 kW; solar
energy supplies 38% and 91%
demand in winter and summer
respectively
Ileri [66]
PCM (calcium chloride
hexahydrate) integrated
to solar collector
PCM 1500 kg with collector area of 30 m2
for laboratory building of 75 m2
¯oor area
Storage eciency 40±60%; mean
collector eciency, heat pump
COP, system COP and storage
eciency are 70%, 4.5%, 4.0%
and 60% respectively
Comakli et al. [78±80]
Gas/solar-driven heat
pumps
For sea water puri®cation, refrigeration
and power generation
COP of 0.75 and thermal e-
ciency of 16%
Nguyen et al. [75]
Solar-assisted heat
pump (SAHP)
Simulation of two serial SAHP compared
with conventional heat pump and solar
air heater
Saving of 21% or 41% of annual
total costs; operating with R134a
increase COP by 50% and 4% of
heat pump and collector e-
ciency; reduction of collector
area by 25% and 50%
Abou-Ziyan et al. [68]
Solar-
assisted
CHP
Solar collector-chemical
reactor as receiver/reac-
tor
Solar collector of 20 m2
paraboloidal dish
solar concentrator and U-shaped tubular
chemical reactor
Capacity 15 kW chem† Meirovitch et al. [69]
(continued on next page)
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1509

Table 5 (continued)
and application
Author(s)
Solar energy storage
and transport system
Ammonia based using directly irradiated
catalyst ®lled tubes (receiver/reactor)
Maximum power level per reac-
tor tube pair is 390 W at mass
¯ow of 0.12 g/s for 750°C, 680 W
at 0.21 g/s at 800°C
Lovegrove and Luzzi
[70]
Solar/adsorption heat
pump (zeolite-water
pair)
Adsorbent metal integrated system COP between 0.06 and 0.13 using
23.3 kg of zeolite 4A
Tather and Erdem-
Senatalar [54]
1510
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(with built-in cold storage), rapid cold generation and product freezing, continuous cold gener-
ation in a chamber and self-contained cold generation and storage, cooling for electronic devices
and calculators, and instant ice cubes for household use.
Two applications showing the advantage of storage function allows the design of appropriate
CHP con®gurations [40]. For thermal comfort in electrical vehicle application, speci®c en-
ergy de®ned as the ratio of energy extracted to evaporator or energy released by reactor and/
or condenser to the amount of ammonia used is about 93 and 46 Wh/kg of hot and cold pro-
duction.
For applications of air-conditioning in automobiles, a solid±gas accumulator (MnCl2(6/2)/
NH3) in electric vehicles which provides heating and cooling (for di€erent seasons) has been
evaluated by Gardie and Goetz [15]. This system uses MnCl2 of 2.3 kg in a reactor for 1 kW h of
heat storage. During production phase, cold is produced by evaporation to cool in summer and
heat is produced by the reaction to heat in winter. Regeneration takes place when the automobile
is stopped or during the charging of batteries. On the other hand, for individual air-conditioning,
during the production phase, cold is provided by the decomposition of BaCl2 (8NH3). Then in
regeneration phase, decomposition of MnCl2 (6NH3) takes place. The two solid±gas reactors are
closed and are then ready for cold production. This enables cold production at 10±20°C, which is
sucient for direct air-conditioning [40].
Adsorption chiller developed by NAK [63] using silica gel/water as working pair is rated at 50±
430 kW and has a COP varying between 0.46 and 0.65. Heat at 85°C is used to produce chilled
water at 9°C. This single-e€ect machine could use waste heat for commercial air-conditioning
applications.
The scale up from laboratory scale or experimental unit (prototype) necessitates the develop-
ment of an operating method to control the CHPs cycling. Studies to predict the performance and
comparison with pilot scale units have been described [21,45,64].
Suda [65] notes that commercialization of metal hydride systems (for heat pumps and refrig-
erating purposes) should take into account the limitation to actual power generated by the design.
For instance, some of the factors to be considered are overall mass of the reactor system, reactor
con®guration, and operating temperatures of the heat source, heat sink, heat or cold to be gene-
rated, and a combination of the hydriding materials selected.
Solar assisted absorption systems with R22-DEGDME [66] and NH3±LiNO3, and NH3±
NaSCN [67] have been studied. Performance of solar assisted heat pump using R22, R404a and
R134a as working ¯uids was compared with conventional heat pump and solar air heater by
Abou-Ziyan et al. [68].
Chung et al. [60] show that the CHPs, where heat is stored in chemical substances do not have
losses due to temperature di€erences. In addition, low-temperature source such as solar thermal
energy could be upgraded to satisfy the requirements at higher level by exothermic reaction. Solar
collector (20 m2
paraboloidal dish solar concentrator) directly integrated to a U-shaped tubular
chemical reactor as receiver/reactor (15 kWchem) was developed for reforming of methane [69]. By
this way, thermochemical conversion of solar energy into chemical energy and sensible heat could
be integrated into a self-regulatory system.
A design of ammonia based thermochemical solar energy storage and transport system has
been developed by Lovegrove and Luzzi [70] using directly irradiated catalyst ®lled tubes
(receiver/reactor). The study shows the technical feasibility for a 20-m2
paraboloidal dish

concentrator with Ni catalyst. Experimental studies have also shown that simple tube and tube
counter ¯ow heat exchanger are adequate for obtaining higher storage eciencies.
Tamainot-Telto and Critoph have presented preliminary results of concentrating (CPC) solar
energy collector with absorber containing carbon and ammonia at the focus [71]. The experi-
mental setup is designed to produce a cooling power of up to 120 W.
Ito et al. [72] have investigated a solar assisted heat pump system using hybrid photovol-
taic (PV) and thermal panels as an evaporator for heat pump. Evaporator with multiple ®ns
(convective-type evaporator) is placed in parallel with the ¯at-plate collectors (radiative-type
evaporator) over a total area of 3.24 m2
. Polycrystalline silicon PV is bonded to the surface of
the collector plates, which are made of copper. Electricity produced by PV is supplied to com-
pressor of heat pump, while a part of waste heat produced is used for raising evaporation
temperature. COP of six obtained is higher than that of a conventional heat pump during day-
time.
Hu [73] has developed a mathematical model for simulating a non-valve solar powered carbon/
methanol adsorption refrigeration system and studied the phases of temperature change and
methanol migration. The results of the study compared well with experimental observations and
the model is a useful tool to analyze the e€ect of daily operations in a non-valve system.
3. Areas of further research
Being an environmental friendly technology, CHPs for various applications need to be inves-
tigated as they could o€er a viable solution for ecient thermal energy use. They could help to
utilize renewable energy (e.g. solar energy, biomass, etc.) and waste heat over a wide range of
temperatures.
The potential research areas for development of CHP are summarized in Table 6. The domains
for CHP application in industries include pulp and paper, chemicals, textiles, and food and drink
[7]. Meunier [4] noted two areas for CHP development: low eciency applications with simple
technology (e.g. domestic appliance, short-term storage, waste heat assisted air-conditioning) and
high ecient applications (industrial). Solar-assisted CHP could be used for small (residential)
and large (i.e. industry, building, commercial, agriculture) scale applications. The possible ac-
tivities (experimental and theoretical studies) in building and industrial sector and techniques
(new or those which have been already successfully applied) have been noted.
Besides the improvement of technology for solid±gas or liquid±gas CHP itself, integration with
other system requires further research. Both theoretical (modeling, simulation, evaluation/veri®-
cation techniques) and experimental (laboratory scale, scale transposition to prototype and pilot)
studies are required to improve the overall performance (e.g. eciencies, given power/work) to
suit particular applications. Scale transposition from laboratory scale (experimental unit and
prototype) to practical application is also an area of work, which could result in the development
of Ôgeneral design proceduresÕ.
At present research on upgrading of solar energy by liquid±gas CHP are being done [60,72]. A
CHP component (e.g. reactor, adsorber, condenser, and evaporator) might be coupled directly to
solar collector as well as an integral evaporator/condenser to the storage unit of solar system for
optimizing the system size.

Table 6
Possible research areas for solar assisted CHPs
Application Experimental work Theoretical work
Building
Hot water and steam production by heat recovery
from air-conditioning units, other sources, etc. for
in-house use (e.g. laundry)
Hot and cold production for speci®c purpose (e.g.
supplying hot and cold water)
Air-conditioning
Verify the performance of new de-
signs of solar-CHP in laboratory and
then scale to prototype
Improvement of one component by modi®cation of
the existing con®guration(s), for example, increase
the heat exchange area at heat exchanger
To achieve better heat transfer and kinetics
To reduce the limitation of mass transfer
To obtain better overall heat transfer coecient
Design the new coupling technique between solar
components and CHP components Modi®cation of
solar collector to merge with the reactor or adsorber
directly as well as to merge the storage tank with
other CHP components (e.g. condensor, adsorber,
evaporator)
Develop Ôgeneral design procedureÕ
Industry
Thermal energy storage by CHP from waste heat
Dehumidi®cation
Drying of agricultural products Replacement of the
steam usage by medium temperature steam/water
production (150±250°C)
Refrigeration rooms by coupled solar/CHP/ab-
sorption chiller
High temperature process supplied by solar-metal
hydrides system
Carry out the experimentation for
whole year and verify with the pre-
dicted performance from simulation
based on available meteorological
data
Testing the prototype
Validate Start-stop control strate-
gies for solar/CHPs system
Comparison of performance of
di€erent coupled combination under
identical and di€erent conditions
The simulation tools could be TRNSYS, MATLAB
and any other user-developed programs
Examine thermodynamical limits of performance
analytically and graphically
Develop second law analysis of the new design and
determination of the exergetic eciency
Develop nomograms to estimate the characteristics
and sizing of the designed system Develop the scale
transposition procedure for e€ective sizing of the
practical system for commercialization
Forecasting the performance of the solar-assisted
CHP under di€erent climate conditions using neural
network
Energy and economic comparison of di€erent con-
®gurations (arrangement)
W.
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1513

Research on solar±heat pump±energy storage system for speci®c applications depend on re-
quirements. This could be for: (1) only one component such as reactor, evaporator, condenser,
heat exchanger of CHP module and energy storage unit; (2) more than one component consid-
ering either individually or considering their interrelationship among sub-system (sub-module);
(3) energy transportation network; (4) control system; and (5) the complete assembly. For
example, di€erent combinations to integrate solar system, heat pumps and energy storage could
be considered:
1. Input is solar energy only; heat pumps act as thermal energy storage (TES) to upgrade energy
and storage purpose (Fig. 5a).
Fig. 5. (a) Integrated solar and heat pump for temperature upgrading and storage, (b) integrated solar, heat pump and
upgrading of waste heat temperature, and (c) integrated solar, heat pump, and waste heat for storage and upgrading.

2. Input is both solar energy (at solar collector) and/or waste heat (for heat pump) to upgrade en-
ergy (Fig. 5b).
3. Input is solar energy (at solar collector). The TES of solar thermal system is also available (e.g.
water tank, PCM). Heat pump could be integrated with the solar collector for temperature up-
grading of solar collector output. Waste heat and other alternate source (if needed) could sup-
ply heat to heat pump unit. Heat from alternate source could be also supplied to storage unit
(Fig. 5c).
Table 7 summarizes results of some recent theoretical and experimental studies on heat pump
and refrigeration giving information on the speci®c power produced (heating or cooling) by
various researchers. Srivastava and Eames [24] also note that though signi®cant strides have been
made during the 1990s especially in adsorbent bed technologies, improvements need to be made
in order to compete with conventional absorption and vapor compression technologies. For
example, a recent simulation study [23] using monolithic carbon and ammonia as working pair
has shown that a cooling power of up to 1 kW/kg is possible, and could be a target for appli-
cations.
Acknowledgements
W. Wongsuwan is grateful to the French Government, and the Postgraduate Technical Studies
Programme sponsored by the European Commission DG1/B of the European Union for their
®nancial support in conducting this study.
Table 7
Summary of recent theoretical and experimental studies on heat pump and refrigeration
Author Application Speci®c power
(W/kg)
Theoretical/
experimental
System studied
Poyelle et al. [20] Air-conditioning 135 Experimental Zeolite±water
600 Theoretical
Pons et al. [83] Heat pump 34.5 Experimental Zeolite (graphite)±
water
Ben Amar et al. [84] Heat pump 150 Theoretical Na Zeolite±Water
AC±ammonia
170 Theoretical
Balat and Spinner [85] Heat pump 654/759 Experimental
(synthesis/
decomposition)
CaCl2/CH3NH2
Lai [86] Refrigeration 125.6 Theoretical Zeolite±water
Pons and Szarzynski [87] Cooling 100 Theoretical Zeolite NaX±water
Ortel and Fisher [88] Cooling 10 Experimental Methanol±silica gel
Vasiliev et al. [89] Cooling 300 Experimental ``Buso®t'' (AC ®-
ber)±NH3 ``Buso®t''
(AC ®ber)±CaCl2/
NH3
330

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