| Compared with the shallow ground source heat pump(SGSHP),the mid-deep ground source heat pump(MDGSHP)has the advantages less occupied ground surface area,higher heat extraction capacity,and higher system energy efficiency.However,when a MDGSHP operates for a long time,the heat extraction capacity of its borehole heat exchanger(MBHE)will significantly decrease due to the decrease of its surrounding ground temperature.For a multi-well mid-deep borehole heat exchanger(MMBHE)with relatively small borehole spacing,the heat influence among boreholes is relatively apparent,which is prone to the decrease of heat extraction capacity.This thesis proposes applying heat storage during non-heating seasons to alleviate the above problems.The heat transfer models for heat extraction and storage of single-well mid-deep borehole heat exchanger(SMBHE)and MMBHE are established,and verified with experimental data reported in the literature.The results show that the established models have high accuracy.Based on the model of SMBHE,the effects of operating time,borehole depth,annual heat storage time,heat storage fluid inlet temperature,and heat storage fluid flow rate on the heat extraction capacity of the SMBHE are studied.The results show that the heat extraction capacity of SMBHE increases with the increase of operation life,borehole depth,annual heat storage time,and heat storage fluid flow rate.However,the increase rate of heat extraction capacity gradually decreases with the increase of annual heat storage time and heat storage fluid flow rate.Furthermore,heat storage can improve the heat extraction capacity of the MBHE only when the heat storage fluid inlet temperature is greater than the critical heat storage temperature.The heat extraction capacity of the SMBHE increases approximately linearly with the increase of the heat storage fluid inlet temperature.Based on the model of MMBHE,the effects of borehole layout,borehole amount,borehole spacing,annual heat storage time,heat storage fluid inlet temperature,and per borehole heat storage fluid flow rate on the heat extraction capacity of MMBHE are studied.The results show that the layout of boreholes only has a little impact on the heat extraction capacity of the MMBHE.With the borehole spacing increasing,the heat extraction increase with heat storage is less than that without heat storage,and the heat extraction improvement induced by heat storage becomes less.The influence of annual heat storage time,heat storage fluid inlet temperature,and per borehole heat storage fluid flow rate of MMBHE is similar to that of SMBHE.In addition,the critical heat storage fluid temperature of MMBHE is much lower than the borehole bottom temperature,and is also lower than the critical heat storage temperature of SMBHE under the operation condition.The contrast ratio(ξ)is proposed to compare the heat storage effects of MMBHE and SMBHE under the same conditions.With the increase of the annual heat storage time and the heat storage fluid inlet temperature,ξ gradually decreases,and the influence of MMBHE heat storage becomes less and less.With the increase of the per borehole heat storage fluid flow rate,ξ increases apparently first,and gradually tends to the quasi-equilibrium state when the flow rate is larger than 5 kg/s.Based on the power consumption of the pump,the heat extraction improvement rate(γ)and the heat storage yield ratio(δ)are proposed to evaluate the economy of heat storage.According to economy evaluation,the recommended annual heat storage time range and fluid flow rate ranges are 3-4 months and 4-6 kg/s,respectively.When the thermal influence among boreholes becomes stronger,heat extraction capacity improves more,i.e.,the higher profits the heat storage yields. |
