Heat Storage Using High Temperature Borehole Heat Exchangers

Author

Jack HorvathFollow

Date of Award

8-2025

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Environmental Engineering and Earth Science

Committee Chair/Advisor

Larry Murdoch

Committee Member

Ron Falta (Co-Advisor)

Committee Member

Scott Brame

Abstract

The use of renewable energy sources such as wind and solar power is growing, and with it comes an increased need to store excess energy when it is generated and release it on demand. Moderate temperature thermal energy storage in the subsurface is widely used to heat buildings, but high temperatures, greater than 120℃, are required to power generators that convert stored heat to electricity. High temperature heat can be generated from solar collectors or other means and then transferred to the subsurface using borehole heat exchangers (BHE). A BHE consists of a closed-loop pipe system used to circulate a heat-transfer fluid through a borehole filled with insulating or thermally conductive material. The heat-transfer fluid can be either hotter or cooler than the subsurface when entering the BHE, so the system can be used to store or recover heat. This approach has the potential to store energy and generate electricity, but subsurface applications at high temperatures are unproven.

The objective of this project was to characterize the performance of a borehole heat exchanger operating at temperatures above 120℃. This was accomplished by developing high temperature borehole heat exchangers (ht-BHEs) and testing them in the lab and with theoretical analyses. A variety of BHE designs were evaluated and U-Bend and coaxial closed-loop-pipe designs were selected for testing in the laboratory. The thermal resistance between the fluid piping and borehole wall, known as borehole thermal resistance (BTR), was used to evaluate each design. While the borehole annulus was filled with sand, the BTR of the U-Bend and coaxial BHE designs was similar, between 0.10 and 0.13 Km/W. The sand was replaced with thermally conductive grout created by mixing calcium aluminate cement with granular graphite in a 6:1 ratio by weight to give a material with a thermal conductivity of 2.6 W/m℃. The BTR using the thermally conductive grout was 0.04 Km/W, roughly 1/4 to 1/3 of the BTR using sand in the annulus. These results show that thermally conductive grout can enhance a BHE’s ability to transfer heat. The U-Bend design is easier to fabricate than the coaxial design, so it was used alongside the thermally conductive grout on a field scale test.

Prior to field implementation, thermal response tests using a heat transfer fluid at temperatures up to 250℃ were conducted. This was to ensure all components that were used in the field were functional under these elevated temperatures. These tests were conducted in dry and variably saturated conditions, which could be present in surrounding geology on a field scale. Through this phase of laboratory testing, insulating and soil vapor extraction components were evaluated.

Results from the laboratory experiments demonstrated that the materials used in the creation of lab-scale ht-BHEs could operate at high temperatures while maintaining minimal thermal resistances. Insulating and soil vapor extraction components tested in the laboratory were able to decrease the loss of thermal energy and therefore would increase the efficiency of a ht-BHE.

Numerical models were used to simulate high-temperature laboratory experiments to interpret data and estimate material properties. COMSOL was used to simulate heat transfer in dry conditions that include the fluid flow in the ht-BHEs. TOUGH2 was used to simulate tests in variably saturated conditions with simplified borehole geometries to evaluate heat transfer with boiling and condensation in surrounding pore spaces. The use of these models verified that the thermal properties of borehole annulus materials were suitable for inclusion in a field scale test. They also highlighted the effect insulation and pore waters have on the ability to raise the subsurface to elevated temperatures.

Using information obtained from laboratory tests and modeling software, a high temperature borehole thermal energy storage (ht-BTES) system was designed. Construction was completed in Seneca, SC, where an array of ht-BHEs were deployed in the upper 10m of the vadose zone. Components and materials from laboratory tests were used in the construction of the field scale ht-BTES system to evaluate the ability of the system to inject, store, and recover thermal energy from the shallow subsurface.

Recommended Citation

Horvath, Jack, "Heat Storage Using High Temperature Borehole Heat Exchangers" (2025). All Theses. 4614.
https://open.clemson.edu/all_theses/4614