Study On Heat Transfer Mechanism And Key Technologies Of Medium And Deep "DCHE+HP" System Based On "3E"

Geothermal energy is one of the main ways to develop clean energy.By 2020,the installed capacity of direct utilization of global geothermal energy will be 108 million kilowatts,an increase of 52%over 2015.The utilization of geothermal energy will be 1020887TJ/year(about 283.58billion kilowatt hours/year),an increase of 72.3%over 2015.China has huge geothermal reserves and is widely distributed.The domestic geothermal industry started relatively late,but has developed rapidly.The country has introduced relevant incentive measures for geothermal development,but currently there are still problems with high investment,low thermal efficiency,difficulty in reinjection,and groundwater pollution in geothermal exploitation.This article mainly focuses on the heat transfer mechanism and engineering practice technology of underground coaxial casing heat exchanger(DCHE)and downhole heat pump(HP)systems in mid to deep geothermal formations.Based on the finite difference method,the control equation and boundary equation of the heat transfer model are discretization by using the numerical simulation method;At the same time,the 1100h actual operation monitoring data of the geothermal heating system in a residential area were collected for verification and analysis;On this basis,numerical simulation analysis was conducted on the energy transfer and conversion characteristics of the system.The results showed that the COP of the coaxial casing heat exchanger(DCHE)in the middle and deep geothermal underground system increased by about 5%compared to the shallow system;On the basis of simulation,the effects of factors such as drilling depth,thermal conductivity,size,material selection of the central pipe,heat exchange medium flow rate,temperature,and cementing materials on the extracted thermal power were discussed.The optimal design and operating parameters were selected and applied to a geothermal heating project.After operating in a heating season,the system operates stably,with a comprehensive performance coefficient of 4.33,which is about 5%higher than the average level,verifying the advantages of the underground DCHE system compared with the shallow geothermal heat pump and similar medium and deep heating system.Finally,the efficiency,efficiency,environmental protection and economy of the middle and deep"DCHE+HP"heating system are evaluated.The efficiency of the medium and deep"DCHE+HP"heating system is 28.96%.Compared with gas boiler and air source heat pump,the incremental investment of this system is 105 yuan/m~2 and 62 yuan/m~2,respectively.Compared with the gas boiler heating system,the incremental investment payback period is 3.93 years,and compared with the air source heat pump heating system,the incremental investment payback period is 2years.The investment in the whole life cycle is more economical.