Comparison of influence factors on horizontal ground heat exchanger performance through numerical simulation and gray correlation analysis

Abstract

Horizontal ground heat exchanger (HGHE) is a cost effective method to exploit geothermal energy due to its shallow installation depth, which meanwhile causes its performance sensitive to climatic conditions. However, quantitative effects of climatic conditions on the HGHE performance still remain unclear. Influence degrees of key factors on the HGHE performance are not yet studied. Therefore, we do not know which factors should be priorities for the HGHE design. To this end, influence degrees of all key factors, including injection mass flow rate, operation modes, heat storage, subsurface water flow, soil thermal conductivity, installation depth and especially atmospheric conditions (precipitation and air temperature), on the HGHE performance are comprehensively analyzed and compared through numerical simulation and gray correlation analysis. Results indicated that non-consideration of climatic conditions could underestimate HGHE outlet temperature in summer and winter. The intermittent operation mode showed much better performance than continuous one. Increasing installation depth and improving grout thermal conductivity were both beneficial for HGHE performance. The most significant impact factors on the HGHE performance were groundwater flow velocity > circulation flow rate > precipitation > air temperature. Based on simulation results, subsurface water flow, injection mass flow rate and climatic conditions should be priorities for the optimal design of HGHE.