Seasonal changes in thermal process based on thermal response test of borehole heat exchanger

Abstract

The ground-source heat pump (GSHP) system is one of the most energy-efficient cooling and heating systems popularly used worldwide. The subsurface thermal properties, especially the apparent thermal conductivity, are applied in designing an optimal system. A thermal response test (TRT) is usually conducted once to estimate the apparent thermal conductivity. However, seasonal changes in the apparent thermal conductivity obtained from the TRT have not been sufficiently discussed. Subsurface conditions such as groundwater flow and temperature distribution can vary seasonally because of natural or artificial causes. Therefore, to optimally design a GSHP system, an evaluation of the variation of apparent thermal conductivity caused by seasonal changes in subsurface conditions such as groundwater flow and temperature distribution is imperative. We performed TRT with temperature monitoring three times in January, May, and October, and analyzed the results using numerical simulation. The apparent thermal conductivity in January (winter season) was estimated to be approximately 10%–15% greater than that in May and October owing to the rapid groundwater flow in the partial aquifer, assumed to be because of pumping well operation for snow melting. To evaluate the rate of increase of the apparent thermal conductivity caused by the partial groundwater flow, numerical simulations were performed using a three-dimensional single borehole heat exchanger model. It was found that the parameter of dispersivity plays an important role in the numerical simulation of the thermal process with groundwater flow, and the rate of increase of thermal conductivity has a linear relationship with the mass flow rate, regardless of the partial aquifer thickness and dispersivity. This finding may aid in the estimation of apparent thermal conductivity when considering seasonal changes in groundwater.