The NSF Sustainable Energy pathways (SEP) Program, under the umbrella of the NSF Science, Engineering and Education for Sustainability (SEES) initiative, will support the research program of Prof. John McCartney and co-workers at the University of Colorado at Boulder, and Prof. Ning Lu and co-workers at the Colorado School of Mines. The goal of this project is to understand the fundamental multi-physics processes, engineering challenges, environmental impacts, and implementation strategies for soil borehole thermal energy storage (SBTES) of heat collected from solar-thermal panels. The thermo-hydraulic properties of unsaturated soils and associated coupled heat, water, and vapor flow processes will be engineered to form a heat pipe, enhancing heat transfer. This is a departure from conventional borehole or aquifer thermal energy storage systems, which rely on conduction or water extraction/injection to transfer heat into or from the subsurface. The objective of this research is to seek the optimum scalable efficiency of energy injection into SBTES systems and subsequent extraction for direct use in building heating or electricity generation. To reach this objective, specific tasks include: (1) constructing a field-scale test facility to evaluate the efficiency of heat injection and withdrawal for different borehole configurations, (2) evaluating coupled water, vapor and heat flow processes and potential environmental impacts within densely-instrumented soil tanks, (3) characterizing the nonlinear transport properties of unsaturated soils from the different tests, (4) validation and establishment of a scalable numerical model to examine the long-term operation, efficiency, and environmental impact of SBTES systems, and (5) exploration of engineering approaches to enhance the heat exchange efficiency.
A parallel effort will be to assess implementation strategies for residential-, community-, and industrial-scale subsurface energy storage through evaluation of policies and user experience from exploratory SBTES sites. These sites have established high efficiencies of energy recovery, but usage trends, barriers to implementation, or socio-economic issues related to different policy strategies have not been fully analyzed. Preliminary estimates indicate that SBTES systems have low capital cost compared to other energy storage solutions, permitting rapid cost recovery through energy savings. Low environmental impact is expected because SBTES are closed-loop systems and because groundwater flow will not significantly affect thermal migration in the vadose zone. This project will contribute to generation of a workforce with a broad set of skills that can be applied to emerging renewable energy technologies, including hydrology, civil engineering, thermodynamics, environmental impact analyses, and energy policy. The researchers will build upon their track records of recruiting and retaining students from diverse backgrounds, will incorporate student exchange and active participation in tasks at both universities, and will define tracks established courses for effective training. Communities which have successfully implemented SBTES systems will be used as case histories, forming the basis of short courses which will be given by the investigators to communities and policy makers to ensure wider spread implementation of this technology.
SBTES systems are expected to play an important role in reducing the amount of electricity or natural gas required to heat residential and commercial buildings by integrating renewable heat sources with a sustainable thermal energy storage solution. By considering the improvements in heat transfer investigated in this research, the efficiency of energy recovery can be improved to provide scalable, sustainable direct use heating for communities. In addition, in some cases extracted heat may also be converted into electricity using thermal-electricity technologies (binary cycle power plants), reducing greenhouse gas emissions. SBTES systems could provide a key element in balancing the cost and efficiency of renewable energy technologies such as solar-thermal panels, influencing the fate of these technologies. Although these energy sources are renewable, energy is often generated at times when it is not necessary or in locations far from where it is needed. SBTES systems can be implemented in nearly any location in the US to provide a sustainable storage solution. The potential for SBTES systems to be scaled to different applications provides an important strategy for overcoming socio-economic concerns with the up-front costs of these systems.