Thermo-economic Analysis And Optimization Of Power Generation System Combined Of Low Grade Heat And LNG

With environmental and energic problems constantly emerging,the utilization of clean and renewable energy has been receiving increasing attention which is the eternal topic of sustainable development.Limited by the level of science and technology,lots of low-temperature heat resources in China have not been effectively exploited,resulting in energy waste and environmental pollution.As a burgeoning energy source,liquefied natural gas(LNG)will release a large amount of cold energy during practical application.In order to make full use of the low-temperature thermal energy and the cold energy of LNG,the power generation system combined of low grade heat and LNG is constructed by coupling the low-temperature organic Rankine cycle(ORC)with the LNG direct expansion system.The thermo-economic performances of the system are studied.The main research contents and conclusions are as follows:A preliminary selection mechanism of organic working fluids is established based on the characteristics of low-temperature thermal energy and LNG cold energy.Ten organic substances,R600,isobutene,R600a,propane,R32,R152a,R161,R1234ze,R245fa,and R134a,are selected as working fluids according to their thermophysical,economic,safe,and environmental performances.The influences of the key operating parameters and heat source conditions on the thermo-economic performance of the system are carried out through the MATLAB simulation platform.For different evaluation criteria,the evaporating temperature,superheating temperature,pinch temperature,and LNG vaporization pressure achieving the maximum goal are different.Lower condensing temperatures are beneficial to the system performance.The condensing temperature and the evaporating temperature have a great impact on the system performances.The increase of heat source temperature has a positive effect on the system performance.The matching between the working fluid and the heat source can be optimized by the parameter optimization.The particle swarm optimization algorithm is utilized to optimize the system with the optimization targets of both the maximum exergy efficiency and the minimum electricity production cost.The results show that the optimal parameters obtained under different optimization goals are different,and the optimal working fluid is also different.On this basis,a thermo-economic evaluation index F(X)considering both the thermal and the economic performances is established.As is shown clearly,under typic,al parameters,R32 is the optimal working fluid when F(X)reaches 0.3311,and the corresponding optimal parameters are mostly between the results of those two single-target cases.The F(X)s of all the working fluids increase with the increase of heat source temperature.For the heat source temperature range from 75°C to 125°C,R32 is the optimal working fluid.For 150°C,the optimal working fluid is R161.The optimal fluids are those with both suitable critical temperatures and low normal boiling temperatures.Based on the power generation systerm_driven by both_low grade heat and LNG,two two-stage ORC systems,the parallel and the cascade,are constructed.Both structures can obviousely improve the thermo-economic performance of the system.In the parallel system,the influence of the low-temperature fluid on the thermo-economic performance is far greater than that of the high-temperature fluid.Hence,it is a good choice to determine the low-temperature fluid first and then select the same fluid as high-temperature fluid.For the cascade system,a superheated high-temperature cycle with a saturated low-temperature cycle is the best form.When the heat source temperature is between 75°C and 1250C,the parallel system performance is outstanding.When the heat source temperature reaches 1500C,the performance of the cascade system performs better.The best working fluid combination is R161/R32 when the.relevant thermo-economic performanceindexF(X)is 0.3835.