Adiabatic salt energy storage
Efficient energy storage is provided by using a working fluid flowing in a closed cycle including a ganged compressor and turbine, and capable of efficient heat exchange with heat storage fluids on a hot side of the system and on a cold side of the system. This system can operate as a heat engine by transferring heat from the hot side to the cold side to mechanically drive the turbine. The system can also operate as a refrigerator by mechanically driving the compressor to transfer heat from the cold side to the hot side. Heat exchange between the working fluid of the system and the heat storage fluids occurs in counter-flow heat exchangers. In a preferred approach, molten salt is the hot side heat storage fluid and water is the cold side heat storage fluid.
1. Energy storage and retrieval apparatus comprising:
a compressor;
a first heat storage unit;
a turbine that is a separate unit from the compressor;
a second heat storage unit; and
a working fluid that flows in a closed cycle including, in sequence, the compressor, the first heat storage unit, the turbine, and the second heat storage unit, wherein the first heat storage unit and the second heat storage unit are in the same closed cycle;
wherein the compressor and the turbine have a common mechanical shaft such that both the compressor and the turbine rotate together upon flow of the working fluid through the closed cycle;
wherein the first and second heat storage units are both capable of exchanging heat with the working fluid;
wherein the apparatus reversibly operates as both (i) a heat engine to provide mechanical work from heat and (ii) as a refrigerator to use mechanical work to store heat; and
wherein the working fluid flows through, in sequence, the compressor, the first heat storage unit, the turbine, and the second heat storage unit when the apparatus operates as the refrigerator and when the apparatus operates as the heat engine.
2. The apparatus of claim 1 , wherein the first heat storage unit comprises:
a first hot ( 1 -H) heat storage tank;
a first cold ( 1 -C) heat storage tank;
a first heat storage fluid capable of flowing between the 1 -H and 1 -C heat storage tanks to store or release heat; and
a first counter-flow heat exchanger, wherein the first heat storage fluid and the working fluid flow in opposite directions.
3. The apparatus of claim 2 , wherein the first heat storage fluid comprises molten salt.
4. The apparatus of claim 3 , wherein the molten salt comprises a eutectic mixture of sodium nitrate and potassium nitrate.
5. The apparatus of claim 2 , wherein the second heat storage unit comprises:
a second hot ( 2 -H) heat storage tank;
a second cold ( 2 -C) heat storage tank;
a second heat storage fluid capable of flowing between the 2 -H and 2 -C heat storage tanks to store or release heat; and
a second counter-flow heat exchanger, wherein the second heat storage fluid and the working fluid flow in opposite directions.
6. The apparatus of claim 5 , wherein the second heat storage fluid comprises water.
7. The apparatus of claim 6 , wherein the second heat storage fluid further comprises an antifreeze compound.
8. The apparatus of claim 1 , further comprising a radiator operatively coupled to the apparatus to dissipate waste heat that may be generated during operation of the apparatus.
9. The apparatus of claim 1 , wherein a minimum pressure of the working fluid in the closed cycle is about one atmosphere.
10. The apparatus of claim 1 , wherein a minimum pressure of the working fluid in the closed cycle is greater than ten atmospheres.
11. The apparatus of claim 1 , wherein a minimum pressure of the working fluid in the closed cycle is about 200 atmospheres.
12. The apparatus of claim 1 , wherein the working fluid is air.
13. The apparatus of claim 1 , wherein the working fluid is Ar.
14. A method for storing and releasing energy, the method comprising:
providing a system comprising a closed cycle for a working fluid such that the working fluid flows through, in sequence, a compressor, a first heat storage unit, a turbine that is a separate unit from the compressor, and a second heat storage unit, wherein the first heat storage unit and the second heat storage unit are in the same closed cycle;
wherein the compressor and the turbine have a common mechanical shaft such that both the compressor and the turbine rotate together upon flow of the working fluid through the closed cycle, and wherein the first and second heat storage units are both capable of exchanging heat with the working fluid;
reversibly operating the system in a refrigerator mode and a heat engine mode, wherein in the refrigerator mode, mechanical work provided by the common mechanical shaft is used to transfer heat energy from the second heat storage unit to the first heat storage unit, and wherein in the heat engine mode, heat energy transferred from the first heat storage unit to the second heat storage unit is used to rotate the common mechanical shaft; and
wherein, the working fluid flows through, in sequence, the compressor, the first heat storage unit, the turbine, and the second heat storage unit in both the refrigerator mode and the heat engine mode.
15. The method of claim 14 , further comprising controlling the temperature difference between fluid elements exchanging heat such that an entropy generation in the first and second heat storage units is minimized.
16. The method of claim 14 , wherein an entropy generated in the first and second heat storage units is negligible when compared to an entropy generated by the compressor.
17. The method of claim 14 , wherein the first heat storage unit or the second heat storage unit comprises a heat storage fluid.
18. The apparatus of claim 1 , wherein the first heat storage unit or the second heat storage unit comprises a heat storage fluid.
19. The apparatus of claim 1 , wherein the first heat storage unit and the second heat storage unit comprise a heat storage fluid.
20. The apparatus of claim 1 , wherein the second heat storage unit comprises a storage fluid held at about ambient pressure.