20-38. Developing regional- and national- scale methods for assessing underground thermal energy storage, focusing on aquifer and reservoir thermal energy storage

CLOSED

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Background: Underground Thermal Energy Storage (UTES) is a broad class of thermal-energy-storage methods that capitalizes upon the insulation capacity of geologic materials to limit thermal energy loss during a storage period. Heat is stored by two mechanisms: (1) the heat contained in the stored injected water (i.e., specific heat) and (2) conductive heat exchange with solid materials in the subsurface. Subsurface materials can be natural geologic deposits or buried engineered materials with high heat capacity and high heat exchange rate surrounded by insulating geologic deposits.

Both heating and cooling energy can be stored, and the source of both can be waste heat or any other ambient or generated source. For example, solar heating can supplement summer heat, and heat exchange with rivers could further cool injected water. Due to the ubiquitous nature of porous geologic materials, UTES can be used to supply a significant fraction of U.S. energy needs.

UTES systems may be open (e.g., Aquifer Thermal Energy Storage [ATES]; Reservoir Thermal Energy Storage [RTES]) or closed (e.g., Borehole Thermal Energy Storage [BTES]), with a myriad of operational configurations. Open systems allow fluids to flow into geologic formations, and closed systems have no fluid exchange (only conductive heat exchange). For this proposal, we propose to focus our evaluation of UTES technologies on open systems (i.e., RTES and ATES), but evaluation of closed systems (e.g., BTES) is possible. On the typical storage/release timescale (e.g., seasonal), closed systems only rely upon the subsurface to insulate the volume of heated or cooled water, so thermal storage and release capacity is dominantly a function of the engineered container and not the natural environment. 

The primary advantage of UTES over other energy storage systems (e.g., electro-chemical batteries) is long charge/discharge cycle (months to years) and the potential to store very large quantities of energy.  Electrical batteries for example cannot effectively store energy for longer than days to a few weeks. UTES is limited by the “reservoir” size and the rate at which stored heat (hot or cold) is lost via conduction to surrounding geology.  For example, ATES “leaks” heat faster because groundwater sweeps heat away, but RTES loses heat much more slowly via conduction.  Recoverable heat tends to increase over time as surrounding geologic materials are heated/cooled to operational temperatures, and to decrease as storage-cycle time increases (because heat loss continues for the duration of the storage period).

Description of the Research Opportunity: This research opportunity focuses on thermal energy storage in open systems, where heat and fluids are exchanged with geologic media (e.g., ATES and RTES).  In an RTES system, it is assumed that the heat storage reservoir is comprised of permeable strata that underlies and is in poor hydraulic connection with overlying potable aquifers. ATES would store heat in the overlying aquifer. These systems differ in a number of ways, but for the purposes of this research, a primary distinction is that heat injected into ATES systems is swept away from the point of injection, so must be captured downgradient; and heat injected in RTES systems uses lower quality water, but stored hot or cold water is nearly stagnant, resulting in different operational constraints.

There are a range of significant research contributions that can be made, with a few suggested here:

1. Develop simulation tools and approaches to extend the utility of existing and newly developed regional groundwater flow models to estimate local ATES resources.  New tools and methods are needed to update existing groundwater models to represent subsurface thermal characteristics and to develop high-resolution inset models capable of representing and optimizing ATES operations.
2. Develop tools to compare ATES to RTES for a district-heating/cooling area, taking into account subsurface geologic and hydrogeologic conditions, background water quality and temperature, geothermal gradient, and other factors, to allow end-users to evaluate options (i.e., multi-resource assessment modules).  Demonstration of these tools for underserved communities is encouraged.
3. Employ optimization techniques with groundwater flow and heat transport models to identify optimum and sustainable ATES and RTES operational practices.
4. Create regional (e.g., the Illinois basin) maps of ATES or RTES potential based on subsurface conditions and numerical analyses.
5. Develop new metrics to allow comparison of ATES or RTES resources for different regions (e.g., how do you evaluate the efficacy of RTES when comparing cooling in Arizona to heating in Alaska?).  New metrics might include heat recovery over time, total storage capacity, etc.  Analyses might consider regional base-heating/cooling loads, heat storage/extraction cycle times, etc.
6. Combine knowledge from previous USGS studies (e.g., groundwater, CO₂ storage, etc.) to make maps of ATES and RTES potential for a region. Whereas regions have heterogeneous geologic (subsurface) properties, research could focus on ranking the importance of properties, any needs for accurate characterization, etc. Geophysical data (e.g., well logs and seismic reflection) may be included, when appropriate.
7. Consider geochemical properties of the working fluid (e.g., native fluid in the reservoir), and how heating/cooling may alter the reservoir? Are there concerns/benefits of pore clogging and associated porosity/permeability reduction?
8. Consider statistical methods appropriate for resource assessments (e.g., similar to USGS hydrocarbon and CO2 storage assessments), that allow for quantification of a range and uncertainty of ATES and RTES resource estimates.

The preceding list of questions is intended to encourage creative thinking, but this list is far from comprehensive. Applicants are encouraged to think creatively about research needs and novel approaches. Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas.

Proposed Duty Station: Portland, Oregon or Mounds View, Minnesota

Areas of PhD: Geology, geophysics, mathematics, engineering, statistics, computer sciences, physical scientists, or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).

Qualifications: Applicants must meet the qualifications for one of the following: Research Engineer, Research Geologist, Research Geophysicist, Research Hydrologist, Research Physical Scientist.

(This type of research is performed by those who have backgrounds for the occupations stated above.  However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Human Resources Office Contact: Paj Shua Cha, 650-439-2455, pcha@usgs.gov