SURFACE MINE CLOSURE THAT INCLUDES SEASONAL THERMAL ENERGY STORAGE AND RELATED ASSETS

Precise statistics are hard to find, but in the US, metal miners move and store 1-2 billion tonnes of mine waste rock or overburden annually. Coal miners move an additional 1-2 billion tonnes (assuming an average strip ratio of 2-3). Perhaps 50 billion tonnes of historical mine waste rock dot the US. Worldwide numbers are perhaps 5 to 10 times those of US numbers.

Surface mine sites offer massive (literally) opportunities to study, design and build seasonal thermal energy storage and related assets as part of the mine closure process. Earthen materials available in which to store heat and with which to insulate against heat loss include: overburden at a coal or lithium mine, a heap leach system at a gold mine, coal bottom ash from a coal-fired thermal generation plant, and tailings from iron and copper ore. Physical properties of the earthen materials available for manipulation include mass/volume/density, heat capacity, thermal conductivity, particle size distribution, mix of lithologies, porosity, permeability, etc. The earthen materials can be arranged physically to build patterned porous media that may include enhanced permeability fluid flow pathways, low permeability encapsulating layers, etc. Hundreds of surface mine sites with millions of tonnes of available earthen materials are in play.

Consider this simple example: A rock-insulated thermal energy storage heat reservoir with dimensions of 125 m x 125 m x 20 m can contain 650,000 tonnes of crushed rock. If this reservoir is heated to 50-90°C and surrounded by rocks at 15°C, ~83% of the heat is retained after 6 months, assuming that the surrounding rock provides insulation equivalent to 40 cm fiberglass. This can yield approximately 4.73 x 10⁶ kWh of stored thermal energy, enough energy to heat approximately 2.4 acres of indoor greenhouse buildings, double-glazed, metal frames.