A consolidated calcium chloride-expanded graphite compound for use in sorption refrigeration systems

Transcription

1 Carbon 45 (2007) A consolidated calcium chloride-expanded graphite compound for use in sorption refrigeration systems R.G. Oliveira *, R.Z. Wang Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai , China Received 21 March 2006; accepted 6 September 2006 Available online 17 October 2006 Abstract A consolidated composite reactive bed for refrigeration sorption systems made from expanded graphite powder impregnated with CaCl 2 was produced and tested. The composite material was compressed under a pressure of 10 MPa to enhance its heat transfer properties. Experimental results showed that this material could incorporate 0.90 kg of NH 3 per kg of salt and that the agglomeration phenomenon was avoided. The blocks with apparent density of 0.56 g cm 3 and 35% of expanded graphite ensured a good refrigerant mass transfer with a negligible pressure drop between the inner and outer part of the blocks. However, the heat transfer still need some improvements, as the temperature difference inside the blocks could reach 15 C during the decomposition phase. The average specific cooling power during the synthesis phase was 306 and 194 W per kg of salt at the average evaporation temperatures of 2.7 and 18.3 C, respectively. The calculated coefficient of performance under different generation temperatures and global conversions ranged from 0.28 to 0.46, and it was not very sensitive to the increase of the generation temperature. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Sorption refrigeration and heat pump machines are heat-powered systems, which can use a large range of heat source temperatures. In the case where ammoniates or aminoderivatives are employed as sorbents, the reacting temperature can range from 50 C to 300 C [1]. The possibility to use temperature sources from 50 to 150 C may be useful in energy saving applications, as those employing solar energy or waste heat. Furthermore, these systems can be ecological friendly, as they normally do not utilize CFCs and HCFCs as refrigerants. For evaporation temperatures equal to or below 0 C, activated carbon is the physical adsorbent most employed [2 5]. When considering the sorption capacity, metallic salts are a better alternative than the physical adsorbents, * Corresponding author. Tel./fax: address: rgra_br@yahoo.com (R.G. Oliveira). as the amount of refrigerant incorporated can be as high as 1.05 kg kg 1 [6]. However, salts and granular physical adsorbents have low thermal conductivity, which increase the reaction time and decrease the specific cooling power. Furthermore, salts may agglomerate after consecutives decompositions and synthesis, which can reduce the conversion capacity. Wang et al. [7] mixed CaCl 2 with activated carbon and they avoided the agglomeration and obtained constant adsorption capacity. Lu and coworkers [8] also used a mixture of CaCl 2 and activated carbon in an adsorption icemaker, which had a specific cooling power ranging from to W kg 1 according to the operation condition employed. Vasiliev et al. [9] developed a solar-electric refrigerator where the salt was mixed with carbon fibres to increase the heat and mass transfer performance and avoid the agglomeration phenomenon. Apart from activated carbon or carbon fibres, expanded graphite is another carbonaceous material that can be used as additive in reactive salt beds. One of the pioneers in the utilization of mixtures of expanded graphite and salt was /$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi: /j.carbon

2 R.G. Oliveira, R.Z. Wang / Carbon 45 (2007) Coste et al. [10]. They assessed the heating output of heat pumps using different salt, and with different proportions of expanded graphite. Later, Mauran et al. [11] patented a process of manufacturing consolidated blocks impregnated with metallic salts. A heat pump using the consolidated expanded graphite block impregnated with CaCl 2 achieved a heating output of 770 kw m 3, when the refrigerant employed was methylamine. Several other studies [12 16] showed that the expanded graphite when employed as inert material could avoid the agglomeration of the salt and improve the heat and mass transfer in the reactive bed of chemical heat pumps. In the study performed by Wang and coworkers [17], the effective thermal conductive measured was about 9 W m 1 K 1, when the content of expanded graphite was 20%. Han and Lee [18] identified the thermal conductivity of several salts impregnated in expanded graphite blocks and measured values close to 50 W m 1 K 1 when the amount of expanded graphite was 70%. These authors showed that there is a great variation of the thermal conductivity among the blocks with different proportion of expanded graphite and densities. The presented work aimed to design and test a consolidated composite reactive bed to be employed in sorption refrigeration machines. The reactive bed was made from expanded graphite powder impregnated by CaCl 2. Different from the patent applied by Mauran et al. [11], and from other studies [12 16], in the presented work, no vacuum chamber was used in the impregnation process, which occurred in the expanded graphite powder before the manufacturing of the blocks, as explained in Section 2.1. The conversion capacity of the composite blocks reacting with ammonia was assessed by measuring the variation of the level of this refrigerant in the evaporator/condenser during the synthesis and decomposition phases. The temperature profiles in different positions of the bed and the pressure drop within the bed were analyzed to identify which was the limiting factor in the conversion rate. The specific cooling power (SCP) and the coefficient of performance (COP) Table 1 Chemical composition of the expandable graphite C (%) H (%) N (%) S (%) Remainder (%) obtained with the consolidated bed were calculated for different operation conditions. 2. Experimental set-up and results 2.1. Preparation of the expanded graphite and the consolidated composite blocks The first step to prepare the consolidated composite blocks was the expansion of the expandable graphite by heating treatment. The expandable graphite used was the type KP80 (Mesh 80), which the chemical composition is presented in Table 1. In the experiments performed by Wang et al. [17], the expanded graphite underwent expansion at the temperature of 300 C. But according to Han et al. [19] the temperature of the treatment influences the degree of expansion and these authors stressed the importance of expand the graphite at temperatures higher than 600 C, to ensure a proper expansion. Han and coworkers [19] also noted that the graphite expanded at temperatures above 700 C had the lowest densities and at least twice the porosity of the expanded graphite treated at 500 C. In this work, several temperatures and heating time (from 300 to 700 C and 5 to 120 min) were tested to assess which condition would be used to produce the expanded graphite employed in the manufacturing of the composite blocks. The apparent density and the residual mass fraction of the expanded graphite were the figures of merit used to choose the expanding conditions. The apparent density was obtained using a proprietary standard method similar to that described by Han and coworkers [19]. The residual mass fraction (x) was calculated with the following equation: x ¼ m EdG 100% ð1þ m ElG where m EdG and m ElG are the mass of expanded and expandable graphite, respectively. As can be seen in Table 2, the apparent density of the expanded graphite decreases with the heating temperature. At the temperature of 600 C, the apparent specific volume was 96.2 ml g 1, which was much lower than the ml g 1 informed by the producer of the expandable graphite (Qingdao Tianhe Graphite Co. Ltd.). When the heating temperature was increased to 700 C, an apparent specific volume of ml g 1 was obtained, but at an expense of a great reduction in the sample mass. As the content of carbon in the expandable graphite was 86.38%, any value equal to or lower would imply that carbon was burned. When larger samples were heated, the results, for the same expansion condition used in the lighter samples were different, and the sample with larger mass always presented higher density and higher residual mass fraction. Considering the heating temperature and time of 700 C and 5 min, respectively, the sample with 50 g presented an apparent density almost twice the apparent density of the 2 g sample. The increase of the heating time of the 50 g sample at 700 C did not caused a pronounced reduction of the apparent density, which suggests that the highest expansion of the Table 2 Heat treatment conditions and results Initial mass (g) Temperature ( C) Time (min) Apparent density (kg m 3 ) Apparent specific volume (ml g 1 ) Residual mass fraction (%) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± 0.9

3 392 R.G. Oliveira, R.Z. Wang / Carbon 45 (2007) graphite happens in the first moment when it was submitted to the heat treatment. For this reason, all parts of the sample should be uniformly heated and reach the heating temperature almost instantaneously when submitted to the heating treatment. From the obtained results, the expansion conditions chosen to produce the expanded graphite used in the composite blocks were: heating temperature of 700 C and heating time of 10 min. After the heat treatment, the expanded graphite was mixed with a 60% salt solution and dried for 15 h at 100 C to allow the impregnation of the CaCl 2 Æ 2H 2 O in the expanded graphite, and then, kept for more 7 h at 250 C, to calcinate the CaCl 2 Æ 2H 2 O into CaCl 2. The final proportion between expanded graphite and dry salt was 7/13, which corresponded to a graphite mass fraction in the compound (f g, Eq. (2)) of 0.35: m EdG f g ¼ ð2þ m EdG þ m S where m S is the mass of dry salt. The composite powder was inserted in a mold and compressed during about 30 s under a pressure of 10 MPa to enhance the heat transfer properties of the compound. This procedure was done because according to Wang et al. [17], the consolidated compound can have a thermal conductivity about 30 times higher than the unconsolidated powder. The composite powder was weighted before being introduced in the mold and the desired apparent density of the expanded graphite in the block (q EdGÆB, Eq. (3)) was achieved when the volume of the mixture during the compression time was reduced to a prespecified value. The blocks were produced to have an q EdGÆB equal to 0.20 g cm 3 and the average apparent density of the blocks (q B, Eq. (4)) was 0.56 ± 0.01 g cm 3 : q EdGB ¼ m EdG ð3þ V B q B ¼ m EdG þ m S ð4þ V B where V B is the volume of the consolidated composite block. The reactor employed in the experiments was filled with four blocks, as shown in Fig. 1. Assuming that the porosity of the bed (e) can be calculated as suggested by Han and Lee [20], then, the block when the salt incorporated 8 mol of ammonia (CaCl 2 Æ 8NH 3 ) had a porosity of The conversion capacity of the consolidated block Once the reactor was filled with the blocks, it was heated to 97 C and connected to a vacuum pump to be degassed for 3 h. The experiments to identify the conversion capacity were performed in the test rig shown in Fig. 2. The reactions for the decomposition and synthesis phases were CaCl 2 8NH 3 () CaCl 2 4NH 3 þ 4NH 3 ð5þ CaCl 2 4NH 3 () CaCl 2 2NH 3 þ 2NH 3 ð6þ The synthesis and decomposition reactions take place when the salt is removed from the equilibrium condition of temperature and pressure. The equilibrium lines for the reactions described in Eqs. (5) and (6), and the constraint conditions at the synthesis and decomposition phases are presented in Fig. 3. The synthesis constraint 1 and the decomposition constraint were not kept constant during the experiments, and the value showed in Fig. 3 is the value at the end of the experiment. The evolution of these temperature constraints is shown in Section 2.3. Fig. 1. Consolidated composite blocks. Fig. 2. Experimental test rig (a), and thermocouple position in the upper block (b).

4 R.G. Oliveira, R.Z. Wang / Carbon 45 (2007) Fig. 3. Clausius Clapeyron diagram. To assess the global conversion (X), the volume of ammonia desorbed or absorbed was measured by the displacement of a magnetostrictive level sensor positioned in the evaporator/condenser. The relative measuring error of the sensor was less than 0.05% [8]. It is assumed that a global conversion equal to 1 represents that 6 mol of NH 3 reacted with 1 mol of CaCl 2 (0.919 kg of ammonia per kg of dried salt). The assessed global conversions in the decomposition and synthesis phases described by the reactions of Eqs. (5) and (6) were ± and ± 0.002, respectively. Such values of conversion indicate that the expanded graphite matrix in the proportion and density used helped to avoid the agglomeration phenomena and thus, to promote an almost complete reaction (98% of conversion) without attenuation effect after several synthesis and decomposition cycles. Although the agglomeration was avoided, the expansion of the blocks was observed and the inner radius changes from 73 mm to 60 mm. The apparent density of the blocks decreased from 0.56 to about 0.34 g cm 3 and the porosity increased to Such an expansion of the block was necessary because the void volume inside the block was not enough to incorporate the adsorbed ammonia Heat and mass transfer in the consolidated compound Fig. 4. Temperature in the reactor during decomposition (a), and synthesis phases (b). As is possible to see in Fig. 4a, that when the heat transfer fluid reaches temperatures higher than 62 C, the temperature inside the reactor remains almost constant for several minutes which means that the decomposition from CaCl 2 Æ 8NH 3 to CaCl 2 Æ 4NH 3 started and the heat released from the heat transfer fluid was used to supply the desorption heat instead of the sensible heat. The small increase in the global conversion observed before the heat transfer fluid was at 62 C(Fig. 5a), could be caused by the desorption of ammonia from the expanded graphite matrix, as at this temperature, the equilibrium pressure in any point of the bed, for the reaction described in Eq. (5), was not higher than the condensation pressure. The amount of fluid desorbed by the expanded graphite matrix corresponded, in this case, to 0.04 kg kg 1. When the reaction described by Eq. (5) finished, the temperature in different points of the bed started to increase again until reached a temperature high enough to start the reaction described by Eq. (6). The part of the bed in contact with the wall (T2, Fig. 2b) completed the first reaction 10 min earlier than the part of the bed closer to the gas channel (T5). The temperature of the bed during the first 5 min of the synthesis was almost uniform and the conversion was little (Figs. 4b and 5b). From this moment, due to the temperature conditions, the bed around T2 could have both reactions occurring simultaneously. Around T5, during the follow 5 min only the reaction to synthesize CaCl 2 Æ 4NH 3 was occurring, and as the amount of heat released by this reaction started to decrease, the temperature started to drop. From about 20 to 35 min, the temperature around T5 was almost constant, which implies that the reaction to synthesize CaCl 2 Æ 8NH 3 was occurring and it was releasing heat to the adsorbent bed. It is possible to observe in Fig. 4 that once the reaction was finished in a portion of the bed, the temperature at this point start to increase or decrease, according to the kind of reaction, steadily. This interruption of the temperature change during the heating or cooling of the reactive bed was due to the fact that the heat transfer in the bed was not enough to provide heat as fast it was consumed by the reactions. In the parts of the bed closer to the wall, and thus with faster heat supply, the effect of the reaction heat was less pronounced and the interruption of the temperature change was smother. The differential pressure sensor showed negligible pressure difference in all experiments, and in the experiment with constraint pressure of 3.6 bar, the maximum pressure drop between the inner and outer part of the bed was 52 mbar, with an average value of 17 mbar. Such a result indicates that the mass transfer in the bed did not influenced the rate of chemical reaction and the heat transfer, although

5 394 R.G. Oliveira, R.Z. Wang / Carbon 45 (2007) Fig. 5. Global conversion evolution during decomposition (a), and synthesis phases (b). Fig. 6. Specific cooling power. improved, was the limitation factor in the length of the synthesis and decomposition phases Specific cooling power From the assessed variation of global conversion with the time (X(t)), it was possible to calculate the specific cooling power (SCP) during the cooling phase, which is described in Eq. (7): SCP ¼ cm NH 3 h vl ðt EV Þ M S X ðtþ t t 0 h vl ðt EV Þ¼ h vlrðt CR T EV Þ ðt CR T R Þ where c is the number of moles of refrigerant consumed per mol of salt to convert CaCl 2 Æ 2NH 3 into CaCl 2 Æ 8NH 3 ; M NH3 is the molar mass of ammonia; M S is the molar mass of the dry salt; h vl (T EV ) is the enthalpy of vaporization (J kg 1 ), h vlær is the enthalpy of vaporization at the reference temperature (J kg 1 ), t is the synthesis time, t 0 is the initial time of the synthesis reaction, T CR is the critical temperature for ammonia ( C), T EV is the evaporation temperature ( C); and T R is the reference temperature equal to 0 C. The SCP was calculated for all the length of the synthesis phase and this procedure allows the identification of the reaction time that produces the highest SCP. In the experiment where the initial evaporation temperature was 0 C, the temperature of the heat transfer fluid and the bed decreased to the set point temperature simultaneously to the synthesis reaction (Fig. 4b). The reaction in this situation was almost finished even before the heat transfer fluid had reached the constraint temperature of 30 C. As can be seen in Fig. 6a, the highest SCP, which was 415 W kg 1, was obtained at 35 min of reaction. At this moment, the heat transfer fluid temperature was still about 42 C, the evaporation temperature had dropped to 5 C and the global conversion was As the reaction continued, the specific cooling power started to decrease because the reaction rate was slowed, and when 97% of the conversion was completed, the average SCP during the synthesis phase was 306 W kg 1. It is expected that if ð7þ ð8þ the heat transfer fluid was kept at 30 C during the whole cooling phase, the temperature of the bed would had dropped faster and higher SCP could be obtained. During the experiment where evaporation temperature was set to 15 C (Fig. 6b), the heat transfer fluid and the bed were first cooled to the constraint temperature of 30 C and then, the composite bed started to incorporate the refrigerant. The highest SCP was 255 W kg 1 at the synthesis time of 40 min. The global conversion at this moment was 0.48 and the evaporation temperature had dropped to 22 C. When 97% of the conversion was completed, the average SCP over the synthesis phase was 194 W kg 1. The difference between the amount of global conversion at the highest values of SCP for the experiments with constraint evaporation temperature of 0 and 15 C were due to the different cooling pattern of each experiment, and it can be explained as follows. The rate of global conversion can be described as a function of equilibrium drop (DP Eq or DT Eq ), the Arrhenius factor (Ar) and the vacant sites for reaction (f(x)), in a generic form presented in equation (Eq. (9)): dx dt ¼ f ðx ÞArDP Eq ð9þ Mazet et al. [15] suggested that f(x) can have the form of (1 X) n, which physically implies that the rate of conversion decreases with the amount of conversion. The equilibrium drop can be described by Eq. (10) and the equilibrium pressure (P Eq ) is a function of the bed temperature, as shown in Eq. (11). For such a reason, the furthest is the equilibrium pressure from the constraint pressure, or the bed temperature from the equilibrium temperature, the fastest will be the reaction: DP Eq ¼ 1 P Eq P C ð10þ lnðp Eq Þ¼ DH S RT þ DS R ð11þ where P C is the constraint pressure (Pa); DH S is the reaction enthalpy equal to 41,413 and 42,268 J mol 1 for the reactions in Eqs. (5) and (6), respectively; DS is the reaction entropy equal to and J mol 1 K 1 for the reactions in Eqs. (5) and (6), respectively; and R is the gas constant (J mol 1 K 1 ) [21].

6 R.G. Oliveira, R.Z. Wang / Carbon 45 (2007) When the temperature and pressure of the bed is kept constant, it is expected that the refrigerant can be absorbed faster at the beginning of the reaction, and as the reaction proceeds, the absorption rate is reduced. But as the equilibrium drop also influences the evolution of the reaction, the increase of this drop will also increase the reaction rate. Thus, these two factors influence the SCP during the synthesis phase. The reaction that started with evaporation temperature of 15 C occurred when the equilibrium pressure of the bed was as further as possible from the constraint pressure, and thus, as the conversion amount started to increase, the reaction rate started to decrease and also the SCP. For the experiments where evaporation temperature started at 0 C, while the advancement of the global conversion reduced the rate of chemical reaction, the increase of the temperature and pressure drop, enhanced the reaction rate, and the influence of these two factors together caused the decrease of SCP only when the global conversion was more advanced Coefficient of performance The COP of a system employing the consolidated composite adsorbent was calculated at the two evaporation temperatures used in the experiments. Furthermore, it was considered how the COP would change at different generation temperatures (T G ) and different total global conversions (DX). Eqs. (12) (16) together with the follow assumptions were used to calculate the COP: (I) The temperature at the beginning of the decomposition phase (T i ) was 30 C. (II) Both synthesis and decomposition would have the same total global conversion. (III) The physical properties of the salt and expanded graphite were constant at the considered temperature range, except the specific heat of the salt that changed according to the equilibrium temperature, and thus, with the amount of ammonia absorbed by the salt. (IV) The number of moles of ammonia absorbed by the salt changed according to the temperature of the bed: 8 mol from T i until the equilibrium temperature for the reaction described in Eq. (5) (T Eq84 ); 4 mol from T Eq84 until the equilibrium temperature for the reaction described in Eq. (6) (T Eq42 ); and then 2 mol until T G. (V) The evaporation temperature was constant (0 or 15 C). (VI) Only the heat consumed by the consolidated blocks was computed. (VII) There was no heat loss to the environment. (VIII) The reaction heat (DH S ) was a weighted average of the reactions heat relative to Eqs. (5) and (6). This assumption did not produce significant error, as the difference between the enthalpy of each reaction is less than 3%. (IX) The mass of ammonia in the gas phase inside the reactor was negligible compared to the mass of the salt and the mass of the expanded graphite. Q S ¼ n S C ps8 ðt Eq84 T i Þþn S C ps4 ðt Eq42 T Eq84 Þ þ n S C ps2 ðt G T Eq42 Þ Q EG ¼ n SM S f g ð1 f g Þ C p EdG ðt G T i Þ Q R ¼ n S cdh S DX Q E ¼ n S cm NH3 h vl DX Q E ð12þ ð13þ ð14þ ð15þ COP ¼ ð16þ ðq S þ Q EG þ Q R Þ where n S is the number of moles of salt; C ps8 is the specific heat of CaCl 2 Æ 8NH 3 equal to 610 J C 1 mol 1 [22]; C ps4 is the specific heat of CaCl 2 Æ 4NH 3 equal to 360 J C 1 mol 1 [22]; C ps2 is the specific heat of CaCl 2 Æ 2NH 3 equal to 190 J C 1 mol 1 [22]; and C pedg is the specific heat of the expanded graphite equal to 610 J C 1 kg 1 [23]. In the considered conditions, the consolidated adsorbent could produce a COP ranging from 0.36 to 0.46 when the global conversion was equal or higher than 0.5 (Fig. 7). The influence of the generation temperature on the COP was more pronounced with the decrease of the conversion, due to the reduction of the proportion between the reaction heat load (Q R ) and the total heat load (Q T ). The lowest conversions also produced the lowest COP. At global conversion equal to 0.25, there was an increase of 9.6% in Q T when T G changed from 105 to 150 C, and Q R corresponded to 58.3% and 53.3% of Q T, respectively. When the global conversion was 1, Q R at T G of 150 C was 82% of Q T. At this condition, Q T was only 3.5% higher than at T G of 105 C. Different from what was observed by in research with physical adsorbents [24 28], the reduction of the evaporation temperature and the generation temperature did not reduce the COP. This is due to the fact that in the physical adsorption systems, like those employing activated carbon as sorbent, higher generation temperatures result in higher desorbed and thus, adsorbed mass. Furthermore, at a specific generation temperature, lower evaporation temperatures produce lowers adsorbed mass. However, in chemical adsorption, as long as the system is removed from the equilibrium and no mass transfer problem exists, it can be expected that the absorption and desorption capacity will not change at different values of constraint temperatures. Thus, in this case, the increase of the temperature generation enlarges the sensible heat without produce gain of desorbed/ absorbed mass, which by consequence reduces the COP. As the decrease of the evaporation temperature does not decrease the absorption capacity, but increases the enthalpy of vaporization of the refrigerant, higher COP can be expected at lower evaporation temperatures. 3. Conclusions Fig. 7. COP. A consolidated composite reactive bed made from expanded graphite impregnated with CaCl 2 was manufactured for utilization in sorption refrigeration systems. Experimental results showed that the expanded graphite avoided agglomeration of the salt and produced a

7 396 R.G. Oliveira, R.Z. Wang / Carbon 45 (2007) compound with stable absorption capacity of 0.90 kg of NH 3 per kg of CaCl 2. A simple design reactor using such a consolidated adsorbent can produce a SCP of 415 and 255 W kg 1 at the average evaporation temperatures of 2.7 and 18.3 C, respectively. The assessed COP ranged from 0.36 to 0.46 when the global conversion was equal or higher than 0.5, and the increase of the generation temperature in the range studied does not produce significantly decrease in this figure, which is an advantage considering that higher temperature lifts can reduce the cycle time and thus, increase the SCP. Acknowledgements This work was supported by National Science Fund for Distinguished Young Scholars of China under the Contract No , Shanghai Shuguang Training Program for the Talents under the Contract No. 02GG03. References [1] Spinner B. Ammonia-based thermochemical transformers. Heat Recov Syst CHP 1993;13(4): [2] Tamainot-Telto Z, Critoph RE. Adsorption refrigerator using monolithic carbon ammonia pair. Int J Refrig 1997;20(2): [3] Wang RZ. Adsorption refrigeration research in Shanghai Jiao Tong University. Renew Sust Energy Rev 2001;5(1):1 37. [4] Li M, Wang RZ, Xu YX, Wu JY, Dieng AO. Experimental study on dynamic performance analysis of a flat-plate solar solid-adsorption refrigeration for ice maker. Renew Energy 2002;27(2): [5] Oliveira RG, Silveira Jr V, Wang RZ. Experimental study of mass recovery adsorption cycles for ice making at low generation temperature. Appl Therm Eng 2005;26: [6] Wang LW, Wang RZ, Wu JY, Wang K, Wang SG. Adsorption ice makers for fishing boats driven by the exhaust heat from diesel engine: choice of adsorption pair. Energy Convers Manage 2004;45: [7] Wang LW, Wang RZ, Wu JY, Wang K. Compound adsorbent for adsorption ice maker on fishing boats. Int J Refrig 2004;27: [8] Lu ZS, Wang RZ, Wang LW, Chen CJ. Performance analysis of an adsorption refrigerator using activated carbon in a compound adsorbent. Carbon 2006;44(4): [9] Vasiliev LL, Mishkinis DA, Antukh AA, Vasiliev LL. A solar and electrical solid sorption refrigerator. Int J Therm Sci 1999;38(3): [10] Coste C, Crozat G, Mauran S. Gaseous-solid reaction. EUA patent , [11] Mauran S, Lebrun M, Prades P, Moreau M, Spinner B, Drapier C. Active composite and its use as reaction medium. USA patent , [12] Balat M, Spinner B. Optimization of a chemical heat pump energetic density and power. Heat Recov Syst CHP 1993;13(3): [13] Han JH, Lee KH, Kim DH, Kim H. Transformation analysis of thermochemical reactor based on thermophysical properties of graphite MnCl 2 complex. Ind Eng Chem Res 2000;39(11): [14] Huang HJ, Wu GB, Yang J, Dai YC, Yuan WK, Lu HB. Modeling of gas solid chemisorption in chemical heat pumps. Sep Purif Technol 2004;34: [15] Mazet N, Amouroux M, Spinner B. Analysis and experimental study of the transformation of a non-isothermal solid/gas reacting medium. Chem Eng Commun 1991;99: [16] Lu HB, Mazet N, Spinner B. Modelling of gas solid reaction coupling of heat and mass transfer with chemical reaction. Chem Eng Sci 1996;51(15): [17] Wang K, Wu JY, Wang RZ, Wang LW. Effective thermal conductivity of expanded graphite CaCl 2 composite for chemical adsorption chillers. Energy Convers Manage 2006;47(13 14): [18] Han JH, Lee KH. Effective thermal conductivity of graphite-metalic salt complex for chemical heat pumps. J Thermophys Heat Transfer 1999;13(4): [19] Han JH, Cho KW, Lee KH, Kim H. Porous graphite matrix for chemical heat pumps. Carbon 1998;36(12): [20] Han JH, Lee KH. Gas permeability of expanded graphite-metallic salt composite. Appl Therm Eng 2001;21(4): [21] Neveu P, Castaing J. Solid gas chemical heat pumps: Field of application and performance of the internal heat of reaction recovery process. Heat Recov Syst CHP 1993;13(3): [22] Hosatte-Ducassy S, Rheault F. Kinetics and modelling of CaCl 2 NH 3 reactions. In: Extended symposium of solid sorption refrigeration, Paris, p [23] Bejan A. Transferência de calor. São Paulo SP: Edgard Blücher LTDA; p [24] Critoph RE. Evaluation of alternative refrigerant-adsorbent pairs for refrigeration cycles. Appl Therm Eng 1996;16(11): [25] Teng Y, Wang RZ, Wu JY. Study of the fundamentals of adsorption systems. Appl Therm Eng 1997;17(4): [26] Szarzynski S, Feng Y, Pons M. Study of different internal vapour transports for adsorption cycles with heat regeneration. Int J Refrig 1997;20(6): [27] Wang RZ, Wu JY, Xu YX, Wang W. Performance researches and improvements on heat regenerative adsorption refrigerator and heat pump. Energ Convers Manage 2001;42(2): [28] Wang RZ. Performance improvement of adsorption cooling by heat and mass recovery operation. Int J Refrig 2001;24: