Multi-objective Optimization Of Low Grade Energy Utilization Systems Based On Genetic Algorithm

Efficient,reasonable and deep recycling of industrial waste heat resources has important significance of improving the efficiency of energy.The actual performance of the low grade energy conversion system directly decides the waste heat recovery efficiency of high and low,the optimization of its performance is very important.In most cases,multiple target of the system should be comprehensive considerated in order to meet the actual needs.In this paper,the temperature of industrial flue gas as the heat source is below 200?.Performance analysis and multi-objective optimization of the organic Rankine(ORC),Kalina system 11(KCS-11)and thermally regenerative electrochemical cycle(TREC)have been investigated based on non-dominated sorting genetic algorithm(NSGA-II)and TOPSIS(technique for order preference by similarity to an ideal solution)decision making method.Some useful results are drawn that can be helepful for actual application,optimization of design parameters and selection of cycle working medium of ORC,Kalina and TREC devices.Considering both power output and exergy efficiency,the impacts of the working fluids and heat source inlet temperature on the power output,thermal efficiency,exergy efficiency and exergy destruction of the ORC system have been investigated.Results reveal that the power output and exergy efficiency could not achieve their maximum values simultaneously,multi-objective optimization results can better coordinate the relationship between the two objectives.Under the multi-objective optimal condition,the optimal working medium are R245 fa,n-pentane and R245 ca.When R245 fa as the working medium,the power output and exergy efficiency only reduced by 0.37% and 4.22% respectively compared with the maximum.A higher evaporation temperature of the ORC system leads to a lager output power and exergy destruction.A working fluid with a higher critical temperature leads to a higher thermal efficiency and exergy efficiency.Kalina cycle has better performance than the ORC system in theory,the internal and external exergy efficiencies are adopted to analyze the impacts of the evaporating pressure and ammonia fraction of the ammonia-water mixture on the performance of the KCS-11.Multi-objective optimization results reveal that there exist optimal combination of the evaporating pressure and ammonia fraction leading to the maximum external,internal and total exergy efficiencies.Under the optimal condition,the power output of the system only reduced by 0.7% compared with the maximum,and exergy efficiency is the intermediate value.The Pareto frontier obtained by Multi-objective optimization shows that the curves of Pnet—?ex and ?ex—Pnet are both 4 order polynomial.In addition,the TREC also has a great potential for application in low grade waste heat recovery.In this article,we systematically investigate thermally regenerative electrochemical refrigerator(TRER)performance based on the finite time analysis,considering the irreversibility of finite-rate heat transfer and heat leakage.The general expressions of some important parameters of TRER are derived.The ? figure of merit considers both the coefficient of performance(COP)and cooling load rate,which can be seen as a compromise between maximum cooling load rate and maximum COP.Based on this,the traditional region between both can be divided into two more specific performance regions that represent two different operating demands.Under the maximum ? figure of merit,the impacts of the cell material's characteristics,heat conductance of heat exchangers,and heat reservoir temperature ratio on the corresponding cooling load rate and COP are analyzed in detail.Results reveal that materials with larger isothermal coefficient,specific charging/discharging capacity,lower internal resistance(less than 0.05?),and specific heat correspond to higher cooling load rate and extracted COP.Better heat exchange performance of the regenerator increases cooling load rate and power input;however,it does not guarantee a higher corresponding COP,there exist optimal heat conductance(Kh=40 WK-1? Kc=160 WK-1)and specific charging/discharging capacity(41.7mA·hg-1)leading to the maximum COP.Whats more,an optimal analysis of an integrated utilization system consisting of organic Rankine cycle(ORC)and thermally regenerative electrochemical cycle(TREC)subsystems has been conducted with the maximum power output and exergy efficiency as the objective functions.The results show that the cascade system significantly improves the efficiency of the comprehensive recovery of waste heat.The output power of the ORC subsystem is lower than the TREC subsystem,but its thermal efficiency and exergy efficiency were significantly higher than the latter.Compared with the maximum output power optimization,the total output power of the multi-objective optimization only reduced by 1.16%,and the corresponding exergy efficiency increased by 2.23%;Compared with the maximum exergy efficiency optimization,the total exergy efficiency only reduced by 2.06%,while the output power increased by 19.27%.Finally,in terms of the application of the TRER devices,a mathematic model of a thermoelectric generators(TEG)powered electrochemical refrigeration system consisting of a thermoelectric module used to convert waste heat into electricity and a thermally regenerative electrochemical refrigeration(TRER)has been proposed.Results reveal that the type HZ-2 thermoelectric module are at a disadvantage of the thermoelectric conversion performance,however,its smaller size and weight are more suitable for the application in automobile,satellite and other fields.