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Adsorption heat pumps
Heat Recovery Systems Vol. 6, No, 4, pp. 277-284. 1986
0198-7593/86 $3.00 + .DO Pergamon Journals Ltd
Printed in Great Britain.
A D S O R P T I O N HEAT P U M P S SEMRA ULKU Dokuz Eyliil Oniversitesi, Miih.-Mim. Fak., Makina Miih. B61., Bornova-lzmir, Turkey (Received 10 December 1985)
Abstract--Adsorption heat pumps have sparked considerable attention in recent years. In this paper general outlines for adsorption heat pumps are presented and the results of the work related to an adsorption heat pump which has been built and tested are given. The hermetically sealed heat pump system mainly consists of a packed bed of adsorbent, a condenser and an evaporator. A natural zeolite and water vapour pair is used as the working material. Heat of adsorption and heat of condensation are utilized for heating and heat of evaporation is utilized for cooling purposes.
NOMENCLATURE m q,, x' Ax'
COP COP' COW
a~to AH~ P
Q R
amount of dry adsorbate [kg] isosteric heat of adsorption [kJ/kg adsorbate] amount adsorbed [kg/kg dry adsorbent] cycled amount of adsorbate [kg]. Carnot cycle performance coefficient real cycle performance coefficient limiting performance coefficient heat of adsorption [kJ/kg adsorbate] heat of evaporation [kJ/kg] pressure [kPa] heat exchanged [kJ] specific gas constant [kJ/(kg adsorbate)K]
Indices a
C, con d el) v
hp ref A H L
adsorption, construction material condenser desorption evaporator evaporation heat pump refrigerator adsorber high low
INTRODUCTION Adsorption heat pump systems have gained considerable attention after the demonstration presented by D. I. Tchernev [1, 2] for space heating and cooling, refrigeration and ice manufacturing. Devices extracting heat from a low temperature level (TL) and giving off this heat at a higher temperature level (TH) are called heat pumps. In heat pumps attention is focused on the utilization of the heat delivered at the high temperature level rather than the heat extracted at the low temperature level; if the extracted heat region is the prime concern then the device is called a refrigerator. It is obvious that the combination of heating and cooling duties in the same system would be economically desirable. HEAT
PUMPS
From the second law of thermodynamics, the transfer of heat from a low temperature to a high temperature source is possible only if there exists a third energy source available. This energy may be available in the form of mechanical or electrical energy as in the case of mechanically driven compression heat pumps or in the form of heat as in the case of thermally driven sorption (adsorption and absorption) heat pumps. Compression heat pumps are driven by the energy produced from the primary energy with a low efficiency and the requirement of high grade energy in the form of shaft work for the compressor is the greatest disadvantage for them. Whereas 277
278
SEMRAOLK¢~
thermally driven heat pumps can be operated directly by heat without conversion to mechanical energy. In the last few years an increasing interest in sorption beat pumps has emerged due to their advantages related to primary energy consumption, lack of compressor, lack of vibration and noise problems, expected long service life, etc. Performance of a heat pump is measured by the ratio of the useful function provided by the device to the required input to the device; and this ratio is called coefficient of performance. For a compression heat pump and a refrigerator from the first law of thermodynamics:
Q.
COPhp = --~
Q.
1
Q . _ QL
QL
QH COP=r = Q__.A= QL = 1 W Q H - QL Qn 1 QL
and from the second law of thermodynamics: COPhp ~<
1
TL
1 ---
T.
COP=r ~<
l
T. --- 1 T, are obtained. Where equality sign indicates the maximum efficiency obtained from the ideal reversible Carnot heat pump operating between Tn and T. temperatures, Since it is not possible to operate a heat pump according to Carnot cycle, it would be worthwhile to define an efficiency as the ratio of the COP of the actual heat pump to that of Carnot heat pump o ~ r a t i n g between the same temperature levels: COP~, q = COPc~.ot" Thermal heat pumps operate on the principle of sorption, by holding the vapours and gases in solid or liquid materials at low temperatures and giving them off when the temperature is raised, In sorption processes there is always a generation of heat which is called Hea t of Sorption, For the operation of this heat pump, it is necessary to have a third heat source available at a temperaturc Tz higher than T~ and Tc. Heating is provided at temperatures TA and Tc and cooling is provided at temperature TL. Again from first and second law analysis: QL+Qz=Q~+Qc
or
QL 0- z + ~ . ~ - ~ u TL
Tz
TA
"
*c
~ligh t e m p e r t u r e source (Tz)
I I
Qz
QA (T.)
I W-O. - QL
W~ - - - "- ~ (OnLy for ~ al~moe~on heat. ~ pump)
)
/
oc ~ -
..... i, ~ J. : I. ,,
TA
. ~ t u r e ~ I ~" , I
I QL,, , mree
(TO
]
Fig. 1. Compression heat pump.
c ,,0,
II
Fig. 2. Sorpfion heat pump.
sources
Adsorption heat pumps
279
Highest. t.emperoture source ( Tz ) Medium t.emperoture source (TA)
Low t,emperoture source (Tc )
Lowest. t,emperoture [
I source (TL)
]
Fig. 3. Sorption heat pump as two systems
Sorption heat pump operating according to Carnot cycle can be considered as if it was made of two systems; operating between T z - Tc and between TA -- TL temperatures. First one operates as heat engine receives Qz amount of heat from the highest temperature source (Tz), rejects heat at a medium temperature (Tc). In the second system QL is transferred from the lowest temperature source (TL) to the medium temperature source (TA) using the work produced in the first system. From first and second laws for each system:
Qz-Qc+ W=0 Qz Tz
Qc>. 0 Tc ~"
- W+Qt-QA=O QL TL
QA >10. TA
Then for sorption heat pump or refrigerator operating according to Carnot cycles between 4 temperature levels, maximum performance coefficients (Carnot performance c~tficients) come
out as: COP~r =
1-Tc/Tz Ta/Tz- I
COPhp = 1 -+
1-Tc/Tz TA/Tz- 1"
If there exists only one temperature level at which heat is rejected (Tc = TA = T#): COP~f -
1 - TxlTz
T./TL1 -
COPhp = 1 4
1 T.IT~
T.ITt-
1
are obtained for maximum performance coefficients. Although principles of sorption heat pumps can be outlined independently of the materials (sorbate and sorbent) used, the choice of the most appropriate pair for the prevailing conditions is one of the significant problems. There exist quite a number of investigations using various couples (ammonia-water, active carbon-methanol, zeolite 13X-water . . . . ) [1--6] as working materials. Among these, studies related to absorption heat pumps (iiquid-vapour as working material) are more than the studies related to adsorption heat pumps (solid-vapour as working material). The most significant differences between the conventional absorption heat pump and adsorption heat pump systems are the immobility of the sorbent and the discontinuity in the operation of the latter if elaborate design has not been carried out.
280
SEMRA I~LKU
ADSORPTION HEAT PUMPS
Adsorption Adsorption involves contact of natural or synthetic materials of m~crocrystalline structure (adsorbent) with either liquids or gases (adsorbate). Selective combination of adsorbent and adsorbate occurs on the pore surfaces, physical adsorptmn is a readily reversible phenomena, and it is caused by the molecular interaction forces between adsorbate and adsorbent: whereas chemical adsorption involves a chemical interaction between an adsorbate and an adsorbent. Equilibrium adsorption characteristics of a gas or vapour upon a solid can be presented by adsorption isotherms ( x = f ( P ) r ) ; adsorption isobars (x =f(T)e); or adsorption isosters (P =f(T)x). Adsorption processes are accompanied by heat evolution, and the heat of adsorpt|on is larger than the latent heat of vaporization. For physical adsorption it is of the order of heat of sublimation. Heat of adsorption can be calculated using adsorption isosters applying Clausius Clapeyron equation: d In P d(l/T)~
AHo R
- q~, R
where q,, corresponds to isosteric heat of adsorption and it varies with the amount adsorbed: integral heat of adsorption can be computed from the relation: AH~ =
AH, dx.
It is also possible to determine AH~ calorimetrically.
Working materials and zeolite-water vapour parr Many adsorbent-adsorbate pairs can be considered as working material in adsorption heat pump systems. The main criteria in the selection of the pair would include affinity of the pair for each other, toxicity, thermal and chemical stability, corrosiveness, thermal conductivity, diffusivity, heat of adsorption, heat of evaporation, availability and cost. Activated alumina, activated carbon, activated clays, silicagel and crystalline zeolites are the common commercially available adsorbents. Activated carbon, activated alumina, and silicagei don't have an ordered crystal structure and their pores are nonuniform: whereas zeolites have uniform pore size and they have selectivity for adsorption. Zeolites are crystalline hydrated alumina silicates of group I and group II dements. There exist more than 150 species of synthetic and 40 species of mineral zeolite. They have a high internal surface area available for adsorption due to channels or pores. Surface area of a zeolite particle reaches to 800--1000m2/gm. They can sorp polar and nonpolar molecules, salts and metals if appropriate conditions are provided. Factors including pressure, temperature, particle size and conditions of dehydration and desorption influences the rate of adsorption by the specified zeolite. Zeolites, depending on their structure, are capable of adsorbing large quantities of water vapour with high heats of adsorption, almost independently of the concentration; and since the latent heat of vaporization for water is largest among the common adsorbates, zeolite-water vapour pair seems a promising pair for heat pump applications. EXPERIMENTAL
Experimental set up A simple ftow diagram of the adsorption heat pump system is shown in Fig. 4. The system mainly consists of a packed bed of adsorbent, a condenser and an evaporator. The adsorbent bed is made of two steel pipes having diameters 0.1 and 0.036 m, and lengths 1.5 and 1.68 m. They are placed coaxially, the inner one is punched and is connected to the condenser and the evaporator (Fig. 4). A heating strip and copper cooling coil, surrounding the outer pipe, are used for the control of the bed temperature. The annular place between these two pipes are filled with i0 kg natural zeolite granules with known properties.
Adsorption hea t pumps
281
Adsorbent bed
ondenser
~
~\\\\\\\\\\\\\\\\\\-~
I"/o=o,e00o.--
Fig. 4. Experimental system flow diagram.
The condenser consists of a copper coil immersed in a constant temperature water tank and a graduated glass vessel for the measurement of the condensed amount. The evaporator is simply a graduated glass cylinder in a constant temperature bath. For the measurement of temperatures, copper constantan thermocouples are inserted in the bed, in the evaporator and in the condenser at various locations. For the measurement of pressures, vacuum gauges are used. Principle of operation and analysis of the system
The In P - 1/T diagram is very helpful in analysing the system. The adsorption isosters for the used natural zeolite-water vapour pair are plotted on the In P - l i T diagram (Fig. 5). The completely reversible cycle can be presented by two isosteric and two isobaric processes. x - y Isosteric heating process. When the zeolite bed, with the connections to the condenser and evaporator closed, is heated, its temperature rises from Tx to T;. and water vapour pressure rises from P, to P2. Pz and P2 are the pressures determined by the temperatures of the evaporator and condenser respectively. At state Y desorption starts and the connection to the condenser is opened. y - z Isobaric desorption process. With further heating water vapour is desorbed and flows to the condenser at constant pressure, and the amount adsorbed by the bed decreases. After the bed has reached the maximum temperature (Tz), determined by the highest temperature source, desorption stops and connection to the condenser is closed. z - w Isosteric cooling process. The hot zeolite bed is cooled down so the temperature of the bed drops (7",,) and vapour pressure of the adsorbate is allowed to decrease down to the pressure determined by the evaporator temperature. The connection between the bed and the evaporator is opened and adsorption starts. Reol cycle
....
/ / H20//
200 .
.
.
.
.
.
.
.
.
.
.
.
/
9.o93po8
//0.115
.
IOOi
//
50 .
.
.
l
I0 0
/
.
I
,, 50
IOO (°C)
(_
iT, tso
/X'(kg H20/kg zeolite]
t
i
TitS-)) 200
Fig. 5. Adsorption isosters and adsorption heat pump cycles on In P
-
I/T
diagram.
282
SEMRAOLKU
w-x Isobaric adsorption process. With further cooling of the adsorbent bed, the water vapour coming from the evaporator is adsorbed and adsorbed amount of water vapour increases from x~ to x2 at constant pressure p~, determined by the evaporator. When the temperature of the bed: is decreased down to T~, the cycle is completed. It is possible to obtain the relationships between the temperatures T,,o., T,~,, Tx, T,,, T., T~ from the application of Clausius Clapeyron equation between P~ and P2 pressures for Vapour-liquid phase and vapour-solid phases along xt and x2 isosters, assuming Alia and AH, as constant in the related ranges.
A..( '\L,
't
(' ') 1
1
For the cycle considered it is possible to utilize the heat released in the bed and in the condenser for heating purposes, and also to utilize the heat extracted in the evaporator for cooling and refrigeration purposes. One of the real cycles with Tco~= 53°C, T,~,= 27°C, Tx = 69°C and T~ = 123°C is shown in Fig. 5.
Performance coe~cients The coefficients of the performances can be given as: copr
= Qutiti~d
agiv~ll COP'~r =
COP[,p = COP,+h =
xQy + yQ~ Q~o.+ ,Qw+ wQ~ .~Qy+ vQ,
Q,~. + Q~o.+ zQ,~+ wQx
xQ,,+,,Q:
where
Q~,=m.Ax"AH~,+
m.Ax'.CpdT on
Qco. = m "Ax" AH,.(T~o.) .~Q.,,=
fi
[m.(C,:+ x;.G..)+m,.C~ldr
yQ~= f:[m'(C,:+ x'C..,)+ma'C~dT + ~m'AHodx' :Qw = ~Q~ =
[m'(C,.+ x ; ' G . , ) + m . ' C M d r
f.:
[m.(G:+x'.G.)+mo.C,,.tdr +
f
m.AHodx'.
Heat of adsorption as a function of amount of fluid adsorbed and tempc~ture is calculated from adsorption isosters (Fig. 6) and used in the calculation of the performance coefficients. For the limiting case, the ideal thermodynamieal performance codl~ie.nts would reach to:
An,, (T~,)
COl~r = - AHo (Ta) COP~h, =
AH,, (T~o~)+ AH, (T~) AH~ (re)
Adsorption heat pumps
283
4OOO A
32OO o ::T- 2800
.~,
01
~
2400
I
2000
I
I
I
I
1
O.0~ 0 . 0 4 0 . 0 6 008 0 . 1 0 0.12 Amount absorbed (kg H z 0 / k g dry zeolite)
I 0.14
Fig. 6. Heat of adsorption as a function of amount of fluid adsorbed.
coP Q5
2--
0.4 tO
"G Q3
0
O.2
, / /
,r I
I00 Maximum temperature Tz ('C)
°"
1
Fig. 7. Performance coefficients and amount of water cycled as a function o f maximum (T:) temperature.
RESULTS For the cycle presented (Too, = 53°C, T~, = 27°C, Tx = 69°C, T~ = 123°C) the performance coefficients (COP r) for cooling and heating were obtained as: 0.34 for cooling and 1.33 for heating cases. The amount of water vapour cycled was 0.016 kg water/kg dry zeolite, and the total cooling and heating loads obtained were 372 kJ and 1449 kJ respectively. By increasing the hot source temperature and by lowering the evaporator pressure, it is possible to increase the cycled amount of water vapour and so the loads. By increasing 7". temperature up to 200°C, cooling load has increased to 1237 kJ and heating load has increased to 4276 kJ. The change of efficiencies with 7"= temperature are given Fig. 7.
CONCLUSIONS So far not so much development has been done on adsorption heat pumps, although they might become one of the most effective means for utilizations of waste heat and solar energy. Optimization of heat and mass transfer together with the development of more suitable adsorbent-adsorbate pairs will promote large scale application of these devices. From the results of the experimental work of this study and from the literature it can be suggested that water vapour and the local natural zeolite (from Turkey) system may have
284
SEMRA ~JLKr~
a p p l i c a t i o n s for c o o l i n g a n d h e a t i n g p u r p o s e s a n d further d e v e l o p m e n t is expected in the feasibility by a c t i v a t i n g the m i n e r a l for the i m p r o v e m e n t o f the a d s o r p t i o n capacity. Acknowledgement--The author is grateful to S. Niknam and E. De~irmen for the experimental work
REFERENCES 1. D. I. Tchernev, Solar app. of natural zeotites. In Proc. Natural Zeolites Oc. Prop. Use (Edited by L. B. S, and F. A. Mumpton). Pergamon Press, Oxford (1976). 2. D. I. Tchernev, Use of zeolites solar cooling, In Proc. 5th Int. Conf. on Zeolites, Italy (t980L 3. P. Maier-Laxhuher, M. Rothmeyer, G. Alefeld, Zeolite heat pump and heat storage. In Int. Con£ Energy Storage. BHRA Fluid Engineering, Cranfieid, U.K. (1983). 4. G. Alef©ld, H. C. Bauer, P. Maier-Laxhuber and M. Rothmeyer, A zeolite heat pump, heat transformer and heat accumulator. In Int. Conf. on Energy Storage, Brighton, U.K., 29 Apri-I May, 1981. BHRA Fluid Engineering, Cranfield, U.K. (1981). 5. F. Mcunier, B. Misehler, J. J. Guillominot, M. H. Simonot, On the use of zeolites 13X--H20 intermiltent cycle for the application to solar climatization of buildings, tn Sun 1I, Proc. Ont. Cong. of the lnL Sol. Eng~ Soc. (Edited by K. B6er), p. 619. Atlanta, 1979.
0198-7593/86 $3.00 + .DO Pergamon Journals Ltd
Printed in Great Britain.
A D S O R P T I O N HEAT P U M P S SEMRA ULKU Dokuz Eyliil Oniversitesi, Miih.-Mim. Fak., Makina Miih. B61., Bornova-lzmir, Turkey (Received 10 December 1985)
Abstract--Adsorption heat pumps have sparked considerable attention in recent years. In this paper general outlines for adsorption heat pumps are presented and the results of the work related to an adsorption heat pump which has been built and tested are given. The hermetically sealed heat pump system mainly consists of a packed bed of adsorbent, a condenser and an evaporator. A natural zeolite and water vapour pair is used as the working material. Heat of adsorption and heat of condensation are utilized for heating and heat of evaporation is utilized for cooling purposes.
NOMENCLATURE m q,, x' Ax'
COP COP' COW
a~to AH~ P
Q R
amount of dry adsorbate [kg] isosteric heat of adsorption [kJ/kg adsorbate] amount adsorbed [kg/kg dry adsorbent] cycled amount of adsorbate [kg]. Carnot cycle performance coefficient real cycle performance coefficient limiting performance coefficient heat of adsorption [kJ/kg adsorbate] heat of evaporation [kJ/kg] pressure [kPa] heat exchanged [kJ] specific gas constant [kJ/(kg adsorbate)K]
Indices a
C, con d el) v
hp ref A H L
adsorption, construction material condenser desorption evaporator evaporation heat pump refrigerator adsorber high low
INTRODUCTION Adsorption heat pump systems have gained considerable attention after the demonstration presented by D. I. Tchernev [1, 2] for space heating and cooling, refrigeration and ice manufacturing. Devices extracting heat from a low temperature level (TL) and giving off this heat at a higher temperature level (TH) are called heat pumps. In heat pumps attention is focused on the utilization of the heat delivered at the high temperature level rather than the heat extracted at the low temperature level; if the extracted heat region is the prime concern then the device is called a refrigerator. It is obvious that the combination of heating and cooling duties in the same system would be economically desirable. HEAT
PUMPS
From the second law of thermodynamics, the transfer of heat from a low temperature to a high temperature source is possible only if there exists a third energy source available. This energy may be available in the form of mechanical or electrical energy as in the case of mechanically driven compression heat pumps or in the form of heat as in the case of thermally driven sorption (adsorption and absorption) heat pumps. Compression heat pumps are driven by the energy produced from the primary energy with a low efficiency and the requirement of high grade energy in the form of shaft work for the compressor is the greatest disadvantage for them. Whereas 277
278
SEMRAOLK¢~
thermally driven heat pumps can be operated directly by heat without conversion to mechanical energy. In the last few years an increasing interest in sorption beat pumps has emerged due to their advantages related to primary energy consumption, lack of compressor, lack of vibration and noise problems, expected long service life, etc. Performance of a heat pump is measured by the ratio of the useful function provided by the device to the required input to the device; and this ratio is called coefficient of performance. For a compression heat pump and a refrigerator from the first law of thermodynamics:
Q.
COPhp = --~
Q.
1
Q . _ QL
QL
QH COP=r = Q__.A= QL = 1 W Q H - QL Qn 1 QL
and from the second law of thermodynamics: COPhp ~<
1
TL
1 ---
T.
COP=r ~<
l
T. --- 1 T, are obtained. Where equality sign indicates the maximum efficiency obtained from the ideal reversible Carnot heat pump operating between Tn and T. temperatures, Since it is not possible to operate a heat pump according to Carnot cycle, it would be worthwhile to define an efficiency as the ratio of the COP of the actual heat pump to that of Carnot heat pump o ~ r a t i n g between the same temperature levels: COP~, q = COPc~.ot" Thermal heat pumps operate on the principle of sorption, by holding the vapours and gases in solid or liquid materials at low temperatures and giving them off when the temperature is raised, In sorption processes there is always a generation of heat which is called Hea t of Sorption, For the operation of this heat pump, it is necessary to have a third heat source available at a temperaturc Tz higher than T~ and Tc. Heating is provided at temperatures TA and Tc and cooling is provided at temperature TL. Again from first and second law analysis: QL+Qz=Q~+Qc
or
QL 0- z + ~ . ~ - ~ u TL
Tz
TA
"
*c
~ligh t e m p e r t u r e source (Tz)
I I
Qz
QA (T.)
I W-O. - QL
W~ - - - "- ~ (OnLy for ~ al~moe~on heat. ~ pump)
)
/
oc ~ -
..... i, ~ J. : I. ,,
TA
. ~ t u r e ~ I ~" , I
I QL,, , mree
(TO
]
Fig. 1. Compression heat pump.
c ,,0,
II
Fig. 2. Sorpfion heat pump.
sources
Adsorption heat pumps
279
Highest. t.emperoture source ( Tz ) Medium t.emperoture source (TA)
Low t,emperoture source (Tc )
Lowest. t,emperoture [
I source (TL)
]
Fig. 3. Sorption heat pump as two systems
Sorption heat pump operating according to Carnot cycle can be considered as if it was made of two systems; operating between T z - Tc and between TA -- TL temperatures. First one operates as heat engine receives Qz amount of heat from the highest temperature source (Tz), rejects heat at a medium temperature (Tc). In the second system QL is transferred from the lowest temperature source (TL) to the medium temperature source (TA) using the work produced in the first system. From first and second laws for each system:
Qz-Qc+ W=0 Qz Tz
Qc>. 0 Tc ~"
- W+Qt-QA=O QL TL
QA >10. TA
Then for sorption heat pump or refrigerator operating according to Carnot cycles between 4 temperature levels, maximum performance coefficients (Carnot performance c~tficients) come
out as: COP~r =
1-Tc/Tz Ta/Tz- I
COPhp = 1 -+
1-Tc/Tz TA/Tz- 1"
If there exists only one temperature level at which heat is rejected (Tc = TA = T#): COP~f -
1 - TxlTz
T./TL1 -
COPhp = 1 4
1 T.IT~
T.ITt-
1
are obtained for maximum performance coefficients. Although principles of sorption heat pumps can be outlined independently of the materials (sorbate and sorbent) used, the choice of the most appropriate pair for the prevailing conditions is one of the significant problems. There exist quite a number of investigations using various couples (ammonia-water, active carbon-methanol, zeolite 13X-water . . . . ) [1--6] as working materials. Among these, studies related to absorption heat pumps (iiquid-vapour as working material) are more than the studies related to adsorption heat pumps (solid-vapour as working material). The most significant differences between the conventional absorption heat pump and adsorption heat pump systems are the immobility of the sorbent and the discontinuity in the operation of the latter if elaborate design has not been carried out.
280
SEMRA I~LKU
ADSORPTION HEAT PUMPS
Adsorption Adsorption involves contact of natural or synthetic materials of m~crocrystalline structure (adsorbent) with either liquids or gases (adsorbate). Selective combination of adsorbent and adsorbate occurs on the pore surfaces, physical adsorptmn is a readily reversible phenomena, and it is caused by the molecular interaction forces between adsorbate and adsorbent: whereas chemical adsorption involves a chemical interaction between an adsorbate and an adsorbent. Equilibrium adsorption characteristics of a gas or vapour upon a solid can be presented by adsorption isotherms ( x = f ( P ) r ) ; adsorption isobars (x =f(T)e); or adsorption isosters (P =f(T)x). Adsorption processes are accompanied by heat evolution, and the heat of adsorpt|on is larger than the latent heat of vaporization. For physical adsorption it is of the order of heat of sublimation. Heat of adsorption can be calculated using adsorption isosters applying Clausius Clapeyron equation: d In P d(l/T)~
AHo R
- q~, R
where q,, corresponds to isosteric heat of adsorption and it varies with the amount adsorbed: integral heat of adsorption can be computed from the relation: AH~ =
AH, dx.
It is also possible to determine AH~ calorimetrically.
Working materials and zeolite-water vapour parr Many adsorbent-adsorbate pairs can be considered as working material in adsorption heat pump systems. The main criteria in the selection of the pair would include affinity of the pair for each other, toxicity, thermal and chemical stability, corrosiveness, thermal conductivity, diffusivity, heat of adsorption, heat of evaporation, availability and cost. Activated alumina, activated carbon, activated clays, silicagel and crystalline zeolites are the common commercially available adsorbents. Activated carbon, activated alumina, and silicagei don't have an ordered crystal structure and their pores are nonuniform: whereas zeolites have uniform pore size and they have selectivity for adsorption. Zeolites are crystalline hydrated alumina silicates of group I and group II dements. There exist more than 150 species of synthetic and 40 species of mineral zeolite. They have a high internal surface area available for adsorption due to channels or pores. Surface area of a zeolite particle reaches to 800--1000m2/gm. They can sorp polar and nonpolar molecules, salts and metals if appropriate conditions are provided. Factors including pressure, temperature, particle size and conditions of dehydration and desorption influences the rate of adsorption by the specified zeolite. Zeolites, depending on their structure, are capable of adsorbing large quantities of water vapour with high heats of adsorption, almost independently of the concentration; and since the latent heat of vaporization for water is largest among the common adsorbates, zeolite-water vapour pair seems a promising pair for heat pump applications. EXPERIMENTAL
Experimental set up A simple ftow diagram of the adsorption heat pump system is shown in Fig. 4. The system mainly consists of a packed bed of adsorbent, a condenser and an evaporator. The adsorbent bed is made of two steel pipes having diameters 0.1 and 0.036 m, and lengths 1.5 and 1.68 m. They are placed coaxially, the inner one is punched and is connected to the condenser and the evaporator (Fig. 4). A heating strip and copper cooling coil, surrounding the outer pipe, are used for the control of the bed temperature. The annular place between these two pipes are filled with i0 kg natural zeolite granules with known properties.
Adsorption hea t pumps
281
Adsorbent bed
ondenser
~
~\\\\\\\\\\\\\\\\\\-~
I"/o=o,e00o.--
Fig. 4. Experimental system flow diagram.
The condenser consists of a copper coil immersed in a constant temperature water tank and a graduated glass vessel for the measurement of the condensed amount. The evaporator is simply a graduated glass cylinder in a constant temperature bath. For the measurement of temperatures, copper constantan thermocouples are inserted in the bed, in the evaporator and in the condenser at various locations. For the measurement of pressures, vacuum gauges are used. Principle of operation and analysis of the system
The In P - 1/T diagram is very helpful in analysing the system. The adsorption isosters for the used natural zeolite-water vapour pair are plotted on the In P - l i T diagram (Fig. 5). The completely reversible cycle can be presented by two isosteric and two isobaric processes. x - y Isosteric heating process. When the zeolite bed, with the connections to the condenser and evaporator closed, is heated, its temperature rises from Tx to T;. and water vapour pressure rises from P, to P2. Pz and P2 are the pressures determined by the temperatures of the evaporator and condenser respectively. At state Y desorption starts and the connection to the condenser is opened. y - z Isobaric desorption process. With further heating water vapour is desorbed and flows to the condenser at constant pressure, and the amount adsorbed by the bed decreases. After the bed has reached the maximum temperature (Tz), determined by the highest temperature source, desorption stops and connection to the condenser is closed. z - w Isosteric cooling process. The hot zeolite bed is cooled down so the temperature of the bed drops (7",,) and vapour pressure of the adsorbate is allowed to decrease down to the pressure determined by the evaporator temperature. The connection between the bed and the evaporator is opened and adsorption starts. Reol cycle
....
/ / H20//
200 .
.
.
.
.
.
.
.
.
.
.
.
/
9.o93po8
//0.115
.
IOOi
//
50 .
.
.
l
I0 0
/
.
I
,, 50
IOO (°C)
(_
iT, tso
/X'(kg H20/kg zeolite]
t
i
TitS-)) 200
Fig. 5. Adsorption isosters and adsorption heat pump cycles on In P
-
I/T
diagram.
282
SEMRAOLKU
w-x Isobaric adsorption process. With further cooling of the adsorbent bed, the water vapour coming from the evaporator is adsorbed and adsorbed amount of water vapour increases from x~ to x2 at constant pressure p~, determined by the evaporator. When the temperature of the bed: is decreased down to T~, the cycle is completed. It is possible to obtain the relationships between the temperatures T,,o., T,~,, Tx, T,,, T., T~ from the application of Clausius Clapeyron equation between P~ and P2 pressures for Vapour-liquid phase and vapour-solid phases along xt and x2 isosters, assuming Alia and AH, as constant in the related ranges.
A..( '\L,
't
(' ') 1
1
For the cycle considered it is possible to utilize the heat released in the bed and in the condenser for heating purposes, and also to utilize the heat extracted in the evaporator for cooling and refrigeration purposes. One of the real cycles with Tco~= 53°C, T,~,= 27°C, Tx = 69°C and T~ = 123°C is shown in Fig. 5.
Performance coe~cients The coefficients of the performances can be given as: copr
= Qutiti~d
agiv~ll COP'~r =
COP[,p = COP,+h =
xQy + yQ~ Q~o.+ ,Qw+ wQ~ .~Qy+ vQ,
Q,~. + Q~o.+ zQ,~+ wQx
xQ,,+,,Q:
where
Q~,=m.Ax"AH~,+
m.Ax'.CpdT on
Qco. = m "Ax" AH,.(T~o.) .~Q.,,=
fi
[m.(C,:+ x;.G..)+m,.C~ldr
yQ~= f:[m'(C,:+ x'C..,)+ma'C~dT + ~m'AHodx' :Qw = ~Q~ =
[m'(C,.+ x ; ' G . , ) + m . ' C M d r
f.:
[m.(G:+x'.G.)+mo.C,,.tdr +
f
m.AHodx'.
Heat of adsorption as a function of amount of fluid adsorbed and tempc~ture is calculated from adsorption isosters (Fig. 6) and used in the calculation of the performance coefficients. For the limiting case, the ideal thermodynamieal performance codl~ie.nts would reach to:
An,, (T~,)
COl~r = - AHo (Ta) COP~h, =
AH,, (T~o~)+ AH, (T~) AH~ (re)
Adsorption heat pumps
283
4OOO A
32OO o ::T- 2800
.~,
01
~
2400
I
2000
I
I
I
I
1
O.0~ 0 . 0 4 0 . 0 6 008 0 . 1 0 0.12 Amount absorbed (kg H z 0 / k g dry zeolite)
I 0.14
Fig. 6. Heat of adsorption as a function of amount of fluid adsorbed.
coP Q5
2--
0.4 tO
"G Q3
0
O.2
, / /
,r I
I00 Maximum temperature Tz ('C)
°"
1
Fig. 7. Performance coefficients and amount of water cycled as a function o f maximum (T:) temperature.
RESULTS For the cycle presented (Too, = 53°C, T~, = 27°C, Tx = 69°C, T~ = 123°C) the performance coefficients (COP r) for cooling and heating were obtained as: 0.34 for cooling and 1.33 for heating cases. The amount of water vapour cycled was 0.016 kg water/kg dry zeolite, and the total cooling and heating loads obtained were 372 kJ and 1449 kJ respectively. By increasing the hot source temperature and by lowering the evaporator pressure, it is possible to increase the cycled amount of water vapour and so the loads. By increasing 7". temperature up to 200°C, cooling load has increased to 1237 kJ and heating load has increased to 4276 kJ. The change of efficiencies with 7"= temperature are given Fig. 7.
CONCLUSIONS So far not so much development has been done on adsorption heat pumps, although they might become one of the most effective means for utilizations of waste heat and solar energy. Optimization of heat and mass transfer together with the development of more suitable adsorbent-adsorbate pairs will promote large scale application of these devices. From the results of the experimental work of this study and from the literature it can be suggested that water vapour and the local natural zeolite (from Turkey) system may have
284
SEMRA ~JLKr~
a p p l i c a t i o n s for c o o l i n g a n d h e a t i n g p u r p o s e s a n d further d e v e l o p m e n t is expected in the feasibility by a c t i v a t i n g the m i n e r a l for the i m p r o v e m e n t o f the a d s o r p t i o n capacity. Acknowledgement--The author is grateful to S. Niknam and E. De~irmen for the experimental work
REFERENCES 1. D. I. Tchernev, Solar app. of natural zeotites. In Proc. Natural Zeolites Oc. Prop. Use (Edited by L. B. S, and F. A. Mumpton). Pergamon Press, Oxford (1976). 2. D. I. Tchernev, Use of zeolites solar cooling, In Proc. 5th Int. Conf. on Zeolites, Italy (t980L 3. P. Maier-Laxhuher, M. Rothmeyer, G. Alefeld, Zeolite heat pump and heat storage. In Int. Con£ Energy Storage. BHRA Fluid Engineering, Cranfieid, U.K. (1983). 4. G. Alef©ld, H. C. Bauer, P. Maier-Laxhuber and M. Rothmeyer, A zeolite heat pump, heat transformer and heat accumulator. In Int. Conf. on Energy Storage, Brighton, U.K., 29 Apri-I May, 1981. BHRA Fluid Engineering, Cranfield, U.K. (1981). 5. F. Mcunier, B. Misehler, J. J. Guillominot, M. H. Simonot, On the use of zeolites 13X--H20 intermiltent cycle for the application to solar climatization of buildings, tn Sun 1I, Proc. Ont. Cong. of the lnL Sol. Eng~ Soc. (Edited by K. B6er), p. 619. Atlanta, 1979.