Working Fluid Selection And Parameter Optimization Of Combined Heat And Cold Energy Power Generation System

There are many high-energy-intensely industries in China.These industries not only have low energy efficiencies,but also discharge a large amount of waste heat,resulting in environmental pollution.The reutilization of these waste heat resources can alleviate not only the energy crisis,but also the environmental problems.With the gradual implementation of the "coal to gas" policy,China's natural gas consumption has increased year by year.there is a lot of cold energy in the liquefied natural gas.Organic Rankine Cycle(ORC)can combine the low-temperature thermal heat and the cold exergy of LNG to generate electricity,which is one of the effective ways to solve China's energy problems and environmental pollution.This article focused on the combination of waste heat and cold energy utilizations.Eleven organic fluids are selected as the working fluids,and the maximum exergy efficiency is used as the objective function.At the foundation of simple single-stage ORC,the two-stage ORC power generation systems are constructed.The main parameters that affect system performance are analyzed and optimized,obtain the optimal operating conditions and maximum efficiency.At the same time,the performance of the system and the corresponding optimal parameters were analyzed under different heat source temperatures,different natural gas distribution pressures and different lower condensing pressures.The main conclusions are as follows:(1)Due to the critical temperature limit,the optimal evaporation temperature for each cycle of the two stage parallel system is the critical temperature of the working fluid,and the optimum expander inlet temperature of each cycle is the upper limit of the parameter optimization or slightly higher than the critical temperature of the working fluid.The optimal condensation temperature for low temperature cycle is the normal boiling point temperature of the working fluid,and the optimal condensation temperature for high temperature cycle is higher than the normal boiling point temperature of the working fluid.In the two-stage cascade system,the optimal evaporation temperature for high temperature cycle,the optimal expander inlet temperature for high temperature cycle and the optimal condensation temperature for low temperature cycle are the same as that of the parallel system.The difference is that there is an optimal condensing temperature in the high temperature cycle of the cascade system.In the two structures,the influence of low temperature fluid on system efficiency is greater than that of high temperature fluid.The optimal LNG evaporation pressure is generally greater than 5MPa.(2)The exergy efficiencies increase with the heat source temperature rise.For different heat source temperatures,the variation trends of parameters are similar as the case of the heat source temperature of 150?.In the parallel system,when the heat source temperature is 50?,the optimum evaporation temperature is about 20? lower than the heat source temperature.At 100?,the optimal evaporation temperature is 25-40? lower than the heat source temperature.At 150?,the optimum evaporation temperature for the fluid which critical temperature below 115? is its critical temperature.When the heat source temperature increases to 200?,the optimum evaporation temperature is the critical temperature or near critical temperature of the working fluid.In the cascade system,with the increase of the heat source temperature,the optimal evaporation temperature of the high temperature cycle gradually becomes the critical temperature of the working fluid.(3)The bigger the distribution pressures for natural gas at the outlet of the direct expander,the lower the system efficiency and the smaller the net output work of the system.Under different natural gas distribution pressure,the efficiency for the parallel system is always slightly better than the efficiency of the cascade system.In the cascade system,with the increase of the natural gas distribution pressure,the temperature of the natural gas at the outlet of the direct expander continuously rises.(4)The efficiencies of the system increase with the decrease of condensing pressure lower limits.When the lower limit of the condensing pressure declines from the normal pressure to 0.001 MPa,the efficiencies of the two systems only increase of 1.6%.Therefore,the two-stage system does not need to decline the condensation pressure to vacuum in order to get a low condensation temperature,which can be easily obtained by utilizing fluids of low normal boiling temperatures.