Study On Coupling Mechanism Of Dry-hot Rock Comprehensive Utilization System

In this paper,the energy comprehensive utilization system applied to dry hot rock geothermal resources is proposed according to the idea of energy cascade utilization,namely,full-flow screw spray system coupled with organic Rankine cycle system(FFORC)and Single flash system coupled with organic Rankine cycle system(SFORC).According to the relative temperature of the geothermal hot water outlet temperature and the heating temperature,the FFORC system can adopt three operation strategies:electric-electric-heat,electric-heat and heat-electric.The SFORC system can adopt two operational strategies: electric-electric-heat and electric-heat-electric.Using the first law of thermodynamics and the second law,the thermoelectric coupling mechanism of the two systems is analyzed and the physical model is established.The net output work,the second law efficiency,the heat transfer area for unit output work(APR),the turbine size factor(SF)and the thermoelectric ratio are selected as performance evaluation indicators for the system.Under the certain hot water outlet temperature,the heating temperature ranges covered by different operating strategies of FFORC system and SFORC system are obtained.In this paper,nine kinds of pure working fluids such as R245 fa,R236fa,R123,R245 ca,n-pentane,R134 a,R227ea,R21 and R114 were selected as the candidate working fluids.The optimal selection schemes of working fluids with net output power,the second law efficiency,the heat transfer area for unit output work(APR),the turbine size factor(SF)as optimization targets were obtained.The influence of control variables on system operation under different operating strategies of FFORC system and SFORC system is analyzed and optimized.Comparing the performance of different operating strategies of the same system at the same heating temperature,the optimal operating strategy of the system and the corresponding control variable value scheme are obtained in the covered heating temperature range.It is found that the FFORC system has higher net output power,the second law efficiency and lower APR value under the electric-electric-thermal strategy;the electro-thermal strategy has the highest thermoelectric ratio and the highest output heat capacity;the thermo-electric strategy provides hot water with the highest temperature,of which the thermoelectric ratio is the largest.It is not affected by the heating temperature,thus the adaptability is higher.The SFORC system’s electric-electric-thermal strategy covers a larger heating temperature range,has a stronger ability to output thermal energy,and has an APR lower than electric-thermal-electric strategy.The electric-thermal-electric strategy has a larger net output power,a lower SF value,a wider range of thermoelectric ratios,and greater adaptability to the demand side.The influence of geothermal water outlet temperature and dryness on the optimal performance of the system was analyzed,and the performance of FFORC system and SFORC system was compared.The rise of the geothermal source outlet temperature increases the heating temperature range of the system,the net output power and the helium efficiency of the system increase,and the optimal APR and the optimum SF value decrease.The rise in geothermal water outlet temperature increases the energy efficiency and economy of the system.The increase in the dryness of the geothermal hot water outlet increases the net output power,reduces the optimal APR and SF values for both systems and also reduces the adjustable range of thermoelectric ratio.The increase in dryness increases the maximum efficiency of the FFORC system,while the maximum efficiency of the SFORC system decreases.The FFORC system has higher efficiency and lower SF value than the SFORC system.However,in terms of net output power and APR,the SFORC system is superior to the FFORC system.From the perspective of heating temperature,the FFORC system has better overall performance in the lower heating temperature range,while the SFORC system has better overall performance in the higher heating temperature range...