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A conceptual design and performance analysis of a triple-effect solid–gas thermochemical sorption refrigeration system with internal heat recovery
Chemical Engineering Science 64 (2009) 3376 -- 3384
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A conceptual design and performance analysis of a triple-effect solid–gas thermochemical sorption refrigeration system with internal heat recovery T.X. Li, R.Z. Wang ∗ , J.K. Kiplagat, L.W. Wang, R.G. Oliveira Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C L E
I N F O
Article history: Received 23 June 2008 Received in revised form 13 November 2008 Accepted 16 April 2009 Available online 24 April 2009 Keywords: Sorption Refrigeration Triple-effect Heat recovery Thermochemical Cycle
A B S T R A C T
A conceptual design of a triple-effect solid–gas thermochemical sorption refrigeration system using three kinds of reactive salts and ammonia as working pairs is presented. In the proposed system, two internal heat recovery processes were employed to enhance the energy utilization efficiency. The adsorption heat of a high-temperature salt was recovered for the regeneration process of a middle-temperature salt, while the adsorption heat released by the middle-temperature salt was used to regenerate a low-temperature salt. The presented sorption refrigeration system can produce three cooling-effects in one cycle, at the expense of only one heat input at high temperature. The coefficient of performance (COP) of the system can be improved by 146–200% compared to that obtained with a conventional sorption refrigeration system. When the sensible heats of the reactant, the refrigerant and the metallic part of the reactors were considered, theoretical results showed the calculated COP employing the triple-effect sorption cycle varied between 0.75 and 0.97 with the mass ratio between the metallic part of the reactor and the reactive salt. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since the recognition of rational utilization of thermal energy and the concerns about ecological problem, heat-powered green refrigeration technologies have been widely developed. These technologies include the liquid–gas absorption refrigeration, solid–gas thermochemical sorption refrigeration and solid–gas physical sorption refrigeration (Pons et al., 1999). The operation principle of these refrigeration systems is based on the thermal effects of reversible physicochemical reaction processes. The coefficient of performance (COP), defined as the ratio of the cold production to the heat consumption, is usually used to evaluate the performance of these heat-powered sorption systems. In comparison with liquid–gas absorption refrigeration system, solid–gas sorption refrigeration system has some distinct advantages such as high storage capacity or energy density, few problems of corrosion and crystallization. Moreover, solid sorption refrigeration system has a very wide range of working temperatures. It can effectively utilize the heat source temperature ranged from 50 to 300 ◦ C (Wang and Oliveira, 2006). However, the low COP is the main drawback for these solid sorption refrigeration systems, which has been still limiting the extensive application of sorption refrigeration technology. In order to
∗ Corresponding author. Tel./fax: +86 21 34206548. E-mail address: [email protected] (R.Z. Wang). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.04.021
become a realistic alternative to the vapor compression refrigeration systems, solid sorption refrigeration systems must exhibit high performance. One of the promising approaches used to improve the system performance is the utilization of heat recovery strategy between different reactors, which can reduce the amount of heat consumption. These advanced sorption refrigeration cycles include the cascading cycle (Meunier, 1986), thermal wave cycle (Shelton et al., 1990), forced convection cycle (Critoph, 1996), heat regeneration cycle (Douss et al., 1988), heat and mass recovery cycle (Wang, 2001), etc. The above-mentioned sorption refrigeration cycles are mainly focused on the recovery of sensible heat as a method to increase the COP. In addition, a combined double-way thermochemical sorption refrigeration cycle was proposed with the view of improving the system performance (Li et al., 2009). Both the evaporation heat of refrigerant during the adsorption process and the desorption heat of a low-temperature salt (LTS) during the resorption process were employed to provide useful cold. Experimental results showed that a system operating with such double-way sorption cycle can have two useful cold productions during one cycle at the expense of only one heat input at high temperature. In order to further improve the COP, Neveu and Castaing (1993) proposed a double-effect thermochemical sorption refrigeration system using two different reactive salts. In this case, an internal heat recovery process was used to reclaim the adsorption heat of one salt for the regeneration of the other salt. Later, Sorin et al. (2002) developed a new heat recovery method where both the adsorption heat of the reactant and the condensation heat of refrigerant were
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
reclaimed to regenerate the second reactant, but the operating pressure was as high as 77 bar. Moreover, double-effect thermochemical resorption refrigeration systems can also benefit from the internal heat recovery as proposed by Spinner (1993) and Goetz et al. (1997). To accomplish such a task, four reactors and three different reactive salts were used. The adsorption heat of the reactor filled with a high-temperature salt (HTS) was utilized in the desorption process of the reactor filled with a middle-temperature salt. Recently, Li et al. (2007) developed a novel multi-mode, multi-salt and multieffect thermochemical sorption refrigeration system, in which the internal heat recovery strategy, the adsorption process and the resorption process were combined to further improve the COP. During one cycle, the system could have four cold productions at the expense of only one high-temperature heat input. Numerical analysis performed by Oliveira et al. (2008) showed that the advanced multieffect sorption system could have a COP as high as 1.1. This value was almost 3 times the COP of the conventional single-effect thermochemical sorption cycle and 1.5 times that of the thermochemical resorption cycle. However, the multi-effect system would create a vacuum if a low refrigeration temperature is required during the resorption process, and the large variation of working pressure during the adsorption and resorption phases complicates the operation control and reduces the reliability of the machine. The objective of this paper is to present a triple-effect thermochemical sorption refrigeration system, in which three kinds of reactive salts were used as the reactants and ammonia was utilized as the refrigerant. In the proposed system, the working pressure was always higher than the atmospheric pressure, and two internal heat recovery processes were employed to improve the energy utilization efficiency and thereby reduce the heat consumption. The conceptual design, the operation principle and the performance analysis of the proposed sorption system are presented. 2. The conceptual design of a triple-effect thermochemical sorption refrigeration system 2.1. Single-effect thermochemical sorption refrigeration system Conventional single-effect thermochemical sorption refrigeration system has been widely discussed and has now reached the stage of pre-industrial prototype manufacture. As shown in Fig. 1, such a basic system consists of a solid–gas reactor filled with a reactive salt, a condenser and an evaporator. A sorption cycle mainly involves two operating processes. One is the refrigerant desorption and
L/G
3377
condensation process, in which the refrigerant is desorbed from a reactive salt and condenses in the condenser. The other is the refrigerant adsorption and evaporation process, in which the reactive salt adsorbs the refrigerant from the evaporator and evaporation heat of the refrigerant produces a cooling-effect. These two monovariant reaction processes, also known as decomposition and synthesis reactions, only take place when the reactive salts are placed outside of the thermodynamic equilibrium states, and the reaction rates are imposed by the driving equilibrium drop of temperature (T − Teq ) or pressure (P − Peq ). 2.2. Double-effect thermochemical sorption refrigeration system A double-effect thermochemical sorption refrigeration system was developed and its performance was improved by employing internal heat recovery strategy. The component of the basic system consists of two solid–gas reactors, a condenser and an evaporator as shown in Fig. 2. Two different reactive salts are utilized in the doubleeffect sorption system. One reactor is filled with a high-temperature salt and the other is filled with a middle-temperature salt (MTS). The HTS reactor was first heated by an external heat source at a high temperature Td and the refrigerant was desorbed to the condenser, while the MTS reactor was cooled by a heat sink fluid and the refrigerant was adsorbed from the evaporator. Secondly, the working modes of two reactors were interchanged. An internal heat recovery process occurred to reclaim the adsorption heat of the HTS which was sufficiently used to regenerate the MTS. Thus, two cooling-effects could be obtained at the expense of only one heat input at high temperature in the double-effect sorption cycle. 2.3. Triple-effect sorption thermochemical refrigeration system The working principle of the proposed triple-effect thermochemical sorption refrigeration system is illustrated in Fig. 3. It mainly consists of three solid–gas reactors, a condenser and an evaporator. Three kinds of reactive salts were utilized in the presented sorption system to realize two internal heat recovery processes between different reactors. One reactor was filled with a high-temperature salt, the second reactor was filled with a middle-temperature salt, and the third reactor was filled a low-temperature salt. The operating mode of the triple-effect sorption cycle can be divided into two phases: In the first phase (Fig. 3a), desorption of the HTS in reactor 1 occurs by supplying a heat input (Qdes1 ) from an external heat source at a high temperature Td1 . The desorbed refrigerant condenses in
S/G
Qcond Pc
Qdes
Qcond Qdes
LnP
ΔTdes Condenser
S/G reactor
Qads Pe
ΔTads
Te
Qads
Qevap
Qevap Tc
Td -1/ T
Evaporator
S/G reactor
Fig. 1. Schematic diagram of the single-effect thermochemical sorption refrigeration system.
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L/G
Qcond
S/G2
S/G1
Pc
Qdes1
Qcond Qdes1
LnP
ΔTdes Condenser
S/G reactor 1
Qads2 Pe
ΔTads
Q evap Te
Tc
Td1
Evaporator
-1/ T
L/G
Qads2
Qevap
S/G reactor 2
S/G2S/G1
Qcond Pc
Qevap
Qads1
Qdes2 LnP
ΔTdes Evaporator
S/G reactor 1
Qads1 Pe ΔTads
Qevap Te
Tc
Qdes2
Qcond Trec
-1/ T
Condenser
S/G reactor 2
Fig. 2. Schematic diagram of the double-effect thermochemical sorption refrigeration system.
the condenser by rejecting heat (Qcond ) at Tc and then flows into the evaporator. At the same time, the MTS in reactor 2 adsorbs the refrigerant from the evaporator, and the evaporation heat of adsorbed refrigerant produces the first cooling-effect at Te by extracting heat (Qevap ) from a chilled medium. During this phase, an internal heat recovery process between the MTS and the LTS occurs, where the adsorption heat (Qads2 ) released during the synthesis reaction of MTS is used to regenerate the LTS in reactor 3. During the second phase (Fig. 3b), the working modes of the reactors are interchanged. Both the HTS and the LTS reactors perform adsorption while the MTS reactor undergoes desorption. The LTS in reactor 3 adsorbs the refrigerant from the evaporator and produces the second cooling-effect. At the same time, another cooling-effect is obtained during the adsorption process of the HTS in reactor 1. The reaction heat (Qads1 ) released during the synthesis reaction of the HTS is transferred for the use in the regeneration process of the MTS. During the two internal heat recovery processes, the LTS and the MTS reactors are heated by using the recovered thermal energy and the operating pressures increase from synthesis level Pe to decomposition level Pc . In these processes, no additional external heat inputs are required during the desorption processes of the LTS and the MTS. In this proposed sorption system, only the HTS in reactor 1 requires a high-temperature heat supply from an external heat source. In comparison with conventional sorption refrigeration system, the presented sorption system has the distinct advantage of larger cooling capacity. This is because the triple-effect sorption system can
have three cold productions at the expense of only one heat input at high temperature.
3. Performance analysis In order to achieve the internal heat recovery processes between the different reactors in the proposed triple-effect system, the sorption working pairs were selected so as to match the reaction between different reactive salts. The choice of the reactive salts suitable for the system was based on the thermodynamic characteristics of the salt presented in the Clapeyron diagram as shown in Fig. 4, in which metal chlorides–ammonia are selected as the potential working pairs. A group of working pairs of NH3 , SrCl2 , MnCl2 and NiCl2 were chosen and used to assess the performance of the proposed tripleeffect sorption system. NiCl2 was used as the high-temperature salt, MnCl2 was the middle-temperature salt, and SrCl2 was the lowtemperature salt. The operation of the triple-effect thermochemical sorption refrigeration cycle was based on the following reactions: ⎧ SrCl2 · 1NH3 + 7NH3 ⎪ ⎪ ⎪ ⎪ ⇔ SrCl2 · 8NH3 + HRL , HRL = 40.6 kJ mol−1 ⎪ ⎪ ⎨ MnCl2 · 2NH3 + 4NH3 −1 ⎪ ⎪ ⇔ MnCl2 · 6NH3 + HRM , HRM = 46.6 kJ mol ⎪ ⎪ ⎪ ⎪ ⎩ NiCl2 · 2NH3 + 4NH3 ⇔ NiCl2 · 6NH3 + HRH , HRH = 56.7 kJ mol−1
(1)
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
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Qdes1
Qcond L/G
S/G3
S/G2 S/G1
Qcond
Condenser
Pc LnP
Q des3
S/G reactor 1
Qdes1 ΔTdes Qevap
Qads2
Qads2 Pe
Evaporator
S/G reactor 2
Qevap
Te
Trec1
Tc
Td1
Qdes3
Qcond
-1/ T Condenser
S/G reactor 3
Qevap L/G
Qcond
Qads1
S/G3 S/G2 S/G1 Evaporator
Pc
S/G reactor 1
LnP
Qdes2 Qdes2
Qcond Qads1
Q ads3 Pe
Condenser Qevap Te
Tc
Trec2
S/G reactor 2
Qevap
Qads3
-1/ T Evaporator
S/G reactor 3
Fig. 3. Schematic diagram of the triple-effect thermochemical sorption refrigeration system.
Moreover, two groups of working pairs, (1) NH3 , SrCl2 , MnCl2 and (2) NH3 , SrCl2 , NiCl2 were used to evaluate the system performance of the double-effect sorption cycle. The Clapeyron diagrams of the double- and the triple-effect sorption cycles are shown in Figs. 5–7, respectively. The latent heat of vaporization of ammonia (Hevap ) at 0 ◦ C was taken as 21.4 kJ mol−1 . Neglecting the sensible heats of the reactants, the refrigerant, the heat transfer fluid and the metallic reactors, the maximum theoretical coefficient of performance (COPi ) was evaluated by using the reaction enthalpies of refrigerant and metallic salt. It was assumed that the cycle mass of ammonia among different reactors was same in the double- and the triple-effect sorption cycles.
(1) For single-effect thermochemical sorption refrigeration cycle Fig. 4. Equilibrium lines of metal chloride–ammonia in the Clapeyron diagram. 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).
COPi = Hevap / HR
(2)
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Tcooling water
Trecovery
Tgeneration
14.40 14.20 101 °C
30 °C
Pc 14.00
178 °C
Ln[P(Pa)]
13.80 13.60 13.40 13.20 13.00
144 °C
72 °C
0 °C
Pe
12.80 NH3
12.60 12.40 -4.0
-3.8
MnCl2(6/2)
SrCl2(8/1)
-3.6
-3.4
-3.2
-3.0 -2.8 -1000/ T (K)
-2.6
-2.4
-2.2
-2.0
Fig. 5. Clapeyron diagram of the double-effect thermochemical sorption refrigeration system using working pairs: NH3 , SrCl2 , MnCl2 .
Tcooling water
Trecovery
Tgeneration
14.40 14.20 101 °C
30 °C
Pc 14.00
245 °C
Ln[P(Pa)]
13.80 13.60 13.40 13.20 13.00
Pe
0 °C
72 °C
208 °C
NH3
SrCl2(8/1)
NiCl2(6/2)
12.80 12.60 12.40 -4.0
-3.8
-3.6
-3.4
-3.2
-3.0 -2.8 -1000/ T (K)
-2.6
-2.4
-2.2
-2.0
-1.8
Fig. 6. Clapeyron diagram of the double-effect thermochemical sorption refrigeration system using working pairs: NH3 , SrCl2 , NiCl2 .
(2) For double-effect thermochemical sorption refrigeration cycle COPi = 2 · Hevap / HR
(3)
(3) For triple-effect thermochemical sorption refrigeration cycle COPi = 3 · Hevap / HR
(4)
The ideal COPi of the single-effect sorption cycle (NiCl2 –NH3 , MnCl2 –NH3 ), the double-effect sorption cycle (NiCl2 –SrCl2 –NH3 , MnCl2 –SrCl2 –NH3 ), and the triple-effect sorption cycle (NiCl2 – MnCl2 –SrCl2 –NH3 ) is shown in Fig. 8. The sorption systems using MnCl2 as the reactant had a higher COPi than those utilizing NiCl2 as the reactant in the single- and the double-effect sorption cycles. This was because the former reactant has a lower reaction enthalpy than the latter. The double-effect sorption cycle had a higher COPi
compared with the single-effect sorption cycle. Moreover, the tripleeffect sorption refrigeration cycle had the highest COPi among the three kinds of cycles. The improvement in the ideal COPi obtained with the proposed triple-effect cycle varied between 23–50% and 146–200% compared with the double-effect and the single-effect sorption cycles, respectively. In order to obtain a more realistic estimation of the possible COP achieved by the proposed sorption cycle, the sensible heats of the reactive salts, the refrigerant and the metallic part of the reactors were considered. For different reactors, a same cycle mass of ammonia was assumed to determine the mass of each metallic salt complex required as shown in Table 1. The useful cold produced inside the evaporator during one sorption cycle was calculated using liq
Qevap = [n · X · Hevap − Cp-NH MNH3 (Tc − Te )] 3
(5)
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
Tcooling water
3381
Trec2
Trec1 Tgeneration
14.40 14.20 Pc
30 °C
14.00
101 °C
178 °C
245 °C
Ln[P(Pa)]
13.80 13.60 13.40 13.20 13.00
Pe
0 °C
72 °C
144 °C
208 °C
12.80 NH3
12.60 12.40 -4.0
-3.8
MnCl2(6/2)
SrCl2(8/1)
-3.6
-3.4
-3.2
-3.0 -2.8 -1000/ T (K)
-2.6
NiCl2(6/2)
-2.4
-2.2
-2.0
-1.8
Fig. 7. Clapeyron diagram of the triple-effect thermochemical sorption refrigeration system using working pairs: NH3 , SrCl2 , MnCl2 , NiCl2 .
NiCl2-MnCl2-SrCl2-NH3
1.2
Triple-effect cycle
1.4
Double-effect cycle (MnCl2)
0.6 MnCl2-NH3 0.4
Single-effect (NiCl2)
1.0
COP
Ideal COP
Single-effect (MnCl2)
NiCl2-SrCl2-NH3
0.8
Double-effect cycle (NiCl2)
1.2
MnCl2-SrCl2-NH3
1.0
NiCl2-NH3
0.8
0.6
0.4 0.2 0.2 0.0 Single-effect cycle
Double-effect cycle
Multi-effect cycle 0
2
Fig. 8. Ideal COPi of the single-effect, double-effect and triple-effect sorption cycles.
4
6
8
10
12
14
R Fig. 9. COP with different mass ratio R between the metallic part of the reactor and the salt.
Table 1 Mass of metallic salt complex necessary to desorb/adsorb 1 kg of ammonia.
Qdes-MTS = [n · X · HRM + Cploaded -MTS MMTS (Trec-HM − Trec-ML )
Parameters
Value
Mass of SrCl2 · 1NH3 Mass of MnCl2 · 2NH3
1.48 kgsalt kgNH 3 −1 2.35 kgsalt kgNH
Mass of NiCl2 · 2NH3
2.41kgsalt kgNH
−1
−1
3
3
+ Cp-r MR-MTS (Trec-HM − Trec-ML )] Qdes-HTS = [n · X
· HRH + Cploaded -HTS MHTS (Td
(7)
− Trec-HM )
+ Cp-r MR-HTS (Td − Trec-HM )]
(8)
During the synthesis phase, the reaction heats released by the MTS reactor and the HTS reactor were calculated using During the decomposition phase, the desorption heats required for the LTS, MTS and HTS reactors were calculated from
+ Cp-r MR-MTS (Trec-HM − Trec-ML )]
Qdes-LTS = [n · X · HRL + Cploaded -LTS MLTS (Trec-ML − Ta-LTS ) + Cp-r MR-LTS (Trec-ML − Ta-LTS )]
Qads-MTS = [n · X · HRM + Cpunloaded -MTS MMTS (Trec-HM − Trec-ML )
(6)
(9)
Qads-HTS = [n · X · HRH + Cpunloaded -HTS MHTS (Td − Trec-HM ) + Cp-r MR-HTS (Td − Trec-HM )]
(10)
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T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
The expected COP of the triple-effect sorption cycle can therefore be calculated from the following: COP = 3 ×
Qevap Qdes-HTS
(11)
Fig. 9 shows the effect of mass ratio (R) on the COP of the proposed triple-effect sorption cycle. The parameter R is defined as the mass ratio of the metallic part of the reactor to the reactive salt. It can be seen that the mass ratio R had a strong influence on the system performance. The COP decreased significantly as the mass ratio R increased. In the range of mass ratio R from 0 to 15, the COP of the proposed triple-effect cycle varied between 0.97 and 0.75. For most of the solid–gas sorption systems with optimized design, the mass ratio (R) between the metallic part of the reactor and the reactive salt is about 5. The corresponding COP obtained with the single-effect sorption cycle using NiCl2 –NH3 , the doubleeffect sorption cycle using NiCl2 –SrCl2 –NH3 , and the triple-effect sorption cycle using NiCl2 –MnCl2 –SrCl2 –NH3 were 0.21, 0.53 and 0.88, respectively. Thus, the system performance can be significantly
Triple-effect cycle
1.2
Double-effect cycle (MnCl2)
R=5
Double-effect cycle (NiCl2)
1.0
Single-effect (MnCl2) Single-effect (NiCl2)
0.6
0.4
0.2
0.0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Global conversion X
0.8
0.9
1.0
Fig. 10. COP with different global conversion X.
4000 3500
Heat production of MnCl2 salt
5000
Heat production of NiCl2 salt
during the synthesis phase
4500
during the synthesis phase
4000 3000
3500
2500 2000
Heat consumption of SrCl2 salt during the decomposition phase
1500
Q / ( kJ )
Q / ( kJ )
COP
0.8
improved by the application of the proposed triple-effect sorption cycle than the conventional single- and double-effect sorption cycles. The global conversion (X), which represents the percentage of the metallic salt that has reacted with the refrigerant, was used to evaluate the system performance of the proposed triple-effect sorption cycle. Fig. 10 shows the effect of the global conversion X on the COP when the mass ratio R was 5. Generally, the COP increased significantly with the increase in the global conversion X. The COP of the proposed triple-effect sorption cycle varied between 0.09 and 0.88 when the global conversion X ranged from 0.1 to 1. However, high global conversion usually requires a considerable long cycle time because the reaction rate decreases with the conversion time, and the cooling power of the system could be largely reduced. Thus, taking a more realistic situation where all the reactions were assumed 85% complete, the COP obtained with the proposed triple-effect sorption cycle was as high as 0.84. This figure represents an improvement of 342% and 68%, compared with the single-effect sorption cycle (NiCl2 –NH3 ) and the double-effect sorption cycle (NiCl2 –SrCl2 –NH3 ), respectively. During the internal heat recovery process, the reaction heat released by the HTS reactor was reclaimed and used to regenerate the MTS reactor, while the reaction heat released by the MTS was utilized for the desorption of the LTS reactor. The proposed internal heat recoveries are only possible if the total amount of heat released by one reactor during the synthesis phase is equal to or higher than the total heat input required during the regeneration phase of the other reactor. The feasibility of the two internal heat recovery processes between the different reactors was evaluated. The heat released/adsorbed during these adsorption and desorption processes was calculated at the global conversion X of 0.85. The variation of the heat production and heat consumption with mass ratio (R) for the double-effect sorption system is showed in Fig. 11. It shows that the amount of the reaction heat produced by the HTS was higher than the heat consumed by the MTS for both groups of working pairs analyzed. Thus, the reaction heat consumed by the MTS during the decomposition phase can be fully supplied from the reaction heat released by the HTS. Fig. 12 shows the variation of the heat production and heat consumption with mass ratio (R) for the proposed triple-effect sorption system. During the first internal heat recovery process (Fig. 12a), the heat produced by the HTS salt (NiCl2 ) during the synthesis phase was recovered for use in the regeneration process of the MTS salt (MnCl2 ). During the second internal heat recovery process (Fig. 12b), the adsorption heat released by the MTS salt (MnCl2 ) during the
3000 2500 Heat consumption of SrCl2 salt
2000
during the decomposition phase
1500
1000
The amount of heat difference
500
1000 500
0
2
4
6
8 R
10 12 14
The amount of heat difference 0
2
4
6
8 R
10 12 14
Fig. 11. Heat production and heat consumption for the double-effect sorption system: (a) working pair: MnCl2 , SrCl2 –NH3 and (b) working pair: NiCl2 , SrCl2 –NH3 .
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
4500
5000 Heat production of NiCl2 salt
4500
Heat production of MnCl2 salt
4000
during the synthesis phase
4000
during the synthesis phase
3500
3500
3000
3000 2500
Heat consumption of MnCl2 salt
2000
during the decomposition phase
1500 The amount of heat difference between HTS and MTS
1000
Q / ( kJ )
Q / ( kJ )
3383
2500 Heat consumption of SrCl2 salt
2000
during the decomposition phase
1500
The amount of heat difference between MTS and LTS
1000 500
500 0
2
4
6
8 R
10 12 14
0
2
4
6
8
10 12 14
R
Fig. 12. Heat production and heat consumption for the triple-effect sorption system: (a) the first internal heat recovery and (b) the second internal heat recovery.
synthesis phase was recovered and used to regenerate the LTS salt (SrCl2 ). It was observed that the amounts of heat difference during the two internal heat recovery processes were higher than 250 kJ, which implies that the two internal heat recovery processes between different reactive salts were feasible. However, it is necessary to take some measures to keep the energy balance, and the extra heat released must be eliminated by a heat sink to ensure proper operation of the proposed system.
4. Conclusion A triple-effect solid–gas thermochemical sorption refrigeration cycle with internal heat recovery is proposed to improve the system performance. Three types of reactive salts were used as the reactants and ammonia was utilized as the refrigerant in the presented system. The adsorption reaction heat of the high-temperature salt was recovered and used during the regeneration process of the middle-temperature salt, while the adsorption reaction heat released by the middle-temperature salt was used to regenerate the lowtemperature salt. Theoretical results showed that the two internal heat recovery processes between different reactors were feasible. The ideal COPi of the proposed sorption system can be improved by 146–200% compared to that obtained with the conventional sorption system. The calculated COP employing the triple-effect sorption cycle varied between 0.75 and 0.97 with the mass ratio between the metallic part of the reactor and the reactive salt. The proposed tripleeffect sorption cycle can produce three cooling-effects per cycle at the expense of one heat input at high temperature. Hence, it has a larger cooling capacity than the conventional sorption refrigeration cycle.
HRL HRM HTS LTS MTS MHTS MLTS MMTS MNH3 MR n Pc Pe Peq Qads Qcond Qdes Qevap R
T Ta Tc Td Te Teq Trec Trec-HM Trec−ML X
reaction enthalpy of LTS, kJ mol−1 reaction enthalpy of MTS, kJ mol−1 high-temperature salt low-temperature salt middle-temperature salt mass of HTS, kg mass of LTS, kg mass of MTS, kg mass of ammonia, kg mass of metallic reactor, kg number of moles of cycled refrigerant condensation pressure, Pa evaporation pressure, Pa equilibrium pressure, Pa adsorption heat, kJ condensation heat, kJ desorption heat, kJ evaporation heat of refrigerant, kJ mass ratio between the metallic mass of the reactor and the mass of reactive salt equilibrium temperature drop, ◦ C adsorption temperature, ◦ C condensation temperature, ◦ C desorption temperature, ◦ C evaporation temperature, ◦ C equilibrium temperature, ◦ C heat recovery temperature, ◦ C heat recovery temperature between HTS and MTS heat recovery temperature between MTS and LTS global conversion
Subscripts Notation COP Cp-HTS Cp-LTS Cp-MTS Cp-NH3 Cp-r Hevap HR HRH
coefficient of performance specific heat of HTS, kJ kg−1 ◦ C−1 specific heat of LTS, kJ kg−1 ◦ C−1 specific heat of MTS, kJ kg−1 ◦ C−1 specific heat of ammonia, kJ kg−1 ◦ C−1 specific heat of metallic reactor, kJ kg−1 ◦ C−1 vaporization enthalpy of refrigerant, kJ mol−1 reaction enthalpy of reactant, kJ mol−1 reaction enthalpy of HTS, kJ mol−1
ads des i
adsorption desorption ideal
Superscripts
liq loaded unloaded
liquid state loaded state unloaded state
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Acknowledgment This work was supported by the Key Project of the Natural Science Foundation of China under the contract no. 50736004. References Critoph, R.E., 1996. Performance estimation of convective thermal wave adsorption cycles. Applied Thermal Engineering 16, 429–437. Douss, N., Sun, L.M., Meunier, F., 1988. Predictive model and experimental results for a two-adsorber solid adsorption heat pump. Industrial and Engineering Chemistry Research 27, 310–316. Goetz, V., Spinner, B., Lepinasse, E., 1997. A solid–gas thermochemical cooling system using BaCl2 and NiCl2 . Energy 22, 49–58. Li, T.X., Wang, R.Z., Oliveira, R.G., Wang, L.W., 2007. Performance analysis of an innovative multi-mode, multi-salt and multi-effect chemisorption refrigeration system. A.I.Ch.E. Journal 53, 3222–3230. Li, T.X., Wang, R.Z., Oliveira, R.G., Kiplagat, J.K., Wang, L.W., 2009. A combined doubleway chemisorption refrigeration cycle based on adsorption and resorption processes. International Journal of Refrigeration 32, 47–57. Meunier, F., 1986. Theoretical performances of solid adsorbent cascading cycles using the zeolite–water and active carbon–methanol pairs: four case studies. Heat Recovery Systems and CHP 6, 491–498.
Neveu, P., Castaing, J., 1993. Solid–gas chemical heat pumps: field of application and performance of the internal heat recovery. Heat Recovery Systems and CHP 13, 233–251. Oliveira, R.G., Wang, R.Z., Li, T.X., 2008. Transient analysis of a chemisorption air conditioning system operating under different kinds of cycle. Industrial and Engineering Chemistry Research 47, 1102–1110. Pons, M., Meunier, F., Cacciola, G., Critoph, R.E., Groll, M., Puigjaner, L., Spinner, B., Ziegler, F., 1999. Thermodynamic based comparison of sorption systems for cooling and heat pumping. International Journal of Refrigeration 22, 5–17. Shelton, S.V., Wepfer, J.W., Miles, D.J., 1990. Ramp wave analysis of the solid/vapor heat pump. ASME Journal of Energy Resources Technology 112, 69–78. Sorin, M., Spinner, B., Stitou, D., 2002. Thermodynamic techniques for the conceptual design of thermochemical refrigerators using two salt materials. Chemical Engineering Science 57, 4243–4251. Spinner, B., 1993. Ammonia-based thermochemical transformers. Heat Recovery Systems and CHP 13, 301–307. Wang, R.Z., 2001. Performance improvement of adsorption cooling by heat and mass recovery operation. International Journal of Refrigeration 24, 602–611. Wang, R.Z., Oliveira, R.G., 2006. Adsorption refrigeration—an efficient way to make good use of waste heat and solar energy. Progress in Energy and Combustion Science 32, 424–458.
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A conceptual design and performance analysis of a triple-effect solid–gas thermochemical sorption refrigeration system with internal heat recovery T.X. Li, R.Z. Wang ∗ , J.K. Kiplagat, L.W. Wang, R.G. Oliveira Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
A R T I C L E
I N F O
Article history: Received 23 June 2008 Received in revised form 13 November 2008 Accepted 16 April 2009 Available online 24 April 2009 Keywords: Sorption Refrigeration Triple-effect Heat recovery Thermochemical Cycle
A B S T R A C T
A conceptual design of a triple-effect solid–gas thermochemical sorption refrigeration system using three kinds of reactive salts and ammonia as working pairs is presented. In the proposed system, two internal heat recovery processes were employed to enhance the energy utilization efficiency. The adsorption heat of a high-temperature salt was recovered for the regeneration process of a middle-temperature salt, while the adsorption heat released by the middle-temperature salt was used to regenerate a low-temperature salt. The presented sorption refrigeration system can produce three cooling-effects in one cycle, at the expense of only one heat input at high temperature. The coefficient of performance (COP) of the system can be improved by 146–200% compared to that obtained with a conventional sorption refrigeration system. When the sensible heats of the reactant, the refrigerant and the metallic part of the reactors were considered, theoretical results showed the calculated COP employing the triple-effect sorption cycle varied between 0.75 and 0.97 with the mass ratio between the metallic part of the reactor and the reactive salt. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Since the recognition of rational utilization of thermal energy and the concerns about ecological problem, heat-powered green refrigeration technologies have been widely developed. These technologies include the liquid–gas absorption refrigeration, solid–gas thermochemical sorption refrigeration and solid–gas physical sorption refrigeration (Pons et al., 1999). The operation principle of these refrigeration systems is based on the thermal effects of reversible physicochemical reaction processes. The coefficient of performance (COP), defined as the ratio of the cold production to the heat consumption, is usually used to evaluate the performance of these heat-powered sorption systems. In comparison with liquid–gas absorption refrigeration system, solid–gas sorption refrigeration system has some distinct advantages such as high storage capacity or energy density, few problems of corrosion and crystallization. Moreover, solid sorption refrigeration system has a very wide range of working temperatures. It can effectively utilize the heat source temperature ranged from 50 to 300 ◦ C (Wang and Oliveira, 2006). However, the low COP is the main drawback for these solid sorption refrigeration systems, which has been still limiting the extensive application of sorption refrigeration technology. In order to
∗ Corresponding author. Tel./fax: +86 21 34206548. E-mail address: [email protected] (R.Z. Wang). 0009-2509/$ - see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2009.04.021
become a realistic alternative to the vapor compression refrigeration systems, solid sorption refrigeration systems must exhibit high performance. One of the promising approaches used to improve the system performance is the utilization of heat recovery strategy between different reactors, which can reduce the amount of heat consumption. These advanced sorption refrigeration cycles include the cascading cycle (Meunier, 1986), thermal wave cycle (Shelton et al., 1990), forced convection cycle (Critoph, 1996), heat regeneration cycle (Douss et al., 1988), heat and mass recovery cycle (Wang, 2001), etc. The above-mentioned sorption refrigeration cycles are mainly focused on the recovery of sensible heat as a method to increase the COP. In addition, a combined double-way thermochemical sorption refrigeration cycle was proposed with the view of improving the system performance (Li et al., 2009). Both the evaporation heat of refrigerant during the adsorption process and the desorption heat of a low-temperature salt (LTS) during the resorption process were employed to provide useful cold. Experimental results showed that a system operating with such double-way sorption cycle can have two useful cold productions during one cycle at the expense of only one heat input at high temperature. In order to further improve the COP, Neveu and Castaing (1993) proposed a double-effect thermochemical sorption refrigeration system using two different reactive salts. In this case, an internal heat recovery process was used to reclaim the adsorption heat of one salt for the regeneration of the other salt. Later, Sorin et al. (2002) developed a new heat recovery method where both the adsorption heat of the reactant and the condensation heat of refrigerant were
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
reclaimed to regenerate the second reactant, but the operating pressure was as high as 77 bar. Moreover, double-effect thermochemical resorption refrigeration systems can also benefit from the internal heat recovery as proposed by Spinner (1993) and Goetz et al. (1997). To accomplish such a task, four reactors and three different reactive salts were used. The adsorption heat of the reactor filled with a high-temperature salt (HTS) was utilized in the desorption process of the reactor filled with a middle-temperature salt. Recently, Li et al. (2007) developed a novel multi-mode, multi-salt and multieffect thermochemical sorption refrigeration system, in which the internal heat recovery strategy, the adsorption process and the resorption process were combined to further improve the COP. During one cycle, the system could have four cold productions at the expense of only one high-temperature heat input. Numerical analysis performed by Oliveira et al. (2008) showed that the advanced multieffect sorption system could have a COP as high as 1.1. This value was almost 3 times the COP of the conventional single-effect thermochemical sorption cycle and 1.5 times that of the thermochemical resorption cycle. However, the multi-effect system would create a vacuum if a low refrigeration temperature is required during the resorption process, and the large variation of working pressure during the adsorption and resorption phases complicates the operation control and reduces the reliability of the machine. The objective of this paper is to present a triple-effect thermochemical sorption refrigeration system, in which three kinds of reactive salts were used as the reactants and ammonia was utilized as the refrigerant. In the proposed system, the working pressure was always higher than the atmospheric pressure, and two internal heat recovery processes were employed to improve the energy utilization efficiency and thereby reduce the heat consumption. The conceptual design, the operation principle and the performance analysis of the proposed sorption system are presented. 2. The conceptual design of a triple-effect thermochemical sorption refrigeration system 2.1. Single-effect thermochemical sorption refrigeration system Conventional single-effect thermochemical sorption refrigeration system has been widely discussed and has now reached the stage of pre-industrial prototype manufacture. As shown in Fig. 1, such a basic system consists of a solid–gas reactor filled with a reactive salt, a condenser and an evaporator. A sorption cycle mainly involves two operating processes. One is the refrigerant desorption and
L/G
3377
condensation process, in which the refrigerant is desorbed from a reactive salt and condenses in the condenser. The other is the refrigerant adsorption and evaporation process, in which the reactive salt adsorbs the refrigerant from the evaporator and evaporation heat of the refrigerant produces a cooling-effect. These two monovariant reaction processes, also known as decomposition and synthesis reactions, only take place when the reactive salts are placed outside of the thermodynamic equilibrium states, and the reaction rates are imposed by the driving equilibrium drop of temperature (T − Teq ) or pressure (P − Peq ). 2.2. Double-effect thermochemical sorption refrigeration system A double-effect thermochemical sorption refrigeration system was developed and its performance was improved by employing internal heat recovery strategy. The component of the basic system consists of two solid–gas reactors, a condenser and an evaporator as shown in Fig. 2. Two different reactive salts are utilized in the doubleeffect sorption system. One reactor is filled with a high-temperature salt and the other is filled with a middle-temperature salt (MTS). The HTS reactor was first heated by an external heat source at a high temperature Td and the refrigerant was desorbed to the condenser, while the MTS reactor was cooled by a heat sink fluid and the refrigerant was adsorbed from the evaporator. Secondly, the working modes of two reactors were interchanged. An internal heat recovery process occurred to reclaim the adsorption heat of the HTS which was sufficiently used to regenerate the MTS. Thus, two cooling-effects could be obtained at the expense of only one heat input at high temperature in the double-effect sorption cycle. 2.3. Triple-effect sorption thermochemical refrigeration system The working principle of the proposed triple-effect thermochemical sorption refrigeration system is illustrated in Fig. 3. It mainly consists of three solid–gas reactors, a condenser and an evaporator. Three kinds of reactive salts were utilized in the presented sorption system to realize two internal heat recovery processes between different reactors. One reactor was filled with a high-temperature salt, the second reactor was filled with a middle-temperature salt, and the third reactor was filled a low-temperature salt. The operating mode of the triple-effect sorption cycle can be divided into two phases: In the first phase (Fig. 3a), desorption of the HTS in reactor 1 occurs by supplying a heat input (Qdes1 ) from an external heat source at a high temperature Td1 . The desorbed refrigerant condenses in
S/G
Qcond Pc
Qdes
Qcond Qdes
LnP
ΔTdes Condenser
S/G reactor
Qads Pe
ΔTads
Te
Qads
Qevap
Qevap Tc
Td -1/ T
Evaporator
S/G reactor
Fig. 1. Schematic diagram of the single-effect thermochemical sorption refrigeration system.
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L/G
Qcond
S/G2
S/G1
Pc
Qdes1
Qcond Qdes1
LnP
ΔTdes Condenser
S/G reactor 1
Qads2 Pe
ΔTads
Q evap Te
Tc
Td1
Evaporator
-1/ T
L/G
Qads2
Qevap
S/G reactor 2
S/G2S/G1
Qcond Pc
Qevap
Qads1
Qdes2 LnP
ΔTdes Evaporator
S/G reactor 1
Qads1 Pe ΔTads
Qevap Te
Tc
Qdes2
Qcond Trec
-1/ T
Condenser
S/G reactor 2
Fig. 2. Schematic diagram of the double-effect thermochemical sorption refrigeration system.
the condenser by rejecting heat (Qcond ) at Tc and then flows into the evaporator. At the same time, the MTS in reactor 2 adsorbs the refrigerant from the evaporator, and the evaporation heat of adsorbed refrigerant produces the first cooling-effect at Te by extracting heat (Qevap ) from a chilled medium. During this phase, an internal heat recovery process between the MTS and the LTS occurs, where the adsorption heat (Qads2 ) released during the synthesis reaction of MTS is used to regenerate the LTS in reactor 3. During the second phase (Fig. 3b), the working modes of the reactors are interchanged. Both the HTS and the LTS reactors perform adsorption while the MTS reactor undergoes desorption. The LTS in reactor 3 adsorbs the refrigerant from the evaporator and produces the second cooling-effect. At the same time, another cooling-effect is obtained during the adsorption process of the HTS in reactor 1. The reaction heat (Qads1 ) released during the synthesis reaction of the HTS is transferred for the use in the regeneration process of the MTS. During the two internal heat recovery processes, the LTS and the MTS reactors are heated by using the recovered thermal energy and the operating pressures increase from synthesis level Pe to decomposition level Pc . In these processes, no additional external heat inputs are required during the desorption processes of the LTS and the MTS. In this proposed sorption system, only the HTS in reactor 1 requires a high-temperature heat supply from an external heat source. In comparison with conventional sorption refrigeration system, the presented sorption system has the distinct advantage of larger cooling capacity. This is because the triple-effect sorption system can
have three cold productions at the expense of only one heat input at high temperature.
3. Performance analysis In order to achieve the internal heat recovery processes between the different reactors in the proposed triple-effect system, the sorption working pairs were selected so as to match the reaction between different reactive salts. The choice of the reactive salts suitable for the system was based on the thermodynamic characteristics of the salt presented in the Clapeyron diagram as shown in Fig. 4, in which metal chlorides–ammonia are selected as the potential working pairs. A group of working pairs of NH3 , SrCl2 , MnCl2 and NiCl2 were chosen and used to assess the performance of the proposed tripleeffect sorption system. NiCl2 was used as the high-temperature salt, MnCl2 was the middle-temperature salt, and SrCl2 was the lowtemperature salt. The operation of the triple-effect thermochemical sorption refrigeration cycle was based on the following reactions: ⎧ SrCl2 · 1NH3 + 7NH3 ⎪ ⎪ ⎪ ⎪ ⇔ SrCl2 · 8NH3 + HRL , HRL = 40.6 kJ mol−1 ⎪ ⎪ ⎨ MnCl2 · 2NH3 + 4NH3 −1 ⎪ ⎪ ⇔ MnCl2 · 6NH3 + HRM , HRM = 46.6 kJ mol ⎪ ⎪ ⎪ ⎪ ⎩ NiCl2 · 2NH3 + 4NH3 ⇔ NiCl2 · 6NH3 + HRH , HRH = 56.7 kJ mol−1
(1)
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
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Qdes1
Qcond L/G
S/G3
S/G2 S/G1
Qcond
Condenser
Pc LnP
Q des3
S/G reactor 1
Qdes1 ΔTdes Qevap
Qads2
Qads2 Pe
Evaporator
S/G reactor 2
Qevap
Te
Trec1
Tc
Td1
Qdes3
Qcond
-1/ T Condenser
S/G reactor 3
Qevap L/G
Qcond
Qads1
S/G3 S/G2 S/G1 Evaporator
Pc
S/G reactor 1
LnP
Qdes2 Qdes2
Qcond Qads1
Q ads3 Pe
Condenser Qevap Te
Tc
Trec2
S/G reactor 2
Qevap
Qads3
-1/ T Evaporator
S/G reactor 3
Fig. 3. Schematic diagram of the triple-effect thermochemical sorption refrigeration system.
Moreover, two groups of working pairs, (1) NH3 , SrCl2 , MnCl2 and (2) NH3 , SrCl2 , NiCl2 were used to evaluate the system performance of the double-effect sorption cycle. The Clapeyron diagrams of the double- and the triple-effect sorption cycles are shown in Figs. 5–7, respectively. The latent heat of vaporization of ammonia (Hevap ) at 0 ◦ C was taken as 21.4 kJ mol−1 . Neglecting the sensible heats of the reactants, the refrigerant, the heat transfer fluid and the metallic reactors, the maximum theoretical coefficient of performance (COPi ) was evaluated by using the reaction enthalpies of refrigerant and metallic salt. It was assumed that the cycle mass of ammonia among different reactors was same in the double- and the triple-effect sorption cycles.
(1) For single-effect thermochemical sorption refrigeration cycle Fig. 4. Equilibrium lines of metal chloride–ammonia in the Clapeyron diagram. 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).
COPi = Hevap / HR
(2)
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Tcooling water
Trecovery
Tgeneration
14.40 14.20 101 °C
30 °C
Pc 14.00
178 °C
Ln[P(Pa)]
13.80 13.60 13.40 13.20 13.00
144 °C
72 °C
0 °C
Pe
12.80 NH3
12.60 12.40 -4.0
-3.8
MnCl2(6/2)
SrCl2(8/1)
-3.6
-3.4
-3.2
-3.0 -2.8 -1000/ T (K)
-2.6
-2.4
-2.2
-2.0
Fig. 5. Clapeyron diagram of the double-effect thermochemical sorption refrigeration system using working pairs: NH3 , SrCl2 , MnCl2 .
Tcooling water
Trecovery
Tgeneration
14.40 14.20 101 °C
30 °C
Pc 14.00
245 °C
Ln[P(Pa)]
13.80 13.60 13.40 13.20 13.00
Pe
0 °C
72 °C
208 °C
NH3
SrCl2(8/1)
NiCl2(6/2)
12.80 12.60 12.40 -4.0
-3.8
-3.6
-3.4
-3.2
-3.0 -2.8 -1000/ T (K)
-2.6
-2.4
-2.2
-2.0
-1.8
Fig. 6. Clapeyron diagram of the double-effect thermochemical sorption refrigeration system using working pairs: NH3 , SrCl2 , NiCl2 .
(2) For double-effect thermochemical sorption refrigeration cycle COPi = 2 · Hevap / HR
(3)
(3) For triple-effect thermochemical sorption refrigeration cycle COPi = 3 · Hevap / HR
(4)
The ideal COPi of the single-effect sorption cycle (NiCl2 –NH3 , MnCl2 –NH3 ), the double-effect sorption cycle (NiCl2 –SrCl2 –NH3 , MnCl2 –SrCl2 –NH3 ), and the triple-effect sorption cycle (NiCl2 – MnCl2 –SrCl2 –NH3 ) is shown in Fig. 8. The sorption systems using MnCl2 as the reactant had a higher COPi than those utilizing NiCl2 as the reactant in the single- and the double-effect sorption cycles. This was because the former reactant has a lower reaction enthalpy than the latter. The double-effect sorption cycle had a higher COPi
compared with the single-effect sorption cycle. Moreover, the tripleeffect sorption refrigeration cycle had the highest COPi among the three kinds of cycles. The improvement in the ideal COPi obtained with the proposed triple-effect cycle varied between 23–50% and 146–200% compared with the double-effect and the single-effect sorption cycles, respectively. In order to obtain a more realistic estimation of the possible COP achieved by the proposed sorption cycle, the sensible heats of the reactive salts, the refrigerant and the metallic part of the reactors were considered. For different reactors, a same cycle mass of ammonia was assumed to determine the mass of each metallic salt complex required as shown in Table 1. The useful cold produced inside the evaporator during one sorption cycle was calculated using liq
Qevap = [n · X · Hevap − Cp-NH MNH3 (Tc − Te )] 3
(5)
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
Tcooling water
3381
Trec2
Trec1 Tgeneration
14.40 14.20 Pc
30 °C
14.00
101 °C
178 °C
245 °C
Ln[P(Pa)]
13.80 13.60 13.40 13.20 13.00
Pe
0 °C
72 °C
144 °C
208 °C
12.80 NH3
12.60 12.40 -4.0
-3.8
MnCl2(6/2)
SrCl2(8/1)
-3.6
-3.4
-3.2
-3.0 -2.8 -1000/ T (K)
-2.6
NiCl2(6/2)
-2.4
-2.2
-2.0
-1.8
Fig. 7. Clapeyron diagram of the triple-effect thermochemical sorption refrigeration system using working pairs: NH3 , SrCl2 , MnCl2 , NiCl2 .
NiCl2-MnCl2-SrCl2-NH3
1.2
Triple-effect cycle
1.4
Double-effect cycle (MnCl2)
0.6 MnCl2-NH3 0.4
Single-effect (NiCl2)
1.0
COP
Ideal COP
Single-effect (MnCl2)
NiCl2-SrCl2-NH3
0.8
Double-effect cycle (NiCl2)
1.2
MnCl2-SrCl2-NH3
1.0
NiCl2-NH3
0.8
0.6
0.4 0.2 0.2 0.0 Single-effect cycle
Double-effect cycle
Multi-effect cycle 0
2
Fig. 8. Ideal COPi of the single-effect, double-effect and triple-effect sorption cycles.
4
6
8
10
12
14
R Fig. 9. COP with different mass ratio R between the metallic part of the reactor and the salt.
Table 1 Mass of metallic salt complex necessary to desorb/adsorb 1 kg of ammonia.
Qdes-MTS = [n · X · HRM + Cploaded -MTS MMTS (Trec-HM − Trec-ML )
Parameters
Value
Mass of SrCl2 · 1NH3 Mass of MnCl2 · 2NH3
1.48 kgsalt kgNH 3 −1 2.35 kgsalt kgNH
Mass of NiCl2 · 2NH3
2.41kgsalt kgNH
−1
−1
3
3
+ Cp-r MR-MTS (Trec-HM − Trec-ML )] Qdes-HTS = [n · X
· HRH + Cploaded -HTS MHTS (Td
(7)
− Trec-HM )
+ Cp-r MR-HTS (Td − Trec-HM )]
(8)
During the synthesis phase, the reaction heats released by the MTS reactor and the HTS reactor were calculated using During the decomposition phase, the desorption heats required for the LTS, MTS and HTS reactors were calculated from
+ Cp-r MR-MTS (Trec-HM − Trec-ML )]
Qdes-LTS = [n · X · HRL + Cploaded -LTS MLTS (Trec-ML − Ta-LTS ) + Cp-r MR-LTS (Trec-ML − Ta-LTS )]
Qads-MTS = [n · X · HRM + Cpunloaded -MTS MMTS (Trec-HM − Trec-ML )
(6)
(9)
Qads-HTS = [n · X · HRH + Cpunloaded -HTS MHTS (Td − Trec-HM ) + Cp-r MR-HTS (Td − Trec-HM )]
(10)
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T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
The expected COP of the triple-effect sorption cycle can therefore be calculated from the following: COP = 3 ×
Qevap Qdes-HTS
(11)
Fig. 9 shows the effect of mass ratio (R) on the COP of the proposed triple-effect sorption cycle. The parameter R is defined as the mass ratio of the metallic part of the reactor to the reactive salt. It can be seen that the mass ratio R had a strong influence on the system performance. The COP decreased significantly as the mass ratio R increased. In the range of mass ratio R from 0 to 15, the COP of the proposed triple-effect cycle varied between 0.97 and 0.75. For most of the solid–gas sorption systems with optimized design, the mass ratio (R) between the metallic part of the reactor and the reactive salt is about 5. The corresponding COP obtained with the single-effect sorption cycle using NiCl2 –NH3 , the doubleeffect sorption cycle using NiCl2 –SrCl2 –NH3 , and the triple-effect sorption cycle using NiCl2 –MnCl2 –SrCl2 –NH3 were 0.21, 0.53 and 0.88, respectively. Thus, the system performance can be significantly
Triple-effect cycle
1.2
Double-effect cycle (MnCl2)
R=5
Double-effect cycle (NiCl2)
1.0
Single-effect (MnCl2) Single-effect (NiCl2)
0.6
0.4
0.2
0.0 0.1
0.2
0.3
0.4 0.5 0.6 0.7 Global conversion X
0.8
0.9
1.0
Fig. 10. COP with different global conversion X.
4000 3500
Heat production of MnCl2 salt
5000
Heat production of NiCl2 salt
during the synthesis phase
4500
during the synthesis phase
4000 3000
3500
2500 2000
Heat consumption of SrCl2 salt during the decomposition phase
1500
Q / ( kJ )
Q / ( kJ )
COP
0.8
improved by the application of the proposed triple-effect sorption cycle than the conventional single- and double-effect sorption cycles. The global conversion (X), which represents the percentage of the metallic salt that has reacted with the refrigerant, was used to evaluate the system performance of the proposed triple-effect sorption cycle. Fig. 10 shows the effect of the global conversion X on the COP when the mass ratio R was 5. Generally, the COP increased significantly with the increase in the global conversion X. The COP of the proposed triple-effect sorption cycle varied between 0.09 and 0.88 when the global conversion X ranged from 0.1 to 1. However, high global conversion usually requires a considerable long cycle time because the reaction rate decreases with the conversion time, and the cooling power of the system could be largely reduced. Thus, taking a more realistic situation where all the reactions were assumed 85% complete, the COP obtained with the proposed triple-effect sorption cycle was as high as 0.84. This figure represents an improvement of 342% and 68%, compared with the single-effect sorption cycle (NiCl2 –NH3 ) and the double-effect sorption cycle (NiCl2 –SrCl2 –NH3 ), respectively. During the internal heat recovery process, the reaction heat released by the HTS reactor was reclaimed and used to regenerate the MTS reactor, while the reaction heat released by the MTS was utilized for the desorption of the LTS reactor. The proposed internal heat recoveries are only possible if the total amount of heat released by one reactor during the synthesis phase is equal to or higher than the total heat input required during the regeneration phase of the other reactor. The feasibility of the two internal heat recovery processes between the different reactors was evaluated. The heat released/adsorbed during these adsorption and desorption processes was calculated at the global conversion X of 0.85. The variation of the heat production and heat consumption with mass ratio (R) for the double-effect sorption system is showed in Fig. 11. It shows that the amount of the reaction heat produced by the HTS was higher than the heat consumed by the MTS for both groups of working pairs analyzed. Thus, the reaction heat consumed by the MTS during the decomposition phase can be fully supplied from the reaction heat released by the HTS. Fig. 12 shows the variation of the heat production and heat consumption with mass ratio (R) for the proposed triple-effect sorption system. During the first internal heat recovery process (Fig. 12a), the heat produced by the HTS salt (NiCl2 ) during the synthesis phase was recovered for use in the regeneration process of the MTS salt (MnCl2 ). During the second internal heat recovery process (Fig. 12b), the adsorption heat released by the MTS salt (MnCl2 ) during the
3000 2500 Heat consumption of SrCl2 salt
2000
during the decomposition phase
1500
1000
The amount of heat difference
500
1000 500
0
2
4
6
8 R
10 12 14
The amount of heat difference 0
2
4
6
8 R
10 12 14
Fig. 11. Heat production and heat consumption for the double-effect sorption system: (a) working pair: MnCl2 , SrCl2 –NH3 and (b) working pair: NiCl2 , SrCl2 –NH3 .
T.X. Li et al. / Chemical Engineering Science 64 (2009) 3376 -- 3384
4500
5000 Heat production of NiCl2 salt
4500
Heat production of MnCl2 salt
4000
during the synthesis phase
4000
during the synthesis phase
3500
3500
3000
3000 2500
Heat consumption of MnCl2 salt
2000
during the decomposition phase
1500 The amount of heat difference between HTS and MTS
1000
Q / ( kJ )
Q / ( kJ )
3383
2500 Heat consumption of SrCl2 salt
2000
during the decomposition phase
1500
The amount of heat difference between MTS and LTS
1000 500
500 0
2
4
6
8 R
10 12 14
0
2
4
6
8
10 12 14
R
Fig. 12. Heat production and heat consumption for the triple-effect sorption system: (a) the first internal heat recovery and (b) the second internal heat recovery.
synthesis phase was recovered and used to regenerate the LTS salt (SrCl2 ). It was observed that the amounts of heat difference during the two internal heat recovery processes were higher than 250 kJ, which implies that the two internal heat recovery processes between different reactive salts were feasible. However, it is necessary to take some measures to keep the energy balance, and the extra heat released must be eliminated by a heat sink to ensure proper operation of the proposed system.
4. Conclusion A triple-effect solid–gas thermochemical sorption refrigeration cycle with internal heat recovery is proposed to improve the system performance. Three types of reactive salts were used as the reactants and ammonia was utilized as the refrigerant in the presented system. The adsorption reaction heat of the high-temperature salt was recovered and used during the regeneration process of the middle-temperature salt, while the adsorption reaction heat released by the middle-temperature salt was used to regenerate the lowtemperature salt. Theoretical results showed that the two internal heat recovery processes between different reactors were feasible. The ideal COPi of the proposed sorption system can be improved by 146–200% compared to that obtained with the conventional sorption system. The calculated COP employing the triple-effect sorption cycle varied between 0.75 and 0.97 with the mass ratio between the metallic part of the reactor and the reactive salt. The proposed tripleeffect sorption cycle can produce three cooling-effects per cycle at the expense of one heat input at high temperature. Hence, it has a larger cooling capacity than the conventional sorption refrigeration cycle.
HRL HRM HTS LTS MTS MHTS MLTS MMTS MNH3 MR n Pc Pe Peq Qads Qcond Qdes Qevap R
T Ta Tc Td Te Teq Trec Trec-HM Trec−ML X
reaction enthalpy of LTS, kJ mol−1 reaction enthalpy of MTS, kJ mol−1 high-temperature salt low-temperature salt middle-temperature salt mass of HTS, kg mass of LTS, kg mass of MTS, kg mass of ammonia, kg mass of metallic reactor, kg number of moles of cycled refrigerant condensation pressure, Pa evaporation pressure, Pa equilibrium pressure, Pa adsorption heat, kJ condensation heat, kJ desorption heat, kJ evaporation heat of refrigerant, kJ mass ratio between the metallic mass of the reactor and the mass of reactive salt equilibrium temperature drop, ◦ C adsorption temperature, ◦ C condensation temperature, ◦ C desorption temperature, ◦ C evaporation temperature, ◦ C equilibrium temperature, ◦ C heat recovery temperature, ◦ C heat recovery temperature between HTS and MTS heat recovery temperature between MTS and LTS global conversion
Subscripts Notation COP Cp-HTS Cp-LTS Cp-MTS Cp-NH3 Cp-r Hevap HR HRH
coefficient of performance specific heat of HTS, kJ kg−1 ◦ C−1 specific heat of LTS, kJ kg−1 ◦ C−1 specific heat of MTS, kJ kg−1 ◦ C−1 specific heat of ammonia, kJ kg−1 ◦ C−1 specific heat of metallic reactor, kJ kg−1 ◦ C−1 vaporization enthalpy of refrigerant, kJ mol−1 reaction enthalpy of reactant, kJ mol−1 reaction enthalpy of HTS, kJ mol−1
ads des i
adsorption desorption ideal
Superscripts
liq loaded unloaded
liquid state loaded state unloaded state
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