Study On Two-Stage Adsorption Refrigeration Cycle Driven By Low-Temperature Heat Source

Adsorption refrigeration is considered as kind of green refrigeration technology owing to its ability to be driven by low-grade heat source, which grants it promising development on application of industrial waste heat and solar energy. Conventional single-stage chemical adsorption refrigeration cycle has two main disadvantages, namely requirements of higher temperature heat source and lower temperature cooling media, when applied under freezing condition driven by low-grade heat source. To solve those problems, a two-stage adsorption refrigeration cycle is proposed based on adsorption refrigeration process and resorption process. Calcium chloride and barium chloride are chosen as middle temperature reacting salt and low temperature reacting salt respectively to construct two-stage adsorption refrigeration cycle with CaCl₂/BaCl₂-NH3 as working pair. Through study of cycle dynamic characteristics and test of refrigeration performance of the two-stage adsorption refrigeration cycle, main conclusions are obtained as follows:(1) Two-stage adsorption refrigeration cycle was proposed based on adsorption refrigeration process and resorption process, and in the cycle, regeneration processes of middle temperature reacting salt and low temperature reacting salt were finished by two-stage desorption process. The two-stage adsorption refrigeration cycle can lower down the temperature requirements of heat source and cooling media.(2) Calcium chloride and barium chloride are chosen as middle temperature reacting salt and low temperature reacting salt respectively for better reacting dynamic characteristic performance. As to choice of additive, corrosion performances of adsorbents with activated carbon and expanded graphite as additive and without additive were studied by experimental comparison. The results showed that with coexistence of oxygen and water, CaCl2/expanded graphite would make more serious corrosion to stainless steel, but all of the adsorbents barely made corrosion to stainless steel without oxygen or water. Expanded graphite was chosen as additive considering better adsorption performance of composite adsorbent and actual working condition of nonexistence of oxygen and water.(3) Cycle dynamic characteristic analysis of two-stage adsorption refrigeration cycle with as CaCl₂/BaCl₂-NH3 working pair showed that maximum equilibrium desorption temperature could be lowered down by nearly 30%, compared single-stage chemical adsorption refrigeration cycle with CaCl₂-NH₃ as working pair when the cooling temperature is 30℃. Even in summer when the cooling temperature is as high as 35℃, maximum desorption temperature of the two-stage cycle is just 75℃. Thus, the two-stage adsorption refrigeration cycle is suitable of utilization of heat source between 80℃and 100℃all over the year.(4) Performance test of two-stage adsorption refrigeration cycle with CaCl₂/BaCl₂-NH3 as working pair showed that maximum cycle desorption quantity of CaCl₂ was 0.870kg/kg when heating temperature, cooling temperature and evaporating temperature were 90℃, 25℃and 5℃, respectively and minimum cycle desorption quantity of CaCl₂ was 0.121kg/kg when heating temperature, cooling temperature and evaporating temperature were 70℃, 30℃and -25℃. Especially, cycle adsorption quantity of CaCl₂ was between 0.302kg/kg and 0.552kg/kg when heating temperature, cooling temperature and evaporating temperature were 70～90℃, 30℃and -20℃, respectively.(5) Prediction of coefficient of performance and specific cooling power of the two-stage adsorption refrigeration cycle showed that, coefficient of performance was about 0.23⁰.26, irrespective of heat capacity of adsorbers and sensible heat of adsorbents and heat transfer fluid. Considering a two-stage adsorption refrigerator with CaCl₂/BaCl₂-NH3 as working pair whose cooling power is 3kW, specific cooling power is 139.5～184.6W/kg when heating temperature, cooling temperature and evaporating temperature were 85℃, 25～35℃and -25～5℃, respectively.