Researches On Performance Optimization And Dynamic Characteristics Of A Combined CO₂ Power And Cooling System Driven By The Waste Heat Of Flue Gas

Population growth and economic development have continuously increased energy consumption worldwide,which aggravates energy shortage and environmental pollution.Therefore,the rational utilization of waste heat is the key to improve the energy conversion efficiency.CO₂ has the excellent properties of non-corrosive,non-toxic and non-flammable.It has been widely used as the working fluid in power cycle,refrigeration cycle and heat pump system.Therefore,aiming at the efficient recovery of waste heat,the system design,performance optimization and dynamic simulation for the combined CO₂ power and refrigeration cycle are carried out in this paper.The main contents and conclusions are summarized as follows:（1）With the aim to recover the waste heat,a novel system was proposed to combine the supercritical CO₂（S-CO₂）power cycle and the transcritical CO₂ refrigeration cycle.The low-temperature recuperator and cooler are shared by two sub-cycles.On this basis,thermodynamic and economic models were established,and the effects of cooler pressure,evaporation temperature and turbine inlet temperature on system performances were studied.The results show that under the design condition,thermal efficiency and exergy efficiency of the system are 42.42%and 39.05%,respectively.The average cost per unit of products is 9.28$/GJ.Furthermore,the parameter analysis indicates that under lower evaporation temperature and higher cooler pressure,the lower net work and the higher cooling capacity are obtained.（2）In order to evaluate the performance of different combined systems,thermo-economic performances of four systems（the proposed system and reference systemⅠ,Ⅱ,Ⅲ）and influences of key parameters on the performance were analyzed.Thereafter,key parameters were optimized by a multi-objective optimization algorithm NSGA-Ⅱ,so as to simultaneously maximize the total output（the sum of net output work and cooling capacity）and minimize the total cost.Under basic operating conditions,the highest total output and the lowest cost are obtained by the reference systemⅢand reference systemⅠ,respectively.The optimal results show that the proposed system gets the highest product（7345.4k W）,while the reference systemⅠrequires the lowest cost（27.51$/h）.Overall,the performance of the proposed system is better than other reference systems.（3）To reduce the operating pressure of CO₂ combined system,thermodynamic performances of CO₂/R600a and CO₂/R601a system were studied.At the same time,effects of key parameters such as CO₂ mass fraction,turbine inlet temperature and evaporator outlet temperature on the performance were analyzed.Taking the maximum net work as optimal objective,the system performances of pure CO₂ and mixtures under different condenser outlet temperatures were compared.The results illustrate that under basic operating conditions,the net work of CO₂/R600a and CO₂/R601a are 2717.93k W and 1145.98k W respectively.The comparison results reveal that the highest specific work is always obtained by CO₂/R600a system.（4）Dynamic models of basic S-CO₂ power cycle,recuperator cycle and recompression cycle were established.When the conditions of heat and cold sources appear step changes,dynamic characteristics of these three systems were clarified and compared.It’s found that the response time of basic S-CO₂ power cycle is the fastest when the cold source temperature and the mass flow rate of heat and cold sources vary.However,when the heat source temperature suddenly decreases,the adjustment time for the recompression cycle is the shortest.Including 83 Figures,26 Tables and 117 References.