Vertical ground heat exchanger pressure loss – Experimental comparisons and calculation procedures

Vertical ground heat exchangers (GHEs) used with ground source heat pump systems have been constructed in a variety of configurations including single U-tubes, double U-tubes, quadruple U-tubes, and coaxial-type. Selections of vertical GHE other than single U-tubes are often motivated by a desire to decrease the borehole thermal resistance and therefore reduce the required amount of drilling and/or improve the thermal performance of the system. As with conventional heat exchanger design, the hydraulic performance – i.e. how much pressure loss is incurred – is also very important to the overall system performance. Yet, relatively little attention has been paid to this aspect of the design. In this paper, we present experimental measurements of pressure loss versus flow for four types of vertical GHE in a 200 m deep borehole: a single U-tube, double U-tubes connected in parallel, and in series, a coaxial heat exchanger, and a single U-tube with internal heat transfer enhancing ridges. Procedures for calculating the pressure loss in these configurations are tested. While standard procedures are adequate for the smooth U-tubes, the internally-ridged U-tube required a new correlation. For the coaxial heat exchanger, seemingly minor fittings that intruded into the annulus were shown to be important for calculating the pressure loss. Finally, the effects of using fiber optic cable assemblies inserted into the pipe are investigated and correlations for the excess pressure loss are given