Comparison And Optimization Of Different Lng Cold Energy-Low Temperature Thermal Energy Combined Power Generation Systems

Organic Rankine Cycle is an effective way to comprehensively utilize LNG cold energy and low-temperature thermal energy on a large scale.The efficiency of conventional single-stage Rankine cycle is relatively low.In order to improve system efficiency,ten Rankine cycle improvement systems are proposed and studied in this paper.Different systems,working fluids and parameters are optimized under different heat source temperatures and natural gas pipeline pressures.The main contents and conclusions are as follows:(1)Ten configurations,including the regenerative RC,reheat RC,reheat-regenerative RC,parallel two-stage RC,cascade two-stage RC and theses RCs combined with direct expansion cycle(DEC)are investigated.Eight organic substances,including ethane,R32,R125,propane,ammonia,R134 a,R152a and isobutane are selected as working fluids.Main parameters such as turbine inlet temperature,condensation temperature,LNG vaporization pressure,regenerative degree,and reheat pressure ratio are optimized by the particle swarm optimization(PSO)algorithm with the objective of maximum exergy efficiency.(2)The exergy efficiency increases with the increase of heat source temperature and with the decrease of pipeline pressure.In most cases,the efficiency of reheat-regenerative RC±DEC system is always higher than those of other RC systems.The maximum efficiency of 36.15% is achieved by the reheat-regenerative RC+DEC under the heat source temperature of 180℃ and the pipeline pressure of 0.6MPa.The efficiencies of regenerative RC±DEC are close to or even higher than those of two-stage RCs.The efficiency improvement by the addition of DEC decreases with the increase of pipeline pressure and increases with the increase of the normal boiling point temperature(NBPT)of working fluid.(3)Fluid with low NBPT achieves higher efficiency when the heat source temperature and pipeline pressure are low,while fluid with high critical temperature achieves higher efficiency when the heat source temperature and pipeline pressure are high.Critical temperature of working fluid is less important in regenerative RC±DEC and reheat-regenerative RC ± DEC containing the regenerator.Critical temperature is more important in reheat RC±DEC.For cascade two-stage RC±DEC,low NBPT is more essential for low temperature cycle I and high critical temperature is more important for high temperature cycle II.Moreover,dry and isentropic working fluids are more suitable for regenerative RC±DEC while wet working fluids are better for reheat RC±DEC.(4)In general,the optimal evaporation temperature increases with the increase of heat source temperature,the optimal turbine inlet temperature is usually its upper limit,and the optimal condensation temperature increases with the increase of pipeline pressure.In cascade two-stage RC ± DEC,the evaporation temperature of low temperature cycle is the same as that of turbine inlet temperature,which is equal to the high temperature cycle condensation temperature minus the heat transfer temperature difference.The regenerative degree and reheat pressure ratio change obviously with different working conditions and working fluids.For higher heat source temperature and for isentropic or dry working fluids,the regenerative degree and reheat pressure ratio are higher.(5)The exergy input of LNG is much higher than that of heat source.The exergy input of LNG is fixed and the exergy input of heat source increases with the increase of heat source temperature.The exergy loss of LNG increases with the increase of pipeline pressure.The condenser and auxiliary heat exchanger account for the highest proportion of total exergy destruction.