Operation Characteristics And Performance Optimization Of Small Systems Based On CO₂ Transcritical Cycle

Based on CO₂ transcritical cycle,two methods are proposed to improve the performance of small CO₂ system.One method is the optimization of system operation parameters,the other method is the use of azeotropic mixture refrigerant.Moreover,the test room of CO₂system performance and the novel defrosting device are introduced simply.Finally,the theoretical design of"dual temperature zone"refrigerator system is carried out to enlarge the application range of CO₂ refrigeration technology.A thermodynamic model based on small CO₂ water heater is proposed to calculate the optimal operation parameters.Under the design condition,the optimal combination of capillary tube geometry and refrigerant charge is predicted by the model.According to the results of theoretical calculation,a small CO₂ water-source heat pump water heater was designed and relevant experimental tests were conducted to validate simulation results and the experiment results showed that the best combination of capillary tube length and refrigerant charge is determined as 3.9m and 270g when the capillary tube inner diameter is1mm,the evaporation temperature is 3 ^oC and the outlet temperature of the gas cooler is 34^oC.The model had relatively good accuracy,so can provide researchers theoretical guidance on the optimal refrigerant charge and design of capillary tube geometry in a small transcritical CO₂ cycle.The work fluid R41 is selected to blend with CO₂ as as an azeotropy refrigerant.CO₂/R41 mixture refrigerant is evaluated for application in three different systems including a refrigerated cabinet,an air-source heat pump water heater and a water-source heat pump water heater based on optimal operation condition.The effects of CO₂/R41 mixture as an azeotropy refrigerant on system performance are analyzed under various working conditions.The results show that CO₂/R41 mixture may be a good choice to substitute for pure CO₂ in the studied systems because of its stable chemical properties,low optimal high pressure,high system COP,low compression ratio,low discharge temperature,high refrigerating capacity per unit,and high heating capacity per unit.Furthermore,the exergy efficiencies of the three studied systems are improved to more than 23%with the proposed CO₂/R41?0.5/0.5?mixture refrigerant.The refrigerated cabinet has almost the same exergy efficiency compared to the heat pump water heater with CO₂/R41?0.5/0.5?mixture refrigerant,which implies a potential usage of this kind refrigerant in refrigerated cabinet.In order to provide experimental platform for the performance test of small CO₂ system in the future,one constant temperature room is designed based on a cold storage and it has good temperature control accuracy.Then the novel device based the evaporator of constant temperature room has good effect.At last,a double-door refrigerator system adopted CO₂ transcritical cycle with two temperature areas is designed theoretically,which can provide ideas for the expansion of the application scope of CO₂ refrigeration technology.