Three reservoir electric thermal energy storage system

1. A method for operating a pumped thermal energy storage (“PTES”) system, the method comprising:

circulating a working fluid through a working fluid circuit, the working fluid having a mass flow rate and a specific heat capacity; and

balancing a product of the mass and the specific heat capacity of the working fluid on a high-pressure side of a recuperator and a low-pressure side of the recuperator as the working fluid circulates through the working fluid circuit, the balancing including:

splitting the working fluid into a first portion and a second portion on the high-pressure side of the recuperator;

bypassing the first portion around the high-pressure side of the recuperator;

cooling the first portion during the bypass;

circulating the second portion through the recuperator; and

combining the cooled first portion with the second portion after the second portion exits the recuperator.

2. The method of claim 1 , wherein cooling the first portion during the bypass includes:

circulating the working fluid through the bypass; and

transferring heat between the working fluid and a medium temperature thermal reservoir, including:

in a charging phase, transferring heat from the working fluid to the medium-temperature thermal reservoir; and

in a generating phase, transferring heat from the medium-temperature thermal reservoir to the working fluid.

3. The method of claim 1 , wherein the first portion of the working fluid comprises approximately 40% of the working fluid portion and the second portion comprises approximately 60% of the working fluid portion.

4. The method of claim 1 , wherein:

circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO 2 );

the first portion of the working fluid comprises approximately 40% of the working fluid portion; and

the second portion comprises approximately 60% of the working fluid portion.

5. The method of claim 1 , wherein circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO 2 ).

6. The method of claim 1 , wherein balancing a product of the mass and the specific heat capacity of the working fluid on the high-pressure side of the recuperator and the low-pressure side of the recuperator includes:

in a charging phase:

circulating approximately 60% of the working fluid comprised of carbon dioxide (CO 2 ) through the high-pressure side of the recuperator; and

circulating 100% of the working fluid comprised of CO 2 through the low-pressure side of the recuperator; and

in a generating phase:

circulating approximately 60% of the working fluid comprised of CO 2 through the high-pressure side of the recuperator; and

circulating 100% of the working fluid comprised of CO 2 through the low-pressure side of the recuperator.

7. The method of claim 1 , wherein balancing the product of the mass and the specific heat capacity of the working fluid on the high-pressure side of the recuperator and the low-pressure side of the recuperator as the working fluid circulates through the working fluid circuit includes reducing the mass flow rate on the high-pressure side of the recuperator.

8. The method of claim 1 , further comprising exchanging heat between the second portion of the working fluid on the high-pressure side of the recuperator and a combined portion of the working fluid on the low-pressure side of the recuperator.

9. A pumped thermal energy storage (“PTES”) system, comprising:

a medium temperature thermal reservoir; and

a working fluid circuit, including:

a recuperator having a high-pressure side and a low-pressure side, the product of the mass and the specific heat capacity of a working fluid is balanced on the high-pressure side and the low-pressure side when the working fluid circulates through the working fluid circuit;

a split point on the high-pressure side of the recuperator at which the working fluid is split into a first portion and a second portion; and

a bypass by which the first portion of the working fluid bypasses the high-pressure side of the recuperator while the second portion of the working fluid circulates through the high-pressure side of the recuperator.

10. The system of claim 9 , wherein the working fluid is carbon dioxide (CO 2 ).

11. The system of claim 9 , wherein:

the bypass includes a heat transfer between the first portion and the medium temperature thermal reservoir;

the first portion is less than the second portion; and

the working fluid circuit further comprises:

a combination point on the high-pressure side of the recuperator where the first portion combines with the second portion upon the second portion exiting the recuperator.

12. The system of claim 9 , wherein the first portion is approximately 40% of the total working fluid and the second portion is approximately 60% of the total working fluid.

13. The system of claim 9 , wherein:

the working fluid is carbon dioxide (CO 2 );

the first portion is approximately 40% of the total working fluid; and

the second portion is approximately 60% of the total working fluid.

14. The system of claim 9 , further wherein, in operation, heat is exchanged between the second portion of the working fluid on the high-pressure side of the recuperator and a combined portion of the working fluid on the low-pressure side of the recuperator.

15. A method for operating a pumped thermal energy storage (“PTES”) system, the method comprising:

circulating a working fluid through a working fluid circuit; and

reducing a mass flow rate of the working fluid on a high-pressure side of a recuperator to balance a product of the mass and the specific heat of the working fluid on the high-pressure side and a low-pressure side of the recuperator while circulating the working fluid, the reducing including:

splitting the working fluid into a first portion and a second portion on the high-pressure side of the recuperator, the first portion being less than the second portion;

bypassing the high-pressure side of the recuperator with the first portion of the working fluid; and

circulating the second portion of the working fluid through the high-pressure side of the recuperator.

16. The method of claim 15 , wherein bypassing the high-pressure side of the recuperator includes:

upon transferring heat between the working fluid and a high-temperature reservoir in a charging phase, bypassing the high-pressure side of a recuperator with the first portion of the working fluid and transferring heat from the first portion to a medium-temperature reservoir during the bypass while the second portion circulates through the recuperator, the first portion being less than the second portion; and

upon exiting a pump in a generating phase, bypassing the high-pressure side of the recuperator with a third portion of the working fluid while transferring heat from the medium-temperature reservoir to the third portion during the bypass while circulating a fourth portion of the working fluid through the recuperator.

17. The method of claim 15 , further comprising exchanging heat between the second portion of the working fluid on the high-pressure side of the recuperator and a combined portion of the working fluid on the low-pressure side of the recuperator.

18. The method of claim 15 , wherein reducing the mass flow rate of the working fluid on the high-pressure side of the recuperator to balance the product of the mass and the specific heat of the working fluid on the high-pressure side and the low-pressure side of the recuperator while circulating the working fluid further includes:

transferring heat to and from the first portion during the bypass; and

combining the first portion with the second portion after the second portion exits the recuperator.

19. The method of claim 18 , wherein transferring heat to and from the first portion during the bypass includes transferring heat between the working fluid and a medium temperature thermal reservoir, further including:

in a charging phase, transferring heat from the working fluid to the medium temperature thermal reservoir; and

in a generating phase, transferring heat from the medium temperature thermal reservoir to the working fluid.

20. The method of claim 15 , wherein circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO 2 ).

21. The method of claim 15 , wherein the first portion of the working fluid comprises approximately 40% of the total working fluid and the second portion comprises approximately 60% of the total working fluid.

22. The method of claim 15 , wherein:

circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO 2 ); and

the first portion of the working fluid comprises approximately 40% of the working fluid portion and the second portion comprises approximately 60% of the working fluid portion.

23. A pumped thermal energy storage (“PTES”) system, comprising:

a low-temperature thermal reservoir;

a high-temperature thermal reservoir;

a medium-temperature thermal reservoir; and

a working fluid circuit configurable for a charging phase and a generating phase of a PTES operating cycle and through which, in use, a working fluid circulates, the working fluid circuit including:

a recuperator;

when in the charging phase:

an expander positioned between the recuperator and the low-temperature reservoir; and

a charge compressor positioned between recuperator and the high-temperature heat reservoir; and

when in the generating phase:

a pump positioned between recuperator and the low-temperature heat reservoir; and

a power turbine positioned between recuperator and the high-temperature heat reservoir; and

a bypass, by which, in both the charging phase and the generating phase, a first portion of the working fluid bypasses the high-pressure side of the recuperator and flows through the medium-temperature thermal reservoir, the medium-temperature thermal reservoir transferring heat between the working fluid and the medium-temperature thermal reservoir, while a second portion of the working fluid circulates through the recuperator.

24. The system of claim 23 , wherein the working fluid is carbon dioxide (CO 2 ).

25. The system of claim 23 , wherein the first portion is approximately 40% of the total working fluid and the second portion is approximately 60% of the total working fluid.

26. The system of claim 23 , wherein:

the working fluid is carbon dioxide (CO 2 );

the first portion is approximately 40% of the total working fluid; and

the second portion is approximately 60% of the total working fluid.

27. A method for operating a pumped thermal energy storage (“PTES”) system, the method comprising:

circulating a high-heat capacity working fluid through a working fluid circuit including a recuperator; and

reducing a mass flow rate of the working fluid on a high-pressure side of the recuperator such that the product of mass and the specific heat capacity is the same on both the high-pressure side and a low-pressure side of the recuperator, including:

in a charging phase:

bypassing the recuperator with a first portion of the working fluid and transferring heat from the first portion to a medium-temperature reservoir during the bypass while circulating a second portion of the working fluid through the recuperator;

circulating a second portion through the recuperator while the first portion bypasses the recuperator and transferring heat from the second portion while circulating through the recuperator; and

circulating both the first portion and the second portion together through an expander after the first portion bypasses the recuperator and the second portion circulates through the recuperator; and

in a generating phase:

bypassing the recuperator with a third portion of the working fluid and transferring heat from the third portion to a medium-temperature reservoir during the bypass while circulating a fourth portion of the working fluid through the recuperator;

circulating a fourth portion through the recuperator while the third portion bypasses the recuperator and transferring heat to the fourth portion while circulating through the recuperator; and

circulating both the third portion and the fourth portion together through a high-temperature reservoir after the third portion bypasses the recuperator and the fourth portion circulates through the recuperator.

28. The method of claim 27 , wherein the working fluid is carbon dioxide (CO 2 ).

29. The method of claim 28 , wherein:

the first portion represents approximately 40%;

the second portion represents approximately 60% of the mass flow rate of the working fluid in the charging phase; and

the third portion represents approximately 40% and the fourth portion represents approximately 60% of the mass flow rate of the working fluid in the generating phase.

30. The method of claim 27 , wherein:

the first portion represents approximately 40% and the second portion represents approximately 60% of the mass flow rate of the working fluid in the charging phase; and

the third portion represents approximately 40% and the fourth portion represents approximately 60% of the mass flow rate of the working fluid in the generating phase.