Characterization of Coaxial Borehole Heat Exchangers by Dimensionless Numbers

We analyze a vertical coaxial borehole heat exchanger (CBHE) using two dimensionless numbers. One dimensionless number, Nr, characterizes the heat flow from the rock to the flowing fluid in the annulus and the other, Nw, characterizes the insulation between the flowing fluids in the inner tube and the annulus. Based on an analytical solution for a CBHE, the outlet temperature and the heat production are expressed as weighted sums of two temperature differences. It is shown that the outlet temperature, the heat production, and the weighting functions have four regimes defined by the two dimensionless numbers. High outlet temperature and high heat production take place in two different regimes. An optimal operation of the CBHE, which gives a reasonable heat production and a noticeable increase in the fluid temperature, is when 2更多