Systems and methods for long-duration thermal energy storage

Abstract

The invention provides systems and methods for energy storage for providing low temperature, long-duration thermal energy storage. The system provides for subterranean thermal energy storage having a sufficiently high energy density and the potential for sufficiently low energy loss at a storage timescale of months to allow grid-scale energy buffers of the size needed to manage seasonal energy supply and demand mismatches inherent with widely available renewable energy sources (primarily wind and solar).

1 . A method for energy storage and exchange, the method comprising:
 receiving and storing thermal energy in one or more subterranean thermal reservoirs; and   maintaining an average temperature differential between the one or more subterranean reservoirs, including the thermal energy stored therein, and a surrounding local subterranean environment to thereby achieve low energy loss during a duration of storage of the thermal energy.   
     
     
         2 . The method of  claim 1 , wherein maintaining an average temperature differential comprises adjusting a temperature of the one or more subterranean thermal reservoirs and/or the thermal energy stored therein relative to an ambient temperature of the surrounding local subterranean environment. 
     
     
         3 . The method of  claim 2 , wherein adjusting a temperature of the one or more subterranean thermal reservoirs and/or the thermal energy stored therein comprises heating or cooling the one or more subterranean thermal reservoirs and/or the thermal energy stored therein. 
     
     
         4 . The method of  claim 3 , wherein at least a first subterranean thermal reservoir is maintained at a temperature greater than an ambient temperature of the surrounding local subterranean environment and at least a second subterranean thermal reservoir is maintained at a temperature less than an ambient temperature of the surrounding local subterranean environment. 
     
     
         5 . The method of  claim 1 , wherein the average temperature differential is 300 kelvin (K) or less. 
     
     
         6 . The method of  claim 1 , wherein the thermal energy is stored in a thermal storage medium associated with the one or more subterranean thermal reservoirs. 
     
     
         7 . The method of  claim 6 , wherein the thermal storage medium comprises at least one of a solid medium and a liquid medium. 
     
     
         8 . The method of  claim 7 , wherein the thermal storage medium is selected from the group consisting of soil, sand, rock, composite material comprising fine and/or coarse aggregate and a binder, and an aqueous solution. 
     
     
         9 . The method of  claim 8 , wherein the thermal storage medium comprises water-backfilled crushed rock. 
     
     
         10 . The method of  claim 1 , wherein the one or more subterranean thermal reservoirs comprises an insulating region comprising a medium for minimizing heat loss by way of convection and/or conduction. 
     
     
         11 . The method of  claim 1 , wherein the low energy loss is energy loss of no greater than 10% over a storage duration of 100 days. 
     
     
         12 . The method of  claim 1 , wherein the thermal energy is provided to the one or more subterranean thermal reservoirs via one or more heat pumps operably associated therewith, wherein the one or more heat pumps are configured to convert input energy in the form of at least one of electrical energy, mechanical work, and industrial waste heat into thermal energy to be stored. 
     
     
         13 . The method of  claim 12 , wherein maintaining an average temperature differential between the one or more subterranean reservoirs, including the thermal energy stored therein, and a surrounding local subterranean environment is based, at least in part, on operation of the one or more heat pumps. 
     
     
         14 . The method of  claim 12 , wherein the one or more heat pumps operate in accordance with a thermodynamic cycle including at least one of vapor compression cycle and vapor absorption cycle. 
     
     
         15 . The method of  claim 12 , wherein operation of the one or more heat pumps provide for a chemical reaction. 
     
     
         16 . The method of  claim 12 , further comprising converting stored thermal energy into at least one of electrical energy and mechanical work via one or more heat engines operably associated with the one or more subterranean thermal reservoirs. 
     
     
         17 . The method of  claim 16 , wherein the one or more heat engines operate in accordance with a thermodynamic cycle selected from the group consisting of an organic Rankine cycle, Stirling cycle, and Brayton cycle. 
     
     
         18 . The method of  claim 16 , wherein the one or more heat pumps and the one or more heat engines utilize heat exchange fluid for effectuating heat exchange with the one or more subterranean thermal reservoirs. 
     
     
         19 . The method of  claim 18 , wherein each of the one or more subterranean thermal reservoirs comprises a primary heat exchanger simultaneously coupling the one or more heat pumps and the one or more heat engines to the thermal energy via a heat exchange fluid. 
     
     
         20 . The method of  claim 19 , wherein the primary heat exchanger of each subterranean thermal reservoir comprises a grid of boreholes through which heat exchange fluid is in contact with thermal storage medium storing the thermal energy. 
     
     
         21 . An energy storage and exchange system comprising:
 one or more subterranean thermal reservoirs comprising thermal storage medium configured to receive and retain thermal energy, wherein an average temperature differential between the one or more subterranean reservoirs, including the thermal energy stored therein, and a surrounding local subterranean environment is maintained to thereby achieve low energy loss during a duration of storage of the thermal energy.   
     
     
         22 . The system of  claim 21 , wherein the average temperature differential is 300 kelvin (K) or less. 
     
     
         23 . The system of  claim 21 , wherein the one or more subterranean thermal reservoirs have a combined volume greater than 1,000 cubic meters. 
     
     
         24 . The system of  claim 21 , wherein the thermal storage medium comprises at least one of a solid medium and a liquid medium. 
     
     
         25 . The system of  claim 24 , wherein the thermal storage medium is selected from the group consisting of soil, sand, rock, composite material comprising fine and/or coarse aggregate and a binder, and an aqueous solution. 
     
     
         26 . The system of  claim 25 , wherein the thermal storage medium comprises water-backfilled crushed rock. 
     
     
         27 . The system of  claim 21 , wherein the one or more subterranean thermal reservoirs comprises an insulating region comprising a medium for minimizing heat loss by way of convection and/or conduction. 
     
     
         28 . The system of  claim 21 , wherein the low energy loss is energy loss of no greater than 10% over a storage duration of 100 days. 
     
     
         29 . The system of  claim 21 , further comprising one or more heat pumps operably associated with the one or more subterranean thermal reservoirs, wherein the one or more heat pumps are configured to convert input energy in the form of at least one of electrical energy, mechanical work, and industrial waste heat into thermal energy to be stored. 
     
     
         30 . The system of  claim 29 , wherein operation of the one or more heat pumps maintains the average temperature differential between the one or more subterranean reservoirs, including the thermal energy stored therein, and the surrounding local subterranean environment. 
     
     
         31 . The system of  claim 30 , wherein the one or more heat pumps are configured to adjust a temperature of the one or more subterranean thermal reservoirs and/or the thermal energy stored therein relative to an ambient temperature of the surrounding local subterranean environment. 
     
     
         32 . The system of  claim 31 , wherein the one or more heat pumps are configured to heat or cool the one or more subterranean thermal reservoirs and/or the thermal energy stored therein. 
     
     
         33 . The system of  claim 32 , wherein the one or more heat pumps are configured to operate in accordance with a thermodynamic cycle including at least one of vapor compression cycle and vapor absorption cycle to heat or cool the one or more subterranean thermal reservoirs and/or the thermal energy stored therein. 
     
     
         34 . The system of  claim 32 , wherein operation of the one or more heat pumps provide for a chemical reaction to heat or cool the one or more subterranean thermal reservoirs and/or the thermal energy stored therein. 
     
     
         35 . The system of  claim 32 , wherein at least a first subterranean thermal reservoir is maintained at a temperature greater than an ambient temperature of the surrounding local subterranean environment and at least a second subterranean thermal reservoir is maintained at a temperature less than an ambient temperature of the surrounding local subterranean environment. 
     
     
         36 . The system of  claim 29 , further comprising one or more heat engines operably associated with the one or more subterranean thermal reservoirs, wherein the one or more heat engines are configured to convert stored thermal energy into at least one of electrical energy and mechanical work. 
     
     
         37 . The system of  claim 36 , wherein the one or more heat engines are configured to operate in accordance with a thermodynamic cycle selected from the group consisting of an organic Rankine cycle, Stirling cycle, and Brayton cycle. 
     
     
         38 . The system of  claim 36 , further comprising one or more primary heat exchangers associated with a respective one of the one or more subterranean thermal reservoirs, wherein each primary heat exchanger simultaneously couples the one or more heat pumps and the one or more heat engines to stored thermal energy via heat exchange fluid. 
     
     
         39 . The system of  claim 38 , wherein the one or more heat pumps and the one or more heat engines are configured to utilize heat exchange fluid for effectuating heat exchange with the one or more subterranean thermal reservoirs. 
     
     
         40 . The system of  claim 38 , wherein each primary heat exchanger comprises a grid of boreholes through which heat exchange fluid is in contact with thermal storage medium storing the thermal energy.