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Study on heat and mass recovery in adsorption refrigeration cycles
Applied Thermal Engineering 21 (2001) 439±452
www.elsevier.com/locate/apthermeng
Study on heat and mass recovery in adsorption refrigeration cycles T.F. Qu, R.Z. Wang*, W. Wang School of Power & Energy Engineering, Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai, 200030, PR China Received 23 August 1999; accepted 14 March 2000
Abstract An adsorption air conditioner has been developed and some operation results are summarized. Mass recovery process is proposed to improve the performance. Performance predictions are presented and show that mass recovery can play an important role to better the performance of adsorption refrigeration cycle. Coecient of performance might be increased or decreased with mass recovery process due to dierent working conditions. Cooling capacity can be signi®cantly increased with mass recovery process. The cycle with mass and heat recovery has a relatively higher improvement. It can also be seen that the cycle time will be much shorter and it will certainly enhance the cycle with higher cooling/heating power. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Mass recovery; Heat recovery; Adsorption; Refrigeration cycles
1. Introduction Solid sorption refrigeration is environmental friendly as it employs safe refrigerants like water, ammonia, methanol, etc. The cycle can be driven by low-grade temperature thermal energy and usually without moving parts. As a result, it has special advantages in energy saving and environment protection. A great deal of research work has been done over many years, and it is developing more and more rapidly nowadays. * Corresponding author. Tel.: +86-21-6281-3250; fax: +86-21-6293-3250. E-mail address: [email protected] (R.Z. Wang). 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 0 - 8
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Nomenclature X concentration (kg/kg) speci®c heat (J/kg K) Cp P pressure (Pa) T temperature (K) D variation of variable enthalpy of adsorption (J/kg) Ha enthalpy of desorption (J/kg) Hd a2, a3, g2, g3 points on the thermodynamic cycle heat capacity ratio of bed metal to activated carbon Rm heat capacity ratio of heat-transfer ¯uid to activated carbon Rf As we know, however, the main drawbacks of adsorption cycles are its low coecient of performance (COP), small speci®c cooling power (SCP) and long cycle time. These are mainly caused by the poor heat and mass transfer rate in the adsorption bed. Therefore, research work is being carried out at present, which mainly focuses on how to enhance the cycle performance, for which there are several means such as [1±5]: 1. 2. 3. 4.
®nd new better working pairs and improve the adsorption performance, propose new types of adsorption cycle, intensify the heat and mass transfer in the adsorbent bed, decrease the cycle time.
In proposing new types of cycles, much has been discussed besides the basic cycle, such as two-beds continuous cycle, continuous heat recovery cycle, thermal wave cycle, multieect cycle, etc. [1,6±8]. Among these types of cycles, the basic cycle is suitable for recovering heat from intermittent heat sources, like solar energy; two-beds continuous cycle and continuous heat recovery cycle may be used to recover low-grade thermal energy and will provide continuous cooling eect, and the heat recovery cycle has a higher thermal eciency; thermal wave cycle has the highest theoretical coecient of performance, but is dicult to be realized in a prototype system; multieect cycle can reach a high thermal eciency but the system will be a complicated one and will prevent it from wide use. We propose the adsorption cycle with mass recovery and also the systhetical cycle with mass recovery and heat recovery. We are assured that the performance will be signi®cantly improved with these processes. Our research results about the eect of mass recovery and heat recovery on the adsorption cycle are presented in this paper. 2. Adsorption refrigeration cycles 2.1. Basic cycle The basic adsorption cycle for refrigeration/heat pumping consists of four processes (see
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Fig. 1. Diagram of the intermittent and heat regeneration cycle.
Fig. 1). In the ®rst process a2±g1, the adsorbent bed receives heat from high-temperature heating ¯uid and reaches a high-pressure equal to the condensing pressure. The hightemperature thermal ¯uid is the high-temperature heat source. When the bed is being heated, it desorbs the refrigerant, which is condensed in a condenser. The refrigerant releases heat to a high-temperature heat sink and the process g1±g2 is an isobaric process. After that the bed gives heat to a second heat sink and is cooled down and depressurized to a1 by cooling systems. It readsorbs refrigerant vapor in the process a1±a2, which causes liquid evaporation in the evaporator; there the refrigerant extracts heat from low temperature heat source. The schematic system is shown in Fig. 2. It is often called an intermittent cycle as we can see that the evaporating process is going on only half cycle. It can be applied for recovering solar
Fig. 2. Schematic system for basic cycle. The dashed line represents thermal ¯uid ¯ow.
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energy application, etc. When there are two adsorbent beds, we can easily operate them out of phase. The two-beds continuous cycle employs two adsorbent beds to provide continuous cooling power. These are in fact two parallel basic cycles that operate out of phase. Therefore, it is not much dierent from the basic one from this point of view. All the thermal energy received by the adsorbent bed is supplied by the heating system. Heat must be recovered internally to improve the cycle performance. 2.2. Heat recovery cycle There are also two beds in this type of cycle, which are operated out of phase. At the end of each half cycle, one adsorbent bed is cold at point a2 and the other is at g2 with hightemperature. Heat can be easily recovered between the two adsorbent beds because of this temperature dierence. By circulating the thermal ¯uid between the two adsorbent beds adiabatically, the energy eciency can be increased signi®cantly [9,10]. In an ideal heat recovery process, the two beds will reach the same temperature, one is at point e and the other is e ' (see Fig. 1). The system is shown schematically in Fig. 3. Therefore the thermal load for heating can be decreased by Qa2ÿg1ÿe. This is the recovered thermal energy, which can reach about 30% or so of the total necessary heat input of the basic cycle. 2.3. Cycle with mass recovery In a two-bed cycle, the two beds are working out of phase. At the end of each half cycle, one bed is hot with high-pressure Pc and the other is cold with low-pressure Pe (a2 and g2 in Fig. 4, respectively). After that the high-pressure adsorber needs to be cooled down and
Fig. 3. Schematic system of heat recovery cycle. The dashed line represents thermal ¯uid ¯ow.
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Fig. 4. Diagram of mass recovery process and combined cycle with heat and mass recovery.
depressurized while the low-pressure one needs to be heated up and pressurized. Then the two adsorbent beds may be interconnected directly with a simple device and the refrigerant vapor will ¯ow from the high-pressure bed to the low-pressure one. This is called the mass recovery process. It is presented in Fig. 5. The pressure of Adsorber 1 decreases due to mass out¯ow and this will again cause desorption of Adsorber 1. Meanwhile, the pressure of Adsorber 2
Fig. 5. Schematic system of mass recovery/heat and mass recovery cycle. The dashed line represents thermal ¯uid ¯ow; the solid line and valve between the beds processes mass recovery and the dashed line between beds processes heat recovery.
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increases due to mass in¯ow and will cause further adsorption. The process is maintained until the two beds reach the same pressure, Pm (see Fig. 4). Then the connection is broken and each bed goes on with heating and cooling process just as in the basic cycle. This mass recovery process is expected to accelerate the circulation and enhance the cycle cooling power, for it only involves direct mass ¯ow and the pressure balance is much faster than temperature balance via heat-transfer medium. 2.4. Synthetical cycle with heat and mass recovery We can also use a heat regeneration process after the mass recovery process because there is also a large temperature dierence between the two beds. The thermal load can be further decreased so as to improve the cycle performance. In Fig. 4, this process begins with a3 and g3 and ends at e±e ' that have the common temperature. The system scheme is also shown in Fig. 5. The process of mass recovery enlarges dx, the concentration change of adsorbate in a cycle as we can see from the P±T±x diagram in Fig. 4. In this discussion the mass recovery process is assumed as adiabatic. The process is divided into two parts operated in two beds as a2±a3 and g2±g3. The process can be simulated and calculated utilizing the following model [8,11]. There are several assumptions, described later, to facilitate the calculation of the model. 1. The vapors desorbed from the high-pressure bed is entirely readsorbed by the low-pressure bed. That is: DXa2 ±a3 DXg2 ±g3
1
2. The beds are adiabatic during the process. The temperature variation is caused by sorption or desorption. ÿ ÿ Cpc Xa2 ±a3 Cpa Ta3 ÿ Ta2 DHa DXa3 ±a2 2 ÿ ÿ Cpc Xg2 ±g3 Cpa Tg3 ÿ Tg2 DHd DXg3 ÿg2
3
3. The ®nal pressure of the two beds is equal to each other, Pg3 Pa3
4
By supposing a ®nal pressure at ®rst, then calculating the temperature and concentration change of each bed, the supposed pressure can be modi®ed if necessary to ful®ll Eq. (1) until a satisfactory result is derived.
3. System description In our research group, an adsorption air conditioner has been developed in which activated carbon±methanol has been adopted as adsorption pair. It employs two-plate ®nned shell and
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tube heat exchanger as the adsorber and is operated in heat recovery cycle. The design scheme is shown also in Fig. 3. The designed heat capacity ratio between adsorber mass plus thermal ¯uid and adsorbent bed is less than 3, which utilizes water as the heat-transfer ¯uid. Each of the adsorber was embedded with 26 kg activated carbon. Detailed description of the system is presented elsewhere by Wang et al. [12]. Some operation parameters and test results are summarized in Table 1 as test No. 1 and No. 2. The evaporating temperature of the two operation conditions are 68C and 6.18C, in which we get 3.46 and 3.7 kW cooling power, respectively. The respective COPs are 0.37 and 0.34. In order to improve the system performance more and to help develop a new adsorption system, we propose utilizing cycles with mass recovery process and heat and mass recovery combined process. And, furthermore, the inert heat capacity of bed metal and heat-transfer ¯uid is another barrier in promoting the system performance. In the present system the heat capacity ratios of bed metal and thermal ¯uid to activated carbon in this operation are Rm 1:35 and Rf 1:498, respectively. If we employ oil as the heat-transfer ¯uid, we will have Rm 1:35 and Rf 0:5: The system performance could be further improved. For the present case, the system COP will be increased to 0.44 and 0.39 in Table 1 as No. 3 and No. 4, respectively. The in¯uence of the new process still remains unknown. Therefore, predictions and analysis are necessary to reveal the potential of improvement. 4. Performance results We calculated the cycle performances of the dierent cycles mentioned earlier. Activated carbon (ACYK)±methanol is the working pair, and D±A equation is employed to calculate the concentration change of the cycle, which is described as following: X X0 exp ÿ K TS =T ÿ 1n 5 The adsorption properties of ACYK on methanol are shown in Wang et al. [3]. There does exist the in¯uence of heat capacity of the metal bed and the heat-transfer ¯uid (HTF), and researchers are always trying to minimize the heat capacity ratio of the bed and Table 1 Summarized test performances of the adsorption air conditioner with respect to two operation conditionsa,b No.
Th (8C)
T (min)
Td (8C)
Ta (8C)
Tc (8C)
Te (8C)
Qf (kW)
SCP (W/kg)
COP
1 2 3 4
100 110 100 110
40 40 ± ±
97.8 106 ± ±
44.2 45.7 ± ±
26.8 28.7 ± ±
6.1 6.0 ± ±
3.46 3.70 ± ±
151 161 151 161
0.37 0.34 0.44 0.39
a
No. 1 and No. 2 are cases of water as HTF while No. 3 and No. 4 are calculated COP when oil is employed as HTF. b Th is the heat source temperature; Td is the desorption temperature; Tc is the condensing temperature, Te is the evaporation temperature; T is the cycle time, Qf is the averaged refrigeration power, SCP is the adsorption pair cooling power.
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HTF to the activated carbon. That is the extreme case to avoid inert heat capacity. As a result, we ®rst calculated the cycle performance without taking into account the heat capacity of the bed metal and HTF, then we took them into account and compared the results with former ones. Calculated COPs of dierent cycles are shown in Fig. 6. The operating conditions are: adsorption and condensing temperature Ta2 Tc 308C, evaporating temperature Te 58C, the heat recovery and mass recovery processes are realized to the maximum extent. Each curve represents the ratio of the cycle COP to that of the basic one. Therefore, the basic cycle is not shown in the ®gure. It can be easily seen that the cycle with synthetical process of mass recovery and heat recovery has the largest COP, which is at most 1.35 times of the basic one. It is of great potential. The second largest COP occurs with the heat recovery cycle and is 1.25 times the basic cycle. The strangest thing is that the COP of the cycle with mass recovery process is reduced by 1% compared with the basic cycle. The answer can be expressed as given in the following paragraph. Taking the low-pressure bed at a2, for example, it changes to a3 after the mass recovery process. The concentration is enlarged. Then comes the isosteric pressurization process and the bed is heated to g1 '. The result of mass recovery process is to move a2 to a3 and g1 to g1 '. We can see that the concentration change of the basic cycle is enlarged by dx a2 ±a3 and the cooling capacity is also enlarged as a result. The desorption heat is enlarged by qg1 0 ÿg1 at the same time. It also should be noted that the sensible heat of the isosteric process is reduced. Whether the COP will be decreased or increased is determined by the variation of cooling capacity and the required heat input. For this reason, we also calculated the variation of cooling capacity and heat input as curves 1 and 2 shown in Fig. 7. It is found that the enlarged heating load is slightly larger than the cooling capacity. This is in fact because the desorption heat being
Fig. 6. COP ratio of dierent cycles to basic one. 1 Ð Heat and mass recovery cycle, 2 Ð heat recovery cycle, 3 Ð mass recovery cycle.
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slightly larger than the heat of adsorption. In the above example, the desorption heat derived from Clausius±Clapeyron equation is 1.43 MJ/kg when the adsorbent bed is at 908C, meanwhile the heat of desorption at evaporating pressure is 1.24 MJ/kg. It should be noted that the respective cooling capacity is smaller than the latent heat at the same temperature, which is 1.16 MJ/kg. And the sensible heat is only decreased by 32.7 kJ, which is rather small in magnitude. In this case we do not take into consideration the heat capacity of thermal ¯uid and bed metal. If we take them into account the ®gure could be increased and of course will better the performance. From the ®gure, when the maximum desorption temperature is 1108C, the cooling capacity of mass recovery increases about 13.4% and the heating load increases about 14.2%. That is why the COP of the cycle with mass recovery is lower. And what is more, the amount of mass transferred from one bed to another is of importance. Amount of recovered mass is directly a causal factor in increasing the cooling capacity. In fact, the percentage of mass recovery to the total percentage of mass transfer is the same as the percentage of the enlarged cooling capacity of the cycle. Hence, curve 1 in Fig. 7 also represents the amount of the mass recovery as percentage of the total mass transfer during the desorption/adsorption processes. The other two curves in Fig. 7 show the amount of heat recovered in the cycle with combined process of mass recovery and heat recovery and of the heat recovery cycle. The percentage of heat recovered from the combined cycle is 25.4% at 1108C desorption temperature and that of the heat recovery cycle is 18%, so the synthetical cycle has the highest COP. From the above analysis, we can see that mass recovery process can increase the cycle cooling capacity signi®cantly, but not the cycle COP. Heat recovery is necessary to enhance the
Fig. 7. The percentage of various types of thermal energy. 1 Ð Increase in cooling capacity with mass recovery, 2 Ð increase in heat input with mass recovery, 3 Ð percentage heat recovered with heat and mass recovery cycle, 4 Ð percentage heat recovered with heat recovery cycle.
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cycle COP. Indeed, the combined cycle with mass recovery and heat recovery has the highest COP. Fig. 8 shows the ratio of coecients of performance of the cycle with mass recovery cycle at dierent evaporating temperatures to that of the basic cycle. The adsorption temperature and condensing temperature are also Ta Tc 308C: We can see that the COP of the mass recovery cycle changes from a higher value to lower than the basic cycle at certain adsorption and desorption temperatures when the evaporating temperature rises. When the evaporating temperature is low, the refrigerant has a large latent heat, therefore cooling capacity of the mass recovery cycle increases more than the heating load and the COP also increases. But when the evaporating temperature is high, the latent heat decreases and the cooling capacity increases less than the heating load. So the COP will be decreased. We can deduce that the mass recovery process will not absolutely increase the cycle COP. There is a transition evaporating temperature that corresponds to certain adsorption/desorption and condensing temperatures. The pressure balance of the mass recovery process is much faster than the temperature change that occurrs between two the adsorbent beds and the thermal ¯uid, so the cycle period will be decreased to achieve certain quantity of evaporated refrigerant [11]. Considering that the cooling capacity is enlarged to a relatively high extent, we can assert that the cooling/ heating power of the adsorption cycle will be enhanced signi®cantly. It is very useful for waste heat recovery since the cycle COP is not very important in this case. In the previous study, we have researched the in¯uence of heat capacity of the bed metal and heating ¯uid in a real cycle [5]. In order to get clearer information of the mass recovery process, we calculated the COP ratio at 58C evaporating temperature when water is utilized as thermal ¯uid as in our experimental setup [12]. The results are presented in Fig. 9. It is shown that the COP of mass recovery cycle is increased in this situation, taking into account the heat
Fig. 8. Ratio of COP of mass recovery to that of a basic cycle vs. various evaporating temperature.
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Fig. 9. COP ratio of mass recovery to basic cycle when Te 58C. 1 Ð Heat and mass recovery cycle, 2 Ð mass recovery. ±Q± Include heat capacity of the thermal ¯uid and metal adsorber. ÐÐ Without consideration of heat capacity of the thermal ¯uid and metal adsorber.
capacity of metal and thermal ¯uid. It is inferred that mass recovery process does contribute to the enhancement of cycle COP to a small extent though not much. What is more, the synthetical cycle with mass recovery and heat recovery process has a much higher COP than the mass recovery cycle. Some data of COP and cooling capacity per kg activated carbon of dierent cycles are presented in Table 2, which gives us magnitude of the calculation. The footnote `a' represents theoretical calculation without heat capacity of thermal ¯uid and bed metal and the footnote `b' represents it taking into account the heat capacity of ratio of adsorption bed metal and thermal ¯uid to adsorbent which is 2.9. The table expresses the COP variation more clearly. The adsorption and condensing temperatures are: Ta Tc 308C, evaporating temperature Te 58C, desorption temperature Tg2 958C. It can be seen that the heat and mass recovery cycle increases COP by 31.2% and 18.4%, respectively, compared with the basic cycle and heat Table 2 COP of dierent cycles Type
Intermittent cycle
Continuous heat recovery cycle
Mass recovery cycle
Synthetical cycle
COPa Q0a (kJ/kgAC) COPb Q0b (kJ/kgAC)
0.582 129.8 0.431 129.8
0.645 129.8 0.536 129.8
0.580 156.6 0.532 159.3
0.764 156.6 0.593 159.3
a b
Without metal and heating ¯uid heat capacity. With metal and heating ¯uid heat capacity.
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recovery cycle in series (1). In series (2), the ®gures are 37.6% and 10.6%. We can also see that the cooling capacity per kg AC increases 20% and 22%, respectively, with and without consideration of the heat capacity of thermal ¯uid and bed metal. It is worth being adopted in a real system even if there is not much increase in cycle COP. As previously mentioned, once oil is employed as the heat-transfer ¯uid, the total heat capacity ratio of oil and bed metal to AC will be 1.85. We also calculated the relative COP of the two types of cycles. The results are shown in Fig. 10. As we can see, the eect of mass recovery process and the combined process as compared to oil-circled process is not so much as that of the water-circled process. When desorption temperature is 1008C, the enlarged cooling capacities are 19.7% for water and 18.5% for oil. The combined cycle also has better eect as shown in Fig. 10. From the above analysis we can see that the synthetical cycle always has better improvement than other cycles and the performance improvements of new cycles are not so signi®cant for oil-circled process than water-circled process in general, which is notable in further investigation. This is part of the in¯uence of the heat capacity. As we can see, the importance of mass recovery becomes signi®cant when the in¯uence is taken into account. It maybe caused by the exclusion of thermal ¯uid within the recovery process, that is, the negative in¯uence of the heat capacity of the ¯uid is reduced in the cycle, thus system performance is greatly improved. And that is supposed to be the reason why the importance of mass recovery declined when oil is applied as thermal ¯uid instead of water. The negative in¯uence of heat capacity of oil is not as much as that of water, hence, exclusion of thermal ¯uid brings less improvement for oil case than for water case in that more negative in¯uence is eliminated by mass recovery process
Fig. 10. COP ratio of mass recovery to basic cycle using dierent thermal ¯uid when Te 58C. 1 Ð Heat and mass recovery cycle, 2 Ð mass recovery cycle. ±Q± Water, ÐÐ oil.
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when water is adopted. This is shown as the discrepancy of the two curves of curve group 2 in Fig. 10. Furthermore, in the synthetical cycle where heat recovery process is adopted, oil will be better to improve the performance than water in heat recovery (this is already presented in Ref. [5]). As a result, the dierence of the two curves is decreased when both mass recovery and heat recovery are adopted. We can see this from curve group 1 in Fig. 10. Although the improvement in the case of water is slower than in the case of oil, illustrated as relative COP, it should be noted that the oil may have a high COP, as in Table 1, to be the denominator. 5. Conclusions As we can see from the analysis, it is of great potential to improve the design and operation of our adsorption system. Mass recovery can be of great help to improve the performance of the system. Together with heat recovery, it will be more helpful. Listed below are the concluded results of the analysis to improve our system design and operation. 1. The mass recovery process will enhance the cooling capacity per kg AC signi®cantly to about 20%. 2. Whether the mass recovery process will promote the cycle COP or not, depends on the operating conditions. 3. The synthetical cycle with mass recovery and heat recovery has the highest COP among the calculated cycles. It is 30% and 10% higher than the basic cycle and heat recovery cycle, respectively. 4. The mass recovery process will accelerate the adsorption cycle and increase the cycle cooling/heating power. 5. The performance improvements of the new cycles are not so signi®cant for oil-circled process than water-circled process in general due to the in¯uence of heat capacity of construction material and thermal ¯uid.
Acknowledgement This work is partly supported by the State Key Fundamental Research Program under the contract No. G2000026309. References [1] M. Pons, F. Meunier, et al., Thermodynamic based comparison of sorption systems for cooling and heat pumping, Int. J. Refrig. 22 (1) (1999) 5±17. [2] R.Z. Wang, J.Y. Wu, Y.X. Xu, Y. Teng, W. Shi, Experiment on a continuous heat regenerative adsorption refrigerator using spiral plate heat exchanger as adsorbers, Applied Thermal Engineering 18 (1/2) (1998) 13±23. [3] R.Z. Wang, J.P. Jia, Y.H. Zhu, et al., Study on a new solid adsorption refrigeration pair: active carbon± methanol, ASME Journal of Solar Energy Engineering 119 (1997) 214±218.
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[4] G.H.W. van Benthem, G. Cacciola, G. Restaccia, Regenerative adsorption heat pumps: optimization of the design, Heat Recovery Systems & CHP 15 (6) (1995) 531±544. [5] Y. Teng, R.Z. Wang, J.Y. Wu, Study of the fundamentals of adsorption systems, Applied Thermal Engineering 17 (4) (1997) 327±338. [6] R.E. Critoph, Performance estimation of convective thermal wave adsorption cycles, Applied Thermal Engineering 16 (5) (1996) 429±437. [7] M. Pons, Global analysis of adsorption cycles with thermal regeneration, in: Proc. Ab-Sorption'96, Int. Absorption Heat Pump Conference, Montreal, Sept., 1996, pp. 153±160. [8] M. Pons, F. Poyelle, Adsorptive machines with advanced cycles for heat pumping or cooling applications, Int. J. Refrig. 22 (1) (1999) 27±37. [9] F. Poyelle, J.J. Guilleminot, F. Meunier, I. Soide, Experimental test of a gas ®red adsorptive air conditioning system, in: Proc. Ab-Sorption'96, Int. Absorption Heat Pump Conference, Montreal, Sept., 1996, pp. 221±229. [10] M. Pons, Analysis of the adsorption cycles with thermal regeneration based on the entropic mean temperatures, Applied Thermal Engineering 17 (7) (1997) 615±627. [11] S. Szarzynski, Y. Feng, M. Pons, Study of dierent internal vapor transports for adsorption cycles with heat regeneration, Int. J. Refrig. 20 (6) (1997) 390±401. [12] R.Z. Wang, J.Y. Wu, W. Wang, Performance researches and improvements on heat regenerative adsorption regeneration and heat pump, in: Proceedings of the Int. Sorption Heat Pump Conference, March, Munich, Germany, 1999, pp. 631±638.
www.elsevier.com/locate/apthermeng
Study on heat and mass recovery in adsorption refrigeration cycles T.F. Qu, R.Z. Wang*, W. Wang School of Power & Energy Engineering, Institute of Refrigeration & Cryogenics, Shanghai Jiao Tong University, Shanghai, 200030, PR China Received 23 August 1999; accepted 14 March 2000
Abstract An adsorption air conditioner has been developed and some operation results are summarized. Mass recovery process is proposed to improve the performance. Performance predictions are presented and show that mass recovery can play an important role to better the performance of adsorption refrigeration cycle. Coecient of performance might be increased or decreased with mass recovery process due to dierent working conditions. Cooling capacity can be signi®cantly increased with mass recovery process. The cycle with mass and heat recovery has a relatively higher improvement. It can also be seen that the cycle time will be much shorter and it will certainly enhance the cycle with higher cooling/heating power. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Mass recovery; Heat recovery; Adsorption; Refrigeration cycles
1. Introduction Solid sorption refrigeration is environmental friendly as it employs safe refrigerants like water, ammonia, methanol, etc. The cycle can be driven by low-grade temperature thermal energy and usually without moving parts. As a result, it has special advantages in energy saving and environment protection. A great deal of research work has been done over many years, and it is developing more and more rapidly nowadays. * Corresponding author. Tel.: +86-21-6281-3250; fax: +86-21-6293-3250. E-mail address: [email protected] (R.Z. Wang). 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 4 3 1 1 ( 0 0 ) 0 0 0 5 0 - 8
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Nomenclature X concentration (kg/kg) speci®c heat (J/kg K) Cp P pressure (Pa) T temperature (K) D variation of variable enthalpy of adsorption (J/kg) Ha enthalpy of desorption (J/kg) Hd a2, a3, g2, g3 points on the thermodynamic cycle heat capacity ratio of bed metal to activated carbon Rm heat capacity ratio of heat-transfer ¯uid to activated carbon Rf As we know, however, the main drawbacks of adsorption cycles are its low coecient of performance (COP), small speci®c cooling power (SCP) and long cycle time. These are mainly caused by the poor heat and mass transfer rate in the adsorption bed. Therefore, research work is being carried out at present, which mainly focuses on how to enhance the cycle performance, for which there are several means such as [1±5]: 1. 2. 3. 4.
®nd new better working pairs and improve the adsorption performance, propose new types of adsorption cycle, intensify the heat and mass transfer in the adsorbent bed, decrease the cycle time.
In proposing new types of cycles, much has been discussed besides the basic cycle, such as two-beds continuous cycle, continuous heat recovery cycle, thermal wave cycle, multieect cycle, etc. [1,6±8]. Among these types of cycles, the basic cycle is suitable for recovering heat from intermittent heat sources, like solar energy; two-beds continuous cycle and continuous heat recovery cycle may be used to recover low-grade thermal energy and will provide continuous cooling eect, and the heat recovery cycle has a higher thermal eciency; thermal wave cycle has the highest theoretical coecient of performance, but is dicult to be realized in a prototype system; multieect cycle can reach a high thermal eciency but the system will be a complicated one and will prevent it from wide use. We propose the adsorption cycle with mass recovery and also the systhetical cycle with mass recovery and heat recovery. We are assured that the performance will be signi®cantly improved with these processes. Our research results about the eect of mass recovery and heat recovery on the adsorption cycle are presented in this paper. 2. Adsorption refrigeration cycles 2.1. Basic cycle The basic adsorption cycle for refrigeration/heat pumping consists of four processes (see
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441
Fig. 1. Diagram of the intermittent and heat regeneration cycle.
Fig. 1). In the ®rst process a2±g1, the adsorbent bed receives heat from high-temperature heating ¯uid and reaches a high-pressure equal to the condensing pressure. The hightemperature thermal ¯uid is the high-temperature heat source. When the bed is being heated, it desorbs the refrigerant, which is condensed in a condenser. The refrigerant releases heat to a high-temperature heat sink and the process g1±g2 is an isobaric process. After that the bed gives heat to a second heat sink and is cooled down and depressurized to a1 by cooling systems. It readsorbs refrigerant vapor in the process a1±a2, which causes liquid evaporation in the evaporator; there the refrigerant extracts heat from low temperature heat source. The schematic system is shown in Fig. 2. It is often called an intermittent cycle as we can see that the evaporating process is going on only half cycle. It can be applied for recovering solar
Fig. 2. Schematic system for basic cycle. The dashed line represents thermal ¯uid ¯ow.
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energy application, etc. When there are two adsorbent beds, we can easily operate them out of phase. The two-beds continuous cycle employs two adsorbent beds to provide continuous cooling power. These are in fact two parallel basic cycles that operate out of phase. Therefore, it is not much dierent from the basic one from this point of view. All the thermal energy received by the adsorbent bed is supplied by the heating system. Heat must be recovered internally to improve the cycle performance. 2.2. Heat recovery cycle There are also two beds in this type of cycle, which are operated out of phase. At the end of each half cycle, one adsorbent bed is cold at point a2 and the other is at g2 with hightemperature. Heat can be easily recovered between the two adsorbent beds because of this temperature dierence. By circulating the thermal ¯uid between the two adsorbent beds adiabatically, the energy eciency can be increased signi®cantly [9,10]. In an ideal heat recovery process, the two beds will reach the same temperature, one is at point e and the other is e ' (see Fig. 1). The system is shown schematically in Fig. 3. Therefore the thermal load for heating can be decreased by Qa2ÿg1ÿe. This is the recovered thermal energy, which can reach about 30% or so of the total necessary heat input of the basic cycle. 2.3. Cycle with mass recovery In a two-bed cycle, the two beds are working out of phase. At the end of each half cycle, one bed is hot with high-pressure Pc and the other is cold with low-pressure Pe (a2 and g2 in Fig. 4, respectively). After that the high-pressure adsorber needs to be cooled down and
Fig. 3. Schematic system of heat recovery cycle. The dashed line represents thermal ¯uid ¯ow.
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Fig. 4. Diagram of mass recovery process and combined cycle with heat and mass recovery.
depressurized while the low-pressure one needs to be heated up and pressurized. Then the two adsorbent beds may be interconnected directly with a simple device and the refrigerant vapor will ¯ow from the high-pressure bed to the low-pressure one. This is called the mass recovery process. It is presented in Fig. 5. The pressure of Adsorber 1 decreases due to mass out¯ow and this will again cause desorption of Adsorber 1. Meanwhile, the pressure of Adsorber 2
Fig. 5. Schematic system of mass recovery/heat and mass recovery cycle. The dashed line represents thermal ¯uid ¯ow; the solid line and valve between the beds processes mass recovery and the dashed line between beds processes heat recovery.
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increases due to mass in¯ow and will cause further adsorption. The process is maintained until the two beds reach the same pressure, Pm (see Fig. 4). Then the connection is broken and each bed goes on with heating and cooling process just as in the basic cycle. This mass recovery process is expected to accelerate the circulation and enhance the cycle cooling power, for it only involves direct mass ¯ow and the pressure balance is much faster than temperature balance via heat-transfer medium. 2.4. Synthetical cycle with heat and mass recovery We can also use a heat regeneration process after the mass recovery process because there is also a large temperature dierence between the two beds. The thermal load can be further decreased so as to improve the cycle performance. In Fig. 4, this process begins with a3 and g3 and ends at e±e ' that have the common temperature. The system scheme is also shown in Fig. 5. The process of mass recovery enlarges dx, the concentration change of adsorbate in a cycle as we can see from the P±T±x diagram in Fig. 4. In this discussion the mass recovery process is assumed as adiabatic. The process is divided into two parts operated in two beds as a2±a3 and g2±g3. The process can be simulated and calculated utilizing the following model [8,11]. There are several assumptions, described later, to facilitate the calculation of the model. 1. The vapors desorbed from the high-pressure bed is entirely readsorbed by the low-pressure bed. That is: DXa2 ±a3 DXg2 ±g3
1
2. The beds are adiabatic during the process. The temperature variation is caused by sorption or desorption. ÿ ÿ Cpc Xa2 ±a3 Cpa Ta3 ÿ Ta2 DHa DXa3 ±a2 2 ÿ ÿ Cpc Xg2 ±g3 Cpa Tg3 ÿ Tg2 DHd DXg3 ÿg2
3
3. The ®nal pressure of the two beds is equal to each other, Pg3 Pa3
4
By supposing a ®nal pressure at ®rst, then calculating the temperature and concentration change of each bed, the supposed pressure can be modi®ed if necessary to ful®ll Eq. (1) until a satisfactory result is derived.
3. System description In our research group, an adsorption air conditioner has been developed in which activated carbon±methanol has been adopted as adsorption pair. It employs two-plate ®nned shell and
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tube heat exchanger as the adsorber and is operated in heat recovery cycle. The design scheme is shown also in Fig. 3. The designed heat capacity ratio between adsorber mass plus thermal ¯uid and adsorbent bed is less than 3, which utilizes water as the heat-transfer ¯uid. Each of the adsorber was embedded with 26 kg activated carbon. Detailed description of the system is presented elsewhere by Wang et al. [12]. Some operation parameters and test results are summarized in Table 1 as test No. 1 and No. 2. The evaporating temperature of the two operation conditions are 68C and 6.18C, in which we get 3.46 and 3.7 kW cooling power, respectively. The respective COPs are 0.37 and 0.34. In order to improve the system performance more and to help develop a new adsorption system, we propose utilizing cycles with mass recovery process and heat and mass recovery combined process. And, furthermore, the inert heat capacity of bed metal and heat-transfer ¯uid is another barrier in promoting the system performance. In the present system the heat capacity ratios of bed metal and thermal ¯uid to activated carbon in this operation are Rm 1:35 and Rf 1:498, respectively. If we employ oil as the heat-transfer ¯uid, we will have Rm 1:35 and Rf 0:5: The system performance could be further improved. For the present case, the system COP will be increased to 0.44 and 0.39 in Table 1 as No. 3 and No. 4, respectively. The in¯uence of the new process still remains unknown. Therefore, predictions and analysis are necessary to reveal the potential of improvement. 4. Performance results We calculated the cycle performances of the dierent cycles mentioned earlier. Activated carbon (ACYK)±methanol is the working pair, and D±A equation is employed to calculate the concentration change of the cycle, which is described as following: X X0 exp ÿ K TS =T ÿ 1n 5 The adsorption properties of ACYK on methanol are shown in Wang et al. [3]. There does exist the in¯uence of heat capacity of the metal bed and the heat-transfer ¯uid (HTF), and researchers are always trying to minimize the heat capacity ratio of the bed and Table 1 Summarized test performances of the adsorption air conditioner with respect to two operation conditionsa,b No.
Th (8C)
T (min)
Td (8C)
Ta (8C)
Tc (8C)
Te (8C)
Qf (kW)
SCP (W/kg)
COP
1 2 3 4
100 110 100 110
40 40 ± ±
97.8 106 ± ±
44.2 45.7 ± ±
26.8 28.7 ± ±
6.1 6.0 ± ±
3.46 3.70 ± ±
151 161 151 161
0.37 0.34 0.44 0.39
a
No. 1 and No. 2 are cases of water as HTF while No. 3 and No. 4 are calculated COP when oil is employed as HTF. b Th is the heat source temperature; Td is the desorption temperature; Tc is the condensing temperature, Te is the evaporation temperature; T is the cycle time, Qf is the averaged refrigeration power, SCP is the adsorption pair cooling power.
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HTF to the activated carbon. That is the extreme case to avoid inert heat capacity. As a result, we ®rst calculated the cycle performance without taking into account the heat capacity of the bed metal and HTF, then we took them into account and compared the results with former ones. Calculated COPs of dierent cycles are shown in Fig. 6. The operating conditions are: adsorption and condensing temperature Ta2 Tc 308C, evaporating temperature Te 58C, the heat recovery and mass recovery processes are realized to the maximum extent. Each curve represents the ratio of the cycle COP to that of the basic one. Therefore, the basic cycle is not shown in the ®gure. It can be easily seen that the cycle with synthetical process of mass recovery and heat recovery has the largest COP, which is at most 1.35 times of the basic one. It is of great potential. The second largest COP occurs with the heat recovery cycle and is 1.25 times the basic cycle. The strangest thing is that the COP of the cycle with mass recovery process is reduced by 1% compared with the basic cycle. The answer can be expressed as given in the following paragraph. Taking the low-pressure bed at a2, for example, it changes to a3 after the mass recovery process. The concentration is enlarged. Then comes the isosteric pressurization process and the bed is heated to g1 '. The result of mass recovery process is to move a2 to a3 and g1 to g1 '. We can see that the concentration change of the basic cycle is enlarged by dx a2 ±a3 and the cooling capacity is also enlarged as a result. The desorption heat is enlarged by qg1 0 ÿg1 at the same time. It also should be noted that the sensible heat of the isosteric process is reduced. Whether the COP will be decreased or increased is determined by the variation of cooling capacity and the required heat input. For this reason, we also calculated the variation of cooling capacity and heat input as curves 1 and 2 shown in Fig. 7. It is found that the enlarged heating load is slightly larger than the cooling capacity. This is in fact because the desorption heat being
Fig. 6. COP ratio of dierent cycles to basic one. 1 Ð Heat and mass recovery cycle, 2 Ð heat recovery cycle, 3 Ð mass recovery cycle.
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slightly larger than the heat of adsorption. In the above example, the desorption heat derived from Clausius±Clapeyron equation is 1.43 MJ/kg when the adsorbent bed is at 908C, meanwhile the heat of desorption at evaporating pressure is 1.24 MJ/kg. It should be noted that the respective cooling capacity is smaller than the latent heat at the same temperature, which is 1.16 MJ/kg. And the sensible heat is only decreased by 32.7 kJ, which is rather small in magnitude. In this case we do not take into consideration the heat capacity of thermal ¯uid and bed metal. If we take them into account the ®gure could be increased and of course will better the performance. From the ®gure, when the maximum desorption temperature is 1108C, the cooling capacity of mass recovery increases about 13.4% and the heating load increases about 14.2%. That is why the COP of the cycle with mass recovery is lower. And what is more, the amount of mass transferred from one bed to another is of importance. Amount of recovered mass is directly a causal factor in increasing the cooling capacity. In fact, the percentage of mass recovery to the total percentage of mass transfer is the same as the percentage of the enlarged cooling capacity of the cycle. Hence, curve 1 in Fig. 7 also represents the amount of the mass recovery as percentage of the total mass transfer during the desorption/adsorption processes. The other two curves in Fig. 7 show the amount of heat recovered in the cycle with combined process of mass recovery and heat recovery and of the heat recovery cycle. The percentage of heat recovered from the combined cycle is 25.4% at 1108C desorption temperature and that of the heat recovery cycle is 18%, so the synthetical cycle has the highest COP. From the above analysis, we can see that mass recovery process can increase the cycle cooling capacity signi®cantly, but not the cycle COP. Heat recovery is necessary to enhance the
Fig. 7. The percentage of various types of thermal energy. 1 Ð Increase in cooling capacity with mass recovery, 2 Ð increase in heat input with mass recovery, 3 Ð percentage heat recovered with heat and mass recovery cycle, 4 Ð percentage heat recovered with heat recovery cycle.
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cycle COP. Indeed, the combined cycle with mass recovery and heat recovery has the highest COP. Fig. 8 shows the ratio of coecients of performance of the cycle with mass recovery cycle at dierent evaporating temperatures to that of the basic cycle. The adsorption temperature and condensing temperature are also Ta Tc 308C: We can see that the COP of the mass recovery cycle changes from a higher value to lower than the basic cycle at certain adsorption and desorption temperatures when the evaporating temperature rises. When the evaporating temperature is low, the refrigerant has a large latent heat, therefore cooling capacity of the mass recovery cycle increases more than the heating load and the COP also increases. But when the evaporating temperature is high, the latent heat decreases and the cooling capacity increases less than the heating load. So the COP will be decreased. We can deduce that the mass recovery process will not absolutely increase the cycle COP. There is a transition evaporating temperature that corresponds to certain adsorption/desorption and condensing temperatures. The pressure balance of the mass recovery process is much faster than the temperature change that occurrs between two the adsorbent beds and the thermal ¯uid, so the cycle period will be decreased to achieve certain quantity of evaporated refrigerant [11]. Considering that the cooling capacity is enlarged to a relatively high extent, we can assert that the cooling/ heating power of the adsorption cycle will be enhanced signi®cantly. It is very useful for waste heat recovery since the cycle COP is not very important in this case. In the previous study, we have researched the in¯uence of heat capacity of the bed metal and heating ¯uid in a real cycle [5]. In order to get clearer information of the mass recovery process, we calculated the COP ratio at 58C evaporating temperature when water is utilized as thermal ¯uid as in our experimental setup [12]. The results are presented in Fig. 9. It is shown that the COP of mass recovery cycle is increased in this situation, taking into account the heat
Fig. 8. Ratio of COP of mass recovery to that of a basic cycle vs. various evaporating temperature.
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Fig. 9. COP ratio of mass recovery to basic cycle when Te 58C. 1 Ð Heat and mass recovery cycle, 2 Ð mass recovery. ±Q± Include heat capacity of the thermal ¯uid and metal adsorber. ÐÐ Without consideration of heat capacity of the thermal ¯uid and metal adsorber.
capacity of metal and thermal ¯uid. It is inferred that mass recovery process does contribute to the enhancement of cycle COP to a small extent though not much. What is more, the synthetical cycle with mass recovery and heat recovery process has a much higher COP than the mass recovery cycle. Some data of COP and cooling capacity per kg activated carbon of dierent cycles are presented in Table 2, which gives us magnitude of the calculation. The footnote `a' represents theoretical calculation without heat capacity of thermal ¯uid and bed metal and the footnote `b' represents it taking into account the heat capacity of ratio of adsorption bed metal and thermal ¯uid to adsorbent which is 2.9. The table expresses the COP variation more clearly. The adsorption and condensing temperatures are: Ta Tc 308C, evaporating temperature Te 58C, desorption temperature Tg2 958C. It can be seen that the heat and mass recovery cycle increases COP by 31.2% and 18.4%, respectively, compared with the basic cycle and heat Table 2 COP of dierent cycles Type
Intermittent cycle
Continuous heat recovery cycle
Mass recovery cycle
Synthetical cycle
COPa Q0a (kJ/kgAC) COPb Q0b (kJ/kgAC)
0.582 129.8 0.431 129.8
0.645 129.8 0.536 129.8
0.580 156.6 0.532 159.3
0.764 156.6 0.593 159.3
a b
Without metal and heating ¯uid heat capacity. With metal and heating ¯uid heat capacity.
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recovery cycle in series (1). In series (2), the ®gures are 37.6% and 10.6%. We can also see that the cooling capacity per kg AC increases 20% and 22%, respectively, with and without consideration of the heat capacity of thermal ¯uid and bed metal. It is worth being adopted in a real system even if there is not much increase in cycle COP. As previously mentioned, once oil is employed as the heat-transfer ¯uid, the total heat capacity ratio of oil and bed metal to AC will be 1.85. We also calculated the relative COP of the two types of cycles. The results are shown in Fig. 10. As we can see, the eect of mass recovery process and the combined process as compared to oil-circled process is not so much as that of the water-circled process. When desorption temperature is 1008C, the enlarged cooling capacities are 19.7% for water and 18.5% for oil. The combined cycle also has better eect as shown in Fig. 10. From the above analysis we can see that the synthetical cycle always has better improvement than other cycles and the performance improvements of new cycles are not so signi®cant for oil-circled process than water-circled process in general, which is notable in further investigation. This is part of the in¯uence of the heat capacity. As we can see, the importance of mass recovery becomes signi®cant when the in¯uence is taken into account. It maybe caused by the exclusion of thermal ¯uid within the recovery process, that is, the negative in¯uence of the heat capacity of the ¯uid is reduced in the cycle, thus system performance is greatly improved. And that is supposed to be the reason why the importance of mass recovery declined when oil is applied as thermal ¯uid instead of water. The negative in¯uence of heat capacity of oil is not as much as that of water, hence, exclusion of thermal ¯uid brings less improvement for oil case than for water case in that more negative in¯uence is eliminated by mass recovery process
Fig. 10. COP ratio of mass recovery to basic cycle using dierent thermal ¯uid when Te 58C. 1 Ð Heat and mass recovery cycle, 2 Ð mass recovery cycle. ±Q± Water, ÐÐ oil.
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when water is adopted. This is shown as the discrepancy of the two curves of curve group 2 in Fig. 10. Furthermore, in the synthetical cycle where heat recovery process is adopted, oil will be better to improve the performance than water in heat recovery (this is already presented in Ref. [5]). As a result, the dierence of the two curves is decreased when both mass recovery and heat recovery are adopted. We can see this from curve group 1 in Fig. 10. Although the improvement in the case of water is slower than in the case of oil, illustrated as relative COP, it should be noted that the oil may have a high COP, as in Table 1, to be the denominator. 5. Conclusions As we can see from the analysis, it is of great potential to improve the design and operation of our adsorption system. Mass recovery can be of great help to improve the performance of the system. Together with heat recovery, it will be more helpful. Listed below are the concluded results of the analysis to improve our system design and operation. 1. The mass recovery process will enhance the cooling capacity per kg AC signi®cantly to about 20%. 2. Whether the mass recovery process will promote the cycle COP or not, depends on the operating conditions. 3. The synthetical cycle with mass recovery and heat recovery has the highest COP among the calculated cycles. It is 30% and 10% higher than the basic cycle and heat recovery cycle, respectively. 4. The mass recovery process will accelerate the adsorption cycle and increase the cycle cooling/heating power. 5. The performance improvements of the new cycles are not so signi®cant for oil-circled process than water-circled process in general due to the in¯uence of heat capacity of construction material and thermal ¯uid.
Acknowledgement This work is partly supported by the State Key Fundamental Research Program under the contract No. G2000026309. References [1] M. Pons, F. Meunier, et al., Thermodynamic based comparison of sorption systems for cooling and heat pumping, Int. J. Refrig. 22 (1) (1999) 5±17. [2] R.Z. Wang, J.Y. Wu, Y.X. Xu, Y. Teng, W. Shi, Experiment on a continuous heat regenerative adsorption refrigerator using spiral plate heat exchanger as adsorbers, Applied Thermal Engineering 18 (1/2) (1998) 13±23. [3] R.Z. Wang, J.P. Jia, Y.H. Zhu, et al., Study on a new solid adsorption refrigeration pair: active carbon± methanol, ASME Journal of Solar Energy Engineering 119 (1997) 214±218.
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[4] G.H.W. van Benthem, G. Cacciola, G. Restaccia, Regenerative adsorption heat pumps: optimization of the design, Heat Recovery Systems & CHP 15 (6) (1995) 531±544. [5] Y. Teng, R.Z. Wang, J.Y. Wu, Study of the fundamentals of adsorption systems, Applied Thermal Engineering 17 (4) (1997) 327±338. [6] R.E. Critoph, Performance estimation of convective thermal wave adsorption cycles, Applied Thermal Engineering 16 (5) (1996) 429±437. [7] M. Pons, Global analysis of adsorption cycles with thermal regeneration, in: Proc. Ab-Sorption'96, Int. Absorption Heat Pump Conference, Montreal, Sept., 1996, pp. 153±160. [8] M. Pons, F. Poyelle, Adsorptive machines with advanced cycles for heat pumping or cooling applications, Int. J. Refrig. 22 (1) (1999) 27±37. [9] F. Poyelle, J.J. Guilleminot, F. Meunier, I. Soide, Experimental test of a gas ®red adsorptive air conditioning system, in: Proc. Ab-Sorption'96, Int. Absorption Heat Pump Conference, Montreal, Sept., 1996, pp. 221±229. [10] M. Pons, Analysis of the adsorption cycles with thermal regeneration based on the entropic mean temperatures, Applied Thermal Engineering 17 (7) (1997) 615±627. [11] S. Szarzynski, Y. Feng, M. Pons, Study of dierent internal vapor transports for adsorption cycles with heat regeneration, Int. J. Refrig. 20 (6) (1997) 390±401. [12] R.Z. Wang, J.Y. Wu, W. Wang, Performance researches and improvements on heat regenerative adsorption regeneration and heat pump, in: Proceedings of the Int. Sorption Heat Pump Conference, March, Munich, Germany, 1999, pp. 631±638.