Experimental Study On Two-stage Adsorption Refrigeration Cycle Using CaCl₂ /BaCl₂ -NH 3 As Working Pair

As a kind of environmental benign refrigeration technology driven by low-grade heat, adsorption refrigeration is very promising on the application of industrial waste heat and solar energy. Nowadays for the conventional single-stage chemical adsorption refrigeration cycle there are two main restrictions for the freezing working conditions when the heating source temperature is lower than 100oC. One restriction happens on the high temperature salts and they generally need the heat source with the temperature higher than 100oC, and the other restriction happens on the low temperature salt that generally need the condensing temperature lower than 25oC when the heat source temperature is lower than 100oC. In order to overcome the limitation of these restrictions a two-stage adsorption refrigeration cycle which is based on basic adsorption process and resorption process was developed, in which calcium chloride and barium chloride were chosen as the middle and low temperature reacting salt respectively. The adsorption performance of two-stage cycle was tested, and after that a simulation of the system had been finished based on the adsorption equilibrium data. Then the two-stage system was designed and established, and the cycle dynamic characteristics have been studied.There are two different desorption processes in a two-stage adsorption refrigeration cycle, the desorption process between the low temperature salt and the condenser, and the desorption process between the middle temperature salt and low temperature salt. Such processes guarantee the system can work successfully under the condition of low heat source temperature, high condensing temperature, and deep freezing refrigeration temperature.The adsorption performance was analyzed firstly under the condition that calcium chloride and barium chloride were chosen as the middle temperature reacting salt and the low temperature reacting salt respectively. Results showed that: To achieve -20oC refrigeration temperature under the condition of cooling temperature of 30oC, compared with CaCl2-NH3 single stage adsorption refrigeration cycle, the maximum equilibrium desorption temperature of CaCl2/BaCl2-NH3 two-stage cycle decreases from 98oC to 69oC, the drop rate is about 30%. While compared with BaCl2 -NH3 single stage adsorption refrigeration cycle, the minimum equilibrium adsorption temperature of the two-stage cycle increases from 16oC to 43oC, the clime rate is about 269%. Meanwhile the driven temperature can be decreased by 13oC. Accordingly, two-stage adsorption refrigeration is suitable for deep-freezing refrigeration utilization when the heating source is between 80oC and 100oC all the year round.Secondly the adsorption performance of two-stage cycle was tested, and the adsorption equilibrium equations were gotten from the experimental data. Then the two-stage system was designed. The models were established for the designed system by the analysis of heat transfer and adsorption equations. The simulation results showed that: The system was able to use 85oC heating source for the output of -15oC freezing temperature, when the condensing temperature is 30oC. The cooling capacity of the system simulated was 2.6kW. The SCP and COP were 115W/kg and 0.238, respectively.Based on the designed system, we established the CaCl2/BaCl2-NH3 two-stage adsorption refrigeration experimental system. The performance tests were carried out and firstly the optimal cycle time was studied, which was 25~40 minutes, and the corresponding SCP and COP were approximately 100W/kg and 0.127, respectively. Results also showed that better performance could be gotten when the CaCl2 desorption time is longer than CaCl2 adsorption time, the performance of the system increased when the heating source temperature increased and cooling temperature decreased. The cooling power of two-stage adsorption system was 1.12~1.49kW when the heating temperature was 85oC, the condensing temperature was 25oC, and the refrigeration temperature was -15~-5oC. During the experimental process the performance of the system would be influenced significantly if there was incoagulable gas, which could get into the system in a variety of ways. The remarkable characteristic for the influence caused by incoagulable gas is the greatly rising of the system desorption pressure, and it would lead to the bad desorption performance, which consequently would lead to the unpleasant refrigeration performance. Experimental results showed that the cooling power decreased by 10% when the desorption pressure increased by 140kPa.