Study On The THM Coupling Mechanism Of Groundwater Heat Pump And Optimal Design Of Pump/Injection Well

In recent years,groundwater source heat pump technology has been widely used in the construction industry due to its characteristics of stable operation,energy saving and high efficiency,convenient utilization,low cost and environmental impact compared with traditional air conditioning system,and has gradually become an important technology for energy saving and emission reduction in China.The performance and efficiency of groundwater heat pump are closely related to the geological condition and the layout of pumping wells.An improper layout and design will cause the heat transfixion between pumping wells.This phenomenon induces a decline in the efficiency of the heat pump,and may damage the geological environment,thus affecting the service life of heat pump system.Therefore,it is necessary to investigate the influence of groundwater heat pump on the heat aquifer,so as to design and apply groundwater heat pump system reasonably and realize the sustainable development of shallow geothermal energy.In order to investigate the heat-fluid-solid multi-physical couplings in the groundwater heat pump system,this study first deduces a heat-fluid-solid coupling model for the geothermal aquifer in the groundwater heat pump.This model is developed by using the theories of groundwater seepage,heat transfer and porous medium mechanics.This model is implemented within the platform of COMSOL Muti-physics 5.5,a finite element numerical simulation software,and is used to simulate the three stages of the groundwater heat pump(GWHP): cooling in summer,storage in shutdown and heating in winter.The evolutions of temperature,seepage and stress in the underground aquifer are obtained in different operating stages.Further,parametric study is conducted in terms of slow heat transfixion and power output to explore the important parameters affecting the underground heat transfer in the porous media.These parameters include aquifer permeability,porosity,thickness of aquifer,natural groundwater flow speed,pumping rate,pumping temperature difference and pumping well spacing.The system energy output is calculated under different schemes,and the avoiding of premature heat transfixion strategy is discussed.Finally,the layout schemes of one pump and two irrigations,two pumps and two irrigations are simulated.The advantages and disadvantages of these layout schemes are analyzed from two aspects: the variation trend of groundwater temperature and the characteristics of thermal breakthrough.At the same time,the well changing mode is simulated to optimize the heat pump system,reduce the adverse environmental impact and improve the sustainability of the system.These simulation results show that:(1)The heat injection of aquifer will produce the heat-affected area centered on the recharge well.The heat-affected area increases with the extraction time.When the heat-affected area reaches the pumping well,the system establishes a thermal connection and the thermal efficiency will decrease.(2)Aquifer permeability,porosity,aquifer thickness,underground water flow velocity,pumping and irrigation rate,pumping and irrigation temperature difference and pumping and irrigation well spacing are important parameters to the expansion of heat-affected areas.Large temperature difference and small flow rate can avoid premature thermal connection of the system,and the pumping and irrigation wells should be arranged in the area with a large horizontal downstream flow velocity of groundwater.The arrangement of the pumping well and the recharge well should be far away from each other within the allowable scope of space.(3)The layout schemes of one pumping and two irrigations and two pumping and two irrigations are better than the combined well system in alleviating thermal connection and improving system performance.The exchange of pumping and two irrigation wells in different stages can effectively solve the problems of cold and hot accumulation of water layer caused by the long-term operation of the heat pump system,thus enhancing the energy output of the heat pump and reducing the environmental impact.