Analysis of thermal interaction coefficient for multiple borehole heat exchangers in layered soil considering groundwater seepage

Abstract

• A multilayer multiple BHEs numerical model with groundwater seepage is developed. • A method for obtaining the total HTR of a BHE field in stratified soil is proposed. • The δ is expressed as a length in metres and has a range of zero to infinity. • Effects of inlet temperature and inlet velocity on δ can be ignored. Accurately predicting the performance loss caused by thermal interaction between boreholes is essential for estimating the initial investment cost of ground source heat pump systems. In this study, based on the finite volume method and considering groundwater seepage, a multiple Borehole Heat Exchangers (BHEs) heat transfer model in layered soil is established and validated. By introducing the thermal interaction coefficient ( δ ) as the evaluation index of thermal interaction between boreholes, an empirical method is proposed for quickly obtaining the total Heat Transfer Rate (HTR) of a BHE field in stratified soil. For different types of multiple-layer soil, various δ values are compared and the effects of soil thermal conductivity, inlet temperature, inlet velocity, and seepage velocity on δ between boreholes are investigated. Results indicate that in soils with thermal conductivities of 1 W∙m −1 ∙K −1 to 4 W∙m −1 ∙K −1 and seepage velocities of 5 × 10 -7 m·s −1 , where the thickness of each layer does not vary significantly, the δ value for multiple BHEs is largest in the first layer of soil. Besides, the effects of inlet temperature and inlet velocity on δ could be ignored. Greater seepage velocities lead to lower δ values in the soil layer, as well as faster stabilisation times. This results and proposed empirical approach can be used to accurately predict the total HTR of a BHE field in stratified soil and provide directions for optimising the layout of a BHE field.