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heat source in order to provide a large heat power to the second adsorber. According to Pons and Poyelle [44], this is possible because the large amount of energy recovered from the first adsorber is regenerated (according to the thermodynamics second law) by the energy supplied at high temperature. The heat regeneration can be a very efficient heat storage process because about 65% of the total energy received by each adsorber can be internally recovered [44]. Furthermore, the heat transfer fluid circuit can be very simple. It needs only a reversible pump and valves are not necessary. Tchernev and Emerson [101] developed a double bed zeolite-water adsorption air conditioner that employed the heat regeneration process to increase the COP and obtained experimental values between 1.6 and 1.0, according to the ambient temperature that ranged from 27 to 38 °C. The SCP was not sensitive to the ambient temperature and it was 36 W kg-1. In the heat regenerative cycle, the flow rate and the type of heat transfer fluid have great influence on the performance of the adsorption system. High flow rates increase the SCP but decrease the COP [38], thus this variable must be chosen carefully to ensure that the machine will have the expected performance. The machine developed by Tchernev and Emerson [101] could achieve a SCP of 108 W kg-1, but in this case, when the ambient temperature was 27 °C, the COP was reduced to 0.7. In the heat regenerative system developed by Miles and Shelton [102], the working pair used was activated carbon-ammonia, and the COP obtained varied between 1.19 and 0.42, for ambient temperatures from 20 °C to 35 °C. Another kind of heat regenerative cycle was suggested by Critoph [40], in which the refrigerant serves as heat transfer medium. A thermodynamic model presented by the author predicted a COP of 0.95 with an evaporation temperature of 0 °C and a condensation temperature of 42 °C. Wade et al. [34] simulated a regenerative adsorption cycle with eight beds that could recover 76 % of the waste energy from the adsorption process. During this cycle, the hot thermal fluid that left the hot adsorber (Adsorber 1 in Fig. 34) pre-heated the adsorbers 2 to 4, before be completely cooled by the heat sink. The cold heat transfer fluid flowed from the heat sink towards the adsorber at the adsorption phase (Adsorber 5). After remove the heat from the adsorber 5, the heat transfer fluid continued its path to pre-cool the adsorbers 6 to 8, and be pre-heated. Then, the thermal fluid was heated until the generation temperature by an external heat source before enters the adsorber 1. This fluid route was maintained during a specific period, before the adsorbers switch position. Fig. 34. The design of heating/cooling thermal fluid loop with eight adsorbers. The previous work was the base for the design of a prototype with four beds that was used to produce cooling at -136 °C [103]. The energy input necessary to produce 1 W of cooling in this prototype was 76.6 W, which was much lower than the 165 W required in the system without heat regeneration. Neveu and Castaing [36] proposed a heat recovery cycle between two adsorbers filled with different metallic salts. The adsorption chemical reaction inside the first adsorber releases the necessary heat to regenerate the salt placed in the second adsorber. Thus, only the salt in the first adsorber needs external heat supply. The use of MnCl2 in the first adsorber and NiCl2 in the second adsorber could produce a theoretical COP of 0.6 at evaporation temperature of –10 °C, while at this same evaporation temperature, the utilization of SrCl2 and FeCl2 would produce a COP of 0.7. In both cases, the assumed condensation temperature was 40 °C, and ammonia was the refrigerant desorbed and adsorber. A different approach to improve the COP of sorption systems was followed by Satzger et al. [39]. The authors presented the theoretical results that can be expected when absorption and chemical reaction chillers operated together under triple and quadruple effect. The absorption chiller is used in the bottom stage due to temperature restrictions observed in this kind of machine, and the chemical reaction chiller is employed in the topping stage. A system performing the cycle with triple effect in double stage only shows higher COP than a double stage absorption system if the COP of the topping machine is higher than 0.3. When the system operates under the three stages quadruple effect cycle, the theoretical COP can be close to 2.0 if the COP of the chemical sorption chiller is 0.5 or about 1.6 with the COP of the later is 0.3. In both cases, the COP would be higher than the value of 1.2, which was assumed for a double stage absorption chiller. Besides the utilization of heat management cycles, it is possible to employ refrigerant mass recovery between two adsorbent beds to enhance effectively both the cooling power and the COP of adsorption systems. Szarzynski et al. [40], analyzed cycles with refrigerant vapour recovery and concluded that the SCP could be increased by about 20 %. This author did not notice major changes in the COP. Wang [48] compared the COP of adsorptions systems with and without vapour recovery and found that the former could produce a COP from 10 to 100 % higher than the latter. The difference between the COPs was higher at lower generation temperatures. In the experiments carried out by Oliveira et al. [51], the operation of an icemaker with mass recovery enhanced the adsorbed mass with about 37 to 42 % when compared to the operation of the system without this process. The pair employed in the experiments was activated carbon-ammonia, and the generation temperatures were 85 and 115 °C. At the lowest generation temperature, the utilization of the cycle with mass recovery in double stage produced the best results, while at the highest generation temperature, the best results were obtained when the conventional mass recovery was employed. The cycle with mass recovery in double stage studied by these authors inserts the process of mass recovery at the end of the generation/adsorption phase of the conventional double stage cycle to increase the amount of adsorbed mass from the evaporator. 17PDF Image | International Sorption Heat Pump Conference
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