Suitability Evaluation And Aquifer Temperature Field Analysis In Water Source Heat Pump

Ground-water source heat pump system is a highly efficient and energy-saving geothermal energy development technology.In the recent years,it has been widely promoted and utilized in Xi’an.However,the water-source heat pumps lack of systematic planning scheme for site selection or arrange the well pattern at random,which has led to a reduction of recharging efficiency and made the aquifers face thermal pollution at the same time.After long-term operation,the heat exchange efficiency of the system is low,which would affect the utilization efficiency and service life for water source heat pump.In order to solve this problems,the paper evaluates the suitability of confined water source heat pump in the plain area of Xi’an,and also studies characteristics of the aquifer temperature field in different scenarios,some scientific planning and designing schemes of the water source heat pump were provided at last.The main research results are as follows:(1)Establishing a two-level suitability evaluation model of water source heat pump based on general factors and special factors,and using the AHP method,Gini coefficient weighting method,comprehensive index method to evaluate the suitability of general factors at the first level,the one-vote veto method is used to evaluate suitability of the special factors in secondary level evaluation.As a result,the suitable area of water source heat pump was3300.36 km~2 in Xi’an,and the unsuitable area was 6807.64 km~2.(2)On the basis of water source heat pump surveys and pumping tests,the porous medium parameters(hydraulic conductivity,thermal conductivity,specific heat capacity,thermal dispersion)and well group parameters(pumping-recharge ratio,well spacing)were determined in suitable area according to the principles of hydraulics,thermodynamics,ground source heat pump system.It will provide a scientific reference for well-designed of pumping capacity and wells pattern in water source heat pump.(3)A hydro-thermal coupling numerical model was constructed for water source heat pump systems,the temperature field characteristics of aquifer in directional flow and non-directional flow(direction changes with time)were simulated by FEFLOW.The results showed that temperature field’s range to non-directional flow is greater than that of directional flow when recharge wells are on both sides of the pumping well.The former is more likely to cause thermal breakthrough to pumping wells,and also cause more serious thermal pollution to aquifers.(4)Considering special circumstances as unequal thickness aquifers and non-integrated wells in ground-water source heat pump systems,FEFLOW software is used to simulate two scenarios of equal and unequal thickness aquifers,integrated and non-integrated wells for studying temperature field characteristics(ignore natural flow field),and then analyze difference between temperature field.The results show that thermal breakthrough is more likely to occur on smaller thickness side in the unequal thickness aquifer,and the spacing between pumping and recharge wells should be increased in this side.In the non-integrated wells,the horizontal heat transfer rate is much greater than that of vertical direction to aquifer,it is more likely to cause thermal breakthrough and aquifer pollution than integrated wells.