Modelling and Optimization of A Geoexchange Coaxial Bore Heat Exchanger

Abstract

The growing threat of climate change has emphasized the need for energy production methods that generate less emissions than previous processes. Geothermal and geoexchange technology represent an excellent opportunity for clean energy investment due to the low land use intensity that future geothermal and geoexchange projects will command compared to wind and solar. Coaxial borehole heat exchangers can boost the efficiency of geoexchange systems compared to U-Bend BHEs, but are still the subject of research to optimize the design. In this project, coaxial BHE optimization was explored through manipulation of the center pipe diameter, the addition of insulation to the center pipe, and the addition of surface roughness to the outer pipe. DTS testing on the geology of Princeton University was conducted to improve the accuracy of the ground environment in the Ansys Fluent coaxial BHE model used to simulate the effect of the changes to the coaxial BHE design. The results show that manipulation of the center pipe diameter and the addition of insulation produce marginal increases in outlet temperature of the bore for shallow coaxial bores. The DTS testing of the Princeton University geology shows that the geothermal gradient is constant for the first 100 meters of depth, suggesting that bores shorter than this depth cannot leverage the geothermal gradient to improve performance. Simulations within EED show that increases of the center pipe diameter results in small decreases in required bore length to meet the required heating and cooling load of the system.