Thermal Performance Analysis Of A Cascade Heat Exchange Geothermal Power Generation System Based On ORC

With the increasing problem of energy shortage and environmental pollution,how to reduce the dependence on fossil energy is an urgent problem to be solved in the construction of modern and efficient energy system in our country.With the development of energy technology innovation,wide attention has been paid to the utilization of low grade heat,such as renewable solar energy,biomass energy,geothermal energy and industrial waste heat.Organic Rankine cycle(ORC)power generation technology is one of the effective ways to use the low grade heat.Due to the restriction of temperature level,ORC power generation technology has disadvantages of low power generation efficiency and low heat utilization rate when using low grade heat.In order to improve low grade heat utilization ratio using ORC power generation technology,based on the idea of the energy cascade utilization,a cascade heat exchange geothermal ORC power generation system is proposed,and the corresponding mathematical modeling and simulation are conducted.The details and results are listed below:First of all,with net power output as objective function,the cascade ORC system selects zeotropic mixture as circulation medium,the parameters are optimized;subsequently,the exergy analysis of the cascade ORC system is conducted.It is shown that optimal parameters exist that gives the maximum net power output;the main exergy loss of the cascade ORC system comes from the irreversible loss in evaporator and condensor,so concerns should be given to the improvement of heat exchanger when to improve the performance of the cascade ORC system.Secondly,under the same boundary conditions,both the first and the second law of thermodynamics are employed to analyze the thermal performance of two different ORC systems including the basic ORC system and the cascade ORC system using different pure working fluids and zeotropic mixtures,meanwhile,theoretical analysis and basis are provided for the selection of the cascade ORC system working fluid.Results reveal that taking net power output as the objective function,the net power output of the cascade ORC system is better than that of the basic ORC system,no matter using pure working fluids or zeotropic mixtures as circulation medium;using zeotropic mixtures as working fluids improves the thermal performance of the cascade ORC system due to better matches of the temperature profiles of the working fluid and heat source/sink;in the selected working fluids,R601/R236 ea is recommended as most suitable candidate for the cascade ORC system.Finally,the influence of geothermal temperature and structure optimization on the performance of the cascade ORC system is investigated.Results indicate that the type of working fluid has little impact on the net power output of the cascade ORC system when the geothermal inlet temperature is in the range of 363.15~423.15 K,particularly when the geothermal inlet temperature is lower than380K;the installation of heat regeneration exerts no influence on the net power output and exergy efficiency,but the thermal efficiency and the utilization ratio of low grade heat can be improved significantly;the decrease of evaporation side heat load and condensation side heat load leads to smaller exchange area requirement of evaporator and condensor and thus reduces initial investment.