A Comparative Study of Medium Deep Borehole Thermal Energy Storage Systems Using Numerical Modelling

Abstract

In this study the application of Medium Deep Borehole Heat Exchangers (BHEs) as High Temperature Borehole Thermal Energy Storages is presented. Therefore 27 different Borehole Thermal Energy Storage configurations were compared, using a numerical program for modeling the heat transport processes in the BHEs and in the underground. The influence of different design parameters on the storage performance was studied by varying the BHE length, the number of BHEs, the spacing between the BHEs and the inlet temperature during the extraction period. A simplified underground model was assumed and a simplified operation procedure was applied over a period of 10 years of storage operation. The simulation results show, that all the considered storage systems improved their performance with time. There is a strong influence of the studied parameters on the efficiency and the specific heat extraction rate of such storage systems. Depending on the selected parameters, the storage efficiency ranges between about 20 % and 70 % in the 10th year of storage operation. The average specific heat extraction rate for the preset extraction time of 4380 hours reaches values from about 40 W∙m-1 to more than 90 W∙m-1 in the 10th year of operation, which correlates to an annual specific heat extraction of about 175 KWh to more than 390 KWh per meter of BHE.