Study on the best heat transfer rate in thermal response test experiments with coaxial and U-pipe borehole heat exchangers

This paper concerns the modeling of vertical Borehole Heat Exchangers (BHEs) for Ground Source Heat Pump (GSHP) applications. Focus is devoted to the analyses of Thermal Response Test (TRT) simulations aimed at understanding the main factors that influence the ground thermal conductivity and the effective borehole thermal resistance estimations. The conventional infinite line-source (ILS) model does not include any influence of the external heat transfer rate on the BHE/ground property evaluation. Analyses of numerically simulated TRTs show this omission can sometimes produce an error in the estimate of the ground thermal conductivity. The error may be between +/- 10% and +/- 22% for coaxial boreholes (800 m depth), if the ground has a significant geothermal gradient. On the other hand, for single and double U-pipe BHEs the error is less than +/- 5% under similar conditions. The parameter q(ratio) is identified as an indicator of when the error is significant. This parameter is equal to the external heat rate (injection or extraction) divided by a natural heat rate that is related to the geothermal gradient. Errors greater than +/- 10% tend to occur for coaxial boreholes with a center-pipe fluid inlet when vertical bar q(ratio)vertical bar < 1. Under the same conditions but with the annulus as the fluid inlet, the error is less than +/- 6%.