Date of Award
Master of Science (MS)
Environmental Engineering and Earth Science
Dr. Ronald Falta
Dr. Lawrence Murdoch
Dr. Brady Flinchum
Thermal energy storage is a potential method for storing excess energy produced when supply is greater than demand. The use of the subsurface for storing thermal energy has become more recognized as a viable alternative to conventional methods of energy storage. However, high temperature borehole thermal energy storage has yet to be researched in-depth. Therefore, the goal of this project is to determine the feasibility of using the subsurface to store thermal energy at relatively high temperatures.
The focus of this work is to determine what design elements would make a borehole thermal energy storage system most effective and produce the most power. To do so, laboratory experiments assessing different borehole heat exchanger designs as well as analytical and numerical models were used to evaluate different parameters. Once important parameters were understood, a numerical model was built in TOUGH2.1-EOS3 to determine an estimated power production from both small-scale and pilot-scale systems. The borehole thermal resistance (BTR) represents the sum of the resistances inside the borehole between the circulating fluid and the soil (Zhang et al., 2015). The BTR is valuable when designing the BTES system because it can have a significant effect on the system performance and should be minimized to see the best results (Gehlin, 2002). Therefore, by constructing several different models for each size BTES system, the effects of varying borehole spacing, and borehole thermal resistance (BTR) values could be established.
The results show that when borehole heat exchangers are in direct contact with unconsolidated sediment, the BTR is between 0.10 and 0.13 m°C/W. However, when the annulus of the borehole is filled with a thermally enhanced grout the BTR value for the U-bend falls to approximately 0.03 m°C/W. The radial temperature drop required for a unit of heat flux between the BHE and the formation is proportional to the BTR, so a design with a low BTR is more efficient than one with a higher BTR.
Varied BTR values were used within the TOUGH multiphase flow simulations to determine the impact they would have on each system. In the pilot scale simulation, the production of power at the end of a 30-day production cycle was between 30 and 60 W/m. The lower thermal power output correlated with the system that had a higher BTR value and vice versa. Therefore, the BTR is highly important and design choices should be made to keep it as low as possible.
Bicknell, Kayla, "Thermal Energy Storage Using High Temperature Borehole Heat Exchangers in Unconsolidated Materials" (2023). All Theses. 4179.