theoretical studies on adsorption heat transformer using zeolite-water vapour pair

Heat Recovery Systems & CHP Vol. 10, No. 5/6, pp. 527-537, 1990 Printed in Great Britain

08904332/90 $3.00 + .00 Pergamon Press pk:

THEORETICAL STUDIES ON ADSORPTION HEAT TRANSFORMER USING ZEOLITE-WATER

VAPOUR PAIR*

ISI-IWAR CHANDRA and V. S. PATWARDHANaf Chemical Engineering Division, National Chemical Laboratory, Pune 41 I008, India

(Received 17 February 1990)

Abstainer--An adsorption heat transformer can raise the temperature level of a fraction of waste heat by rejecting the remaining heat to a low temperature level. In this work some alternatives in the design of an adsorption heat transformer, such as a 2-tank system, 3-tank system and 4-tank system, are evaluated using zeolite-water vapour as the adsorbent-adsorbate pair. The values of coefficient of performance (COP) are computed for each system for various temperatures of waste heat source at which the heat is available and heat sink at which the heat is delivered.

It is found that an adsorption heat transformer can be used for a gross temperature lift as high as 50°C with a fairly good COP value. Moreover the 4-tank system gives a much improved COP value as compared to the 2-tank and 3-tank systems for the same operating conditions. It is also found that the effect of temperature driving force for heat transfer on the COP value is quite pronounced.

Cl Cs Cv COP

M m ml m2 P eL e[ Po el p Po QA QL Q[ Qo T 7", TA r~ rD r~ rL r~ 7"0 r~ (TL)o AT W X

N O M E N C L A T U R E

heat capacity of water, cal/g, °C heat capacity of dry adsorbent, cal/g, °C heat capacity of water vapour, cal/g, °C coefficient of performance, dimensionless heat of adsorption at 75°C, cal/g of water adsorbed heat of vaporization of water at T L, cal/g water holdup in tank I in 2-tank system after desorption, g amount of water vapour transferring between tanks during adsorption/desorption ( = m l - mz), g amount of water in adsorbent bed after adsorption, g amount of water in adsorbent bed after desorption, g pressure, mbar vapour pressure of water at T L, mbar vapour pressure of water at T~., mbar vapour pressure of water at To, mbar vapour pressure of water at To', mbar partial pressure of adsorbate, mbar adsorbate pressure in single component system, mbar heat delivered to heat sink from adsorbent bed, cal heat absorbed from waste heat source in evaporator, cal heat absorbed from waste heat source in adsorbent bed, cal heat rejected to cooling water in condenser, cal temperature, K atmospheric water temperature, °C temperature of adsorbent bed during adsorption (see Fig. 2), °C temperature of heat sink, °C temperature difference (= T;-T'o), °C equalized temperature of tank 3 and tank 4 in 4-tank system, °C temperature of adsorbent bed during desorption (see Fig. 2)/of evaporating water in evaporator, °C temperature of waste heat source, °C temperature of condensing water vapour in condenser, °C temperature of cooling water, °C gross temperature lift (= T~ - T[.), °C temperature driving force for heat transfer, °C amount of dry adsorbent in each adsorbent bed, g water concentration in adsorbent, g of water/100 g of dry adsorbent

*NCL Communication No. 4831. tTo whom all correspondence should be addressed.

H~S I0 .~ ~ F 52"7

528 I. CHANDRA and V. S. PATWARDHAN

INTRODUCTION

Adsorption systems involving a solid adsorbent and a gas or vapour have several attractive features for both heat pump and heat transformer applications [1-5]. Adsorption cooling has attracted considerable interest for many years, and several studies, both theoretical and experimental, have been reported in the literature [6-9]. However adsorption heat transformers have not received much attention so far [10]. Adsorption heat pumps and heat transformers, especially for industrial applications, need to operate at higher temperatures than adsorption coolers, whereas the usual absorption systems using aqueous electrolyte solutions can lead to corrosion problems. Absorption systems using non-aqueous solvents may lead to excessive operating pressures due to low boiling points. Thus zeolite-water vapour is one of the ideal pairs for high temperature heat transformers. The solid-gas adsorption systems have to operate in an intermittent manner as the solids cannot be easily transported. This puts some limitations on the performance of solid-gas adsorption systems. Here we examine several ways of operating such systems and the corresponding performance characteristics that can be expected on theoretical grounds.

ADSORPTION HEAT TRANSFORMER

Figure 1 shows a schematic diagram of an adsorption heat transformer. It consists of an evacuated hermetically sealed unit consisting of a packed bed of solid adsorbent, a condenser, an evaporator and an intermediate tank for condensed adsorbate. A small centrifugal pump is required for transferring the condensed adsorbate from the intermediate tank to the evaporator which is at a relatively higher pressure. An adsorption heat transformer operates in discontinuous mode, comprising repeated cyclic operations. Each cycle consists of a desorption phase and an adsorption phase.

The In P vs - ( l / T ) diagram can be used for analysing the operation of an adsorption heat transformer. A typical in P vs - ( l / T ) diagram for the zeolite and water vapour is shown in Fig, 2. The operating cycle is shown by the rhombus PQRS comprising two nearly isosteric and two isothermal steps of operations.

Prior to desorption the zeolite bed is at its maximum temperature and water concentration corresponding to point P. During the desorption phase the bed, which is connected to the condenser maintained at temperature To by the flow of cooling water, is switched to the flow of waste heat fluid. Since at point P, the vapour pressure of water in the bed is higher than the condensing

Condenser

Intermediate tonk

Evaporator

~,, centrifugal

pump

OL

L

I\

Fig, I. Schematic diagram of an adsorption heat transformer.

Adsorption heat transformer studies 529

E

W

W tY

10 4 ,

i o =

1o'

101

PL . . . . . . . . . .

I -3"6 -3 .2

!cT ! i c . . V , , , , - 2 " 8 - 2 . 4 - 2 . 0 - 1 "6x10 "3

- I I T ~ K " I

Fig. 2. Adsorption isosters and adsorption heat transformer cycle on a typical LnP vs - 1/T diagram for zeolite (NaX) and water vapour.

pressure of water vapour P0 (corresponding to the condensation temperature To in the condenser), the desorption of water vapour occurs from the bed along line PQ, subsequently bringing down the bed temperature to TL. In this range heat of desorption is supplied by the sensible heat of the bed. Further desorption of water vapour occurs isothermally at temperature TL along line QR and heat of desorption is supplied by the waste heat source. The desorption phase is over at point R where the vapour pressure of water in the bed equalizes with the condensation pressure P0. Thereafter the condensed water from the intermediate tank is pumped into the evaporator which is at a relatively higher pressure.

Desorption is followed by the adsorption phase. The bed is now connected to the evaporator maintained at temperature TL by the flow of waste heat fluid. Since at point R, the vapour pressure of water in the bed is lower than the evaporating pressure of water PL (corresponding to the evaporation temperature TL in the evaporator), the adsorption of water vapour occurs in the bed along lines RS, subsequently raising the bed temperature to T^. Further adsorption of water vapour is carried out isothermally at temperature T^ along line SP by extracting heat of adsorption from the bed by the flow of heat transfer fluid. The adsorption phase is over at point P where the vapour pressure of water in the bed equalizes with the evaporation pressure PL.

Performance efficiency of a heat transformer is given by the fraction of the total input heat which becomes available as the useful heat at a higher temperature. The coefficient of performance (COP) is therefore defined as

COP = Q^ (QL + Q [)"

DESIGN ALTERNATIVES

Some improvements in the basic design are possible in the present developmental stage of adsorption heat transformer. In this work some alternatives in its design, namely the 2-tank system, 3-tank system and 4-tank system are investigated by computing the COP values for different operating temperatures. The adsorbent selected is zeolite AIPO,-5.

Figure 3 shows the adsorption isotherm for zeolite 4A (which is typical of most zeolites) for the adsorption of water vapour at 25°C [11]. This material has a fairly large capacity for the adsorption of water vapour i.e. 28.5 g of water per 100 g of dry material at saturation at 25°C. Figure 3 also shows the adsorption isotherm for zeolite AIPO4-5 for the adsorption of water vapour at 24°C [12]. The synthesis for this zeolite is also described therein [12]. This material has an equally large capacity for the adsorption of water vapour i.e. 28 g of water per 100 g of dry material at saturation at 24°C and is quite stable up to a temperature of ! 50°C. Moreover the S-shape of the adsorption


30

25

(1)

(2 )

o o 20

x

z o 15

5

-0 0 .2 0 .4 0.6 0.8 1.0

RELATIVE PRESSURE , Pi Po

Fig. 3. Water adsorption isotherms; (1) for a typical zeolite 4A at 25°C, and (2) for a~eolite AIPO4-5 at 24°C.

isotherm makes this a unique zeolite where regeneration is possible without l~ving m go to very high vacuum. Therefore the calculations presented in the following sections are based on the AIPO,-5 adsorption isotherm.

2-Tank system

This scheme consists of two tanks only, interconnected to each other for the transfer of water vapour as shown in Fig. 4. Tank 1, which is for water, is provided with septrate arranlements for the flow of waste heat fluid and cooling water for necessary heat transfer. Tank 2, which contains the solid adsorbent, is also provided with separate arrangements for the flow of heat :transfer fluid and waste heat fluid as required.

During the adsorption phase, water vapour is generated in tank 1 by the flow of waste heat at temperature T;. which is adsorbed by the solids in tank 2. The heat of adsorption evolved in tank 2 is extracted at a relatively higher temperature by the flow of heat transfer fluid at temperature T~,. The adsorption is over when equilibrium is attained between the two tanks. Thereafter the desorption phase is carried out by the flow of waste heat fluid at temperature TL' in tank 2 and

Tank I Tank 2

( water ) (Zeol i te )

Fig. 4. 2-Tank system as an adsorption heat transformer.

Adsorpt ion heat t ransformer studies 531

of cooling water at temperature T~ in tank 1. The des0rbed water, vap0ur from tank 2 gets condensed in tank 1. The desorption is over when equilibrium is again attained between the two tanks. The process of adsorption followed by desorption is carried out repeatedly thus extracting heat at a lower temperature T;. and delivering a portion of it at a relatively higher temperature T~.

During the adsorption phase, the temperature of the boiling water in tank 1 is TL (= T ~ - AT) and the temperature of the solids in tank 2 is T^ (= T~ + AT). The amount of water with the solids in tank 2 after the adsorption, m,, is given by the adsorption isotherm of AIPO4-5 in Fig. 3 corresponding to pressure ratio PL/P^ where PL and P^ are the vapour pressures of water at temperatures T L and T^ respectively.

During the desorption phase, the condensing temperature of water vapour in tank 1 is To (= T~ + AT) and the temperature of the solids in tank 2 is TL (----T~.- AT). The amount of water with the solids in tank 2 after the desorption, m2, is given by the adsorption isotherm of AIPO4-5 in Fig. 3 corresponding to pressure ratio Po/PL where P0 and PL are the vapour pressures of water at temperatures To and TL respectively.

The amount of water vapour generated in tank l during the adsorption (or condensed during the desorption) is, m ffi m I - - m 2.

Heat released from tank 2 to the heat sink at temperature T~ during the adsorption is given as:

Q^ff im.H~--m.cv(75-- TL)--(c,.W +c,.m2)(75-- TL)--(c,.W +c,.m,)(T^--75). (1)

It may be noted here that/'/.d refers to the heat of adsorption at 75°C. The temperature of 75°C has been selected arbitrarily. If H.d is available at some other temperature T, then T should replace 75 in this equation.

Heat absorbed in tank 1 from the waste heat source at temperature T~. during the adsorption is given as

QL - - ' - - m "H~ + M .¢j(r L - - To). (2)

Heat absorbed in tank 2 from waste heat source at temperature T~ during the desorption is given as

Q'Lfm'H~d--m'cv(75--TL)--(c, 'W+c, 'm,)(TA--75)--(c, 'W+c, 'm2)(75--TL). (3)

In the present study M, the amount of water in tank 1 after the desorption, is taken as 1.75 times m. This number is arbitrary, but should be such that even at the end of adsorption, there is enough liquid hold up in tank for sufficient heat transfer.

The COP values have been computed for various temperatures of waste heat source T[. and heat sink T~ for the design conditions listed in Table 1. The computed COP values are plotted in Fig. 5 against gross temperature lift (TL)o (= T'A -- T~.) with temperature difference TD ( ffi T~. - Tfi) as a parameter. The COP values computed for AT = 5°C are also shown in Fig. 5.

3-Tank system,

This scheme consists of 3 tanks, interconnected to each other through a solenoid valve for the transer of water vapour as shown in Fig. 6. Tank 1 and tank 2 which are for hot and cold water respectively are provided with arrangements for the flow of waste heat fluid and cooling water for necessary heat transfer. Tank 3 which contains the solid adsorbent is provided with separate arrangements for the flow of heat transfer fluid and waste heat fluid as required.

The 3-tank system is a modification over the 2-tank system. In the 2-tank system the liquid hold up in tank 1 is repeatedly heated and cooled prior to the adsorption and desorption phases respectively. This is avoided in the 3-tank system which may result in an improved COP value.

Table I. List of input design data for the analysis of 2-tank, 3-tank and 4-tank systems

Atmospheric water temperature Temperature of coolin s water Temperature drivins force for heat transfer Heat capacity of water vapour Heat capacity of dry adsorbent Heat of adsorption at 75°C Amount of dry adsorbent in each adsorbent bed

35°C 30°C 0°C and 5°C O.5 cal/g, °C 0.3 cal/g, °C 832.4 cai/s of water 1000g

532 1. CHANDRA and V. S. PATWARDHAN

06 l AT : 0*C

---- AT : 5"C

0'

G. O0" ,IO

O"

0 . I I 20 30 40 50 60 70 80 90 100 (TL)G

Fig. 5, Plot of COP values against gross temperature rift (TL)o with temperature difference T v as a

parameter for 2-tank system.

During the adsorption phase, tank 1 is connected to tank 3. Water vapour which gets generated in tank l by the flow of waste heat fluid at temperature T~., is adsorbed by the solids in tank 3. The heat of adsorption evolved in tank 3 is extracted at a relatively higher temperature by the flow of heat transfer fluid at temperature T~,. The adsorption is over when equilibrium is attained between the two tanks. Thereafter during the desorption phase, tank 2 is connected to tank 3, Water vapour which gets desorbod in tank 3 by the flow of waste heat fluid at temperature T~., condenses in tank 2 due to the flow of cooling water at temperature T~. The desorption is over when

m~ T a , m, ' l ~

TI~ ~ Tonk I

To ~ Tank 2 Tank 3 (cold woter) ( z e o l i t e )

m Fig. 6. 3-Tank system as an adsorption heat transformer.


equilibrium is again attained between the two tanks. The process of adsorption followed by desorption is carried out repeatedly, thus extracting heat at a lower temperature T[. and delivering a portion of it at a relatively higher temperature T~.

In this process, a certain quantity of water gets removed from tank 1 in the form of water vapour during the adsorption. The same quantity of water vapour gets condensed in tank 2 during the desorption. Therefore, after every cycle of operation, this much quantity of water must be added into tank 1 and removed from tank 2 for the continuity of operation.

During the adsorption phase, the temperature of the boiling water in tank 1 is TL (= T;. - AT) and temperature of the solids in tank 3 is T^ (= T~ + AT). The amount of water with the solids in tank 3 after the adsorption, mr, is given by the adsorption isotherm of AIPO4-5 in Fig. 3 corresponding to pressure ratio PL/P^ where PL and PA are respectively the vapour pressures of water at temperatures TL and T^.

During the desorption phase, the condensing temperature of water vapour in tank 2 is To ( = T~ + AT) and the temperature of the solids in tank 3 is TL (= T;. - AT). The amount of water with the solids in tank 3 after the dcsorption, m2, is given by the adsorption isotherm of AIPO4-5 in Fig. 3 corresponding to pressure ratio Po/PL where P0 and PL are respectively the vapour pressures of water at temperatures To and TL.

The amount of water vapour generated in tank 1 during the adsorption (or condensed in tank 2 during the desorption) is, m = m I - m 2.

While heat released from tank 3 to the heat sink at temperature T~ during the adsorption and heat absorbed in tank 3 from the waste heat source at temperature T[. during the desorption are given by equations (1) and (3) respectively, heat absorbed in tank 1 from the waste heat source at temperature T~. during the adsorption is given as:

Q L f m ' H c , + m ' c I ( T L - T=). (4)

The COP values have been computed for various temperatures of waste heat source T{. and heat sink T~ for the design conditions listed in Table 1. The COP values are plotted in Fig. 7 against gross temperature lift (TL)G with temperature difference TD (= T;. - T~) as a parameter. The COP values computed for AT = 5°C are also shown in Fig. 7.

4-Tank system

This scheme consists of 4 tanks, interconnected to each other through two solenoid valves for the transfer of water vapour as shown in Fig. 8. Tank 1 and tank 2 which are for hot and cold water respectively arc provided with arrangements for the flow of waste heat fluid and cooling water for necessary heat transfer. Tank 3 and tank 4 which contain the solid adsorbent are provided with separate arrangements for the flow of heat transfer fluid and waste heat fluid as required. In addition, they are interconnected with a heat transfer coil to interchange the heat by flow of a recirculating fluid through a small centrifugal pump.

The 4-tank system is a modification over the 3-tank system. In the 3-tank system the solids in tank 3, during the adsorption, first get heated to temperature T^ from temperature TL consuming a fraction of water vapour generated in tank 1. In the 4-tank system, by equalization of the temperatures between tank 3 and tank 4, the sensible heat of the solids in the high temperature tank is utilized to raise the temperature of the solids in the low temperature tank and this may result in an improved COP value.

In this scheme when adsorption is carried out in tank 3 by connecting it with tank 1, desorption is carried out in tank 4 by connecting it with tank 2. When the adsorption in tank 3 and the desorption in tank 4 is over, the temperatures of tank 3 and tank 4 are equalized by interchanging heat between the two by the flow of recirculating fluid through the pump. Thereafter tank 3 is put under desorption (by connecting it with tank 2) and tank 4 under adsorption (by connecting it with tank 1) simultaneously. The process of simultaneous adsorption/desorption is carried out repeatedly thus extracting heat at a lower temperature T[. and delivering a portion of it at a relatively higher temperature Tj~.

In this scheme too, a certain quantity of water gets removed from tank 1 in the form of water vapour during the adsorption. The same quantity of water vapour gets condensed in tank 2 during


°61 AT= O°C ----AT : 5°C

0"5

o. 00.4 u

0 . 3

0 " 2 l, I i 20 30 4 0 5 0 6 0 70 8 0 9 0 100

(TL) G

Fig. 7. Plot of COP values against gross temperature lift (TL)G with temperature difference To as a parameter for 3-tank system.

the dcsorption. Therefore after each adsorption/desorption cycle this much quantity of water must be added into tank 1 and remov~ from tank 2 for the continuity of operation.

The amount of water with the solids in either of tank 3 or tank 4 after the adsorption, ml, and after the dcsorption, m2, are given by the adsorption isotherm of AIPO#-5 in Fig. 3 corresponding

T~ I 7 I T°nk -~I \ l ("~, water)

[

T~ ~ Tank 2 = L ~ (cold water)

m

I Tank 3 (zeolite)

Tank 4 (zeolite)

Fig. 8. 4-Tank system as an adsorption heat transformer.


to pressure ratios PL/PA and Po/PL respectively. Po, PL and P^ are respectively the vapour pressures of water at temperatures To (= T~ + AT), TL (= T~. - A n and T^ (ffi T,~ + AT). The amount of water vapour generated in tank 1 during the adsorption (or condensed in tank 2 during the desorption) is, m = m l - m2.

The equalized temperature of tank 3 and tank 4, after the simultaneous adsorption/desorption, is given by

(c," W + c, .m,)T^ + (c,. W + c, "m2)TL Tm= 2.c,. W + c,(m, + m2) (5)

Heat released from either tank 3 or tank 4 to the heat sink at temperature T~ during the adsorption is given as:

QA = r e ' H a d - - m. Cv(75 - TL)-- (c ," W ÷ c~ "m2)(75 -- T~)- (c , . W + c,. m,) (TA -- 75). ' (6)

Heat absorbed in either of tank 3 or tank 4 from the waste heat source at temperature T~. during the desorption is given as

Q~ =m'H,d--m'c~(75-- TL)--(C," W +c,.m,)(T~--75)--(c,. W +cI'm~)(75- TL). (7)

Heat absorbed in tank 1 from the waste heat source at temperature T~. during the adsorption is given by equation 4.

o.61 AT -- O" C

- - - - A T : 5 "C

0 . 4

0"SL \ \ / 3 o

0"3

b50 l l

\\ \

\

\ ,o,,

1 \ l t I

l

I I

\

\90 \ l

I I I

110

l

I I I I I I I I 0 " 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 I00

{ T L ) G

Fig. 9. Plot of COP values against gross temperature Eft (TL) o with temperature difference T D as a parameter for 4-tank system.


Table 2. Comparison of the COP values for 2-tank, 3-tank and ,*-tank systems for a (TL)c value of 50°C.

as taken from Figs 5, 7 and 9

System Tt~ = 50°C TD = 70°C

2-tank 0.321 0.453 3-tank 0.336 0.475 4-tank OA81 0.526

The COP values have been computed for various temperatures of waste heat source T~. and heat sink T~ for the design conditions listed in Table 1. The COP values are plotted in Fig. 9 against gross temperatue lift (TL)G with temperature difference TD(= T[. - T~)) as a parameter. The COP values computed for AT -- 5°C are also shown in Fig. 9.

RESULTS AND DISCUSSION

It is evident from the results of 2-tank, 3-tank and 4-tank systems that the COP value, for a given temperature of waste heat source, decreases at an increasing rate as the gross temperature lift increases. Hence an adsorption heat transformer cannot he used for a high value of gross temperature lift. However the upper limit of gross temperature lift for which an adsorption heat transformer can be used increases as the temperature of the waste heat source increases. It is evident from the figures that an adsorption heat transformer can be used for a gross temperature lift as high as 50°C with a fairly good COP value.

From the results of 2-tank, 3-tank and 4-tank systems, as shown in Figs 5, 7 and 9 respectively, it is evident that although the 3-tank system provides better COP values as compared to the 2-tank system, it is the 4-tank system which provides much improved COP values as compared to both the 2-tank and 3-tank systems. The comparison of COP values for the three systems for a gross temperature lift of 50°C and TD values of 50 and 70°C can be easily made from the values given in Table 2. The COP values are taken from Figs 5, 7 and 9. The improvement in COP values given by the 4-tank system is mainly due to the fact that it partly avoids the sensible heat losses that usually arise during intermittent operation. The importance of avoiding sensible heat losses has been recently brought out for the case of adsorption heat pumps [5].

It is evident from Figs 5, 7 and 9 that the effect of AT on the COP values is quite pronounced. Therefore, for an improved efficiency of adsorption heat transformers, more emphasis should be given to selecting proper designs for the heat transfer/collection systems for the tanks so as to obtain lower values of temperature driving force for heat transfer.

The 4-tank system as shown in Fig. 8 can be thought of in practice due to its improved working efficiency. However it requires addition of water from outside after each adsorption/desorption cycle which is not possible since the unit should be evacuated and hermetically sealed from outside. Therefore a connecting line may be provided betweeen tank 1 and tank 2 which can be opened as and when required for adjusting the water levels in the two tanks.

C O N C L U S I O N S

In order to improve the COP of an adsorption heat transformer, it is very important to minimize sensible heat losses in the tanks that invariably take place during the intermittent operation. The 4-tank system described in this study does this very effectively. It is also equally important to reduce the temperature driving forces for heat transfer by choosing proper designs for the heat transfer/collection systems for the tanks.

R E F E R E N C E S

1. S. Ulku, Adsorption heat pumps, J. Heat Recovery Systems 6, 277-284 (1986). 2. P. Maier-Laxhuber, M. Rothmeyer and G. Alefeld, Zeolite heat pump and heat storago, Secomilnternational Conference

on Energy Storqe, Stratford-upon-Avon, England, 24-26 May (1983). 3. D. Jung, N. Khelifa, E. Lavemann and R. Siznmnn, Energy storalp in zeolitu and apptkation to heatin s and air

conditioning, Proc. Int. Syrup. organized by Boris--Kidric inst. Chem., Ljubljana, Yugoslavia, 3-8 September (1984)


4. T. Asahiua, M. Kosaka, H. Taoda and K. Tajiri, Thermal properties of zeolite as a heat stora~ material of desiccant type, Third Japan Symposium on The~ophysical Proper~ieS~:Na~ Ja.~n (1982).

5. S. V. Shelton, W. J. Wepfer and D. J Mdes, Square wave analysis of the solid-vapour kd~rpuon heat pumps, Heat Recovery Systems & ClIP 9, 233-248 (1989).

6. F. Meunier, Research and development toward new thermochernical cycles for cold production from solar energy using solid adsorbents, First Conference of the Solar Energy Research Centre, Baghdad, 27-30 November (1982).

7. A. Sakoda and M. Suzuki, Fundamental study on solar powered adsorption cooling system, J. Chem. Engng Japan 17, 52-57 (1984).

8. D. I. Tchernev, The use of zeolites for solar cooling, Proc. Fifth International Conference on Zeolites, Naples, Italy, 2--6 June (1980).

9. N. Douss and F. Meunier, Effect of operating temperatures on the coefficient of performance of active carbon-methanol systems, Heat Recovery Systems & ClIP 8, 383-392 (1988).

10. G. Alefeld, H. C. Bauer, P. Maier-Laxhuber and M. Rothmeyer, A zeolite heat pump, heat transformer and heat accumulator, International Conference on Energy Storage, Brighton, England, 29 April-I May (1981).

I 1. D. W. Breck, Zeolite molecular sieves: structure, chemistry and use, Wiley-Interscience, U.S.A. (1974). 12. S. T. Wilson, B. M. Lok, C. A. Messina and E. M. Flanigen, Synthesis of AIPO 4 molecular sieves, Proc. Sixth

International Zeolite Conference, Reno, U.S.A., 12-15 July (1983).

theoretical studies on adsorption heat transformer using zeolite-water vapour pair

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