Study On Heat Transfer Performance Of Borehole Heat Exchanger Under The Influence Of Groundwater Advection

Ground-source heat pump can provide cooling and heating for building airconditioning systems, which is efficiently, energy saving and environmental friendly. Itbrings more opportunity for air conditioning technology to develop in China and haswide application prospect at domestic. However, the design and operationalperformance of ground-source heat pump depends on hydro-geologic condition of theshallow earth. The operating characteristics of ground-source heat pump is closelyrelated to the geotechnical thermal physical properties. Especially the groundwateradvection tends to produce great influence. Therefore, it is necessary to clearly grasphow groundwater advection affect the heat transfer performance between borehole heatexchanger and soil,. This may help to improve the design accuracy of borehole heatexchangers.In order to analyze the heat transfer of borehole with vertical U shape buried pipein the soil with or without groundwater advection, analytical solutions to the soiltemperature change obtained from the infinite line source model (ILS), finite linesource model (FLS), moving infinite line source model (MILS) and moving finite linesource model(MFLS) that based on the line source theory are summarized. And theanalytical solution from these models are calculated by using the mathematical toolMATLAB software, the results of the distribution of isolines of soil temperaturechange are given around the borehole heat exchanger. Comparisons among theanalytical solution based different models indicate that the scope suitable for theanalytical solution from every model. The analytical solution from MFLS model was used to discuss the influnce of such factors as groundwater advection velocity and soilproperties on the heat transfer performance. The results can be referred when boreholeheat exchanger is to be designed to consider the groundwater advection.The heat transfer process of a single borehole heat exchanger was simulatednumerically, treating the soil as porous medium with groundwater advection.Numerical solutions to the temperature field of the soil at different time around theborehole after its continuous heat rejection were compared to the analytical solutionsobtained from the MFLS model. The numerical solution validate the analyticalsolutions each other to single borehole heat exchanger..And then, the analytical solutions of the soil temperature responses to heatrejection of multi-borehole heat exchangers into the soil were deduced based on thesuperposition principle. Thermal disturbance among the multi-boreholes were analyzedand the heat transfer characteristics of the most unfavorable borehole and the mostfavorable borehole among the multi-pipe exchanger were illustrated. Optimal layoutstrategies of the multi-borehole heat exchanger under the influence of groundwateradvection are obtained. It is suggested that the spaces between the boreholes in theflow direction of groundwater should be widened in order to avoid the decrease in heattransfer performance caused by thermal disturbances. Under the influence ofgroundwater advection, the dynamic soil temperature feild around single borehole andmulti-borehole heat exchanger under their intermittent operational mode were analyzedby using the MFLS model and superposition principle. Compared with the resultsunder continuous operational model, it can be found that the thermal accumulation inthe soil becomes weaker under the intermittent operational mode. It means intermittentoperation is very helpful for soil temperature recovery.