Date of Award
5-2025
Document Type
Thesis
Degree Name
Master of Science (MS)
Department
Hydrogeology
Committee Chair/Advisor
Lawrence Murdoch
Committee Member
Scott DeWolf
Committee Member
Ronald Falta
Abstract
The development of renewable power sources like solar panels and wind turbines is crucial to lowering greenhouse gas emissions and slowing the increase in global temperature. These sources are limited by the intermittency of power output depending on factors such as weather or time of day. Storing energy during periods of peak productivity improves the efficiency of renewable sources and makes them more competitive with fossil fuels. The excess energy can be stored underground as heat. At high enough temperatures (>150ºC) and discharge rates (>70 kW) the stored energy can be converted back into electricity. This turns the subsurface into a “geothermal battery.”
Borehole heat exchangers (BHEs) are used to store and recover subsurface thermal energy. Thermal energy can be stored this way for long periods of time (weeks to months). Storage using BHEs is commonly used for seasonal heating of homes and offices. A high temperature BHE (ht-BHE) would store heat at high enough temperatures to generate electricity with an organic Rankine cycle power block. Conventional BHEs operate at lower (<100ºC) temperatures, so the effectiveness of a ht-BHE is unknown.
The objective of this research is to evaluate the performance of prototype designs for high temperature underground thermal energy storage using a ht-BHE. This objective was met with a combination of laboratory experiments and numerical simulations. Three candidate BHE designs were tested. Two heat exchangers used a U-bend geometry, and another used a coaxial pipe. The heat exchangers were placed in an apparatus made from a steel drum filled with sand and heated with fluid temperatures of 40-60ºC. One of the U-bend heat exchangers was surrounded by a borehole filled with cement grout with a graphite admixture. The coaxial heat exchanger and the other U-bend heat exchanger used boreholes filled with sand. Numerical simulations of the experiments that solve for fluid flow and heat transfer were created in a multiphysics modeling software.
The evaluation analyzed the borehole thermal resistance (BTR) and the energy balance of each heat exchanger design. Three methods of calculating BTR were used. In one, the temperature difference between the heat transfer fluid and the borehole wall was divided by the specific heat injection rate. Another solved for BTR through an analysis of the increase in average fluid temperature. A third method did not use experimental data, but instead calculated BTR with the geometric and thermal parameters of the borehole and heat exchanger pipe. The energy balance compared the change in energy storage to the net energy put in or recovered by the heat exchanger pipe to determine how boundary heat loss limited storage. A storage time constant, the characteristic time for energy storage when 63.2% of energy storage had completed, and a specific storage capacity, describing the power stored for a given BHE length and rise in fluid inlet temperature, describing how quickly the apparatus was able to store energy were developed for each apparatus.
The results of the analyses of experimental data and simulations indicates that BTR calculated using the three methods are similar. The BTR of the U-bend heat exchanger placed in grout is 0.06 Km/W using all methods. The BTR using geometric and experimental methods is 0.38 Km/W and 0.37±0.02 Km/W for the U-bend heat exchanger placed in sand and 0.39 Km/W and 0.38±0.03 Km/W for the coaxial heat exchanger. This indicates that the BTR of prototype BHE designs can be estimated using their geometry before collecting experimental data. Of the two methods using experimental data, calculating BTR with only the average fluid temperature has more applicability to field experiments because it requires fewer temperature measurements.
The U-bend heat exchanger in graphite grout stored 5.3 MJ at the quickest rate with a storage time constant of 15 hours. The U-bend BHE in the sand and the coaxial heat exchanger both required more time to equilibrate, with storage time constants of 24 and 20 hours, even though they stored approximately the same amount of energy as the U-bend BHE in grout (4.7 and 4.0 MJ). The difference is attributable a higher specific storage capacity (about 12 W/mºC at peak) in the grouted U-bend heat exchanger than in the other two heat exchangers (about 3.5 W/mºC at peak) during the first 10 hours of heating, and a roughly similar specific storage capacity in all three for the remaining time. Results of numerical simulations further demonstrated that the beginning of heating or recovery is the most crucial period for BHE performance in small scale experiments, as there is limited boundary interaction.
The difference in performance between the three prototype designs is attributable to borehole filling and not pipe geometry. The coaxial pipe and U-bend heat exchanger used in the experiments had almost no difference in performance when their boreholes were both filled with sand. The U-bend heat exchanger in graphite grout outperformed the same geometry enveloped with less thermally conductive sand. Using the grout with a graphite admixture increases cost, but reduces BTR by 84% compared to a heat exchanger with no grout. The U-bend in graphite grout was chosen for use in experiments at higher temperatures.
Recommended Citation
McDaniel, Robert J., "Evaluation of High Temperature Borehole Heat Exchangers for Thermal Energy Storage" (2025). All Theses. 4504.
https://open.clemson.edu/all_theses/4504