Optimizing borehole Heat exchanger spacing to maximize advective heat transfer

University of Minnesota M.S. thesis. September 2013. Major: Water Resources Science. Advisor: Martin O. Saar. 1 computer file (PDF); x, 83 pages.This generalized study provides first order insights into the effect of heat advection on ground source heat pump (GSHP) system operation and the optimization of spacing between borehole heat exchangers (BHE) using groundwater flow and heat transport models. In these systems, there is a threshold Péclet number, the ratio of heat advection rate to heat conduction rate, beyond which the efficiency of heat transfer between the BHEs and the aquifer is significantly increased, thus lowering the temperature drop of the circulating fluid in the BHE and increasing the overall efficiency of the BHE system. This threshold Péclet number depends on the groundwater flow rate and effective thermal diffusivity (among other factors) of the system and, for the given conditions, is approximately 2 with a 1% change in BHE outlet temperature and approximately 11 with a 5% change. In GSHP systems with standardized spacings between BHEs, groundwater heat advection can cause negative thermal interactions between heat exchangers, which can be eliminated and in some situations replaced with positive thermal interactions by optimizing the spacing between BHEs. Above the threshold Péclet number, there is specified spacing between heat exchangers that will allow for the utilization of the previous season's heat injection or extraction, a half year transport distance. For the GSHP system simulated in this study, the BHE spacings for optimization are 6.65 m and 13.8 at groundwater flow velocities of 2.5 x 10-5 m/s and 5 x 10-5 m/s, respectively. It may also be possible to space the heat exchangers at a distance that captures heat after a year and a half of transport (for systems with only slight heat advection dominance), but more simulations are necessary to investigate the results of such a strategy