The objective of this project is to demonstrate that thermal energy can be efficiently stored and recovered from the ground at high temperatures (150-250^oC) using borehole heat exchangers. The project team will use a phased approach for the demonstration, where the first phase focuses on individual borehole heat exchanger design and operation. Following successful completion of that phase, the project team will move to a second phase where an array of multiple borehole heat exchangers would be operated to demonstrate long-term thermal energy storage and recovery. Upon successful completion of this project, a separate follow-on third phase will be recommended, where the high temperature borehole thermal energy storage would be coupled with concentrating solar collectors or a waste heat source and an organic Rankine cycle or steam turbine power block to generate continuous electricity (and heat).

This project focuses on the creation of an underground high temperature borehole thermal energy storage (BTES) system that can be charged and discharged to form a thermal battery. The high temperatures will allow generation of process steam or electricity on demand using heat extracted from the storage system. The heat is stored in the ground by circulating a hot heat transfer oil (an environmentally benign food grade mineral oil or other suitable fluid) through metal pipes that are installed in vertical boreholes in the ground. This is a closed-loop configuration, where the metal pipes are grouted in the boreholes so that there in no direct contact with the groundwater system, and heat transfer to and from the formation occurs by thermal conduction. Natural earth materials have high heat capacities and moderate to low thermal conductivities. These properties allow the for the storage of large amounts of heat with modest heat losses and eliminates the need to actually purchase the (thermal) “battery”.

Department of Defense installations use thermal energy to fuel industrial processes, condition facilities and generate electrical power. Most installations produce thermal energy using natural gas fired boilers, heat pumps or through combined heat and power plants that generate both electricity and heat. Natural gas supplies are subject to interruption during natural disasters and at times of extreme natural gas demand. There is a need for thermal energy storage that is both resilient and cost effective, that can provide thermal energy (or even electrical energy) that is independent of off-base supplies that can be disrupted.

The new method of energy storage that being developed in this project is inherently resilient, because it is located on-site, deep underground. Once the subsurface thermal energy storage system is fully charged (heated), thermal energy can be extracted continuously for a period of weeks or even months, independent of electrical or natural gas supplies. This thermal energy could be used for generating process steam, electricity, or both. The expected capital costs per unit of energy storage for the BTES system are expected to be on the order of $1-$2 per kWh_th. If this heat was converted to electricity with a 20% efficiency (which is the low range of possible efficiency), the equivalent electrical battery cost would be $5-$10 per kWh_e. This is an order of magnitude less expensive than current electro-chemical battery technology costs, and the system could be scaled to practically any size using the same technology.