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.

This paper focuses on characterization of the heat-transfer and water-flow processes in physical models of borehole heat exchanger arrays in unsaturated soil layers. The overall goal is to develop a data set that can be used to validate the coupled thermo-hydraulic flow models needed to simulate the efficiency of heat transfer in soil-borehole thermal energy storage systems. Two bench-scale physical models consisting of a triangular array of vertical heat exchangers within a layer of unsaturated silt were constructed in insulated cylindrical tanks to evaluate the impact of different boundary conditions on the heat-transfer and water-flow processes in the silt during heat injection into the array. In one model, the heat exchangers were placed at a radial location at 26% of the tank radius, while in the other model, the heat exchangers were placed on the inside of the tank wall. During circulation of heated fluid through the heat exchangers, the changes in soil temperature and volumetric water content along the centerline of the array at different depths were measured using dielectric sensors. The thermal conductivity and specific heat capacity of the silt were also monitored using a thermal probe at the center of the silt layer at midheight. Permanent drying was observed for the soil within the array with the smaller spacing, while an increase in water content was observed in the array with a spacing equal to the container diameter. An increase in thermal conductivity of the soil was observed within the array in the case of larger spacing, while the opposite was observed in the case of smaller spacing. The results indicate the possible formation of a convective cell within the larger array as water was driven inward from the heat exchangers. These results highlight the importance of coupled heat transfer and water flow in soil-borehole thermal energy storage systems in the vadose zone.

Subsurface thermal conductivity, thermal gradient, and heat flow are significant parameters when determining the feasibility of utilizing a geologic unit to generate industrial geothermal power. Cores from 18 wells of the subsurface Jurassic Smackover Formation in southwest Arkansas were analyzed at the Arkansas Geological Survey, where thermal conductivity, thermal gradient, and heat-flow values were estimated. Thermal conductivity of several samples was obtained using a KD2 Pro Thermal Analyzer at room temperature. Thermal gradients were estimated from Smackover Formation borehole temperatures, and heat-flow values were calculated from thermal conductivity and thermal gradient values. Average thermal conductivity values for the Smackover Formation are greatest in northeastern Lafayette County at 2.57 W/m·K, followed by southern Columbia and western Calhoun Counties at 2.47 W/m·K each. Northwestern Columbia and northeastern Lafayette Counties exhibit the highest thermal gradient and heat flow, with values averaging 3.51 °C/100 m and 72.3 mW/m 2 , respectively. Interpretation of these parameters confirms that this area exhibits the highest geothermal potential for the Smackover Formation in southwest Arkansas. Investigations further characterizing the Smackover Formation, including in situ thermal properties and borehole temperature measurements, are recommended for future geothermal feasibility studies.

Geothermal energy depends on high subsurface temperature, adequate permeability and fracture volume, and accessible groundwater supply to support heat exchange with surrounding rock. Some regions may have adequate thermal resources but lack the necessary permeability or deep circulating water. Exploitation of such areas for geothermal energy could occur if permeability can be enhanced enough to provide the necessary heat exchange. These improvements to the geothermal reservoir would produce what is termed an “enhanced geothermal system” (EGS). The Snake River Plain (SRP) in southern Idaho is a geological region with high heat flux (~110 mW/m 2 ) that has been recommended as an EGS target. In this study, we consider how the geologic and thermal history of the SRP might influence its EGS potential. We describe the fracture distribution (mean = 28.63 fractures/10 m) in a welded tuff core recovered from one of the few deep boreholes located on the SRP and provide a preliminary discussion of the likely geomechanical behavior under in situ stress. Spatial autocorrelation of fracture features is defined with geostatistical techniques and used in a stochastic simulation of possible structures in other welded tuff reservoirs. Autocorrelation scales for the continuous date are on the order of 70 meters with high subsample scale variability (56 m). Results should aid in designing criteria for a hydraulic fracturing plan that would augment the permeability and connectivity of an SRP reservoir's preexisting fracture network.

Strong reasons exist to hypothesize that phyllosilicates, that is, clay minerals, played a critical catalytic role in the organic synthesis of prebiotic and possibly early biotic compounds and structures. Phyllosilicates would be expected to be abundant at the surface of early Earth (the Hadean) by the hydrous alteration of impact-generated silicate debris. The explorations of Earth, the Moon, and Mars permit reasonable inferences about physical conditions on prebiotic Earth. Also, currently available information allows the definition of necessary steps in prebiotic synthesis in which phyllosilicates may have participated. Consideration of these steps supports the plausibility that such minerals provided catalytic, substrate, and organizational functions for prebiotic and possibly early biotic development of organic structures, leading to formation and replication of ribonucleic acid (RNA) and, in turn, leading to a prebiotic RNA world. Ultimately, prokaryote cells may owe some of their functions to the inherent characteristics of associated phyllosilicates.

Laboratory and numerical cratering experiments into sandstone and quartzite targets were carried out under conditions ranging from pure strength– to pure gravity–dominated crater formation. Numerical models were used to expand the process of crater formation beyond the strength-dominated laboratory impact experiments up to the gravity regime. We focused on the effect of strength and porosity on crater size and determined scaling parameters for two cohesive materials, sandstone and quartzite, over a range of crater sizes from the laboratory scale to large terrestrial craters. Crater volumes and diameters of experimental and modeling data were measured, and scaling laws were then used to determine μ values for these data in the strength and gravity regimes. These μ values range between 0.48 and 0.55 for sandstone and between 0.49 and 0.64 for quartzite. The scaled crater dimensions in numerical models agree quite well with experimental observations. An accurate definition of the strength parameter in pi-group scaling is crucial for predicting the crater size, in particular, in the transitional regime from strength to gravity scaling. We determined an effective strength value that accounts for the weakening of target material due to the accumulation of damage. Using the numerical models, we found an effective strength of 4.6 kPa for quartzite and 3.2 kPa for sandstone, which are almost five orders smaller than the quasi-static experimental strength values that only account for the intact state of the target material.

This study compares results of crater diameter, shape, ejecta morphologies, and interior morphologies in the northern hemisphere of Mars between older results reported from a Viking -derived crater catalog and a new crater catalog obtained from analysis of higher-resolution data sets, primarily Mars Odyssey Thermal Emission Imaging System (THEMIS) daytime infrared and visible data. This report focuses on results from the northern hemisphere, where the new analysis has increased the number of known craters ≥5 km in diameter by 1300 to a total of 14,224. The improved resolution and overall clarity of the new image data sets have improved the classification of ejecta and interior morphologies. This study finds that ~3% of the craters display evidence of oblique impact (through elliptical crater shape and/or asymmetric ejecta blankets), and many show an east-west orientation for the major diameter, indicating impact trajectories approximately parallel to the present-day Martian equator. No strong indication of extended periods of high obliquity or polar wander is recorded within the elliptical crater population. Ejecta morphologies are divided into seven classes, with most fresh craters (92%) displaying one of the three layered ejecta morphologies. Diameter and geographic distributions of the different ejecta morphologies are similar to those reported from Viking analysis. A study of the two types of double layer ejecta craters finds that crater diameter and terrain characteristics are the primary factors leading to the different morphologies. Interior morphologies are divided into nine classes, with most Martian impact craters containing some type of floor deposit from eolian, fluvial, glacial, volcanic, or impact processes. Wall terraces, flat floors, and fractured (chaotic) floors are strongly concentrated in highlands regions. Central peak and summit pit craters are found in similar regions, particularly in the highlands, but floor pit craters are distributed across both highlands and plains units. Target strength appears to be an important factor in the formation of many of these interior morphologies.