Uncertainty analysis of groundwater flow impacts on the performance of a borehole heat exchanger array

Abstract

Shallow geothermal energy has been popularly used for house heating and cooling by combining borehole heat exchangers (BHEs) with a heat pump. The efficiency and sustainability of BHE systems are receiving increasing attention in recent years.  Groundwater flow, among the hydrogeothermal properties of the subsurface, is considered to be a critical positive factor in maintaining BHE efficiency, especially when the groundwater flux is greater than 10-7 m/s. However, in practice, exact information on local groundwater flow is typically not available.  Rough estimates may introduce significant uncertainty in the design an performance of BHE systems. In this study, we implemented an uncertainty analysis of groundwater flow impacts on the seasonal coefficient of performance for a heat pump (SCOP). We first built a numerical model based on an existing geothermal site using FEFLOW. Groundwater flow parameters including groundwater level, flow direction, and groundwater flux within the layered subsurface were assumed to follow a Gaussian distribution in which the mean value is determined from regional hydrogeological conditions. Afterward, we selected the input data sets about groundwater flow by using Sobol sequence sampling, and then employed the selected data sets and the corresponding simulated SCOP in a random forest regression for training and predicting. Preliminary results show that groundwater flux and flow direction have a significant impact on SCOP. The impact of groundwater flow direction is strongly correlated with the arrangement of the BHEs.