Subsurface Thermal Energy Storage for Improved Heating and Air Conditioning
The main objective of this project is to develop a field application of subsurface thermal energy storage (STES) to enhance ground-coupled heat pump efficiency at a Department of Defense (DoD) facility. The field application would provide a proof of concept of an STES application and provide data on key metrics such as reduced energy use, payback period, reduced carbon footprint, and other factors. A second objective is to develop a detailed application manual for the STES technology that will help DoD facility managers determine if STES can be applied at their facility and what variants will provide the most synergy.
With STES, low-cost or waste energy (either heat [positive] or cold [negative]) is harvested when it is produced or when it is available, stored in the subsurface using borehole heat exchangers or water wells, and then utilized when the heat or cold is expensive or difficult to obtain. The anticipated application would consist of two sets of borehole heat exchangers that are tied into a ground source (geothermal) heat pump inside a building. The heat pump operates on a standard refrigeration cycle. During the winter heating season, heat is extracted from water that is circulated through the hot borehole heat exchangers, and it is used to evaporate the working fluid in a heat pump heat exchanger. The heat pump compressor compresses the vapors back into a liquid in a different heat exchanger, releasing the heat into either air or water that is used to warm the building. During the summer cooling season the cycle is reversed, and the heat pump working fluid now evaporates in the heat exchanger that is tied to the building return air (or water), thus absorbing heat and cooling the building. After compression by the heat pump compressor, the refrigerant vapor condenses in a heat exchanger, giving off heat. Water that is circulated through the cold borehole heat exchangers is used to remove this heat from the building by cooling the condensing refrigerant in the heat pump.
The STES geothermal heat pump system to be demonstrated differs from the conventional, single ground-loop, geothermal heat pump operation, because it takes advantage of free or inexpensive sources of heat and cold to create two ground loops (borehole arrays containing heat exchangers), one artificially hot and the other artificially cold. In contrast, a conventional geothermal heat pump system uses heat (cold) from a single ground loop at a constant ambient temperature in both the winter and summer cycles. By using a hot water source in the summer and a cold water source in the winter to maintain the hot and cold ground loops, respectively, the STES geothermal system can achieve a higher heating and cooling efficiency than the conventional geothermal system. Then, net heating of the hot loop and net cooling of the cold loop become benefits.
The STES technology has the potential to reduce DoD heating and cooling energy costs by as much as 30%. At the same time, it allows for the integration of renewable energy into the base infrastructure, and it provides a clear path for reducing base carbon emissions and carbon footprint. As the costs of fossil fuels and carbon emissions rise, the expected benefits increase. (Anticipated Project Completion - 2014)
Points of Contact
Dr. Ronald Falta
Energy and Water
SERDP and ESTCP
- Fact Sheet - Brief project summary with links to related documents and points of contact.
- Final Report - Comprehensive report for every completed SERDP and ESTCP project that contains all technical results.
- Cost & Performance Report - Overview of ESTCP demonstration activities, results, and conclusions, standardized to facilitate implementation decisions.
- Technical Report - Additional interim reports, laboratory reports, demonstration reports, and technology survey reports.
- Guidance - Instructional information on technical topics such as protocols and user’s guides.
- Workshop Report - Summary of workshop discussion and findings.
- Multimedia - On demand videos, animations, and webcasts highlighting featured initiatives or technologies.
- Model/Software - Computer programs and applications available for download.
- Database - Digitally organized collection of data available to search and access.