Study On Working Characteristics Of Decreasing-Pressure Desorption And Working Pairs Of The Soled-Gas Thermochemical Sorption Refrigeration Driven By Low-Temperature Thermal Energy

Recent years, with the increasing attention of the environment and energy issues, and the increasing requirement about improving living condition, exploring and utilizing new energy sources is considered more and more important. For air condition field, how to use the low-grade heat such as solar energy to reduce the energy consumption of air-conditioners is increasingly concerned. Traditional sorption refrigeration is limited to the circulation way so that only barium chloride and strontium chloride were used as adsorbent for solar-powered thermo chemical sorption refrigeration, which restricts many other high temperature sorbents to be utilized in low-grade heat.Considering the problems, the two-stage and low-temperature-powered thermo chemical sorption refrigeration cycle was presented, and reducing the heating temperature of high temperature salts and analyzing the related parameters were targeted. To enhance the heat and mass transfer of salts, expanded graphite was impregnated to the sorbents, and three kinds of consolidated composite sorbent block were made from——MnCl2-expanded graphite, BaCl₂-expanded graphite and SrCl₂-expanded graphite. In the experiment of the two-stage thermo chemical sorption refrigeration cycle, MnCl2-expanded graphite was chosen as high temperature sorbent and SrCl₂-expanded graphite as low temperature sorbent for its better matching ability was showed by the theoretical analysis and experiments of BaCl₂-expanded graphite and SrCl₂-expanded graphite. The main conclusions are as follows: (1) Three kinds of consolidated composite sorbent impregnated with expanded graphite were produced, and the composite materials were compressed under a pressure of 10MPa to enhance their heat transfer properties. Preparation of the expanded graphite and the consolidated composite blocks showed that, heating treatment to graphite at about 700℃for 5 to 10 minutes was perfect, for a proper expansion and the lowest mass loss were achieved that way. During the compressing, well-distributed pressure on the blocks should be noted to ensure the uniform density of the blocks. Meanwhile, heating and degassing the system filled with blocks before experiment were essential, or the steel system materials would be corroded seriously.(2) Contrast experimental results showed that SrCl₂-expanded graphite was more suitable for deep-freezing systems, because there was sorption reaction with ammonia even at evaporation temperature of -20℃. And the maximal conversion mass could reach 0.7kg/kg at the evaporation temperature of -5℃and the cooling water temperature of 30℃, then, when the semi cycle time was 60min, the SCP could reach 220W/kg, which were better than those of BaCl₂-expanded graphite at the same working condition. That meant the less salt mass needed and the smaller scale of systems when using SrCl₂-expanded graphite as sorbent. Besides, according to Clausius-Claperon diagram, the much more generation temperature drop of high temperature salts in the two-stage sorption refrigeration cycle would be theoretically obtained, while using the SrCl₂-expanded graphite as the secondary sorbent, compared with the conventional sorption refrigeration cycles. At the same time, the generation temperature of MnCl2 could fall down to about 100℃.(3) Experimental results of the two-stage sorption refrigeration cycle as MnCl2-expanded graphite being the high temperature sorbent and SrCl₂-expanded graphite the secondary sorbent indicated that, the generation temperature of the high temperature salt was obviously lowered by the two-stage desorption technology, and the process was influenced by several parameters. The much higher the generation temperature was, the much faster the desorption reaction proceeded, and less than 50min was consumed of the first stage desorption process at the generation temperature of 140℃for HTS, and the cooling temperature of 30℃for LTS. Meanwhile, the pressure of the whole system was more affected by the pressure of HTS, which even could impact the reaction of LTS. At the same evaporation temperature, the much higher the generation temperature was, the more the first stage desorption conversion was, and which was good for the long-time operation of systems. But at the same desorption temperature, considering the sorbent fully loaded, the much higher the evaporation temperature was, the less the first stage desorption conversion was, that implied there was a optimization of matching between the desorption temperature and the evaporation temperature. When the evaporation temperature of -30℃, the cooling temperature of 30℃and the conversion percentage of 100%, the generation temperature drop could achieved to 60℃, which was meaningful for enlarging the application field of the low-grade heat like the solar energy.