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ABSTRACT
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 therma lperformance 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.
INTRODUCTION
Geothermal heat pumps (GHP) take advantage of the relatively constant temperature of the ground. In these systems, heat is extracted or rejected to the ground by way of circulating fluid through a buried U-bend loop. The vertical ground loop consists of a high-density-polyethylene (HDPE) U-tube inserted into a borehole (Figure 1). A grout mixture is pumped through a tremie pipe to fill the space between the U-tube and the borehole. Low-permeability grout prevents water and contaminants from migrating along the vertical borehole.
For a geothermal heat pump, heat transfer with the ground is a critical factor in the design. For this reason, the thermal properties of the soil and grout, as well as the gaps between the U-tubes and the borehole, are important. An in-situ test on the ground loop is one method to determine some of these properties. In the ideal test, heat is rejected to the ground at a nearly constant rate, while the entering and discharge fluid temperatures are measured for the ground loop. Often a semilog plot of loop temperature versus time is used to determine the soil thermal conductivity....