| In order to solve the current problems such as energy shortage and environmental pollution,the state put forward two major strategies.Firstly,optimize the structure of China’s energy industry by increasing the proportion of clean energy in primary energy consumption;secondly,improve the efficiency of energy utilizing and develop new energy.Because of the grade is extremely low,most of the low temperature waste heat and new energy like solar energy and geothermal energy are difficult to be got efficient use,which makes lot of waste.And liquefied natural gas（LNG）will release amount of cold exergy during its vaporization process,which is not utilized efficiently either.Utilizing low-grade energy and cold energy of LNG can improve the efficiency of energy utilizing.Organic Rankine cycle（ORC）power generation technology can be used for utilizing low-grade energy,but there are still many deficiencies like lowenergy utilization efficiency In order to improve the efficiency of the ORC power generation system and utilize LNG cold exergy efficiently,this paper propose that utilizing low-grade energy and LNG cold exergy together.A combined system is creatively proposed and multi-objective optimization model has been built.Based on the system and model,the selecting of working fluids,comparison of systems,optimization of parameter,cost and exergy analysis and practical application work have been done in this paper.And many conclusions are obtained:1)Based on the same condition of hot and cold source,cooperation analysis has been done between the novel combined power generation system and the traditional ORC and LNG-ORC power generation using water as cold and hot source.The results show that the novel combined system has better overall performance,greater work ability,lower system cost,and shorter payback period.2)Considering the working conditions at low temperature,thermodynamic properties of working fluids in low and normal temperature cycle,and protection of safety and environmental.20 kinds of working fluids have been taken in account.The results show that the best working fluid for low temperature cycle is propane,and R245fa is the best working fluid for normal temperature cycle.3)The multi-objective optimization analysis of system with four objective functions has been carried out by using MATLAB programming.The condition parameters The optimize parameters are evaporative temperature(Tevap),temperature in internal heat exchanger(Tihe),condensing temperature(Tcond),superheat temperature（δT）,pinch point temperature difference（ΔT）and vapor pressure of LNG(pLNG).The results show that the optimizing Tevap、Tihe andΔT exist,meanwhile there are optimizing temperature range for pLNG.The best work condition is Tevap=387.15K,Tcond=-193.15K,Tihe=309.15K,ΔT=8K,p LNG=3MPa and with no superheat.4)Cost analysis has been done,and acquisition cost and exergy loss for each equipment has been carried out.The results show that the costs proportion of turbine equipment is 40%,and condenser’s account is 25%.And the equipment with most exergy loss is condenser I.At the same time,the heat transfer equipment contributes the greatest exergy loss in the system,which is more than 68%.5)The performance of the single-stage system,the dual-stage system and the combined system under different heat source conditions are analyzed.The results tell us that for the low-grade energy which is below 110℃,the single-stage system are better because of simple structure and low cost.The optimum working fluid is R32 and propane.While for the low-grade energy which is above 110℃,the novel combined system has betterperformance by shorter payback period and higher Wnet.6)Simulations of the combined system using different types of low-grade energy have been carried out.The system utilizing low temperature solar energy can achieve high exergy efficiency at 37%,and the Pt is 5.16 years.Meanwhile,an optimization work using non-azeotropic mixtures has been carried out;the use of propane/ethane and R152a/pentane as working fluis can reduce the exergy loss in condenser I and III obviously.7)Simulations of the combined system using low-temperature flue gas and LNG cold energy have been carried out.In order to reduce exergy loss in heat exchangers,non-azeotropic mixtures and pure working fluids are taken into account.The results show that,non-azeotropic mixtures have lower exhaust flue temperature and narrow interval of work conditions.The use of different working fluids can get different optimization results. |
