Department of War (DoW) facilities, including critical data centers, rely on external power or energy for heating and cooling. This reliance on external energy sources for building temperature and humidity control is a glaring vulnerability in times of crisis and can degrade or delay force readiness and divert critical energy resources from other uses. DoW needs technologies that can reduce peak power consumption, overall energy use, and cooling water consumption to mitigate this weakness. Underground thermal energy storage systems that leverage the relatively constant temperature of the Earth are a very efficient way to store energy over long timescales, but the cost of drilling deep wells often makes them uneconomical. This project will explore the economics of drilling shallower wells with low-cost, readily available equipment and incorporating phase change materials. Durable, Affordable, Rapid ground Thermal Storage (DARTS) is expected to be a low-cost, high-efficiency technology for underground thermal storage. This technology will be applicable in any heat and humidity scenario to any building for storing thermal energy. Energy can be stored from hours to seasons, depending on the threat situation and energy needs. This project will be a desktop/feasibility study to refine the DARTS concept and prepare for a future demonstration.

The project will design low-cost geothermal heat exchangers for geothermal heat pumps and thermal energy networks, applicable for any building and weather. The short-term power needs that typically determine heat exchanger sizing will be decoupled from annual energy needs, reducing heat exchanger sizing and allowing for flexibility in design to fit location and design load requirements. The ground heat exchanger will allow active recharging via a separate system to avoid design limitations from unbalanced loads and extreme heat or cold.

The design objectives of this project are to:

1. Improve heat flux density for both charging and discharging such that boreholes are sized for <30ft/ton via low-cost, enhanced heat exchange geometries and recharge control strategies.
2. Enable multi-timescale thermal storage through additional storage media (latent storage using phase change materials, as well as the sensible storage at multiple temperatures using the ground).
3. Allow active recharge of the ground during advantageous times (due to low-cost energy or favorable thermodynamics) to eliminate unbalanced heating/cooling load design requirements.
4. Design borehole depth to be <30ft to allow use of readily available equipment for excavation.

All four of these improvements to the state-of-the-art will enable smaller borefields with substantially lower installation costs. This project will position the design for low-risk follow-on demonstrations.

Geothermal storage, coupled with a ground-source heat pump, provides heating and cooling efficiency gains over traditional heating, ventilation, and air conditioning systems that result in significant year-round energy and cost savings—with no water use. DARTS can be applied to standalone geothermal systems, incorporated into larger thermal networks to provide load balancing and storage, or it can be retrofitted to improve the performance of existing geothermal systems whose performance has degraded due to thermal drift. Unlike traditional ground heat exchange technologies, DARTS is designed for low-cost installation using readily available equipment and materials, with reduced labor hours to reduce costs by >50% compared to conventional systems. (Anticipated Project Completion - 2027)