Research Of Adsorption Refrigeration System Driven By Low Temperature Heat Source

Adsorption refrigeration technology is environmentally friendly driven by low-grade heat sources.Most of adsorbent-adsorbate pairs are natural.Among them,silica gel-water has wide application prospects in the field of solar air conditioning due to its low desorption temperature.At present,the main shortcomings of silica gel-water adsorption refrigeration system are low energy efficiency and huge unit volume.The commercialization of adsorption refrigeration will be the future research focus of adsorption refrigeration technology.The main work of this paper are the performance study of silica gel composite adsorbent and its application in adsorption refrigeration system,including the adsorption performance and mechanism study of composite adsorbent,the design and construction of adsorption refrigeration system,the thermodynamic analysis of adsorption refrigeration cycle and the performance study of adsorption refrigeration system.The main contents of this paper are as follows:(1)Coarse porous silica gel with different particle diameters is selected as the matrix of composite adsorbent.Calcium chloride is selected as additive.The composite adsorbent is prepared through vacuum impregnation method.The performance characterization parameters of pure silica gel and composite adsorbents are obtained by SEM and BET methods.The composite mechanism of silica gel and calcium chloride as well as the adsorption mechanism of composite adsorbent for water are analyzed.A test platform for adsorbent performance is set up.The dynamic adsorption characteristics of the composite adsorbents are tested by weighing method.The effects of silica gel type and calcium chloride solution concentration on the adsorption properties are studied.The experimental results show that the adsorption capacity of the composite adsorbents are much higher than that of pure silica gel.The adsorption capacity can be increased by 4-10 times,and the thermal conductivity can be increased by 58.2%.According to the experimental results,the optimum concentration of calcium chloride is 40% and the optimum particle diameter of silica gel matrix is 0.5-1.5 mm.(2)The adsorption refrigeration system is designed.First,a two-bed adsorption refrigeration cycle is adopted.The tube-fin heat exchanger is adopted as the adsorbent bed to enhance heattransfer through fins.In order to prevent the refrigerant remaining in the adsorption chamber,the shell size of the adsorption bed is slightly larger than that of the adsorption bed,so that the volume of the clearance is reduced.The finned tube heat exchanger is used as the condenser.In order to reduce the resistance between the evaporator and the adsorbent bed,an immersion evaporator suitable for vacuum system is designed.At the same time,a transparent reservoir is added between the condenser and the evaporator to realize the visual monitoring of the system operation.(3)Taking the adsorbate as the research object,the thermodynamic analysis of the adsorption refrigeration cycle is carried out by Claperon diagram.Compared with the theoretical calculation results in the literature,the COP and SCP of the system can be effectively increased by 5.0% and 5.9% respectively by using composite adsorbent.It is found that increasing the heat source temperature and evaporation temperature can increase the COP and SCP of the cycle.When the heat source temperature is higher than 85 oC,the COP of the adsorption refrigeration cycle reaches its peak value.According to the dynamic adsorption characteristic curve of adsorbent,the refrigeration performance parameters of composite adsorbent are calculated.The results show that the SCP decreases with time because of the decrease of adsorption rate.The SCP increases with the decrease of adsorption temperature and the increase of evaporation temperature.(4)The performance of the adsorption refrigeration system using composite adsorbents is tested.By comparing the dynamic adsorption characteristics of the composite adsorbent and the actual adsorption performance in the system,it is concluded that the adsorption performance of the composite adsorbent in the system is far less than the dynamic adsorption characteristics.The factors influencing the performance of adsorption refrigeration system are analyzed.Increasing the temperature of heat source and chilled water inlet as well as decreasing the temperature of cooling water can improve the refrigeration capacity,COP and SCP of the system.When the heat source temperature is higher than 80 oC,the increase of COP decreases.From the analysis of experimental results,it can be concluded that the influence of heat source temperature on refrigeration capacity and SCP is more significant,and the influence of chilled water inlet temperature on COP of the system is greater.With the increase of refrigeration time,the refrigeration capacity and COP of adsorption refrigeration system increase first,and thendecrease,and the optimal refrigeration time is about 1200 s.With the increase of heat recovery time,the refrigeration capacity and COP of the system increase first and then decrease,so the optimal time is 20 seconds.In conclusion,the adsorption refrigeration system using silica gel composite adsorbent is studied in this paper,which provides theoretical and experimental basis for the application of composite adsorbent in adsorption refrigeration system.