Process Synthesis Of LiBr/NH₃ Cascade Refrigeration System Driven By Waste Heat Based On Multi-objective Optimization

At present,China is facing the situation of surplus industrial waste heat and increasing demand for cold capacity.Aiming at the recovery of industrial low-grade waste heat,the LiBr/NH₃ cascade absorption refrigeration system is designed and simulated by Aspen Plus software.the thermodynamics and economy of the system are discussed,and the optimal scheme of the system is determined by the multi-objective optimization method,and the improvement of the system is given based on the advanced exergy and exergoeconomic analysis method.The cascade absorption refrigeration system proposed in this paper uses low-grade waste heat of 90-150°C to produce cold energy of-40°C.The cascade system includes LiBr/H₂O absorption refrigeration cycle and NH₃/H₂O absorption refrigeration cycle.Based on the experimental data in literature,the Aspen plus physical parameters of the two subsystems are calculated and regressed.On the basis of accurate simulation,the energy,energy and economic analysis of the system are carried out.The thermodynamic analysis of the system takes LiBr evaporator temperature and other parameters as research variables to explore the performance coefficient and exergy destruction of the refrigeration system.The economy of the system considers the investment cost of the equipment and the operation cost of power consumption and water consumption.The results show that there is a conflict between the thermodynamic performance and the economic performance of the cascade absorption system.Taking the total exergy destruction and total annual cost of the system as the objective function,based on the multi-objective optimization method,the optimal solution of the system is determined by non-dominated sorting genetic algorithm and technique for order preference by similarity to an ideal solution.Under the multi-objective optimal solution,the low-grade waste heat of 90-128.3°C is used in LiBr/H₂O absorption refrigeration cycle,while the low-grade waste heat of 128.3-150°C is used in NH₃/H₂O absorption refrigeration cycle.The total exergy destruction and total annual cost of the multi-objective optimal solution are 915 kW and 445245US dollars respectively,which are 4.5%and 5.9%higher than the ideal minimum value,respectively,and better than 11.0%and 14.8%of the single objective optimization.Based on the optimal solution of multi-objective optimization,the method of advanced exergy and exergoeconomic analysis is used to explore the real thermodynamic performance of the system.The advanced exergy and exergoeconomic analysis method divide the exergy destruction of the system into four parts:endogenous,exogenous,avoidable and unavoidable.The results of analysis show that 24.44%,23.8%and 17.68%of investment cost can be avoided.Most of the exergy destruction is endogenous part of the components,especially the thermodynamic performance of generator and heat exchanger needs to be improved.Based on the above analysis,the relevant strategies for the improvement of the system are proposed.