Transient heat transfer in a U-tube borehole heat exchanger

•Proposed analytical model calculates transient vertical temperature profiles in U-tube borehole heat exchanger.•Model is verified with data sets from laboratory and field boreholes.•Model matches both short-time and long-time data in thermal response tests.•Vertical temperature profiles of circulating fluid are generated by the model.•Proposed model estimates the borehole thermal resistance taking into account calculated temperature profiles.

A transient heat transfer model has been development for a thermal response test (TRT) on a vertical borehole with a U-tube. Vertical borehole heat exchangers are frequently coupled to ground source heat pumps, which heat and cool buildings. The model provides an analytical solution for the vertical temperature profiles of the circulating fluid through the U-tube, and the temperature distribution in the ground. The model is verified with data sets from a laboratory sandbox and field TRTs, as well as a previously reported numerical solution. Unlike previous analytical models, the vertical profiles for the circulating fluid are generated by the model without any assumption of their functional form.