M.D. Smith’s research while affiliated with Oklahoma State University and other places

The design of a geothermal heat pump system requires an estimate of soil thermal conductivity. An in-situ test on a bore-hole provides such an estimate, along with an estimate of the borehole resistance. Sometimes electrical power interruptions, running out of fuel, or other equipment problems temporarily disrupt the test and greatly complicate the analysis of test data. This paper develops a method to estimate the elapsed testing time (or recovery time) when the effects of the interruption dissipate sufficiently so that the estimated thermal conductivity is changed by 10% or less. After the power is restored, the method can be used in the field to estimate the required recovery time. Because the test time using standard procedures can be prohibitively long following the interruption, an analytical technique has been developed that shortens the minimum test time for a valid estimate of thermal conductivity. These methods are validated using data sets from a laboratory sandbox. Furthermore, the new method of analysis is able to estimate the soil thermal conductivity from a data set that was previously not interpretable by standard (line-source) methods.

fn-situ tests to determine soil thermal conductivity and borehole resistance are often difficult to interpret due to variability in the heat rate during the test. This paper demonstrates that simply smoothing out fluctuations in temperature data will not remove the distortions caused by variable rates. Instead, an algorithm is developed to properly remove variable heat rate effects. The output of the algorithm is the transient, ground-loop temperature curve that would have been observed if the heat rate would have remained constant. Then, the processed temperature curve can be analyzed by the simple line-source model or alternative models. The algorithm is validated by removing variable-rate effects from a test on a laboratory sandbox with known soil thermal conductivity and borehole resistance. The algorithm successfully removes variable-rate effects in two field borehole tests, where estimates of thermal conductivity based on raw temperature readings vary by as much as a factor of three, depending on the time interval chosen for the analysis.

An in-situ test on a borehole provides a way to estimate soil properties, which are needed to design geothermal heat pump systems. Having sufficient testing time becomes an issue in planning, performing, and interpreting the test. This paper develops a quick method to calculate the minimum testing time necessary to estimate soil thermal conductivity within 10% of the estimated value from a very long test. The quick method is based on an analytical model of the in-situ test expressed in a set of graphs with the least number of independent dimensionless groups. The results indicate minimum testing times can vary by over a factor of 100 among different tests. Even among a cluster of boreholes, application of the quick method demonstrates that minimum testing time varies by a factor of four with grout thermal conductivity and whether or not spacers are placed in between U-tube legs. Therefore, no simple rule for the minimum testing time applies to all cases. Instead, the proposed method, based on a set of graphs, offers a quick estimate of the minimum time to help plan, perform, and interpret borehole tests.

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 (line-source 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.