Despite the superior energy efficiency of geothermal heat pump systems, widespread adoption of the technology is hindered by the higher initial capital cost relative to conventional heating and cooling systems. A promising avenue for reducing first costs is better characterization of salient subsurface properties and the subsequent implementation of more optimal designs for the ground heat exchange component. This study evaluates the potential for geological and pedological models to facilitate optimal design of vertical- and horizontal-based ground heat exchange systems, respectively. Data from well borings and subsurface monitoring sites in Indiana are used for geological model development and the assessment of existing pedological models. Results from performance simulations illustrate how the total design length of a conceptual, commercial-scale vertical ground heat exchange installation could be reduced by 17% as a result of better characterization of the variability in thermal properties gleaned from geological modeling. Given the abundance of publicly available well bore data, the explicit integration of geological modeling during the system design phase appears to be a feasible and promising approach for improving affordability. Results from horizontal ground heat exchange modeling at sites in Indiana that contain in situ monitoring data indicate that the use of design parameters derived from existing pedological models generally leads to overbuilt ground loops that were on average 45% longer than necessary to achieve design specifications. Despite the apparent limitations of existing pedological models, such models have utility in identifying sites that exhibit stable thermal properties and are thus most amenable for optimal design of horizontal ground heat exchange systems.

By using ground source heat pumps to exchange heat with the shallow surface, areas with minimal or no tectonic activity can still be viable resources for low-temperature geothermal energy. This study focuses on generally characterizing the potential exploitation of flooded mines as low-temperature thermal resources within Ohio. These unconventional thermal resources offer large, thermally stable bodies of water, which store relatively more heat than saturated soils and bedrock. The legacy of underground mining, predominantly in the southeastern and eastern portions of Ohio, makes ground source heat pump geothermal energy a potentially valuable resource for the state. Using geographic information system (GIS) software, mines that were either flooded or partially flooded and within 1.6 km of a population center were selected as potential candidates for ground source heat pump exploitation. Physical and thermal parameters were calculated for each of the identified geothermal sites. These include: maximum and minimum residence times of waters within the mines, maximum and minimum recharge to the mines, effective mine volumes, linear groundwater velocities, groundwater flow direction, and percentage of the mine that is flooded. The total theoretical amount of heat extraction or addition per degree change in mine water temperature (Celsius degree) was calculated for each identified mine site, as well as the potential amount of heat either entering or being dissipated in mine waters due to groundwater recharge. This study identified 147 possible mine sites spanning 21 counties that might be used for ground source heat pump installations in Ohio. The mines have an estimated average maximum residence time ranging from 6 to 15 yr and an estimated average minimum residence time ranging from 3.6 to 8.9 yr. It was estimated that, on average, 10 10 kJ °C −1 of heat energy could be extracted from the mines. Overall, this study has shown that abandoned underground mines contain enough stored heat to be used as thermal resources for ground source heat pump systems, and that the number and extent of mines within Ohio could make this type of geothermal resource valuable.

Geoexchange systems utilize the heat capacity of the ground to provide efficient heating and cooling of buildings. West Chester University is developing a 16 MW district geoexchange system in southeast Pennsylvania. The system currently includes 529 borehole heat exchangers installed in fractured gneiss, with an anticipated 1400 borehole heat exchangers when complete. Borefield temperature, heat flux, and electric demand were recorded at 5 min intervals. Mean annual borefield temperature increased 2.1 °C yr −1 between 2011 and 2014, resulting from unbalanced cooling demand from high-occupancy buildings. The greatest recorded daily mean borefield temperature was 34.4 °C, which is significantly greater than ambient ground temperature of 12.8 °C and close to the maximum efficient design temperature of 41 °C for geothermal heat pumps. West Chester University is mitigating this unsustainable increase in temperature by installing a 738 kW cooling tower and heated sidewalks for snow removal. Inverse modeling of borefield temperature revealed an effective thermal conductivity between 1.3 and 1.4 W m −1 °C −1 , which is significantly less than the 2.9 W m −1 °C −1 of the formation, indicating that heat exchange is limited by borehole construction. Despite this performance issue, the system is operating with a coefficient of performance of 3.4 and 4.9 for heating and cooling, respectively. We conclude that thermogeologic investigation of borefield response could lead to significant improvements in efficiency and reduction in the number of wells required to maintain system performance.

Thermal response tests are the industry standard for borehole heat exchanger design in ground-coupled heat pump systems. Two previously conducted thermal response tests in phase 2 of the district-scale ground-coupled heat pump system at Ball State University (BSU) measured a bulk “formation” thermal conductivity K T between 2.6 and 3.0 W m −1 K −1 . Meanwhile, K T from a core recovered near BSU averages 2.2 ± 0.006 and 3.5 ± 0.086 W m −1 K −1 for dry and water-saturated samples, respectively. The range in K T data from saturated samples (1.8–7.2 W m −1 K −1 ) leads to the conclusion that thermal response tests do not capture the vertical and horizontal heterogeneity of heat flux in layered sedimentary aquifers. Characterization of the hydrogeologic environment can be one tool to tune district-scale ground-coupled heat pump systems to the specific on-site conditions that may influence the magnitude and mode of heat transfer. At BSU, temperature ( T ) changes in the groundwater environment at the active phase 1 field through October 2013 support this notion. After constant heat loading, a T increase of 14–18 °C was observed in the central monitoring well. The vertical structure in the T profile of this well may correlate to “thermofacies.” For example, a T spike between 14 and 19.5 m in depth may correspond to a sand and gravel zone in the surficial glacial till, and a T dip at a depth of 70 m agrees with the position of the Brainard Shale—zones of higher permeability and lower measured K T (2.0 W m −1 K −1 ), respectively. Higher measured K T zones, such as the low siliciclastic Silurian Salamonie Limestone and the Ordovician Whitewater Formation, may be target thermofacies for heat deposition and extraction. In contrast, sand and gravel zones within the glacial till may allow for significant thermal loading; however, groundwater advection may reduce the fraction of recoverable thermal load.

Heat transfer in the subsurface can vary with depth due to variations in the thermal conductivity of the geologic medium as well as variations in groundwater flow velocity. However, traditional thermal response tests (TRTs) do not allow for evaluation of the depth variability of heat transfer. We investigate the potential for using discrete-depth heat dissipation tests in open, water-filled boreholes to evaluate variations in heat exchange rate with depth. Heat dissipation tests were initiated at target depths in a test well using an electrical resistance heater. Heat dissipation was monitored by measuring borehole water temperature through time using a fiber-optic distributed temperature sensing system. Temperature data were used to compare the thermal response at different depths in the borehole. To account for both the thermal conductivity of the geologic medium and the groundwater flow velocity, we used a numerical groundwater flow model (MODFLOW) and solute transport model (MT3DMS) to simulate heat dissipation tests. Simulation results indicate the measured response to a heat dissipation test in an open borehole is strongly dependent on the measurement location within the borehole; thus, data are ambiguous when the measurement location is uncontrolled. However, modeling results also indicate that the thermal response of a heat dissipation test as measured at the center of the borehole is sensitive to variations in thermal conductivity and groundwater flow velocity, suggesting that heat dissipation tests are a potentially useful method for characterizing depth variability in thermal properties if a centralized temperature measurement method is used to monitor the tests.