Publication
Seasonal storage of thermal energy has the potential to both significantly reduce the total energy requirements for heating and cooling of buildings, and also allow for the flexibility to store thermal energy from intermittent sources. With approximately half of global energy consumption currently being used for heating and cooling, this represents an important path to reducing greenhouse gas (GHG) emissions. The concept of FTES is to drill two or more wells into a low permeability formation and then generate hydraulic fractures to create flow paths for water to circulate between the wells. Hot or cold thermal energy can then be stored in the surrounding rock mass by circulating hot or cold water through the fractures, which will heat or cool the rock mass. To recover the stored energy, ambient temperature water can be then circulated through the fractures, which will then be heated or cooled by the rock mass. Fractures inherently have a very large ratio of surface area to volume. This allows for very high heat fluxes to and from the rock mass to be achieved despite the relatively low thermal conductivity of most geologic formations. Because large fractures can be made with low-cost equipment and with only inexpensive and environmentally safe materials such as water and sand, the cost to construct even large FTES systems is expected to be quite low. A preliminary estimate is made for the size and operating conditions of a full-scale FTES system capable of heating a commercial building. The paper then presents a dimensional analysis of this system. The scaling parameters that come from this analysis are then used to design an intermediate-scale field test of a fracture thermal energy storage system (FTES) to be conducted at the Sanford Underground Research Facility (SURF) in Lead, SD.