| There are a great amount of low grade heat exergy and liquefied natural gas(LNG)cold exergy in China.Combined utilization of these low grade heat exergy and LNG cold exergy is an effective way to improve the energy utilization rate and protect the ecological environment.Organic Rankine cycle(ORC)could efficiently recover low grade heat exergy and LNG cold exergy on a large scale.In order to reduce exergy destructions in heat transfer processes of ORC system,two improvement techniques: using transcritical system to improve thermal match in steam generator and proposing unconventional condenser to reduce exergy destruction in LNG regasification process,are proposed.The main contents and conclusions are as follows:(1)The mathematic models of simple ORC system,regenerative ORC system and reheat ORC system under subcritical and transcritical operating conditions are established.Ten candidate refrigerants,including R1150,R744,R170,R32,R1270,R290,R152 a,R717,R600 a and R600,are preselected as working fluids.Particle swarm optimization(PSO)algorithm is adopted to optimize the parameters of the systems to obtain the maximum exergy efficiencies,the optimal working fluids and the best parameter combinations under different heat source temperatures and NG distribution pressures.(2)The performance of the simple systems with conventional and unconventional condenser under subcritical and transcritical operating conditions respectively are optimized and analyzed to evaluate the improvement effect of transcritical system and unconventional condenser on system exergy efficiency.The results show that the exergy efficiency values of most working fluids show improvement by using transrcitial system or unconventional condenser,and the improvements become more significant as the heat source temperature increases.However,the two improvement techniques have mutual inhibition effect on the exergy efficiency improvements of working fluids when transcritical system and unconventional condenser are adopted at the same time.Only some fluids with low critical temperatures and low normal boiling point temperatures have further improvement in transcritical system with unconventional condenser,and the absolute value of efficiency increase is lower than the sum increase of the single improvements.(3)The performances of simple system,regenerative system and reheat system under subcritical and transcritical operating conditions are analyzed and compared respectively under different heat source temperatures and NG distribution pressures.The results show that fluids with low critical temperatures achieve higher exergy efficiencies in transcritical systems,and the improvements become more obvious as the heat source temperature increases.While fluids with higher critical temperatures have higher exergy efficiencies in subcritical systems.The maximum exergy efficiencies of regenerative systems and reheat systems are always higher than those of simple systems.The maximum exergy efficiencies of the systems increase with the increase of heat source temperature or the decrease of NG distribution pressure.The transcritical reheat system with unconventional condenser using R290 as working fluid reaches the maximum efficiency of 27.78% under the heat source temperature of200℃ and NG distribution pressure of 3MPa.(4)The exergy losses and destructions of the simple system,regenerative system and reheat system are carried out under subcritical and transcritical operating conditions.The results show that the exergy loss of NG increases with the increase of NG distribution pressure,while the exergy loss of heat source could be ignored.The exergy destruction of the system mainly occurs in LNG regasification process and steam generator.Using unconventional condenser system or reheat system could reduce the exergy destruction in the LNG regasification process,while using transcritical system or regenerative system could improve the thermal match between working fluid and heat source in steam generator. |
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