Thermodynamic Analysis And Multi-objective Optimization Of Combined Power And Cooling System Driven By Low-temperature Heat Sources

Correct development method and efficient utilization of the low-temperature heat sources are of great significance to alleviate the global problems,such as the energy problems,environmental problems and climate changes.In this paper,three novel power/cooling cogeneration systems driven by low-temperature heat sources are proposed,which could provide new solutions for making full use of low-grade heat sources.Mathematical models of these novel systems are written with the Engineering Equation Solver(EES)software under the steady-state conditions.In addition,by means of the cooperative simulation between EES and MATLAB,multi-objective optimizations using NSGA-II method are carried out to obtain the final optimums of these proposed systems.Firstly,a novel combined cooling and power system which combines a conventional ammonia-water power/cooling cycle named GOSWAMI cycle and an ejector refrigeration cycle is proposed and investigated.This new combined system can improve the refrigerating capacity of the conventional power/cooling system,and it can also adjust the cooling capacity to power ratio by changing the proportion of the ammonia-water flow into the turbine and the ejector.The simulation results show that the thermal efficiency and the exergy efficiency are 21.48%and 28.72%,respectively.Exergy analysis shows that exergy destruction of the rectifier increases with the increase of boiler temperature;when the cycle high pressure is high,the exergy destruction mainly occurs in the recovery heat exchanger.Parametric study shows that the cycle high pressure and the boiler temperature have significant effects on the system performance.In the study of multi-objective optimization,according to the TOPSIS decision theory,different final optimization results are selected from the Pareto optimal frontier.Secondly,ammonia absorption combined power/cooling cycle with steam extraction is proposed and investigated.The new system improves the cycle structure of the high pressure side in the combined system and reduces the adverse effects of distillation unit on the thermal performance.Besides,this novel combined system could get more high-grade work at the same time obtain amounts of refrigeration due to the unique coupling mode.Simulation results show that the thermal efficiency and the exergy efficiency are 21.34%and 38.95%,respectively.Exergy analysis shows that the exergy destruction mainly occurs in the recovery heat exchanger.Parametric study shows that the absorber temperature,the cycle high pressure,the temperature at boiler outlet and the extracting ratio have significant effects on the system performance.A multi-objective optimization using NSGA-II method is carried out to obtain the final optimums of the proposed system,and based on the equilibrium point concept,different final optimization results are selected from the Pareto optimal frontier.Finally,by optimizing the cycle structure of the absorption side in the ammonia absorption combined system,a double-absorption combined power/cooling system is proposed and investigated,which not only solves the problem of the mutual restriction between the absorber pressure and the evaporator pressure but also improves the regulation capacity of the system.The influence of the boiler temperature and cycle high pressure,absorber temperature and cycle high pressure,absorber temperature and boiler temperature on the system performance is analyzed.The optimal results show that there is no obvious conflict between the operation cost of the unit and the thermal efficiency as well as the exergy efficiency.By comparing these three cycle modes,it is found that the thermodynamic performance of the double-absorption combined power and cooling system is better,but the operation cost of the unit is the highest.