4572 -- Borehole Thermal Resistance from Line-Source Model of In-Situ Tests PDF

Geothermal heat pump systems exchange heat with the ground, often through a vertical, U-tube, ground heat exchanger. The performance of this U-tube heat exchanger depends on the thermal properties of the soil, as well as grout or backfill in the borehole. In-situ tests provide a means of estimating some of these properties, but the routine analysis (linesource method) estimates only the soil thermal conductivity. This paper extends the line-source method to also give an estimate of borehole resistance. The method is validated in a large, laboratory sandbox and an in-situ field test where thermistors at the grout/soil interface provide an independent estimate of the borehole resistance. In both cases the new method agrees within 10% of the independent estimate. Applications of the model to additional in-situ field tests assess the thermal performance of boreholes with various grouts with and without pipe spacers. The model determines the reduction of in-situ borehole resistance where pipe spacers are installed to force the legs of the U-tube against the borehole wall. With the proposed method no additional data need to be collected beyond the typical data set for an in-situ test.

Units: Dual