Ground heat exchangers with large diameter pipes: What are the benefits?

Abstract

The geothermal industry has recently been opting for large high-density polyethylene pipes to design vertical ground heat exchangers of ground-coupled heat pump system. Thus, we hypothesized that large diameter pipes can help improve heat exchange rate with the subsurface and made this study with the objective of quantifying the benefits gained with U-loop configurations. The finite line source equation and the multipole model were used to evaluate the maximum heat exchange rate that can be achieved with increasing pipe diameter. Sizing calculations were then performed for a school building in Boston. Ground heat exchangers with a single U-pipe having a nominal diameter of 1.25, 1.5 and 2 inches, as well as a double U-pipe, having a nominal diameter of 1.5 inches, were considered. Results highlighted that the double loop is by far the most efficient configuration, followed by the single loop with a 2 inches pipe diameter, respectively providing heat exchange rates that were 16% and 6% greater and total borehole length that were 22 and 9% smaller when compared to single loop with a 1.25 inches diameter. The use of large diameter pipes was shown without a doubt to benefit ground heat exchanger performances.