NSF SEP Project

Project Overview
The overall goal of this NSF project (CMMI 1230237) is to understand the fundamental multi-physics processes, engineering challenges, environmental impacts, and implementation strategies for soil borehole thermal energy storage (SBTES) from renewable energy sources in the shallow subsurface (20 to 50 meters). We are investigating the injection of heat generated from solar-thermal installations into the vadose zone through properly spaced, closed-loop borehole heat exchangers so that the thermal energy may be accessed at a later point for direct use in building heating or electricity generation. The unique thermo-hydraulic properties and coupled heat, water, and vapor flow processes in unsaturated soils are being explored to enhance heat transfer in the vadose zone in a similar manner to that in a heat pipe, leading to more heat transfer than that obtained by conduction. The unsaturated soil surrounding the borehole array also acts as an insulator to minimize lateral loss of stored heat. 

Specific research goals are to: 
  • Characterize the nonlinear transport properties of unsaturated soils from the field and laboratory tests, and of thermal grouts used to backfill the boreholes
  • Evaluate coupled water, vapor and heat flow processes and potential environmental impacts in laboratory-scale simulations within densely-instrumented soil tanks
  • Validation and establishment of scalable numerical models (Tough2 and COMSOL) to examine the long-term operation, efficiency, and environmental impact of SBTES systems
  • Construct a field-scale test facility of closed-loop heat exchangers in boreholes beneath a thermo-hydraulic barrier to evaluate the efficiency of heat injection and withdrawal for different borehole array geometries and fluid flow patterns in the heat exchanger tubing
  • Explore approaches to enhance the heat exchange efficiency (injection and withdrawal)
A parallel but critical goal is to assess the application of community-scale subsurface energy storage through evaluation of policies and user experience from exploratory SBTES sites. The usage of SBTES systems can have impacts on how the system performs (in terms of the efficiency of heat injection and extraction). We are collaborating with the Delta-Montrose Electric Cooperative (DMEA), who have developed a geothermal utility model, and SAIC Canada, who are monitoring the Drake Landing Solar Community (DLSC) in Alberta, to collect data on the performance, usage trends, and efficacy of different policies and implementation strategies of geothermal heat exchange systems.  

Technical Significance:
Results from our research will allow a variety of SBTES system scenarios to be tested that would otherwise be difficult or costly to test in practice. Our work will establish the coupled heat transfer and water flow processes that affect the transient charging and discharging of the SBTES, as well as the ultimate amount of heat that can be stored in the system. Based on the results of our research, benefits to future SBTES systems users, investors and developers come from (1) cost savings from lessons learned in the unique experimental facilities at CU Boulder and Colorado School of Mines, (2) cost savings from using validated models that can be used to simulate field conditions, and (3) gaining more public interest in renewable energy – specifically, in more efficient and environmentally friendly SBTES systems to meet energy demand needs of an ever growing population.