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Experimental study on the performance of double-effect and double-way thermochemical sorption refrigeration cycle
Applied Thermal Engineering 31 (2011) 3658e3663
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on the performance of double-effect and double-way thermochemical sorption refrigeration cycle L. Xu, R.Z. Wang*, T.X. Li, L.W. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 October 2010 Accepted 5 January 2011 Available online 25 January 2011
The feasibility and working performance of a double-effect and double-way thermochemical sorption refrigeration cycle was experimentally investigated. In the test unit, three different metal chlorides were used as high temperature, middle temperature and low temperature salts, respectively. Both the evaporation heat of the refrigerant during adsorption refrigeration process and the decomposition reaction heat of the low temperature salt during resorption refrigeration process were combined to provide useful cold. Experimental results showed that the proposed double-effect and double-way thermochemical sorption refrigeration cycle was feasible, and it can produce four useful cooling-effects during one cycle by using only one heat input at high temperature. The experimental COP was over 1.0 at the chilled temperature of 10e15 C, decomposition temperature of 260 C, and heat sink temperature of 30 C. In addition, the cycle characteristics and the effect of working parameters on the double-effect and doubleway thermochemical sorption refrigeration system were analyzed and discussed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Thermochemical Sorption Refrigeration Double-way Double-effect
1. Introduction At present time, green cooling technology attracts more and more eyes of scientists and researchers all over the world. Due to its cleanness and zero of ozone depletion potential and global warming effect, this technology has and will come into more and more areas of our lives [1,2]. Absorption refrigeration technology and adsorption refrigeration technology are two important streams of green cooling technology. Liquidegas absorption refrigeration technologies have been widely applied [2], while solidegas adsorption refrigeration technologies are still in the R&D stage. However adsorption technologies have the distinct advantages of the wider range of temperature, less requirement on the crystallization and corrosion. Moreover, physical adsorption chiller using silica-gel water working pair can be bought for commercial purpose nowadays. While the thermochemical sorption refrigeration systems have the advantages of larger evaporation heat and bigger reaction coefficient, the physical adsorption chiller still suffers from the big structure dimension and low coefficient of performance of system. Although the low system COP is also the bottleneck of application of thermochemical sorption refrigeration technologies, three directions exist for the possibility of improving it: 1) strengthened heat and mass transfer; 2) proper sorption working
* Corresponding author. E-mail addresses: [email protected], [email protected] (R.Z. Wang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.01.014
pair; 3) more advanced sorption thermodynamic cycle [3]. As for the first point, the utilization of compound adsorbents with porous materials and metal chloride salts can be said to be effective in improvement of COP. The porous materials denote the expanded graphite, carbon fibers, or activated carbon, which provide higher thermal conductivity and larger porosity to enhance the heat and mass transfer. Besides, the application of heat pipes can also increase the efficiency of machine during the internal heat transfer process. Because the choices of sorption working pair are linked with the application of sorption thermodynamic cycle, the selected sorption working pairs must match with the cycle very well. It is true that many advanced cycles have been developed and investigated to improve the COP of systems, like cascading cycle [4], thermal wave cycle [5], forced convection cycle [6], mass recovery cycle [7], heat and mass recovery cycle [8], and multistage cycle [9]. In order to further improve the system COP, Spinner gives out a kind of double-effect thermochemical resorption system using three different salts, by which an internal heat recovery process was used to provide the decomposition heat for a middle temperature salt by the synthesis reaction heat of a high temperature salt [10]. The double-effect and double-way thermochemical refrigeration cycle investigated in this paper is firstly proposed by Li et al., who combine the adsorption, resorption and internal heat recovery together to achieve four cold output at the expense of only one heat input during a complete cycle. The ideal coefficient of performance (COP) can be improved by more than 59e169% if compared to the COP obtained with other kinds of cycle [11]. But whether the
L. Xu et al. / Applied Thermal Engineering 31 (2011) 3658e3663
experimental test complies with the theoretical conclusion needs to be checked by the experimental result.
the high system COP value can be achieved. The expected system COP can reach values between 0.91 and 1.8 [11]. 4. Experimental test unit
2. Choice of sorption working pairs
4.1. Experimental sorption refrigeration prototype
To achieve adsorption refrigeration and resorption refrigeration, the sorption working pairs suitable for the proposed system were selected, in which metal chlorideeammonia are preferred as the potential working pairs according to the equilibrium lines of chlorideeammonia in the ClausiuseClapeyron diagram (Fig. 1). A group of working pairs of NH3, BaCl2, MnCl2 and NiCl2 were chosen and used to assess the performance of the double-effect and double-way sorption system. NiCl2 was used as the high temperature salt (HTS). MnCl2 was used as the middle temperature salt (MTS), and BaCl2 was utilized as the low temperature salt (LTS). In addition, expanded graphite was used as an inert porous additive to enhance the heat and mass transfer of the reactive salts due to its high thermal conductivity and gas permeability [12]. The composite sorbents made from salt and expanded graphite had a graphite mass fraction equal to 0.2.
The photograph of the experimental prototype for the doubleeffect and double-way cycle is shown in Fig. 4. As shown in Fig. 3, it mainly consists of an HTS reactor, an MTS reactor, two LTS reactors (LTS1 and LTS2), a condenser, an evaporator. There are four loops in the unit: ammonia loop, oil loop, condensing water loop and chilled water loop. Corresponding to the description in Fig. 2, the experimental procedure is as the follows: [1] During phase (a), the regeneration process of HTS and the resorption process between LTS1 and MTS and adsorption process between evaporator and LTS2 happen, which require valve A2, A4, A6 are open, valve A1, A3, A5 are close in ammonia loop; and valve O1, O3 are open, valve O2, O4 are closed, in oil loop and valve W1, W2, W5, W6 are open, W3, W4, W7, W8 are closed in condensing and cooling loop. [2] During phase (b), the resorption process between LTS2 and HTS, the adsorption process between the evaporator and LTS1 and the heat recovery process between the HTS and MTS require that the status of valves for those loops take the negative operation, i.e., in ammonia loop, valve A2, A4, A6 are close, valve A1, A3, A5 are open; in oil loop, valve O1, O3 are closed and valve O2, O4 are open; in condensing and chilled loop, valve W1, W2, W5, W6 are closed, W3, W4, W7, W8 are open.
3. Double-effect and double-way sorption refrigeration cycle Fig. 2 shows the schematic diagram of the double-effect and double-way thermochemical sorption refrigeration cycle. The working process mainly consists of two phases: phase (a) and phase (b). During phase (a), nickel chloride-high temperature salt (HTS) is heated by external heat resource to desorb the ammonia to the condenser, at the same time, another adsorption and resorption processes occur, the former of which happens between the barium chloride-low temperature salt (LTS2) and evaporator, and the latter of which happens between the barium chloride-low temperature salt (LTS1) and manganese chloride-middle temperature salt (MTS). During phase (b), the LTS2 connects with the HTS, whose synthesis heat output is used to drive the decomposition of MTS to desorb ammonia to condenser. Simultaneously, the adsorption process takes place between the evaporator and LTS1, the resorption process occur between the LTS2 and HTS. Thus, at the expense of only one heat input, four cold output is available, based on which
L n [P (P a )]
3659
14.50 14.25 14.00 13.75 13.50 13.25 13.00 12.75 12.50 12.25 12.00 11.75 11.50 11.25 11.00 10.75 10.50 10.25 10.00 9.75 9.50
As shown in Fig. 5, all the components of test unit system are listed. The electric oil boiler is connected with the oil loop of test unit to provide the external heat in terms of oil of over 250 C for regeneration process of HTS to desorb ammonia to condenser. The condensing cooling tower provides the heat sink with temperature less than 35 C. The thermostat bath is connected with chilled loop, in which an electric heater is installed. With the help of the power measurement equipment for electric heater installed in thermostat
1
2 3 4
5 67 8
9
10 11 12 13 14
NH 3 L/G
-4.4
-4.2
-4.0
-3.8
-3.6
-3.4
-3.2 -3.0 -1000/T(K)
-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
1=PbCl2 (8/3.25); 2=BaCl2 (8/0); 3=LiCl (4/3); 4=AgCl (3/1.5); 5=CaCl2 (8/4); 6=SrCl2 (8/1); 7=CaCl2 (4/2); 8=CdCl2 (6/2); 9=ZnCl2 (6/2); 10=MnCl2 (6/2); 11=FeCl2 (6/2); 12=CoCl2 (6/2); 13=MgCl2 (6/2); 14=NiCl2 (6/2). Fig. 1. Equilibrium lines of chlorideeammonia in the ClausiuseClapeyron diagram.
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Fig. 2. Schematic diagram of the double-effect and double-way thermochemical sorption refrigeration cycle with internal heat recovery process based on adsorption and resorption processes.
Fig. 3. Working principle of the experimental test unit.
L. Xu et al. / Applied Thermal Engineering 31 (2011) 3658e3663
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Table 1 Measurement equipments and sensors. No.
Parts
Specification
Quantity
Accuracy
1 2 3 4 5
Oil mass flow meter Water mass flow meter Temperature sensor Pressure sensor Power consumption measurer
DN25, PN4.0 DN20 PT100 0e2.6 MPa 8904F
2 4 29 4 1
0.25% 0.1% 0.15 C 0.2% 0.4%
Qar, Qb, Qbr can be acquired by the same method as for the Qa. System COP is calculated as the follows:
COPd:w:d:e: ¼ ðQa þ Qar þ Qb þ Qbr Þ=QdesM Where COPd.w.d.e denotes the system coefficient of performance of double-effect and double-way refrigeration cycle. Fig. 4. Photograph of the experimental prototype.
5. Results and discussion bath, the cooling capacity is measured in terms of power consumption. The details of the measurement equipments and sensors are listed in Table 1. The double-effect and double-way refrigeration cycle consists of two phases: phase (a) and phase (b). During phase (a), the desorption heat Qdes-M, the adsorption refrigeration cooling capacity Qa and the resorption refrigeration cooling capacity Qar need to be calculated from experimental data. During phase (b), the adsorption refrigeration cooling capacity Qb and the resorption refrigeration cooling capacity Qbr need to be calculated from experimental data either. Qdes-M can be available by multiplying the flow rate of oil and specific heat of oil and the average temperature of oil and the time consumed. Qa is the output by multiplying the flow rate of water and specific heat of water and the average temperature of water and the time consumed. The value of Qa can be checked by the power consumption of the electric heater in the constant bath.
5.1. Evolution of cooling power under different chilled temperature At different chilled temperature, evolution of cooling power during phase (a) includes two stages: adsorption stage and resorption stage, as illustrated in Fig. 6. So, each cooling power curve for different refrigeration temperature has two peaks in one phase. One peak happens in evaporator, which corresponds to adsorption stage, another happens in LTS2, corresponding to resorption stage. As shown by Fig. 6, the higher the chilled temperature is, the longer the reaction time is and the deeper the cooling power peaks are, which mean the true cooling power shall be the average value for the curve. For example, the average cooling power for 15 C in Fig. 6 is 1.95 kW, while the peak cooling powers are 6.21 kW and 7.44 kW at 60 s and 1070 s, respectively. Also, the evolution of cooling power during phase (b) is shown in Fig. 7, whose characteristics of cooling power evolutions are similar with the Fig. 6. And the evolution of adsorption process also happens in evaporator, while the evolution of resorption processes occurs in LTS1. Fig. 8 is for the COP variation with the chilled temperature of 15 C, 12 C, 10 C, 8 C and 5 C. Following the increasing of chilled temperature, the COP increases gradually. But COP changes obviously when the chilled temperature locates between 5 C and 8 C.
8
Oil Boiler Pump Test Chiller
7
Cooling power(KW)
Cooling Tower
Pump
6 5 4
Te=12oC
3
Te=15oC
2 1
Pump 0
Te=10oC Te=5oC
Te=8oC 0
400
800
1200
1600
2000
2400
Time(S) Thermostat Bath Fig. 5. Schematic diagram of the testing system.
Fig. 6. Evolution of cooling power during resorption refrigeration and adsorption refrigeration processes using NiCl2eBaCl2eNH3.
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L. Xu et al. / Applied Thermal Engineering 31 (2011) 3658e3663
1.2 15 oC 12 oC 10 oC 8 oC 5 oC
8
6
COP
Cooling power(KW)
1.1
Te=8oC
4
0.9
Te=12oC 2
0.8
Te=10oC Te=15oC Te=5oC
0
1.0
0.7 250
0
400
800
1200
1600
2000
2400
2800
255
Heating
260
265
270
temperature(oC)
Fig. 9. COP variation with the heating temperature under different chilled temperature.
Time(S) Fig. 7. Evolution of cooling power during resorption refrigeration and adsorption refrigeration processes using MnCl2eBaCl2eNH3.
5.3. Pressure evolution of each reactor under different chilled temperature This shows the high performance of the test unit could only be available at from 10 C to 15 C. Because the COPs become steady after three turns of cycles, the COP values after first turn of cycle are different with that after three turns of cycles.
It is very important for the reactor to be ready for valve switching by maintaining the specific pressure level. For adsorption
5.2. Effect of heating temperature on the COP At a specific chilled temperature, different heating temperature and different chilled temperature mean different COP value. Fig. 9 provides the trend of system COP value vs. heating temperature of oil boiler. At specific chilled temperature, following the increasing of heating temperature, the COP increases slightly. And the higher the chilled temperature is, the bigger extent, by which the COP is improved. Because at the chilled temperature of 5 C, the cooling capacity didn’t improve much by the increasing of heating temperature, its COPs maintain almost the same or even smaller if the metallic part of reactor is considered. The COP difference between the 5 C and other chilled temperatures enlarges as the heating temperature becomes bigger, which shows the suitable high system performance is achieved at the chilled temperatures of over 8 C.
2.0 1.8
COP
1.6 1.4
Steady COP value after 1.2
three cycles runs
1.0
COP value after one cycle run
0.8 4
6
8
Refrigeration
10
12
14
16
temperature(oC)
Fig. 8. COP variation with the chilled temperature of 15 C, 12 C, 10 C, 8 C and 5 C.
Fig. 10. Pressure evolution of each reactor under chilled temperature of 12 C.
L. Xu et al. / Applied Thermal Engineering 31 (2011) 3658e3663
1.2
resorption refrigeration process are combined to provide useful cooling. An internal heat recovery strategy was proposed to improve the system COP. The performance of the double-effect and double-way thermochemical sorption refrigeration cycle was investigated, and results obtained were:
Te=15oC Te=12oC
1.1
o
Te=10 C
COP
1.0 0.9
Te=8 oC
0.8
Te=5oC 0.7 0.6 18
20
22
24
26
28
30
32
3663
34
36
38
40
Time(min.) Fig. 11. COP vs. operation time for adsorption process and resorption process under different chilled temperature.
process and resorption process, especially the latter, the pressure level plays a key role in assuring the achievement of required cooling capacity. In Fig. 10, the pressure evolution in every reactor under the chilled temperature of 12 C is the sample of pressure evolution in reactors under other chilled temperatures. In Fig. 10(b), the resorption process between LTS1 and MTS happens in point A, ends in point G. At same time, the regeneration process of HTS occurs in point A in Fig. 10(a). After then, the adsorption process between evaporator and LTS1 happens in point B in Fig. 10(b). After then the heat recovery process happens between the HTS and MTS, which is shown in point C in Fig. 10(b) and point B in Fig. 10(a). Then, the pressure in HTS decreases, the pressure of MTS increase to desorb the ammonia to condenser. Point C in Fig. 10(a) is the switch point for the resorption process between the HTS and LTS2. Point E in Fig. 10(a) is the end point of that resorption process between HTS and LTS2. After then, the adsorption process between LTS2 and evaporator happens in point F in Fig. 10(a). The pressure evolution diagram tells us what is the pressure level needed for the switching of resorption, adsorption and heat recovery, which is very important for the operation of valves and maintaining of high system performance. 5.4. Optimum operation time for adsorption process and resorption process For different chilled temperature and different cooling capacity, the operation time is different. Fig. 11 shows the correlation of COP value with the operation time. There exists optimum reaction time at the chilled temperature of 15 C, 12 C, 10 C, 8 C and 5 C, which are 36 min, 30 min, 28 min, 22 min and 33 min respectively. From the experimental results, we can draw the conclusions that following the increasing of refrigeration temperature, the optimum reaction times increase either, except at the condition of 5 C, which has the different mechanism of explanation of longer reaction time. 6. Discussions and conclusions A double-effect and double-way thermochemical sorption refrigeration system was proposed and experimentally investigated. In this advanced cycle, both the evaporation heat of the refrigerant during adsorption refrigeration process and the decomposition reaction heat of the low temperature salt during
(1) Experimental results showed that the proposed double-effect and double-way thermochemical sorption refrigeration cycle was feasible, and it can produce four useful cooling-effects during one cycle by using only one heat input at high temperature. (2) At the chilled temperature of 10 C, heat sink temperature of 30 C, decomposition temperature of 260 C, the experimental COP obtained with double-effect and double-way cycle was over 1.0. (3) From the point view of cooling capacity, the cooling amount in resorption process is larger than those in adsorption process very much, which shows there are still many potential amounts in adsorption process. This is the direction to improve the system COP value. However, because matching between the reactors are not so perfect and the incomplete reactions in reactors, the experimental results still have difference with the theoretical value. And during the operation of valves, the pressure in reactors is often in positive status and negative status alternatively, which demands a high requirement for the performance of ammonia valves. Except those shortcomings mentioned above, the performance of system is generally stable and able to reach the highest COP of 1.1 experimentally, at the chilled temperature of 15 C, heat sink temperature of 30 C, and decomposition temperature of 260 C. Acknowledgements This work was supported by the Key Project of the Natural Science Foundation of China under the contract No. 50736004. The authors would like to thank Mr. Y.X. Xu, Mr. Y.K. Sun and Mr. N. Yu for their helps in the experiment. References [1] F. Meunier, Solid sorption heat powered cycles for cooling and heat pumping applications, Appl. Therm. Eng. 18 (1998) 715e729. [2] R.Z. Wang, R.G. Oliveira, Adsorption refrigerationdan efficient way to make good use of waste heat and solar energy, Prog. Energ Combust. 32 (2006) 424e458. [3] W. Wongsuwan, S. Kumar, P. Neveu, F. Meunier, A review of chemical heat pump technology and applications, Appl. Therm. Eng. 21 (2001) 1489e1519. [4] S.V. Shelton, J.W. Wepfer, D.J. Miles, Ramp wave analysis of the solid/vapor heat pump, ASME J. Energ. Resour. Technol. 112 (1990) 69e78. [5] F. Meunier, Theoretical performances of solid adsorbent cascading cycles using the zeoliteewater and active carbonemethanol pairs: four case studies, Heat Recov. Syst. CHP 6 (1986) 491e498. [6] R.E. Critoph, Performance estimation of convective thermal wave adsorption cycles, Appl. Therm. Eng. 16 (1996) 429e437. [7] M. Pons, F. Poyelle, Adsorptive machines with advantaged cycles for heat pumping or cooling applications, Int. J. Refrigeration 22 (1999) 27e37. [8] R.Z. Wang, Performance improvement of adsorption cooling by heat and mass recovery operation, Int. J. Refrigeration 24 (2001) 602e611. [9] B.B. Saha, S. Koyama, T. Kashiwagi, A. Akisawa, K.C. Ng, H.T. Chua, Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system, Int. J. Refrigeration 26 (2003) 749e757. [10] B. Spinner, Ammonia-based thermochemical transformers, Heat Recov. Syst. CHP 13 (1993) 301e307. [11] T.X. Li, R.Z. Wang, R.G. Oliveira, L.W. Wang, Performance analysis of an innovative multi-mode, multi-salt and double-effect chemisorption refrigeration system, AIChE J. 53 (2007) 3222e3230. [12] R.G. Oliveira, R.Z. Wang, A consolidated calcium chlorideeexpanded graphite compound for use in sorption refrigeration systems, Carbon 45 (2007) 390e396.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Experimental study on the performance of double-effect and double-way thermochemical sorption refrigeration cycle L. Xu, R.Z. Wang*, T.X. Li, L.W. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 October 2010 Accepted 5 January 2011 Available online 25 January 2011
The feasibility and working performance of a double-effect and double-way thermochemical sorption refrigeration cycle was experimentally investigated. In the test unit, three different metal chlorides were used as high temperature, middle temperature and low temperature salts, respectively. Both the evaporation heat of the refrigerant during adsorption refrigeration process and the decomposition reaction heat of the low temperature salt during resorption refrigeration process were combined to provide useful cold. Experimental results showed that the proposed double-effect and double-way thermochemical sorption refrigeration cycle was feasible, and it can produce four useful cooling-effects during one cycle by using only one heat input at high temperature. The experimental COP was over 1.0 at the chilled temperature of 10e15 C, decomposition temperature of 260 C, and heat sink temperature of 30 C. In addition, the cycle characteristics and the effect of working parameters on the double-effect and doubleway thermochemical sorption refrigeration system were analyzed and discussed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Thermochemical Sorption Refrigeration Double-way Double-effect
1. Introduction At present time, green cooling technology attracts more and more eyes of scientists and researchers all over the world. Due to its cleanness and zero of ozone depletion potential and global warming effect, this technology has and will come into more and more areas of our lives [1,2]. Absorption refrigeration technology and adsorption refrigeration technology are two important streams of green cooling technology. Liquidegas absorption refrigeration technologies have been widely applied [2], while solidegas adsorption refrigeration technologies are still in the R&D stage. However adsorption technologies have the distinct advantages of the wider range of temperature, less requirement on the crystallization and corrosion. Moreover, physical adsorption chiller using silica-gel water working pair can be bought for commercial purpose nowadays. While the thermochemical sorption refrigeration systems have the advantages of larger evaporation heat and bigger reaction coefficient, the physical adsorption chiller still suffers from the big structure dimension and low coefficient of performance of system. Although the low system COP is also the bottleneck of application of thermochemical sorption refrigeration technologies, three directions exist for the possibility of improving it: 1) strengthened heat and mass transfer; 2) proper sorption working
* Corresponding author. E-mail addresses: [email protected], [email protected] (R.Z. Wang). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.01.014
pair; 3) more advanced sorption thermodynamic cycle [3]. As for the first point, the utilization of compound adsorbents with porous materials and metal chloride salts can be said to be effective in improvement of COP. The porous materials denote the expanded graphite, carbon fibers, or activated carbon, which provide higher thermal conductivity and larger porosity to enhance the heat and mass transfer. Besides, the application of heat pipes can also increase the efficiency of machine during the internal heat transfer process. Because the choices of sorption working pair are linked with the application of sorption thermodynamic cycle, the selected sorption working pairs must match with the cycle very well. It is true that many advanced cycles have been developed and investigated to improve the COP of systems, like cascading cycle [4], thermal wave cycle [5], forced convection cycle [6], mass recovery cycle [7], heat and mass recovery cycle [8], and multistage cycle [9]. In order to further improve the system COP, Spinner gives out a kind of double-effect thermochemical resorption system using three different salts, by which an internal heat recovery process was used to provide the decomposition heat for a middle temperature salt by the synthesis reaction heat of a high temperature salt [10]. The double-effect and double-way thermochemical refrigeration cycle investigated in this paper is firstly proposed by Li et al., who combine the adsorption, resorption and internal heat recovery together to achieve four cold output at the expense of only one heat input during a complete cycle. The ideal coefficient of performance (COP) can be improved by more than 59e169% if compared to the COP obtained with other kinds of cycle [11]. But whether the
L. Xu et al. / Applied Thermal Engineering 31 (2011) 3658e3663
experimental test complies with the theoretical conclusion needs to be checked by the experimental result.
the high system COP value can be achieved. The expected system COP can reach values between 0.91 and 1.8 [11]. 4. Experimental test unit
2. Choice of sorption working pairs
4.1. Experimental sorption refrigeration prototype
To achieve adsorption refrigeration and resorption refrigeration, the sorption working pairs suitable for the proposed system were selected, in which metal chlorideeammonia are preferred as the potential working pairs according to the equilibrium lines of chlorideeammonia in the ClausiuseClapeyron diagram (Fig. 1). A group of working pairs of NH3, BaCl2, MnCl2 and NiCl2 were chosen and used to assess the performance of the double-effect and double-way sorption system. NiCl2 was used as the high temperature salt (HTS). MnCl2 was used as the middle temperature salt (MTS), and BaCl2 was utilized as the low temperature salt (LTS). In addition, expanded graphite was used as an inert porous additive to enhance the heat and mass transfer of the reactive salts due to its high thermal conductivity and gas permeability [12]. The composite sorbents made from salt and expanded graphite had a graphite mass fraction equal to 0.2.
The photograph of the experimental prototype for the doubleeffect and double-way cycle is shown in Fig. 4. As shown in Fig. 3, it mainly consists of an HTS reactor, an MTS reactor, two LTS reactors (LTS1 and LTS2), a condenser, an evaporator. There are four loops in the unit: ammonia loop, oil loop, condensing water loop and chilled water loop. Corresponding to the description in Fig. 2, the experimental procedure is as the follows: [1] During phase (a), the regeneration process of HTS and the resorption process between LTS1 and MTS and adsorption process between evaporator and LTS2 happen, which require valve A2, A4, A6 are open, valve A1, A3, A5 are close in ammonia loop; and valve O1, O3 are open, valve O2, O4 are closed, in oil loop and valve W1, W2, W5, W6 are open, W3, W4, W7, W8 are closed in condensing and cooling loop. [2] During phase (b), the resorption process between LTS2 and HTS, the adsorption process between the evaporator and LTS1 and the heat recovery process between the HTS and MTS require that the status of valves for those loops take the negative operation, i.e., in ammonia loop, valve A2, A4, A6 are close, valve A1, A3, A5 are open; in oil loop, valve O1, O3 are closed and valve O2, O4 are open; in condensing and chilled loop, valve W1, W2, W5, W6 are closed, W3, W4, W7, W8 are open.
3. Double-effect and double-way sorption refrigeration cycle Fig. 2 shows the schematic diagram of the double-effect and double-way thermochemical sorption refrigeration cycle. The working process mainly consists of two phases: phase (a) and phase (b). During phase (a), nickel chloride-high temperature salt (HTS) is heated by external heat resource to desorb the ammonia to the condenser, at the same time, another adsorption and resorption processes occur, the former of which happens between the barium chloride-low temperature salt (LTS2) and evaporator, and the latter of which happens between the barium chloride-low temperature salt (LTS1) and manganese chloride-middle temperature salt (MTS). During phase (b), the LTS2 connects with the HTS, whose synthesis heat output is used to drive the decomposition of MTS to desorb ammonia to condenser. Simultaneously, the adsorption process takes place between the evaporator and LTS1, the resorption process occur between the LTS2 and HTS. Thus, at the expense of only one heat input, four cold output is available, based on which
L n [P (P a )]
3659
14.50 14.25 14.00 13.75 13.50 13.25 13.00 12.75 12.50 12.25 12.00 11.75 11.50 11.25 11.00 10.75 10.50 10.25 10.00 9.75 9.50
As shown in Fig. 5, all the components of test unit system are listed. The electric oil boiler is connected with the oil loop of test unit to provide the external heat in terms of oil of over 250 C for regeneration process of HTS to desorb ammonia to condenser. The condensing cooling tower provides the heat sink with temperature less than 35 C. The thermostat bath is connected with chilled loop, in which an electric heater is installed. With the help of the power measurement equipment for electric heater installed in thermostat
1
2 3 4
5 67 8
9
10 11 12 13 14
NH 3 L/G
-4.4
-4.2
-4.0
-3.8
-3.6
-3.4
-3.2 -3.0 -1000/T(K)
-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
1=PbCl2 (8/3.25); 2=BaCl2 (8/0); 3=LiCl (4/3); 4=AgCl (3/1.5); 5=CaCl2 (8/4); 6=SrCl2 (8/1); 7=CaCl2 (4/2); 8=CdCl2 (6/2); 9=ZnCl2 (6/2); 10=MnCl2 (6/2); 11=FeCl2 (6/2); 12=CoCl2 (6/2); 13=MgCl2 (6/2); 14=NiCl2 (6/2). Fig. 1. Equilibrium lines of chlorideeammonia in the ClausiuseClapeyron diagram.
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Fig. 2. Schematic diagram of the double-effect and double-way thermochemical sorption refrigeration cycle with internal heat recovery process based on adsorption and resorption processes.
Fig. 3. Working principle of the experimental test unit.
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Table 1 Measurement equipments and sensors. No.
Parts
Specification
Quantity
Accuracy
1 2 3 4 5
Oil mass flow meter Water mass flow meter Temperature sensor Pressure sensor Power consumption measurer
DN25, PN4.0 DN20 PT100 0e2.6 MPa 8904F
2 4 29 4 1
0.25% 0.1% 0.15 C 0.2% 0.4%
Qar, Qb, Qbr can be acquired by the same method as for the Qa. System COP is calculated as the follows:
COPd:w:d:e: ¼ ðQa þ Qar þ Qb þ Qbr Þ=QdesM Where COPd.w.d.e denotes the system coefficient of performance of double-effect and double-way refrigeration cycle. Fig. 4. Photograph of the experimental prototype.
5. Results and discussion bath, the cooling capacity is measured in terms of power consumption. The details of the measurement equipments and sensors are listed in Table 1. The double-effect and double-way refrigeration cycle consists of two phases: phase (a) and phase (b). During phase (a), the desorption heat Qdes-M, the adsorption refrigeration cooling capacity Qa and the resorption refrigeration cooling capacity Qar need to be calculated from experimental data. During phase (b), the adsorption refrigeration cooling capacity Qb and the resorption refrigeration cooling capacity Qbr need to be calculated from experimental data either. Qdes-M can be available by multiplying the flow rate of oil and specific heat of oil and the average temperature of oil and the time consumed. Qa is the output by multiplying the flow rate of water and specific heat of water and the average temperature of water and the time consumed. The value of Qa can be checked by the power consumption of the electric heater in the constant bath.
5.1. Evolution of cooling power under different chilled temperature At different chilled temperature, evolution of cooling power during phase (a) includes two stages: adsorption stage and resorption stage, as illustrated in Fig. 6. So, each cooling power curve for different refrigeration temperature has two peaks in one phase. One peak happens in evaporator, which corresponds to adsorption stage, another happens in LTS2, corresponding to resorption stage. As shown by Fig. 6, the higher the chilled temperature is, the longer the reaction time is and the deeper the cooling power peaks are, which mean the true cooling power shall be the average value for the curve. For example, the average cooling power for 15 C in Fig. 6 is 1.95 kW, while the peak cooling powers are 6.21 kW and 7.44 kW at 60 s and 1070 s, respectively. Also, the evolution of cooling power during phase (b) is shown in Fig. 7, whose characteristics of cooling power evolutions are similar with the Fig. 6. And the evolution of adsorption process also happens in evaporator, while the evolution of resorption processes occurs in LTS1. Fig. 8 is for the COP variation with the chilled temperature of 15 C, 12 C, 10 C, 8 C and 5 C. Following the increasing of chilled temperature, the COP increases gradually. But COP changes obviously when the chilled temperature locates between 5 C and 8 C.
8
Oil Boiler Pump Test Chiller
7
Cooling power(KW)
Cooling Tower
Pump
6 5 4
Te=12oC
3
Te=15oC
2 1
Pump 0
Te=10oC Te=5oC
Te=8oC 0
400
800
1200
1600
2000
2400
Time(S) Thermostat Bath Fig. 5. Schematic diagram of the testing system.
Fig. 6. Evolution of cooling power during resorption refrigeration and adsorption refrigeration processes using NiCl2eBaCl2eNH3.
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1.2 15 oC 12 oC 10 oC 8 oC 5 oC
8
6
COP
Cooling power(KW)
1.1
Te=8oC
4
0.9
Te=12oC 2
0.8
Te=10oC Te=15oC Te=5oC
0
1.0
0.7 250
0
400
800
1200
1600
2000
2400
2800
255
Heating
260
265
270
temperature(oC)
Fig. 9. COP variation with the heating temperature under different chilled temperature.
Time(S) Fig. 7. Evolution of cooling power during resorption refrigeration and adsorption refrigeration processes using MnCl2eBaCl2eNH3.
5.3. Pressure evolution of each reactor under different chilled temperature This shows the high performance of the test unit could only be available at from 10 C to 15 C. Because the COPs become steady after three turns of cycles, the COP values after first turn of cycle are different with that after three turns of cycles.
It is very important for the reactor to be ready for valve switching by maintaining the specific pressure level. For adsorption
5.2. Effect of heating temperature on the COP At a specific chilled temperature, different heating temperature and different chilled temperature mean different COP value. Fig. 9 provides the trend of system COP value vs. heating temperature of oil boiler. At specific chilled temperature, following the increasing of heating temperature, the COP increases slightly. And the higher the chilled temperature is, the bigger extent, by which the COP is improved. Because at the chilled temperature of 5 C, the cooling capacity didn’t improve much by the increasing of heating temperature, its COPs maintain almost the same or even smaller if the metallic part of reactor is considered. The COP difference between the 5 C and other chilled temperatures enlarges as the heating temperature becomes bigger, which shows the suitable high system performance is achieved at the chilled temperatures of over 8 C.
2.0 1.8
COP
1.6 1.4
Steady COP value after 1.2
three cycles runs
1.0
COP value after one cycle run
0.8 4
6
8
Refrigeration
10
12
14
16
temperature(oC)
Fig. 8. COP variation with the chilled temperature of 15 C, 12 C, 10 C, 8 C and 5 C.
Fig. 10. Pressure evolution of each reactor under chilled temperature of 12 C.
L. Xu et al. / Applied Thermal Engineering 31 (2011) 3658e3663
1.2
resorption refrigeration process are combined to provide useful cooling. An internal heat recovery strategy was proposed to improve the system COP. The performance of the double-effect and double-way thermochemical sorption refrigeration cycle was investigated, and results obtained were:
Te=15oC Te=12oC
1.1
o
Te=10 C
COP
1.0 0.9
Te=8 oC
0.8
Te=5oC 0.7 0.6 18
20
22
24
26
28
30
32
3663
34
36
38
40
Time(min.) Fig. 11. COP vs. operation time for adsorption process and resorption process under different chilled temperature.
process and resorption process, especially the latter, the pressure level plays a key role in assuring the achievement of required cooling capacity. In Fig. 10, the pressure evolution in every reactor under the chilled temperature of 12 C is the sample of pressure evolution in reactors under other chilled temperatures. In Fig. 10(b), the resorption process between LTS1 and MTS happens in point A, ends in point G. At same time, the regeneration process of HTS occurs in point A in Fig. 10(a). After then, the adsorption process between evaporator and LTS1 happens in point B in Fig. 10(b). After then the heat recovery process happens between the HTS and MTS, which is shown in point C in Fig. 10(b) and point B in Fig. 10(a). Then, the pressure in HTS decreases, the pressure of MTS increase to desorb the ammonia to condenser. Point C in Fig. 10(a) is the switch point for the resorption process between the HTS and LTS2. Point E in Fig. 10(a) is the end point of that resorption process between HTS and LTS2. After then, the adsorption process between LTS2 and evaporator happens in point F in Fig. 10(a). The pressure evolution diagram tells us what is the pressure level needed for the switching of resorption, adsorption and heat recovery, which is very important for the operation of valves and maintaining of high system performance. 5.4. Optimum operation time for adsorption process and resorption process For different chilled temperature and different cooling capacity, the operation time is different. Fig. 11 shows the correlation of COP value with the operation time. There exists optimum reaction time at the chilled temperature of 15 C, 12 C, 10 C, 8 C and 5 C, which are 36 min, 30 min, 28 min, 22 min and 33 min respectively. From the experimental results, we can draw the conclusions that following the increasing of refrigeration temperature, the optimum reaction times increase either, except at the condition of 5 C, which has the different mechanism of explanation of longer reaction time. 6. Discussions and conclusions A double-effect and double-way thermochemical sorption refrigeration system was proposed and experimentally investigated. In this advanced cycle, both the evaporation heat of the refrigerant during adsorption refrigeration process and the decomposition reaction heat of the low temperature salt during
(1) Experimental results showed that the proposed double-effect and double-way thermochemical sorption refrigeration cycle was feasible, and it can produce four useful cooling-effects during one cycle by using only one heat input at high temperature. (2) At the chilled temperature of 10 C, heat sink temperature of 30 C, decomposition temperature of 260 C, the experimental COP obtained with double-effect and double-way cycle was over 1.0. (3) From the point view of cooling capacity, the cooling amount in resorption process is larger than those in adsorption process very much, which shows there are still many potential amounts in adsorption process. This is the direction to improve the system COP value. However, because matching between the reactors are not so perfect and the incomplete reactions in reactors, the experimental results still have difference with the theoretical value. And during the operation of valves, the pressure in reactors is often in positive status and negative status alternatively, which demands a high requirement for the performance of ammonia valves. Except those shortcomings mentioned above, the performance of system is generally stable and able to reach the highest COP of 1.1 experimentally, at the chilled temperature of 15 C, heat sink temperature of 30 C, and decomposition temperature of 260 C. Acknowledgements This work was supported by the Key Project of the Natural Science Foundation of China under the contract No. 50736004. The authors would like to thank Mr. Y.X. Xu, Mr. Y.K. Sun and Mr. N. Yu for their helps in the experiment. References [1] F. Meunier, Solid sorption heat powered cycles for cooling and heat pumping applications, Appl. Therm. Eng. 18 (1998) 715e729. [2] R.Z. Wang, R.G. Oliveira, Adsorption refrigerationdan efficient way to make good use of waste heat and solar energy, Prog. Energ Combust. 32 (2006) 424e458. [3] W. Wongsuwan, S. Kumar, P. Neveu, F. Meunier, A review of chemical heat pump technology and applications, Appl. Therm. Eng. 21 (2001) 1489e1519. [4] S.V. Shelton, J.W. Wepfer, D.J. Miles, Ramp wave analysis of the solid/vapor heat pump, ASME J. Energ. Resour. Technol. 112 (1990) 69e78. [5] F. Meunier, Theoretical performances of solid adsorbent cascading cycles using the zeoliteewater and active carbonemethanol pairs: four case studies, Heat Recov. Syst. CHP 6 (1986) 491e498. [6] R.E. Critoph, Performance estimation of convective thermal wave adsorption cycles, Appl. Therm. Eng. 16 (1996) 429e437. [7] M. Pons, F. Poyelle, Adsorptive machines with advantaged cycles for heat pumping or cooling applications, Int. J. Refrigeration 22 (1999) 27e37. [8] R.Z. Wang, Performance improvement of adsorption cooling by heat and mass recovery operation, Int. J. Refrigeration 24 (2001) 602e611. [9] B.B. Saha, S. Koyama, T. Kashiwagi, A. Akisawa, K.C. Ng, H.T. Chua, Waste heat driven dual-mode, multi-stage, multi-bed regenerative adsorption system, Int. J. Refrigeration 26 (2003) 749e757. [10] B. Spinner, Ammonia-based thermochemical transformers, Heat Recov. Syst. CHP 13 (1993) 301e307. [11] T.X. Li, R.Z. Wang, R.G. Oliveira, L.W. Wang, Performance analysis of an innovative multi-mode, multi-salt and double-effect chemisorption refrigeration system, AIChE J. 53 (2007) 3222e3230. [12] R.G. Oliveira, R.Z. Wang, A consolidated calcium chlorideeexpanded graphite compound for use in sorption refrigeration systems, Carbon 45 (2007) 390e396.