Three-reservoir electrical and thermal energy storage system

Abstract

A method for operating a pumped thermal energy storage ("PTES") system includes circulating a working fluid through a working fluid circuit. The working fluid has a mass flow rate and a heat capacity, and balances a product of the mass and heat capacity of the working fluid in 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 PTES system includes a bypass in the working fluid circuit. The bypass causes a first portion of the working fluid to bypass the high pressure side of the recuperator while a second portion of the working fluid circulates through the high pressure side of the recuperator.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to co-pending U.S. Provisional Patent Application No. 63 / 123,266, entitled "3-Reservoir ETES System," filed December 9, 2020, in the name of Timothy Held, and U.S. Nonprovisional Patent Application No. 17 / 546,963, filed December 9, 2021, which is hereby incorporated by reference in its entirety for all purposes, including priority, as if fully set forth herein.

[0002] The present disclosure relates to electrical and thermal energy storage ("ETES") systems, and more specifically, to carbon dioxide ("CO 2 ") based pumped thermal energy storage ("PTES") system. [Background technology]

[0003] PTES systems, sometimes known as electrothermal energy storage systems, are also used for energy storage and regeneration. PTES systems generally use a configurable thermodynamic cycle in which thermal energy is transferred between a hot and cold reservoir via a working fluid in a working fluid circuit. In its simplest form, it consists of a thermodynamic cycle in which the PTES operates as a heat pump in one direction of heat and fluid flow and as a heat engine in the opposite direction of heat and fluid flow. Here, thermal energy is transferred between two reservoirs, one hot and the other cold, as shown in FIG. 1A. The operation of a PTES can be loosely described as including a "charge" phase and a "produce" phase.

[0004] In the "charge" phase of operation, thermal energy is elevated from the low temperature reservoir ("LTR") to the high temperature reservoir ("HTR") by using a heat pump cycle in what is nominally the forward direction. In this process, an electric motor is used to drive a gas compressor, increasing the pressure and temperature of the working fluid. Thermal energy contained in the working fluid is transferred to the high temperature reservoir ("HTR") using an indirect heat exchanger. Thermal energy is further transferred from the working fluid downstream of the indirect heat exchanger to the fluid upstream of the gas compressor in a recuperator heat exchanger. The fluid then expands through a turbine, which drives a shaft used to help drive the compressor. The working fluid at the turbine outlet has a lower pressure and further lower temperature. Heat is transferred from the low temperature reservoir ("LTR") to the working fluid, which returns heat to its initial state at the compressor inlet.

[0005] During the "production" phase of operation, the direction of fluid and heat flow is reversed. The fluid leaving the LTR is compressed, but this time at a much lower inlet and outlet temperature of the "compressor". In effect, the carbon dioxide (CO 2 In the HTR-based type, the fluid may be in a liquid state, so the "compressor" is actually a pump. The fluid is then heated to a relatively high temperature by the HTR and expanded through a turbine to drive a shaft. This turbine work now exceeds the compressor work, and the excess is converted to electricity by a generator and fed back into the power grid. Residual thermal energy from the turbine discharge is transferred to the working fluid upstream of the HTR in a recuperator heat exchanger. Summary of the Invention [Problem to be solved by the invention]

[0006] The techniques disclosed herein reduce the effects of heat capacity mismatch across the recuperator, thereby avoiding lost exergy due to mismatch in temperature-heat transfer ("TQ") gradients, as described below. The techniques disclosed herein also significantly improve cycle performance. As used herein, "exergy" is the maximum useful work possible during the process of bringing a system into equilibrium with a heat reservoir. [Means for solving the problem]

[0007] The presently disclosed technology includes methods and apparatus. A method for operating a pumped thermal energy storage ("PTES") system includes circulating a working fluid through a working fluid circuit, the working fluid having a mass flow rate and a heat capacity, and balancing a product of the mass and heat capacity of the working fluid in 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 PTES system includes a bypass in the working fluid circuit, where a first portion of the working fluid bypasses the high pressure side of the recuperator while a second portion of the working fluid circulates through the high pressure side of the recuperator. [Brief description of the drawings]

[0008] The present disclosure is best understood when read in conjunction with the accompanying drawings, in which it is emphasized that, in accordance with standard industry practice, various features have not been drawn to scale, and in fact the dimensions of various features may be arbitrarily expanded or reduced for clarity of discussion.

[0009] [Figure 1A] FIG. 1A is a process flow diagram of a prior art PTES system during the filling stage of the PTES operating cycle.

[0010] [Figure 1B] FIG. 1B is a process flow diagram of a prior art PTES system during the production stage of the PTES operating cycle.

[0011] [Diagram 2]FIG. 2 is a pressure-enthalpy diagram for a reference PTES cycle with numbers in boxes that are state points for the production phase of the conventional PTES system of FIGS. 1A-1B.

[0012] [Diagram 3] FIG. 3 is a temperature-heat transfer ("TQ") plot of the reference PTES cycle of FIG. 2 for the PTES system of FIGS. 1A-1B.

[0013] [Figure 4A] FIG. 4A is a process flow diagram of a PTES system during the fill stage of the PTES operating cycle, according to one or more embodiments.

[0014] [Figure 4B] FIG. 4B is a process flow diagram of a PTES system during the production stage of the PTES operating cycle, according to one or more embodiments.

[0015] [Diagram 5] A particular example of a control system is shown in Figure 5. By means of the control system, the working fluid circuit of Figures 4A-4B may be configured for the charging stage shown in Figure 4A and the production stage shown in Figure 4B.

[0016] [Figure 6A] FIG. 6A illustrates one implementation of the thermal reservoir of FIGS. 4A-4B as may be seen in one embodiment. [Figure 6B] FIG. 6B illustrates an implementation of the thermal reservoir of FIGS. 4A-4B as may be seen in one embodiment. [Figure 6C] FIG. 6C illustrates an implementation of the thermal reservoir of FIGS. 4A-4B as may be seen in one embodiment. [Figure 6D] FIG. 6D illustrates an implementation of the thermal reservoir of FIGS. 4A-4B as may be seen in one embodiment. [Figure 6E] FIG. 6E illustrates an implementation of the thermal reservoir of FIGS. 4A-4B as may be seen in one embodiment.

[0017] [Figure 6F]FIG. 6F illustrates an implementation of the thermal reservoir of FIGS. 4A-4B as may be seen in one embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] One measure of overall cycle performance is the "round trip efficiency" ("RTE"). This parameter is defined as the amount of electrical energy (kWh) that can be generated during the generation phase divided by the amount of electrical energy consumed during the charging phase. Another important performance parameter is the system capital cost. This can be defined in terms of generation capacity or storage capacity.

[0019] 1A and 1B show a prior art PTES system 100 having a PTES operating cycle. A pressure-enthalpy diagram for this PTES operating cycle is shown in FIG. 2. The states of the working fluid within the boxes shown in FIG. 2 are shown in circles in FIGS. 1A-1B. Thus, the operating states at various times in the operating cycle of the PTES system 100 can be mapped from the pressure-enthalpy diagram in FIG. 2 to the process flow diagram in FIGS. 1A-1B. FIG. 1A shows the charge phase of the operating cycle and FIG. 1B shows the production phase of the operating cycle. The nomenclature shown in Table 1 shall be used in the following discussion and in the accompanying drawings. [Table 1]

[0020] The PTES system 100 includes a working fluid circuit 103, an HTR 106, an LTR 109, and a recuperator RCX. The configuration of the working fluid circuit 103 depends, in part, on whether the PTES system 100 is in the charge or production phase of its operating cycle. As would be understood by one of ordinary skill in the art having the benefit of this disclosure, this configuration is generally a function of programmed control of fluid flow valves. Thus, during the charge phase, some components of the working fluid circuit 103 are activated and some are deactivated by controlling the flow of working fluid through the working fluid circuit 103. Similarly, during the production phase, other components may be activated and other components may be deactivated, again by controlling the flow of working fluid through the working fluid circuit 103. For this reason, the fluid flow valves and controllers have been omitted from FIGS. 1A-1B for clarity.

[0021] In the charging stage, as shown in FIG. 1A, the working fluid circuit 103 includes an expander 115, a charge compressor 112, and an air-cooled cooler ACC1 between the recuperator RCX and the expander 115. The expander 115 is used for an expansion process to expand the working fluid. The charge compressor 112 is used for a compression process to compress the working fluid. In the production stage shown in FIG. 1B, the working fluid circuit 103 includes a pump 118 for the compression process. Similarly, the working fluid circuit 103 includes a power turbine 121 in the production stage. Furthermore, the working fluid circuit 103 excludes the air-cooled cooler ACC1 from the charging stage and includes an air-cooled cooler ACC2 in the production stage. The air-cooled cooler ACC2 is positioned between the LTR 109 and the recuperator RCX.

[0022] In currently used nomenclature, "high temperature" and "low temperature" are interrelated; that is, the HTR operates at a higher temperature than the LTR operates at. The terms "hot" and "cold" are used in conjunction with each other. For example, in some embodiments, the HTR 106 may operate at a temperature in the range of 100-340° C., and the LTR 109 may operate at a temperature in the range of -2-2° C.

[0023] During the charging stage, shown in Figure 1A, heat is transferred from the LTR 109 to the working fluid and heat is transferred from the working fluid to the HTR 106. During the generating stage, shown in Figure 1B, heat transfer occurs in the opposite direction: heat is transferred from the working fluid to the LTR 109 and heat is transferred from the HTR 106 to the working fluid.

[0024] Unlike conventional heat engines, in a theoretically ideal cycle with 100% efficient turbomachinery, there would be no pressure losses and the temperature would be perfectly matched across the heat exchangers, resulting in a 100% RTE for the PTES process. In reality, thermodynamic irreversibilities, pressure losses, and access to finite temperatures through the heat exchangers reduce the RTE value. For the charge phase of the reference cycle shown in Figure 1A, carbon dioxide ("CO 2 " ) and a reasonable set of efficiency values, an RTE of 50-55% can be calculated.

[0025] For example, inefficiencies can arise due to thermodynamic irreversibility. The thermodynamically ideal compression and expansion processes are described as "adiabatic isentropic" devices. In FIG. 2, the expansion and compression processes are represented, for example, by a diagonal line from state 4 to state 5 of the product phase. The term "isentropic" refers to constant entropy processes. In the non-ideal case shown, the compression process is non-isentropic, so that increasing pressure exhibits a less steep slope than the corresponding product expansion process. Because of such irreversibility, the compression process consumes more work than the expansion process.

[0026] Other inefficiencies arise in circulating the working fluid: pressure losses during the processes of adding and rejecting heat (e.g., from state 7 to state 1 in a production process) represent the work lost in the circulating fluid through heat exchangers and piping. Thus, the very act of circulating the working fluid creates inefficiencies.

[0027] Furthermore, a finite temperature difference between the working fluid and the heat storage medium is necessary to facilitate heat transfer between the two materials. Because the direction of heat transfer is reversed between the charging and production stages and the temperature of the reservoir material is fixed, the working fluid temperature during production needs to be lower than the HTR and higher than the HTR, while the working fluid temperature during charging needs to be higher than the HTR and lower than the HTR. Such temperature differences represent a loss of thermodynamic potential and reduce the round trip efficiency of the system.

[0028] Furthermore, the PTES system 100 utilizes internal heat transfer, also known as "recuperation", a process that in the PTES system 100 is carried out by the recuperator RCX. Recuperation is used to increase the temperature of the working fluid entering the compressor 112 during the charge cycle while simultaneously lowering the temperature of the working fluid entering the power turbine 115. Conversely, during the generation cycle, the PTES system 100 uses recuperation to preheat the working fluid before it enters the HTR 106 by extracting residual heat from the exhaust of the turbine 121.

[0029] Heat transfer between the working fluid and the LTR 109 and between the working fluid and the HTR 106 occurs through heat exchangers in the respective thermal reservoirs. For clarity, the heat exchangers are not shown. The heat transfer process through the heat exchangers can be illustrated with a temperature-heat transfer plot, also known as a "TQ plot."

[0030] FIG. 3 is a temperature-heat transfer ("TQ") plot of the reference PTES cycle of FIG. 2 for the PTES system 100 of FIGS. 1A-1B. The TQ plot of FIG. 3 shows how the fluid temperature drops / rises as heat is transferred when the working fluid pressure is high (curve 402) and when the working fluid pressure is low (curve 404). It can be seen that the slope of the TQ curve is proportional to the inverse of the product of the mass flow rate and heat capacity of the fluid. The working fluid for the PTES system 100 of FIGS. 1A-1B is CO 2 It is.

[0031] In the reference version of the cycle, the flow rate of the working fluid through both sides of the recuperator RCX is the same. In the thermodynamically ideal case, the heat capacity of the working fluid would be the same on both sides of the recuperator RCX. In that situation, the two TQ curves 402, 404 would be parallel. As the heat exchanger conductivity ("UA") increases, the two curves will approach each other and, in the limit of infinite UA, will overlap each other.

[0032] However, CO 2 Some working fluids, such as , have heat capacity properties that vary with temperature as well as pressure. Due to the different pressures on both sides of the heat exchanger, the TQ curves are not parallel, but show a "pinch" behavior at one of the "ends" of the heat exchanger. The temperature of the hot stream leaving the heat exchanger is higher than the temperature of the hot stream entering the heat exchanger, even though the amount of heat lost by the hot stream is identical to the amount of heat gained by the cold stream. This temperature difference represents a loss of "thermodynamic potential value" and reduces the performance of the system (more rigorous analysis can be performed using thermodynamic exergy destruction calculation methods to reach the same conclusion). In this case, the fluid leaving the high-pressure end of the recuperator will be at an excessive temperature, and therefore an external heat rejection is required to achieve a nearly completely liquid state at the expander outlet. This heat loss directly affects the cycle performance.

[0033] As discussed above, the disclosed techniques reduce the effect of heat capacity mismatch across the recuperator to avoid exergy losses due to TQ slope mismatch, thereby significantly improving cycle performance. To match the slope of the TQ curve, the mass flow rate of the high heat capacity fluid is reduced so that the product of mass ("m") and heat capacity ("cp") ("mcp") is the same on both sides of the recuperator. The supercritical CO cycle known as the Recompression Brayton Cycle ("RCB Cycle") 2 ("sCO 2 In the power generation cycle, this is done by circulating the high pressure side of the low temperature recuperator using a second "bypass" compressor to produce CO 2This can be achieved by deliberately diverting part of the flow, but since the PTES cycle operates at a much lower temperature than the RCB cycle, this option is not available since compression from the two-phase inlet would be required.

[0034] Instead, during the charging phase, the presently disclosed technology adds a parallel flow path to the high pressure side of the recuperator. 2 Approximately 40% of the flow bypasses the high pressure side of the recuperator and transfers its heat to a third heat transfer medium (the "Medium Temperature Recuperator" or "MTR"). The remaining approximately 60% of the flow proceeds through the recuperator, where the mass times heat capacity of both sides of the recuperator is nearly identical, allowing for even closer temperatures between the fluids. The two streams then recombine before passing through the expander. High Pressure CO 2 The heat extracted from the first 40% is transferred to the heat storage medium.

[0035] During the production cycle, the process is reversed. 2 Approximately 60% of the flow is separated from the pump discharge and its temperature is increased by heat transfer from the MTR medium. 2 The remainder passes through the high pressure side of the recuperator, transferring heat from the recuperator. The streams recombine before being further heated by the hot reservoir material.

[0036] The reduction in exergy destruction significantly improves the system's performance. With comparable hot and cold reservoirs and other pressure, temperature and heat exchanger area constraints, the round trip efficiency ("RTE") of the new cycle improves by 8 percentage points, from approximately 52% to 60%. In addition, the ACC at the charging stage can be omitted.

[0037] Referring again to the drawings, Figures 4A-4B respectively illustrate the charge and production stages of an operating cycle of a PTES system 400 in accordance with one or more embodiments of the presently disclosed technology. Referring collectively to Figures 4A and 4B, the PTES system 400 comprises a configurable working fluid circuit 403, a high temperature reservoir ("HTR") 406, a low temperature reservoir ("LTR") 409, and a mid temperature reservoir ("MTR") 412. The PTES system 400 may be characterized as a "three reservoir system" due to the presence of three reservoirs, the HTR 406, the LTR 409, and the MTR 412.

[0038] HTR406 is so named because it operates at a higher temperature than LTR409 and MTR412. Similarly, LTR409 operates at a lower temperature than HTR406 and MTR412. MTR412 operates at a temperature intermediate between the temperatures at which HTR406 and LTR409 operate. Thus, the terms "high," "medium," and "low," with respect to reservoirs HTR406, LTR409, and MTR412, represent the relative temperatures at which the three reservoirs HTR406, LTR409, and MTR412 operate.

[0039] Each of the thermal reservoirs HTR 406, LTR 409, and MTR 412 includes a thermal storage medium, not shown separately. In the illustrated embodiment, the thermal storage medium is sand, liquid water, and a water / ice mixture, for HTR 406, MTR 412, and LTR 409, respectively. However, the thermal storage medium may be any suitable thermal storage medium, and alternative thermal storage media may be used in alternative embodiments. Each of the thermal reservoirs HTR 406, MTR 412, and LTR 409 may include heat exchangers, piping, pumps, valves, and other controls, not shown separately, for transferring heat between the thermal storage medium and the working fluid during operation of the PTES system 400.

[0040] For example, in the illustrated embodiment of Figures 4A-4B, the HTR 406 may be what is known as a three-tank system, such as the three-tank system 600 shown in Figure 6A. In a three-tank system, during the filling process, the working fluid enters the first heat exchanger HTX1, where it transfers heat to the heat storage medium. The working fluid then enters the second heat exchanger HTX2, where it transfers additional heat to the second heat storage material. The heat storage medium is transferred from the first tank HTRc to the second heat exchanger HTX2, where it receives heat from the working fluid. The additional heat storage material is stored in the second tank HTRm at an intermediate temperature and mixed with the heat storage material. The mixed heat storage material is then transferred to the first heat exchanger HTX1, where it receives heat from the working fluid and is then stored in the third tank HTRh.

[0041] In the production process, the direction of flow is reversed. The working fluid first enters the second heat exchanger HTX2, where it receives heat from the heat storage medium. Then the working fluid enters the first heat exchanger HTX1, where it receives additional heat from the heat storage medium. The heat storage medium is transferred from the third tank HTRh to the first heat exchanger HTX1, where it transfers heat to the working fluid. The cooled heat storage medium is then split into a first part and a second part. The first part of the heat storage material is stored in the second tank HTRm. The second part of the heat storage material is transferred to the second heat exchanger HTX2, where it transfers additional heat to the working fluid. The cooled second part of the heat storage material is stored in the first tank HTRc.

[0042] Similarly, in the illustrated embodiment of Figures 4A-4B, the MTR 412 and LTR 409 may be implemented in a two-tank system, such as the two-tank system 603 shown in Figure 6B. In a two-tank thermal reservoir, during the filling process, the working fluid enters a heat exchanger and transfers heat to a heat storage medium, which may be a liquid such as oil, water, molten salt, or a flowing granular medium such as silica sand or sintered bauxite. The heat storage medium is transported from the first tank, HTRc, to the heat exchanger where it receives heat from the working fluid and is then stored in the second tank, HTRh.

[0043] In the production process, the direction of fluid flow is reversed. The working fluid enters the heat exchanger and receives heat from the heat storage medium. The heat storage medium is transferred from the second tank HTRh to the heat exchanger, where it transfers heat to the working fluid. The cooled heat transfer medium is then stored in the first tank HTRc.

[0044] In alternative embodiments, other types of tank systems may be used. One such tank system is a solid thermocline reservoir 606 shown in FIG. 6C. In a thermocline heat reservoir, during the filling process, the working fluid enters a relatively low temperature heat storage medium. The heat storage medium is typically a solid phase material, through which the working fluid may flow either through pores in the heat storage medium or through tubes or pipes (not shown) embedded within the heat storage medium material. As the working fluid flows through the heat storage medium, heat is transferred from the working fluid to the heat storage medium, increasing its temperature. The working fluid is cooled to a lower temperature and discharged from the heat reservoir.

[0045] In the production process, the direction of the working fluid flow is reversed. The relatively cool working fluid enters the heated thermal storage medium. Heat is transferred from the thermal storage medium to the working fluid, lowering the temperature of the thermal storage medium and increasing the temperature of the working fluid. The heated working fluid is then discharged from the thermal storage medium.

[0046] Another alternative tank system is a thermocline 609 with a heat transfer fluid ("TTF") loop shown in FIG. 6D. Alternatively, during the filling process, the working fluid can transfer heat in a heat exchanger to a fluid heat transfer medium such as oil, water or air. The heat transfer medium can then transfer heat to the heat storage medium by flowing through it. The cooled heat transfer fluid is then transported to the heat exchanger where it is reheated by the working fluid. During the production process, the direction of the working fluid and the heat transfer fluid is reversed.

[0047] Other single-tank thermocline storage systems include the single-tank thermocline storage system 612 in FIG. 6E. In a single-tank thermocline storage system, the working fluid enters a heat exchanger during the filling process, where it transfers heat to a heat transfer fluid. The heated heat transfer fluid is transported to the top of the tank, where thermal stratification occurs, with the hotter fluid remaining in the upper layer due to the lower fluid density of the heat transfer fluid. The cooler heat transfer fluid is removed from the bottom of the tank and transported to the heat exchanger.

[0048] In the production process, the flow direction is reversed: the warmer heat transfer fluid is transported from the top of the tank to the heat exchanger where it heats the working fluid, and the cooled heat transfer fluid is transported to the bottom of the tank where it becomes thermally stratified and remains separated from the warmer heat transfer fluid.

[0049] Other embedded heat transfer surface systems are known, such as system 615 shown in FIG. 6F. In an embedded heat transfer surface thermal storage, a working fluid is transported through a series of tubes, pipes, or other fluid channels immersed in a thermal storage medium at a relatively uniform temperature. In this example, the thermal storage medium may be water or another fluid near its freezing point, and the working fluid may be a liquid near its boiling point or a liquid / vapor mixture at its boiling temperature. During the heat transfer process, heat is transferred from the working fluid to the thermal storage medium, causing the working fluid to boil at a constant temperature while the thermal storage medium freezes from a liquid to a solid. The working fluid leaves the thermal storage as a vapor at or just above its boiling point.

[0050] In the production process, the direction of flow of the working fluid is reversed. The working fluid enters at a pressure such that the boiling point of the working fluid is slightly higher than the freezing point of the heat storage medium. Heat is transferred from the working fluid to the heat storage medium, causing the working fluid to condense into a liquid state and the heat storage medium to melt into a liquid state.

[0051] One or more of the tank systems shown in Figures 6A-6E may be used to implement the HTR 406, MTR 412, and LTR 409 in various alternative embodiments depending on implementation-specific considerations. However, this list is not exhaustive or exclusive. Other tank system designs may also be used.

[0052] 4A-B, common to the configurable working fluid circuit 403 in the charging and production stages is the bypass path 415. The bypass path 415 comprises an MTR 412. As mentioned above and discussed further below, the bypass path 415 allows a portion of the working fluid to bypass the high pressure side of the recuperator RCX.

[0053] 4A specifically illustrates the charge phase of the PTES operating cycle. In the charge phase, the configurable working fluid circuit 403 includes an expander 418 and a charge compressor 421. In the illustrated embodiment, the working fluid is CO 2 In alternative embodiments, alternative working fluids known in the art may be used. The following discussion regarding the charging stage should be considered in conjunction with the operating conditions shown in Table 2. Those skilled in the art having the benefit of this disclosure will appreciate that the values ​​shown in Table 2 are consistent with the operating conditions shown in Table 2 when the working fluid is CO 2 It will be appreciated that this is part of the fact that different implementations of the working fluid may have different values ​​for the operating conditions in Table 2.

[0054] Additionally, in the following discussion, the state of the working fluid at any point in the working fluid circuit 403 during the fill phase of Figure 4A is shown as a number within a circle. Thus, the first state, State 1, is shown as the number 1 within a circle in Figure 4A. To more precisely characterize that state, it can then be mapped back to Table 2. [Table 2]

[0055] This disclosure refers to the "high pressure side" and "low pressure side" of recuperator RCX, i.e., "RCXh" and "RCXl," respectively. In the charging stage shown in FIG. 4A, charging compressor 421 pressurizes the working fluid and provides the motive force for circulating the working fluid in the charging stage. Expander 418 expands or decompresses the working fluid.

[0056] The portion of the working fluid circuit 403 through which the working fluid pressurized by the charge compressor 421 circulates may be referred to as the "high pressure side" of the working fluid circuit 403. Similarly, the portion of the working fluid circuit 403 through which the working fluid expanded by the expander 418 circulates may be referred to as the "low pressure side" of the working fluid circuit 403. Thus, the high pressure side 404 of the working fluid circuit 403 extends from the outlet 422 of the charge compressor 421 to the inlet 419 of the expander 418. The low pressure side 405 extends from the outlet 420 of the expander 418 to the inlet 423 of the charge compressor 421.

[0057] The high pressure side of the recuperator RCX is the side of the recuperator RCX that interacts with the high pressure side 404 of the working fluid circuit 403. In the charging phase shown in FIG. 4A, this would be the side of the recuperator RCX formed by the ports 426, 430, by which the pressurized working fluid circulates through the recuperator RCX. The low pressure side of the recuperator RCX is the side that interacts with the low pressure side 405 of the working fluid circuit 403. The low pressure side of the recuperator RCX is formed by the ports 433, 436, by which the expanded working fluid circulates through the recuperator RCX.

[0058] During the charging phase starting from the recuperator RCX, the working fluid leaves the recuperator RCX and reaches a first temperature T 1 and the first pressure P 1 The working fluid enters the charging compressor 421 at a first state or state 1 of 0.5° C. The charging compressor 421 compresses the working fluid, increasing the temperature and pressure of the working fluid. The working fluid is then heated to a second temperature T 2 and the second pressure P 2 The second temperature and the second pressure each exit the charge compressor 421 at a second state of 1 and the first pressure P1 Higher than.

[0059] The working fluid is then heated to a second temperature T 2 and the second pressure P 2 The working fluid enters the high temperature reservoir HTR 406 at a second temperature T 3 and the third pressure P 3 Upon exiting the HTR 406 at , the pressure and temperature of the working fluid are reduced to a third state.

[0060] The working fluid then reaches point 424 in the working fluid circuit 403 and separates. A first portion of the working fluid enters bypass 415 and a second portion enters line 427. The second portion is cooled to a fourth temperature T 4 and the fourth pressure P 4 With reference to Table 2, the fourth state is at a third temperature and a third pressure, i.e., T 4 =T 3 and P 4 =P 3 It is shown that the fourth state is at a fourth temperature T but different from the third state before separation. The fourth state differs in that it has a lower mass flow rate than the third state, even though the second portion is at the same temperature and pressure as the working fluid in the third state. The second portion then reaches a fourth temperature T 4 and the fourth pressure P 4 In the fourth state, the heat transfer fluid enters the high pressure side of the recuperator RCX through port 426.

[0061] In the recuperator RCX, a heat exchange is carried out between the second part and the circulating working fluid of the low pressure side of the recuperator. This heat exchange is carried out such that the second part is cooled to a fifth temperature T 5 and a slightly lower fifth pressure P 5 The second portion is then cooled to a fifth state at a fifth temperature T 5 and the fifth pressure P 5 In a sixth state, the pressure in the recuperator RCX exits the recuperator RCX on the high pressure side thereof through port 430.

[0062] While the second portion circulates through the recuperator RCX, the first portion enters the bypass 415 in the sixth state. With reference to Table 2, it can be seen that the sixth state is different from the third state before separation. The sixth state differs in that the second portion is at the same temperature and pressure as the working fluid in the fourth state, yet has a lower mass flow rate than the third state. The first portion then circulates at a sixth temperature T 6 and the sixth pressure P 6 Enter MTR412 in the sixth state.

[0063] In the MTR 412, heat is transferred between the medium temperature heat reservoir MTR 412 and a first portion of the working fluid. Recall that the MTR 412 operates at a higher temperature than the LTR 409 and a lower temperature than the high temperature heat reservoir HTR 406. The first portion is then heated to a seventh temperature T 7 and the seventh pressure P 7 In the seventh state, the medium temperature heat reservoir MTR412 is exited.

[0064] After the first portion exits the MTR 412 at a seventh state and the second portion exits the recuperator RCX at a fifth state, the first and second portions merge at point 425. After merging, the working fluid is at an eighth pressure P 8 and the eighth temperature T 8 The junction of the first and second portions, or the "junction portion," then reaches an eighth pressure P 8 and the eighth temperature T 8 The combined portion of the working fluid enters the expander 418 at an eighth temperature T 9 and the ninth pressure P 9 The ninth state is when the inflator 418 exits.

[0065] The working fluid is then heated to a ninth temperature T 9 and the ninth pressure P 9 The working fluid enters the LTR 409 at a ninth temperature T. In the LTR 409, heat is transferred from the LTR 409 to the working fluid. Note that the LTR 409 operates at a lower temperature than the medium temperature heat reservoir MTR 412 and the high temperature heat reservoir HTR 406, as shown in Table 2. The working fluid enters the LTR 409 at a ninth temperature T10 and the tenth pressure P 10 Exit LTR409 in the 10th state.

[0066] As the working fluid leaves the LTR409, it reaches a tenth temperature, T 10 and the tenth pressure P 10 In the tenth state, the recuperator RCX is entered and the first temperature T 1 and the first pressure P 1 The recuperator RCX exits in a first state of 404. In the recuperator RCX, heat is transferred from the working fluid on the high pressure side to the working fluid on the low pressure side of the recuperator RCX. In Table 2, one can see (1) a decrease in temperature of the working fluid on the high pressure side 404 when passing from the fourth state to the fifth state, and (2) an increase in temperature of the working fluid on the low pressure side 405 when passing from the tenth state to the first state. The working fluid then resumes its circulation through the working fluid circuit 403 discussed immediately above.

[0067] 4B, a configuration of the working fluid circuit 403 during the generating phase of the PTES operating cycle is shown. The operating conditions at various points in the working fluid circuit 403 are listed in Table 3. As discussed above, the flow direction of the working fluid through the working fluid circuit 403 is reversed from that during the charging phase of the operating cycle. Note that the expander 419 and charging compressor 421 in the charging phase configuration of FIG. 4A have been replaced with a pump 450 and a power turbine 453, respectively. As can be seen by comparing states 1 and 2 in Table 3 with states 9 and 10 in Table 3, the pump 450 pressurizes the working fluid and the power turbine 453 depressurizes the working fluid.

[0068] Thus, at this stage of the operating cycle, the high pressure side 404 of the working fluid circuit 403 extends from the outlet 451 of the pump 450 to the inlet 454 of the power turbine 453. The low pressure side 405 extends from the outlet 455 of the power turbine 453 to the inlet 452 of the pump 450. Note that the high pressure side 404 comprises the bypass 415, the line 427, point 425 and point 424. The high pressure side 404 of the recuperator RCX is again defined by ports 426, 430 and the low pressure side 405 of the recuperator RCX is again defined by ports 433, 436. [Table 3]

[0069] Starting with the LTR 409, the working fluid exits the LTR 409 at a first temperature T 1 and the first pressure P 1 The working fluid enters the pump 450 at a first state of T 2 and the second pressure P 2 The pump 450 exits in a second state.

[0070] When the working fluid exits pump 450 in the second state, it separates into a first portion and a second portion at point 425. In the charging phase shown in Figure 4A, the two portions of working fluid separate at point 424 and meet at point 425. However, because the flow direction of the working fluid is opposite in the production phase relative to the charging phase, the two portions separate at point 425 and meet at point 424.

[0071] The second portion, after splitting off at point 425, enters line 428. The second portion is heated to a third temperature T 3 and the third pressure P 3 With reference to Table 2, the third state is at a second temperature and a second pressure, i.e., a third temperature T 3 It can be seen that the third state is identical to the second temperature and the second pressure. Nevertheless, the third state differs from the second state in that the mass flow rate is significantly lower than in the second state. The second portion then circulates through the recuperator RCX from its high pressure side 404, entering through port 430 and exiting through port 426. The second portion enters the recuperator RCX in a third state at a third temperature and a third pressure, and exits in a fourth state at a fourth temperature and a fourth pressure.

[0072] The first portion of the working fluid, after separation at point 425, is cooled to a fifth temperature T 5 and the fifth pressure P 5 The first portion enters the bypass 415 at a fifth state T 5 and the fifth pressure P5 The first portion enters the MTR 412 at a fifth state of T 1 . In the MTR 412, heat is transferred from the medium temperature heat reservoir MTR 412 to the first portion. The first portion then reaches a sixth temperature T 6 and the sixth pressure P 6 Exit MTR412 in the sixth state.

[0073] As the first portion of the sixth state and the second portion of the fourth state leave the MTR 412 and the recuperator RCX, respectively, they join at point 424. Note again that in the charging phase shown in FIG. 4A, the two portions of working fluid separate at point 424 and join at point 425. However, because the flow direction of the working fluid is opposite in the production phase relative to the charging phase, the two portions separate at point 425 and join at point 424. The joining portions of the working fluid from point 424 onwards are heated to a seventh temperature T 7 and the seventh pressure P 7 It is in the seventh state.

[0074] The joint then reaches a seventh temperature T 7 and the seventh pressure P 7 The hot water enters the high temperature reservoir HTR 406 at a seventh temperature T 8 and the eighth pressure P 8 The hot water leaves the high temperature reservoir HTR406 in the eighth state.

[0075] The joint portion of the working fluid is then heated to an eighth temperature T 8 and the eighth pressure P 8 More specifically, the combined portion enters the power turbine 453 from the high pressure side 404 of the working fluid circuit 403 through an inlet 454. The power turbine 453 expands the working fluid, cooling the combined portion and reducing its pressure. The combined portion is then cooled to a ninth temperature T 9 and the ninth pressure P 9 The combined portion exits the power turbine 453 at a ninth state. More specifically, the combined portion exits the power turbine and passes through outlet 455 towards the low pressure side 405 of the working fluid circuit 403.

[0076] The combined portion then circulates through the low pressure side of the recuperator RCX via ports 433, 436. In the recuperator RCX, heat exchange is carried out between the second portion of the working fluid entering the recuperator RCX from its high pressure side as described above and the combined portion entering the recuperator RCX from its low pressure side. The combined portion then circulates through the low pressure side of the recuperator RCX at a tenth temperature T 10 and the tenth pressure P 10 The recuperator RCX then exits the junction at a tenth temperature T 10 and the tenth pressure P 10 The air cooler ACC then cools the combined part to an 11th temperature T 11 and 11 pressure P 11 Cool to the eleventh state.

[0077] The joint then has an eleventh temperature T 11 and 11 pressure P 11 The combined portion then leaves the LTR 409 at a first state at a first temperature and a first pressure to be recirculated through the working fluid circuit 403 as just described.

[0078] As mentioned above, the configuration of the actuating fluid circuit 403 between the filling stage shown in Figure 4 and the filling stage shown in Figure 4B may be controlled by fluid flow valves. Such control systems will be readily understood by those skilled in the art, but for completeness, one such control system 500 is shown in Figure 5. The control system 500 may include a number of fluid flow valves 505 and a controller 510 that sends control signals over electrical lines 515.

[0079] The controller 510 includes a processor-based resource 520, which may be, for example, but not limited to, a microcontroller, a microprocessor, an application specific integrated circuit ("ASIC"), an electrically erasable programmable read-only memory ("EEPROM"), or the like. Depending on the implementation of the processor-based resource, the controller 510 may also include a memory 525 encoded with instructions (not shown) executable by the processor-based resource 520 to implement the functions of the controller 510. Furthermore, depending on the implementation of the processor-based resource 520, the memory 525 may be part of the processor-based resource 520 or may be a stand-alone device. For example, the instructions may be firmware stored in a memory portion of a microprocessor or a routine stored in a stand-alone read-only or random access memory chip. Similarly, in some implementations of the processor-based resource 520 (e.g., an ASIC), the memory 535 may be omitted entirely.

[0080] 4A-4B and 5 collectively, a controller such as controller 510 may be used to configure the working fluid circuit 403 between the charging stage shown in FIG. 4A and the charging stage shown in FIG. 4B. The controller 510 may send control signals to the fluid flow valve 505 to control the flow of working fluid. Thus, to configure the working fluid circuit 403 for the charging stage, the controller 510 controls the fluid flow valve 505 to direct the working fluid to the charge compressor 421 and the expander 418 while diverting the working fluid from the power turbine 453 and the pump 450. Conversely, to configure the working fluid circuit 403 for the production stage, the controller 510 controls the fluid flow valve 505 to direct the working fluid to the power turbine 453 and the pump 450 while diverting the working fluid from the charge compressor 421 and the expander 418.

[0081] 4A-4B together, the high pressure side 404 of the working fluid circuit 403 includes a bypass 415 in both the charging and operating phases. Thus, the bypass 415, including a heat exchanger (not shown) of the MTR 412, is on the high pressure side of the recuperator RCX in both phases. The working fluid separates before entering the heat reservoir RCX at point 424 in the charging phase and at point 425 in the production phase. A first portion bypasses the high pressure side of the recuperator RCX via the bypass 415 and a second portion enters the recuperator RCX. The first and second portions then join at point 425 in the charging phase and at point 424 in the production phase after the second portion has passed through the recuperator RCX.

[0082] Separation and merging on the high pressure side 404 of the working fluid circuit 402 occurs at points 424, 425 in the illustrated embodiment. However, whether points 424, 425 are separation or merging points will depend on whether the operating cycle is in the charging or production phase. In the charging phase, point 424 is a separation point, and in the production phase, it is a merging point. Conversely, in the charging phase, point 425 is a merging point, and in the production phase, it is a separation point. It should be noted that in alternative embodiments, there may be separation and merging points in addition to or instead of those disclosed herein. This is especially true when pursuing design goals that are unrelated to the implementation of the techniques disclosed herein.

[0083] The objective is to balance the product of mass and heat capacity of the low pressure side 405 of the recuperator RCX with the product of mass and heat capacity of the high pressure side 404 of the recuperator RCX. The term "balanced" means that the products of mass and heat capacity on both sides of the recuperator RCX are equal. However, for various reasons, this may be difficult to achieve precisely in practice. Thus, the two products are "balanced" when they are "almost," "approximately," or "approximately" equal in the sense that both products are within a margin of error that will cause the overall system to achieve some desired level of operation. The desired efficiency level may be expressed as a range of values ​​that accommodates this type of concern.

[0084] In the same manner, the terms "about," "approximately," and the like, in relation to any quantity in this disclosure indicate that some deviation from the stated amount may be tolerated so long as the actual amount is within a margin of error such that the operation of the overall system achieves some desired level of efficiency. For example, in the illustrated embodiment, the first portion may be 40% and the second portion may be 60% of the total combined working fluid, as discussed in more detail below. 2 In any embodiment employing 40% and 60%, it may be difficult to separate exactly in such proportions. Thus, some deviation may be allowed as long as the ratio is "about" or "approximately" 40% and 60%. The same is true for any other amounts discussed or disclosed herein.

[0085] Those skilled in the art having the benefit of this disclosure will appreciate that both the mass flow rate and heat capacity in any embodiment are implementation specific, depending on factors such as the choice of working fluid implementation. Other factors such as the operating ranges of pumps, expanders, compressors, etc. may affect the operating conditions of various portions of the working fluid circuit. Thus, various amounts of the parameters in Tables 2 and 3 may vary in alternative embodiments that employ different substances as the working fluid or that implement certain devices in different ways.

[0086] As mentioned above, the working fluid in the illustrated embodiment is CO 2 When the working fluids are separated as described above, the first portion is 40% of the total volume of working fluid and the second portion is 60% of the total volume of working fluid. This applies to both the charge and production stages. In alternative embodiments using different working fluids or different mass flow rates, this ratio may be altered to maintain a balance of mass flow rates and heat capacity on both the high and low pressure sides of the recuperator.

[0087] Thus, in a first embodiment, a method for operating a pumped thermal energy storage ("PTES") system is disclosed, the method including circulating a working fluid, the working fluid having a mass flow rate and a heat capacity, through a working fluid circuit, and balancing the product of the mass and 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.

[0088] In a second embodiment, the first embodiment includes separating 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, thereby balancing the product of the mass and 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.

[0089] In the third embodiment, the first portion is cooled during the bypass by circulating the working fluid through the bypass and transferring heat between the working fluid and the medium temperature heat reservoir in the second embodiment. The heat transfer includes transferring heat from the low temperature heat reservoir to the working fluid and transferring heat from the working fluid to the medium temperature heat reservoir and the high temperature heat reservoir in the charging stage, and transferring heat from the high temperature heat reservoir and the medium temperature heat reservoir (high temperature) to the working fluid and transferring heat from the working fluid to the low temperature heat reservoir in the generating stage.

[0090] In a fourth embodiment, the second embodiment may be implemented such that the first portion of the working fluid constitutes 40% of the working fluid and the second portion constitutes 60% of the working fluid.

[0091] In a fifth embodiment, circulating the working fluid through the working fluid circuit comprises circulating carbon dioxide (CO 2 ), where the first portion of the working fluid constitutes 40% of the working fluid and the second portion constitutes 60% of the working fluid.

[0092] In a sixth embodiment, circulating the working fluid through the working fluid circuit comprises circulating carbon dioxide (CO 2 ) may be implemented.

[0093] In a seventh embodiment, the first embodiment may be implemented such that balancing the product of the mass and heat capacity of the working fluid on the high pressure side of the recuperator and the low pressure side of the recuperator includes a charging stage and a generating stage. The charging stage is a stage in which carbon dioxide (CO 2 ) is circulated through the high pressure side of the recuperator, and CO 2 The production step involves circulating 100% of the working fluid comprised of CO 2 60% of the working fluid, consisting of 2 The method includes circulating 100% of the working fluid comprised of through the low pressure side of the recuperator.

[0094] In an eighth embodiment, the first embodiment may be implemented such that balancing the product of the mass and 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.

[0095] In a ninth embodiment, the first embodiment further includes exchanging heat between a second portion of the working fluid on the high pressure side of the recuperator and a confluent portion of the working fluid on the low pressure side of the recuperator.

[0096] In a tenth embodiment, a pumped thermal energy storage ("PTES") system includes a medium temperature thermal storage and a working fluid circuit including a recuperator having a high pressure side and a low pressure side, where the product of the mass and heat capacity of the working fluid is balanced between the high pressure side and the low pressure side as the working fluid circulates through the working fluid circuit.

[0097] In an eleventh embodiment, the working fluid is carbon dioxide (CO 2The tenth embodiment may be implemented as follows:

[0098] In a twelfth embodiment, the tenth embodiment may be implemented such that the working fluid circuit includes a bypass that causes a first portion of the working fluid to bypass the high pressure side of the recuperator while a second portion of the working fluid circulates through the high pressure side of the recuperator.

[0099] In a thirteenth embodiment, the eleventh embodiment may be implemented such that the bypass includes a heat transfer section between the first portion and the medium temperature heat storage. Furthermore, the working fluid circuit includes a separation section on the high pressure side of the recuperator that separates the working fluid into a first portion and a second portion, the first portion being smaller than the second portion and the second portion circulating from the high pressure side of the recuperator through the recuperator, and a junction on the high pressure side of the recuperator where the first portion merges with the second portion as the second portion exits the recuperator.

[0100] In a fourteenth embodiment, the eleventh embodiment may be implemented such that the first portion is 40% of the total amount of working fluid and the second portion is 60% of the total amount of working fluid.

[0101] In a fifteenth embodiment, the working fluid is carbon dioxide (CO 2 ), the first portion being 40% of the total amount of working fluid and the second portion being 60% of the total amount of working fluid.

[0102] In a sixteenth embodiment, the tenth embodiment may be implemented such that, during operation, heat is exchanged between a second portion of the working fluid on the high pressure side of the recuperator and a confluent portion of the working fluid on the low pressure side of the recuperator.

[0103] In a seventeenth embodiment, a method of operating a pumped thermal energy storage ("PTES") system includes 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 the recuperator to balance a product of the mass and heat capacity of the working fluid on the high pressure and low pressure sides of the recuperator while circulating the working fluid. The reducing may include bypassing a first portion of the working fluid around the high pressure side of the recuperator and circulating a second portion of the working fluid through the high pressure side of the recuperator.

[0104] In an eighteenth embodiment, the seventeenth embodiment may be implemented such that bypassing the high pressure side of the recuperator includes having a first portion of the working fluid bypass the high pressure side of the recuperator when transferring heat between the working fluid and the high temperature reservoir during the charging stage, the first portion being smaller than the second portion and transferring heat from the first portion to the intermediate temperature reservoir while the second portion circulates through the recuperator during the bypass, and having a third portion of the working fluid bypass the high pressure side of the recuperator when exiting the pump during the production stage, while circulating a fourth portion of the working fluid through the recuperator while transferring heat from the intermediate temperature reservoir to the third portion during the bypass.

[0105] In a nineteenth embodiment, the seventeenth embodiment may include exchanging heat between a second portion of the working fluid on the high pressure side of the recuperator and a confluent portion of the working fluid on the low pressure side of the recuperator.

[0106] In a twentieth embodiment, the seventeenth embodiment may be implemented such that 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 heat capacity of the working fluid on the high pressure and low pressure sides of the recuperator while circulating the working fluid further includes separating the working fluid into a first portion on the high pressure side of the recuperator and a second portion, the first portion being smaller than the second portion, cooling the first portion during the bypass, and merging the cooled first portion with the second portion after the second portion exits the recuperator.

[0107] In a twenty-first embodiment, the twentieth embodiment may be implemented such that cooling the first portion during the bypass includes transferring heat between the working fluid and the medium temperature heat reservoir. Transferring heat may further include transferring heat from the working fluid to the medium temperature heat reservoir in a charging phase and transferring heat from the medium temperature heat reservoir to the working fluid in a generating phase.

[0108] In a twenty-second embodiment, circulating the working fluid through the working fluid circuit comprises circulating carbon dioxide (CO 2 A seventeenth embodiment may be implemented to include cycling through

[0109] In a twenty-third embodiment, the seventeenth embodiment may be implemented such that the first portion of the working fluid constitutes 40% of the working fluid and the second portion constitutes 60% of the working fluid.

[0110] In a twenty-fourth embodiment, circulating the working fluid through the working fluid circuit comprises circulating carbon dioxide (CO 2 ) wherein the first portion of the working fluid constitutes 40% of the working fluid and the second portion constitutes 60% of the working fluid.

[0111] In a twenty-fifth embodiment, a pumped thermal energy storage ("PTES") system comprises a low temperature storage, a high temperature storage, a mid temperature storage, and a working fluid circuit configurable for charge and production phases of a PTES operating cycle, through which a working fluid circulates during use. The working fluid circuit may comprise a recuperator, and when in the charge phase, an expander positioned between the recuperator and the low temperature storage, and a charge compressor positioned between the recuperator and the high temperature storage, and when in the production phase, a pump positioned between the recuperator and the low temperature storage, a production turbine positioned between the recuperator and the high temperature storage, and a bypass circuit whereby during both the charge and production phases, a first portion of the working fluid bypasses the high pressure side of the recuperator and flows through the mid temperature storage, and the mid temperature storage transfers heat between the working fluid and the mid temperature storage while a second portion of the working fluid circulates through the recuperator.

[0112] In a twenty-sixth embodiment, the working fluid is carbon dioxide (CO 2 The twenty-fifth embodiment may be implemented as follows:

[0113] In the twenty-seventh embodiment, the twenty-fifth embodiment may be implemented such that the first portion is 40% of the total amount of working fluid and the second portion is 60% of the total amount of working fluid.

[0114] In a twenty-eighth embodiment, the working fluid is carbon dioxide (CO 2 ), the first portion is 40% of the total amount of working fluid, and the second portion is 60% of the total amount of working fluid.

[0115] In a twenty-ninth embodiment, a method for operating a pumped thermal energy storage ("PTES") system is provided, the method including 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 heat capacity is the same on both sides of the recuperator. The reducing the mass flow rate includes, during 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 storage vessel while circulating a second portion of the working fluid through the recuperator, the first portion being a smaller portion than the second portion of the working fluid during the bypass, circulating the second portion through the recuperator while the first portion bypasses the recuperator, and transferring heat from the second portion during the circulation through the recuperator, and circulating both the first and second portions together through an expander after the first portion bypasses the recuperator and the second portion has circulated through the recuperator. Reducing the mass flow rate includes, during the generating stage, bypassing the recuperator with a third portion of the working fluid and, while bypassing, transferring heat from the third portion to a medium temperature storage while circulating a fourth portion of the working fluid through the recuperator, where the third portion is a greater portion than the fourth portion; circulating the fourth portion through a heat exchanger while the third portion bypasses the recuperator and transferring heat to the fourth portion while circulating through the recuperator; and circulating both the third and fourth portions together through the high temperature storage after the third portion bypasses the recuperator and the fourth portion has circulated through the recuperator.

[0116] In a 30th embodiment, the working fluid is carbon dioxide (CO 2 The twenty-ninth embodiment may be implemented as follows:

[0117] In a thirty-first embodiment, the thirtieth embodiment may be implemented such that the first portion corresponds to approximately 40% of the mass flow rate of the working fluid during the filling stage, the second portion corresponds to approximately 60% of the mass flow rate of the working fluid during the filling stage, the third portion corresponds to approximately 40% of the mass flow rate of the working fluid during the production stage, and the fourth portion corresponds to approximately 60% of the mass flow rate of the working fluid during the production stage.

[0118] In a thirty-second embodiment, the twenty-ninth embodiment may be implemented such that the first portion corresponds to approximately 40% of the mass flow rate of the working fluid in the filling stage, the second portion corresponds to approximately 60% of the mass flow rate of the working fluid in the filling stage, the third portion corresponds to approximately 40% of the mass flow rate of the working fluid in the production stage, and the fourth portion corresponds to approximately 60% of the mass flow rate of the working fluid in the production stage.

[0119] Those skilled in the art having the benefit of this disclosure may realize still other embodiments of the techniques disclosed herein.

[0120] The features of some embodiments have been outlined above so that those skilled in the art may better understand the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures to carry out the same purposes and / or achieve the same advantages as the embodiments introduced herein. Those skilled in the art should also appreciate that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions and alterations may be made herein without departing from the spirit and scope of the present disclosure.

Claims

1. 1. A method of operating a pumped thermal energy storage ("PTES") system, the method comprising: circulating a working fluid having a mass flow rate and a heat capacity through a working fluid circuit; balancing the product of the mass and the 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.

2. Balancing the product of the mass and the 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 comprises separating 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; and circulating the second portion through the recuperator; The method of claim 1 , further comprising: combining the cooled first portion with the second portion after the second portion exits the recuperator.

3. Cooling the first portion during the bypass comprises: circulating the working fluid through the bypass; Transferring heat between the working fluid and a medium temperature heat reservoir, A charging stage includes transferring heat from the working fluid to a medium temperature heat reservoir; The method of claim 2 , wherein the generating step comprises transferring heat from a medium temperature thermal reservoir to the working fluid.

4. The method of claim 2 , wherein the first portion of the working fluid comprises approximately 40% of the working fluid and the second portion comprises approximately 60% of the working fluid.

5. Circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO ２ ) in circulation, the first portion of the working fluid constituting approximately 40% of the working fluid; The method of claim 1 , wherein the second portion of the working fluid comprises approximately 60% of the working fluid.

6. Circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO ２ 3. The method of claim 2, comprising circulating

7. Balancing the product of the mass and the heat capacity of the working fluid on the high pressure side of the recuperator and the low pressure side of the recuperator comprises: In the filling stage, Carbon Dioxide (CO ２ circulating approximately 60% of the working fluid through the high pressure side of the recuperator; CO ２ and circulating 100% of the working fluid through the low pressure side of the recuperator, the working fluid comprising: In the generation stage, CO ２ circulating approximately 60% of the working fluid through the high pressure side of the recuperator; CO ２ and circulating 100% of the working fluid through the low pressure side of the recuperator.

8. 2. The method of claim 1 , wherein balancing the product of the mass and the 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 comprises reducing the mass flow rate on the high pressure side of the recuperator.

9. 2. 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.

10. 1. A pumped thermal energy storage ("PTES") system comprising: A medium temperature heat storage device; A working fluid circuit comprising: a working fluid circuit including a recuperator having a high pressure side and a low pressure side, wherein the product of the mass and heat capacity of a working fluid is balanced between the high pressure side and the low pressure side as the working fluid circulates through the working fluid circuit.

11. The working fluid is carbon dioxide (CO ２ 11. The system of claim 10, wherein

12. 11. The system of claim 10, wherein the working fluid circuit comprises a bypass whereby a first portion of the working fluid bypasses the high pressure side of the recuperator while a second portion of the working fluid circulates through the high pressure side of the recuperator.

13. The bypass path includes a heat transfer portion between the first portion and the medium temperature heat storage device, The working fluid circuit includes: a separation section on the high pressure side of the recuperator that separates the working fluid into the first portion and the second portion, the first portion being smaller than the second portion, the second portion circulating from the high pressure side of the recuperator through the recuperator; 11. The system of claim 10, further comprising a junction on the high pressure side of the recuperator where the first portion joins the second portion as the second portion exits the recuperator.

14. The system of claim 10 , wherein the first portion is about 40% of a total volume of the working fluid and the second portion is about 60% of a total volume of the working fluid.

15. The working fluid is carbon dioxide (CO ２ ) and the first portion being approximately 40% of a total volume of the working fluid; The system of claim 10 , wherein the second portion is about 60% of a total volume of the working fluid.

16. 11. The system of claim 10, further comprising: during 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.

17. 1. A method of operating a pumped thermal energy storage ("PTES") system, the method comprising: circulating a working fluid through a working fluid circuit; reducing a mass flow rate of the working fluid in a high pressure side of a recuperator to balance the product of the mass and heat capacity of the working fluid in the high pressure and low pressure sides of the recuperator while circulating the working fluid; bypassing the high pressure side of the recuperator with a first portion of the working fluid; circulating a second portion of the working fluid through the high pressure side of the recuperator.

18. Bypassing the high pressure side of the recuperator during a charging phase of transferring heat between the working fluid and a high temperature reservoir, bypassing the high pressure side of the recuperator with a first portion of the working fluid, the first portion being smaller than the second portion, and transferring heat from the first portion to a medium temperature reservoir during the bypass while the second portion circulates through the recuperator; 20. The method of claim 17, comprising: bypassing the high pressure side of the recuperator for a third portion of the working fluid as it exits a pump during a generating stage, while circulating a fourth portion of the working fluid through the recuperator while transferring heat from the intermediate temperature reservoir to the third portion during the bypass.

19. 20. The method of claim 17, 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.

20. Reducing the mass flow rate of the working fluid in the high pressure side of the recuperator to balance the product of the mass and the heat capacity of the working fluid in the high pressure side and the low pressure side of the recuperator while circulating the working fluid comprises separating the working fluid into the first portion and the second portion at the high pressure side of the recuperator, the first portion being smaller than the second portion; transferring heat to and from the first portion during the bypass; 20. The method of claim 17, further comprising: combining the first portion with the second portion after the second portion exits the recuperator.

21. 21. The method of claim 20, wherein transferring heat to and from the first portion during the bypass comprises transferring heat between the working fluid and a medium temperature thermal storage reservoir, The charging step further includes transferring heat from the working fluid to a medium temperature heat reservoir; The method, wherein the generating step further comprises transferring heat from a medium temperature heat reservoir to the working fluid.

22. Circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO ２ 20. The method of claim 17, comprising circulating

23. 20. The method of claim 17, wherein the first portion of the working fluid comprises approximately 40% of the working fluid and the second portion comprises approximately 60% of the working fluid.

24. Circulating the working fluid through the working fluid circuit includes circulating carbon dioxide (CO ２ ) in circulation, 20. The method of claim 17, wherein the first portion of the working fluid comprises approximately 40% of the working fluid and the second portion comprises approximately 60% of the working fluid.

25. 1. A pumped thermal energy storage ("PTES") system comprising: A low temperature heat storage device; A high temperature heat storage device; A medium temperature heat storage device; A working fluid circuit configurable for the charge and production stages of a PTES operating cycle, and through which a working fluid circulates in use, said working fluid circuit comprising: A recuperator; In the filling step, an expander positioned between the recuperator and the low temperature heat reservoir; a charge compressor positioned between the recuperator and the high temperature heat reservoir; In the generating step, a pump positioned between the recuperator and the low temperature heat reservoir; a power turbine positioned between the recuperator and the high temperature heat storage; a working fluid circuit comprising a bypass, wherein the bypass causes a first portion of the working fluid to bypass the high pressure side of the recuperator and flow through the intermediate temperature heat reservoir during both the charging stage and the production stage, and the intermediate temperature heat reservoir transfers heat between the working fluid and the intermediate temperature heat reservoir while a second portion of the working fluid circulates through the recuperator.

26. The working fluid is carbon dioxide (CO ２ 26. The system of claim 25, wherein

27. 26. The system of claim 25, wherein the first portion is about 40% of a total volume of the working fluid and the second portion is about 60% of a total volume of the working fluid.

28. The working fluid is carbon dioxide (CO ２ ) and the first portion being approximately 40% of a total volume of the working fluid; 26. The system of claim 25, wherein the second portion is about 60% of a total amount of the working fluid.

29. 1. A method of 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; reducing the mass flow rate of the working fluid on the high pressure side of the recuperator so that the product of mass and heat capacity is the same on both sides of the recuperator; In the filling stage, bypassing the recuperator with a first portion of the working fluid and, during the bypass, circulating a second portion of the working fluid through the recuperator while transferring heat from the first portion to a medium temperature storage; circulating the second portion through the recuperator while the first portion bypasses the recuperator, and transferring heat from the second portion during circulation through the recuperator; after the first portion bypasses the recuperator and the second portion has circulated through the recuperator, circulating both the first portion and the second portion together through an expander; In the generation stage, bypassing the recuperator with a third portion of the working fluid, and during the bypass, circulating a fourth portion of the working fluid through the recuperator while transferring heat from the third portion to a medium temperature storage tank; circulating a fourth portion through the recuperator while the third portion bypasses the recuperator, and transferring heat to the fourth portion during circulation through the recuperator; circulating both the third and fourth portions together through a hot storage tank after the third portion bypasses the recuperator and the fourth portion has circulated through the recuperator.

30. The working fluid is carbon dioxide (CO ２30. The method of claim 29, wherein

31. the first portion corresponds to approximately 40% of the mass flow rate of the working fluid at the filling stage; the second portion corresponds to approximately 60% of the mass flow rate of the working fluid during the filling stage; 31. The method of claim 30, wherein the third portion represents about 40% of the mass flow rate of the working fluid during the production stage, and the fourth portion represents about 60% of the mass flow rate of the working fluid during the production stage.

32. the first portion corresponds to approximately 40% of the mass flow rate of the working fluid at the filling stage, and the second portion corresponds to approximately 60% of the mass flow rate of the working fluid at the filling stage; 30. The method of claim 29, wherein the third portion represents about 40% of the mass flow rate of the working fluid during the production stage, and the fourth portion represents about 60% of the mass flow rate of the working fluid during the production stage.

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