A preliminary assessment of the effects of groundwater flow on closed-loop ground source heat pump systems

Abstract

A preliminary study has been made of the effects of groundwater flow on the heat transfer characteristics of vertical closed-loop heat exchangers and the ability of current design and in-situ thermal conductivity measurement techniques to deal with these effects. It is shown that an initial assessment of the significance of groundwater flow can be made by examining the Peclet number of the flow. A finite-element numerical groundwater flow and heat transfer model has been used to simulate the effects of groundwater flow on a single closed-loop heat exchanger in various geologic materials. These simulations show that advection of heat by groundwater flow significantly enhances heat transfer in geologic materials with high hydraulic conductivity, such as sands, gravels, and rocks exhibiting fractures and solution channels. Simulation data were also used to derive effective thermal conductivities with an in-situ thermal conductivity estimation procedure. These data were used to design borehole fields of different depths for a small commercial building. The performance of these borehole field designs was investigated by simulating each borehole field using the pre-calculated building loads over a ten-year period. Results of these simulations, in terms of the minimum and peak loop temperatures, were used to examine the ability ofmore » current design methods to produce workable and efficient designs under a range of groundwater flow conditions.« less