Experimental Investigation of Thermal Dispersion under Forced Groundwater Flow through Lab-Scale Tests to Design an Optimal GWHP system

Abstract

Due to the direct use of groundwater as a heat source/sink, the efficiency of GWHP system depends on hydrological and thermal properties of the aquifer in a complex manner. Although a few studies have been conducted to identify key factors affecting the performance of the system, most of them did not focus on thermal properties but flow conditions and well arrangement. However, thermal dispersion was recently recognized for its significance in the high flow field which can be commonly induced by GWHP systems. Therefore, it is needed to further research on how thermal dispersion affects the heat transport in GWHP systems. In this research, a laboratory device simulating the heat transport in saturated porous medium was designed to investigate thermal dispersion behavior under forced groundwater flow conditions. The main goal of this study is to compare the thermal dispersion coefficient in forced flow environments with that in natural flow environments through lab-scale heat tracer tests and further simulations using two different heat sources: (a) a resistor and (b) water injection. The longitudinal/transverse thermal dispersion coefficients under natural flow condition were derived from the tests with a resistor. A linear dependency of the coefficients on flow velocity was examined in this study probably due to the flow conditions being limited to conduction-dominated flow environment (Pet < 1). Thermal dispersion coefficients computed from numerical simulations using the model validated with water-injection tests showed higher values because of the disturbance in flow velocity field. The increase of thermal dispersion coefficients compared to the natural flow condition (without-injection) became larger in the lower regional flow velocity with the higher injection rate. This implicate that the influence of thermal dispersion in navigating thermal plume movement can be significant in the forced flow field, especially when the groundwater flow is slow and the operated injection rate is high. Therefore, it is important to give careful consideration to the adoption of thermal dispersion value when assessing the heat transport with water injection. Otherwise, the oversight of thermal dispersion term may cause a critical error in predicting thermal plume movement in forced flow fields.