Analysis Of Cogeneration System For Waste Heat Recovery Based On Full Loads Of Gas Engines

Gas engines are important primary movers in distributed energy systems.However,the thermal efficiency of gas engine is very low and most of energy is wasted by exhaust.Therefore,it is very significate to recover the exhaust waste heat.Aiming at the practical engineering topic of the variable gas engine loads,working fluid selection of thermal-electric cycle and optimization of cogeneration system based on full gas engine loads are investigated in this project.Focusing on the problems of single index and incomplete investigation for the method of working selection of thermal-electric cycle,the working fluid selection model is established in this article,which covers three levels of system optimization design,off-design simulation and economic analysis.First,the optimization design of systems with different working fluids is investigated.Then,the effects of engine working condition on the cycle performance with different working fluids are analyzed on basis of the optimization design parameters.Finally,the thermal-economic performance analysis of ORC under eight potentially applicable working fluids are conducted on the electricity demand of representative day for an objective hotel.The results show that the part-load adaptability of Toluene is superior to that of Water.Besides,the critical temperature can be utilized to predict the whole working condition performance of thermal-electric cycle.Electricity production cost(EPC^OP)and depreciated payback period(DDP^OP)of Water,considering the practical operation condition of gas engine,are 0.131$/kWh and 8.08 years.EPC^OP and DDP^OP of Toluene are 0.127$/kWh and 6.75 years.These properties denote that using Water as working fluid of thermal-electric cycle is suitable to the building type with steady electricity demand,while Toluene has obvious advantage on the building type with transient electricity demand.Resulting from the same reason above,the straight-chain alkane with high critical temperature should be adopted in the building which has transient electricity demand.However,the straight-chain alkane with low critical temperature suits the building which has steady electricity demand.Focusing on the problem of the exhaust utilization of Water under part-load condition is inadequate.Therefore,electricity-cooling cogeneration system?ECCS?including a steam Rankine cycle?RC?and an absorption refrigeration cycle?ARC?is proposed to improve the part-load performance of Water.The results show that,the equivalent efficiency of cogeneration system reduces by 2.14%when the gas engine load drops from 100%to 40%.Moreover,the improvement of system's total efficiency reduces by only 1.64%.Therefore,the cogeneration system shows excellent part-load adaptability.It should be noted that the absorption refrigeration cycle cannot operate normally under 40%gas engine load due to the crystallization of H2O-LiBr solution.The improvement and optimization of proposed cogeneration system are investigated on basis of the results above.Increasing the mass flow rate of RC working fluid with a suitable superheat degree and decreasing the mass flow rate of ARC solution are favorable for improving the performance of ECCS,especially under low gas engine loads.Decreasing the mass flow rate of ARC solution can efficiently improve the equivalent efficiency of ECCS and avoid the risk of H2O-LiBr solution crystallization.