Adsor-Resor Cycle · A new type adsorption-resorption cycle for the combined power generation and refrigeration driven by low grade heat

A new type adsorption-resorption cycle for the combined power generation and refrigeration driven by low grade heat

Adsorption-resorption cogeneration cycle features the reasonable electricity generation performance and very high refrigeration performance if compared with the conventional cogeneration cycle driven by the low grade heat. In the main phase that supported by the Marie Curie IIF in Newcastle University the feasibility of the cycle had already been verified theoretically. In the return phase of the project, we had researched three aspects of the system and mostly by experiments. Firstly in order to verify the performance of adsorption-resorption cycle the two-stage system for the refrigeration output was studied. Secondly the thermal conductivity and the permeability of the adsorbents for the cycle were tested. Thirdly the cogeneration performance of the cycle was analyzed and the electrical generation system driven by the compressed air was set up. The detailed description for the achievements as follows. (1) On the research work of the two-stage adsorption refrigeration cycle, a test unit with the adsorption and resorption processes was constructed, and in the cycle two different salts were utilized, i.e. CaCl2 and BaCl2. CaCl2 serves as the high temperature salt (HTS), and BaCl2 served as the low temperature salt (LTS). By two types of salts and two different processes, i.e. resorption and adsorption processes, research results showed that the optimal COP and SCP were 0.127 and 100 W/kg, respectively, under the condition of 85°C heating source temperature, 25°C cooling temperature, and -15°C refrigerating temperature. When the refrigeration temperature was -15°C the cycle could be driven by the heat source with the temperature of 75°C. If compared with the similar heating, cooling, and refrigeration conditions in the reference, for which the COP was generally about 0.03-0.04, the COP of the two-stage adsorption cycle was improved by 2-5 times. The lowest driving temperature for freezing refrigeration from the references commonly was higher than 100-120°C, compared with that the system developed could decrease the heat source temperature by 25-55°C, which showed a prospective application for such type of technology for the recovery of the low grade heat. (2) On the research work for the thermal conductivity and permeability, the matrix of the expanded natural graphite (ENG) was investigated for the heat and mass transfer intensification, and the results were compared with the granular adsorbents. Results showed that the thermal conductivity was quite low for pure salts, which is around 0.2-0.35. The best result of 0.29 W/ (mK) had gotten from the NH4Cl which has smallest density in all salts, i.e. 545 kg/m3. Considering for the different application with the temperature of the heat source, NH4Cl, CaCl2, and MnCl2 were the optimum choices for the LTSs (Low temperature salts), MTSs (Middle temperature salts), and HTSs (High temperature salts), respectively. After the ENG was used as the matrix for the compact adsorbent, the thermal conductivity increased with the increasing proportion of ENG. Likewise the permeability increased with the decreasing mass percentage of ENG. The optimal thermal conductivity was 2.13 W/(mK) for the compact adsorbent with ENG as matrix, which was improved by about 10 times if compared with the granular adsorbent. The permeability varied between 10-13-10-11 m2 when the density of compact salts-ENG changed from 400kg/m3 to 550 kg/m3. (3) On the research for the cogeneration cycle, research showed that the cycle features a variable endothermic process which stands for higher adaptability if compared with the traditional Rankine cycle, very little or no ammonia liquid in the system which is a safety feature. Research also showed that the cycle had an electricity generation exergy efficiency of up to 0.69 and a refrigeration coefficient of performance (COP) of up to 0.77. If compared with the Goswami cycle, which was established based on the absorption Kalina cycle for the cogeneration of electricity and refrigeration, the novel resorption cycle kept the merit of the high exergy efficiency for electricity generation, meanwhile, it overcame the limitation of the low refrigeration coefficient of performance (COP) of Goswami cycle, and improved the COP by 10 times. The optimum overall exergy efficiency was as high as 0.9, which is 40%-60% improved if compared with the Goswami cycle under the same working conditions. Sources of waste heat often are dispersed and come in a range of relatively small energy. Therefore, small scale applications have a significant market potential. However, small scale systems usually are required to be simple and have low first cost. To this end, there is also the possibility to eliminate the super-heater and pre-cooler to further simplify the system. Moreover, sacrificing some efficiency due to leaving out the possibility of internal heat recovery, the plant can be reduced to a very simple system with one HTS bed, one LTS bed, and a turbine. Due to the use of consolidated solid sorbents the plant will be very compact, which is of great importance for small scale application.

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