Patent Publication Number: US-11655759-B2

Title: Modular thermal storage

Description:
CROSS REFERENCE 
     This application is a Continuation of U.S. patent application Ser. No. 16/576,357, filed Sep. 19, 2019, which is a Continuation and claims the benefit of U.S. patent application Ser. No. 15/396,641, filed on Dec. 31, 2016 (now U.S. Pat. No. 10,436,109), each of which is hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     In a heat engine or heat pump, a heat exchanger may be employed to transfer heat between a thermal storage material and a working fluid for use with turbomachinery. The heat engine may be reversible, e.g., it may also be a heat pump, and the working fluid and heat exchanger may be used to transfer heat or cold to thermal storage media. 
     SUMMARY 
     A closed thermodynamic cycle power generation or energy storage system, such as a reversible Brayton cycle system, may include at least a working fluid circulated through a closed cycle fluid path including at least two heat exchangers, a turbine, and a compressor. Some systems may include a plurality of closed cycle systems (power subunits), each configured to be connected to or isolated from a shared hot side thermal store and a shared cold side thermal store. 
     Example power generation systems may include a hot side thermal store comprising a first hot thermal storage (“HTS”) tank, a second HTS tank, an HTS medium, an HTS supply stream configured receive a flow of HTS medium from the first HTS tank, an HTS return stream configured to send a flow of HTS medium to the second HTS tank. Example power generation systems may further include a cold side thermal store comprising a first cold thermal storage (“CTS”) tank, a second CTS tank, a CTS medium, a CTS supply stream configured to receive a flow of CTS medium from the second CTS tank, a CTS return stream configured to send a flow of CTS medium to the first CTS tank. Example power generation systems may further include a plurality of power subunits, each power subunit comprising: a generator configured to generate electrical power, a compressor, a hot side heat exchanger, a turbine, a cold side heat exchanger, a working fluid circulating in a closed cycle path, wherein the closed cycle path comprises, in sequence, the compressor, the hot side heat exchanger, the turbine, and the cold side heat exchanger, and a valve arrangement configurable to be in a connected state or an isolated state, wherein in the connected state the valve arrangement is configured to connect the hot side heat exchanger to the HTS supply stream and to the HTS return stream and to connect the cold side heat exchanger to the CTS supply stream and to the CTS return stream, wherein in the isolated state, the valve arrangement is configured to isolate the hot side heat exchanger from the HTS supply stream and from the HTS return stream and to isolate the cold side heat exchanger from the CTS supply stream and from the CTS return stream. Example power generation systems may further include an electrical bus electrically coupled to each generator of each power subunit of the plurality of power subunits, wherein the electrical bus is configured to carry electrical power generated by each generator to an electrical node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically illustrates operation of a pumped thermal electric storage system. 
         FIG.  2    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system in a charge/heat pump mode. 
         FIG.  3    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system in a discharge/heat engine mode. 
         FIG.  4    is a schematic pressure and temperature diagram of the working fluid as it undergoes the charge cycle in  FIG.  2   . 
         FIG.  5    is a schematic pressure and temperature diagram of the working fluid as it undergoes the discharge cycle in  FIG.  3   . 
         FIG.  6    is a schematic perspective view of a closed working fluid system in the pumped thermal system in  FIGS.  2 - 3   . 
         FIG.  7    is a schematic perspective view of the pumped thermal system in  FIGS.  2 - 3    with hot side and cold side storage tanks and a closed cycle working fluid system. 
         FIG.  8    shows a heat storage charge cycle for a water/molten salt system with κ c =0.9 and η t =0.95. The dashed lines correspond to η c =η t =1. 
         FIG.  9    shows a heat storage discharge (extraction) cycle for the water/molten salt system in  FIG.  8    with η c =0.9 and η t =0.95. The dashed lines correspond to η c =η t =1. 
         FIG.  10    shows a heat storage cycle in a pumped thermal system with variable compression ratios between the charge and discharge cycles. 
         FIG.  11    shows roundtrip efficiency contours for a water/salt system. The symbols ⊕ and ⊗ represent an approximate range of present large turbomachinery adiabatic efficiency values. The dashed arrows represent the direction of increasing efficiency. 
         FIG.  12    shows roundtrip efficiency contours for a colder storage/salt system. The symbols ⊕ and ⊗ represent an approximate range of present large turbomachinery adiabatic efficiency values. 
         FIG.  13    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system with a gas-gas heat exchanger for the working fluid in a charge/heat pump mode. 
         FIG.  14    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system with a gas-gas heat exchanger for the working fluid in a discharge/heat engine mode. 
         FIG.  15    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system with a gas-gas heat exchanger for the working fluid in a charge/heat pump mode with indirect heat rejection to the environment. 
         FIG.  16    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system with a gas-gas heat exchanger for the working fluid in a discharge/heat engine mode with indirect heat rejection to the environment. 
         FIG.  17    shows a heat storage charge cycle for a storage system with a gas-gas heat exchanger, a cold side storage medium capable of going down to temperatures significantly below ambient temperature and η c =0.9 and η t t=0.95. 
         FIG.  18    shows a heat storage discharge cycle for a storage system with a gas-gas heat exchanger, a cold side storage medium capable of going down to temperatures significantly below ambient temperature and η c =0.9 and η t t=0.95. 
         FIG.  19    is a schematic flow diagram of hot side recharging in a pumped heat cycle in solar mode with heating of a solar salt solely by solar power. 
         FIG.  20    is a schematic flow diagram of a pumped thermal system discharge cycle with heat rejection to ambient. 
         FIG.  21    is a schematic flow diagram of a pumped thermal system discharge cycle with heat rejection to an intermediate fluid circulated in a thermal bath at ambient temperature. 
         FIG.  22    and  FIG.  23    are pumped thermal systems with separate compressor/turbine pairs for charge and discharge modes. 
         FIG.  24    and  FIG.  25    show pumped thermal systems configured in a combustion heat input generation mode. 
         FIG.  26    is a schematic flow diagram of hot side recharging in a pumped heat cycle through heating by a combustion heat source or a waste heat source. 
         FIG.  27    shows an example of a pumped thermal system with pressure regulated power control. 
         FIG.  28    shows an example of a pumped thermal system with a pressure encased generator. 
         FIG.  29    is an example of variable stators in a compressor/turbine pair. 
         FIG.  30    shows a computer system that is programmed to implement various methods and/or regulate various systems of the present disclosure. 
         FIG.  31    illustrates a power generation system with subunits, according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Example systems and methods are described herein. It should be understood that the words “example” and/or “exemplary” are used herein to mean “serving as an example, instance, or illustration.” Any embodiment or feature described herein as being an “example” or “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or features. The example embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed systems and methods can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein. 
     While various embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions may occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed. It shall be understood that different aspects of the invention can be appreciated individually, collectively, or in combination with each other. 
     It is to be understood that the terminology used herein is used for the purpose of describing specific embodiments, and is not intended to limit the scope of the present invention. It should be noted that as used herein, the singular forms of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. In addition, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     While preferable embodiments of the present invention are shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby. 
     I. Overview 
     A closed thermodynamic cycle power generation or energy storage system, such as a reversible Brayton cycle system, may use a generator/motor connected to a turbine and a compressor which act on a working fluid circulating in the system. Examples of working fluids include air, argon, carbon dioxide, or gaseous mixtures. A closed thermodynamic cycle power generation or energy storage system, such as a reversible Brayton cycle system, may have a hot side and/or a cold side. Each side may include a heat exchanger coupled to one or more cold storage containers and/or one or more hot storage containers. Preferably, the heat exchangers may be arranged as counterflow heat exchangers for higher thermal efficiency. Liquid thermal storage medium be utilized and may include, for example, liquids that are stable at high temperatures, such as molten nitrate salt or solar salt, or liquids that are stable at low temperatures, such as glycols or alkanes such as hexane. For an example in a molten salt and hexane system, the hot side molten salt may include a hot storage at approximately 565° C. and a cold storage at approximately 290° C. and the cold side hexane may include a hot storage at approximately 35° C. and a cold storage at approximately −60° C. 
     Large power generation systems may be slow to ramp up to full power generation. In addition, out-of-phase transient spikes in generated power in such large systems may be disruptive. Instead, it may be desirable to utilize a plurality of power subunits, each power subunit generating a portion of the total maximum power. Each power subunit may include a valve arrangement configurable to be in a connected state or an isolated state with respect to a shared hot side thermal store and a shared cold side thermal store (i.e., to be in use or not in use). 
     II. Illustrative Reversible Heat Engine 
     While various embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions may occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed. It shall be understood that different aspects of the invention can be appreciated individually, collectively, or in combination with each other. 
     It is to be understood that the terminology used herein is used for the purpose of describing specific embodiments, and is not intended to limit the scope of the present invention. It should be noted that as used herein, the singular forms of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. In addition, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     While preferable embodiments of the present invention are shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions will now occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby. 
     The term “reversible,” as used herein, generally refers to a process or operation that can be reversed via infinitesimal changes in some property of the process or operation without substantial entropy production (e.g., dissipation of energy). A reversible process may be approximated by a process that is at thermodynamic equilibrium. In some examples, in a reversible process, the direction of flow of energy is reversible. As an alternative, or in addition to, the general direction of operation of a reversible process (e.g., the direction of fluid flow) can be reversed, such as, e.g., from clockwise to counterclockwise, and vice versa. 
     The term “sequence,” as used herein, generally refers to elements (e.g., unit operations) in order. Such order can refer to process order, such as, for example, the order in which a fluid flows from one element to another. In an example, a compressor, heat storage unit and turbine in sequence includes the compressor upstream of the heat exchange unit, and the heat exchange unit upstream of the turbine. In such a case, a fluid can flow from the compressor to the heat exchange unit and from the heat exchange unit to the turbine. A fluid flowing through unit operations in sequence can flow through the unit operations sequentially. A sequence of elements can include one or more intervening elements. For example, a system comprising a compressor, heat storage unit and turbine in sequence can include an auxiliary tank between the compressor and the heat storage unit. A sequence of elements can be cyclical. 
     Pumped Thermal Systems 
     The disclosure provides pumped thermal systems capable of storing electrical energy and/or heat, and releasing energy (e.g., producing electricity) at a later time. The pumped thermal systems of the disclosure may include a heat engine, and a heat pump (or refrigerator). In some cases, the heat engine can be operated in reverse as a heat pump. In some cases, the heat engine can be operated in reverse as a refrigerator. Any description of heat pump/heat engine systems or refrigerator/heat engine systems capable of reverse operation herein may also be applied to systems comprising separate and/or a combination of separate and reverse-operable heat engine system(s), heat pump system(s) and/or refrigerator system(s). Further, as heat pumps and refrigerators share the same operating principles (albeit with differing objectives), any description of configurations or operation of heat pumps herein may also be applied to configurations or operation of refrigerators, and vice versa. 
     Systems of the present disclosure can operate as heat engines or heat pumps (or refrigerators). In some situations, systems of the disclosure can alternately operate as heat engines and heat pumps. In some examples, a system can operate as a heat engine to generate power, and subsequently operate as a heat pump to store energy, or vice versa. Such systems can alternately and sequentially operate as heat engines as heat pumps. In some cases, such systems reversibly or substantially reversibly operate as heat engines as heat pumps. 
     Reference will now be made to the figures, wherein like numerals refer to like parts throughout. It will be appreciated that the figures and features therein are not necessarily drawn to scale. 
       FIG.  1    schematically illustrates operating principles of pumped thermal electric storage using a heat pump/heat engine electricity storage system. Electricity may be stored in the form of thermal energy of two materials or media at different temperatures (e.g., thermal energy reservoirs comprising heat storage fluids or thermal storage media) by using a combined heat pump/heat engine system. In a charging or heat pump mode, work may be consumed by the system for transferring heat from a cold material or medium to a hot material or medium, thus lowering the temperature (e.g., sensible energy) of the cold material and increasing the temperature (i.e., sensible energy) of the hot material. In a discharging or heat engine mode, work may be produced by the system by transferring heat from the hot material to the cold material, thus lowering the temperature (i.e., sensible energy) of the hot material and increasing the temperature (i.e., sensible energy) of the cold material. The system may be configured to ensure that the work produced by the system on discharge is a favorable fraction of the energy consumed on charge. The system may be configured to achieve high roundtrip efficiency, defined herein as the work produced by the system on discharge divided by the work consumed by the system on charge. Further, the system may be configured to achieve the high roundtrip efficiency using components of a desired (e.g., acceptably low) cost. Arrows H and W in  FIG.  1    represent directions of heat flow and work, respectively. 
     Heat engines, heat pumps and refrigerators of the disclosure may involve a working fluid to and from which heat is transferred while undergoing a thermodynamic cycle. The heat engines, heat pumps and refrigerators of the disclosure may operate in a closed cycle. Closed cycles allow, for example, a broader selection of working fluids, operation at elevated cold side pressures, operation at lower cold side temperatures, improved efficiency, and reduced risk of turbine damage. One or more aspects of the disclosure described in relation to systems having working fluids undergoing closed cycles may also be applied to systems having working fluids undergoing open cycles. 
     In one example, the heat engines may operate on a Brayton cycle and the heat pumps/refrigerators may operate on a reverse Brayton cycle (also known as a gas refrigeration cycle). Other examples of thermodynamic cycles that the working fluid may undergo or approximate include the Rankine cycle, the ideal vapor-compression refrigeration cycle, the Stirling cycle, the Ericsson cycle or any other cycle advantageously employed in concert with heat exchange with heat storage fluids of the disclosure. 
     The working fluid can undergo a thermodynamic cycle operating at one, two or more pressure levels. For example, the working fluid may operate in a closed cycle between a low pressure limit on a cold side of the system and a high pressure limit on a hot side of the system. In some implementations, a low pressure limit of about 10 atmospheres (atm) or greater can be used. In some instances, the low pressure limit may be at least about 1 atm, at least about 2 atm, at least about 5 atm, at least about 10 atm, at least about 15 atm, at least about 20 atm, at least about 30 atm, at least about 40 atm, at least about 60 atm, at least about 80 atm, at least about 100 atm, at least about 120 atm, at least about 160 atm, or at least about 200 atm, 500 atm, 1000 atm, or more. In some instances, a sub-atmospheric low pressure limit may be used. For example, the low pressure limit may be less than about 0.1 atm, less than about 0.2 atm, less than about 0.5 atm, or less than about 1 atm. In some instances, the low pressure limit may be about 1 atmosphere (atm). In the case of a working fluid operating in an open cycle, the low pressure limit may be about 1 atm or equal to ambient pressure. 
     In some cases, the value of the low pressure limit may be selected based on desired power output and/or power input requirements of the thermodynamic cycle. For example, a pumped thermal system with a low pressure limit of about 10 atm may be able to provide a power output comparable to an industrial gas turbine with ambient (1 atm) air intake. The value of the low pressure limit may also be subject to cost/safety tradeoffs. Further, the value of the low pressure limit may be limited by the value of the high pressure limit, the operating ranges of the hot side and cold side heat storage media (e.g., pressure and temperature ranges over which the heat storage media are stable), pressure ratios and operating conditions (e.g., operating limits, optimal operating conditions, pressure drop) achievable by turbomachinery and/or other system components, or any combination thereof. The high pressure limit may be determined in accordance with these system constraints. In some instances, higher values of the high pressure limit may lead to improved heat transfer between the working fluid and the hot side storage medium. 
     Working fluids used in pumped thermal systems may include air, argon, other noble gases, carbon dioxide, hydrogen, oxygen, or any combination thereof, and/or other fluids in gaseous, liquid, critical, or supercritical state (e.g., supercritical CO2). The working fluid can be a gas or a low viscosity liquid (e.g., viscosity below about 500×10 −6  Poise at 1 atm), satisfying the requirement that the flow be continual. In some implementations, a gas with a high specific heat ratio may be used to achieve higher cycle efficiency than a gas with a low specific heat ratio. For example, argon (e.g., specific heat ratio of about 1.66) may be used to substitute air (e.g., specific heat ratio of about 1.4). In some cases, the working fluid may be a blend of one, two, three or more fluids. In one example, helium (having a high thermal conductivity and a high specific heat) may be added to the working fluid (e.g., argon) to improve heat transfer rates in heat exchangers. 
     Pumped thermal systems herein may utilize heat storage media or materials, such as one or more heat storage fluids. The heat storage media can be gases or low viscosity liquids, satisfying the requirement that the flow be continual. The systems may utilize a first heat storage medium on a hot side of the system (“hot side thermal storage (HTS) medium” or “HTS” herein) and a second heat storage medium on a cold side of the system (“cold side thermal storage (CTS) medium” or “CTS” herein). The thermal storage media (e.g., low viscosity liquids) can have high heat capacities per unit volume (e.g., heat capacities above about 1400 Joule (kilogram Kelvin) −1 ) and high thermal conductivities (e.g., thermal conductivities above about 0.7 Watt (meter Kelvin) −1 ). In some implementations, several different thermal storage media (also “heat storage media” herein) on either the hot side, cold side or both the hot side and the cold side may be used. 
     The operating temperatures of the hot side thermal storage medium can be in the liquid range of the hot side thermal storage medium, and the operating temperatures of the cold side thermal storage medium can be in the liquid range of the cold side thermal storage medium. In some examples, liquids may enable a more rapid exchange of large amounts of heat by convective counter-flow than solids or gases. Thus, in some cases, liquid HTS and CTS media may advantageously be used. Pumped thermal systems utilizing thermal storage media herein may advantageously provide a safe, non-toxic and geography-independent energy (e.g., electricity) storage alternative. 
     In some implementations, the hot side thermal storage medium can be a molten salt or a mixture of molten salts. Any salt or salt mixture that is liquid over the operating temperature range of the hot side thermal storage medium may be employed. Molten salts can provide numerous advantages as thermal energy storage media, such as low vapor pressure, lack of toxicity, chemical stability, low chemical reactivity with typical steels (e.g., melting point below the creep temperature of steels, low corrosiveness, low capacity to dissolve iron and nickel), and low cost. In one example, the HTS is a mixture of sodium nitrate and potassium nitrate. In some examples, the HTS is a eutectic mixture of sodium nitrate and potassium nitrate. In some examples, the HTS is a mixture of sodium nitrate and potassium nitrate having a lowered melting point than the individual constituents, an increased boiling point than the individual constituents, or a combination thereof. Other examples include potassium nitrate, calcium nitrate, sodium nitrate, sodium nitrite, lithium nitrate, mineral oil, or any combination thereof. Further examples include any gaseous (including compressed gases), liquid or solid media (e.g., powdered solids) having suitable (e.g., high) thermal storage capacities and/or capable of achieving suitable (e.g., high) heat transfer rates with the working fluid. For example, a mix of 60% sodium nitrate and 40% potassium nitrate (also referred to as a solar salt in some situations) can have a heat capacity of approximately 1500 Joule (Kelvin mole) −1  and a thermal conductivity of approximately 0.75 Watt (meter Kelvin) −1  within a temperature range of interest. The hot side thermal storage medium may be operated in a temperature range that structural steels can handle. 
     In some cases, liquid water at temperatures of about 0° C. to 100° C. (about 273 K-373 K) and a pressure of about 1 atm may be used as the cold side thermal storage medium. Due to a possible explosion hazard associated with presence of steam at or near the boiling point of water, the operating temperature can be kept below about 100° C. or less while maintaining an operating pressure of 1 atm (i.e., no pressurization). In some cases, the temperature operating range of the cold side thermal storage medium may be extended (e.g., to −30° C. to 100° C. at 1 atm) by using a mixture of water and one or more antifreeze compounds (e.g., ethylene glycol, propylene glycol, or glycerol). 
     As described in greater detail elsewhere herein, improved storage efficiency may be achieved by increasing the temperature difference at which the system operates, for example, by using a cold side heat storage fluid capable of operating at lower temperatures. In some examples, the cold side thermal storage media may comprise hydrocarbons, such as, for example, alkanes (e.g., hexane or heptane), alkenes, alkynes, aldehydes, ketones, carboxylic acids (e.g., HCOOH), ethers, cycloalkanes, aromatic hydrocarbons, alcohols (e.g., butanol), other type(s) of hydrocarbon molecules, or any combinations thereof. In some cases, the cold side thermal storage medium can be hexane (e.g., n-hexane). Hexane has a wide liquid range and can remain fluid (i.e., runny) over its entire liquid range (−94° C. to 68° C. at 1 atm). Hexane&#39;s low temperature properties are aided by its immiscibility with water. Other liquids, such as, for example, ethanol or methanol can become viscous at the low temperature ends of their liquid ranges due to pre-crystallization of water absorbed from air. In some cases, the cold side thermal storage medium can be heptane (e.g., n-heptane). Heptane has a wide liquid range and can remain fluid (i.e., runny) over its entire liquid range (−91° C. to 98° C. at 1 atm). Heptane&#39;s low temperature properties are aided by its immiscibility with water. At even lower temperatures, other heat storage media can be used, such as, for example, isohexane (2-methylpentane). In some examples, cryogenic liquids having boiling points below about −150° C. (123 K) or about −180° C. (93.15 K) may be used as cold side thermal storage media (e.g., propane, butane, pentane, nitrogen, helium, neon, argon and krypton, air, hydrogen, methane, or liquefied natural gas). In some implementations, choice of cold side thermal storage medium may be limited by the choice of working fluid. For example, when a gaseous working fluid is used, a liquid cold side thermal storage medium having a liquid temperature range at least partially or substantially above the boiling point of the working fluid may be required. 
     In some cases, the operating temperature range of CTS and/or HTS media can be changed by pressurizing (i.e., raising the pressure) or evacuating (i.e., lowering the pressure) the tanks and thus changing the temperature at which the storage media undergo phase transitions (e.g., going from liquid to solid, or from liquid to gas). 
     In some cases, the hot side and the cold side heat storage fluids of the pumped thermal systems are in a liquid state over at least a portion of the operating temperature range of the energy storage device. The hot side heat storage fluid may be liquid within a given range of temperatures. Similarly, the cold side heat storage fluid may be liquid within a given range of temperatures. The heat storage fluids may be heated, cooled or maintained to achieve a suitable operating temperature prior to, during or after operation. 
     Pumped thermal systems of the disclosure may cycle between charged and discharged modes. In some examples, the pumped thermal systems can be fully charged, partially charged or partially discharged, or fully discharged. In some cases, cold side heat storage may be charged (also “recharged” herein) independently from hot side heat storage. Further, in some implementations, charging (or some portion thereof) and discharging (or some portion thereof) can occur simultaneously. For example, a first portion of a hot side heat storage may be recharged while a second portion of the hot side heat storage together with a cold side heat storage are being discharged. 
     The pumped thermal systems may be capable of storing energy for a given amount of time. In some cases, a given amount of energy may be stored for at least about 1 second, at least about 30 seconds, at least about 1 minute, at least about 5 minutes, at least about 30 minutes, at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, at least about 6 hours, at least about 7 hours, at least about 8 hours, at least about 9 hours, at least about 10 hours, at least about 12 hours at least about 14 hours, at least about 16 hours, at least about 18 hours, at least about 20 hours, at least about 22 hours, at least about 24 hours (1 day), at least about 2 days, at least about 4 days, at least about 6 days, at least about 8 days, at least about 10 days, 20 days, 30 days, 60 days, 100 days, 1 year or more. 
     Pumped thermal systems of the disclosure may be capable of storing/receiving input of, and/or extracting/providing output of a substantially large amount of energy and/or power for use with power generation systems (e.g., intermittent power generation systems such as wind power or solar power), power distribution systems (e.g. electrical grid), and/or other loads or uses in grid-scale or stand-alone settings. During a charge mode of a pumped thermal system, electric power received from an external power source (e.g., a wind power system, a solar photovoltaic power system, an electrical grid etc.) can be used operate the pumped thermal system in a heat pump mode (i.e., transferring heat from a low temperature reservoir to a high temperature reservoir, thus storing energy). During a discharge mode of the pumped thermal system, the system can supply electric power to an external power system or load (e.g., one or more electrical grids connected to one or more loads, a load, such as a factory or a power-intensive process, etc.) by operating in a heat engine mode (i.e., transferring heat from a high temperature reservoir to a low temperature reservoir, thus extracting energy). As described elsewhere herein, during charge and/or discharge, the system may receive or reject thermal power, including, but not limited to electromagnetic power (e.g., solar radiation) and thermal power (e.g., sensible energy from a medium heated by solar radiation, heat of combustion etc.). 
     In some implementations, the pumped thermal systems are grid-synchronous. Synchronization can be achieved by matching speed and frequency of motors/generators and/or turbomachinery of a system with the frequency of one or more grid networks with which the system exchanges power. For example, a compressor and a turbine can rotate at a given, fixed speed (e.g., 3600 revolutions per minute (rpm)) that is a multiple of grid frequency (e.g., 60 hertz (Hz)). In some cases, such a configuration may eliminate the need for additional power electronics. In some implementations, the turbomachinery and/or the motors/generators are not grid synchronous. In such cases, frequency matching can be accomplished through the use of power electronics. In some implementations, the turbomachinery and/or the motors/generators are not directly grid synchronous but can be matched through the use of gears and/or a mechanical gearbox. As described in greater detail elsewhere herein, the pumped thermal systems may also be rampable. Such capabilities may enable these grid-scale energy storage systems to operate as peaking power plants and/or as a load following power plants. In some cases, the systems of the disclosure may be capable of operating as base load power plants. 
     Pumped thermal systems can have a given power capacity. In some cases, power capacity during charge may differ from power capacity during discharge. For example, each system can have a charge and/or discharge power capacity of less than about 1 megawatt (MW), at least about 1 megawatt, at least about 2 MW, at least about 3 MW, at least about 4 MW, at least about 5 MW, at least about 6 MW, at least about 7 MW, at least about 8 MW, at least about 9 MW, at least about 10 MW, at least about 20 MW, at least about 30 MW, at least about 40 MW, at least about 50 MW, at least about 75 MW, at least about 100 MW, at least about 200 MW, at least about 500 MW, at least about 1 gigawatt (GW), at least about 2 GW, at least about 5 GW, at least about 10 GW, at least about 20 GW, at least about 30 GW, at least about 40 GW, at least about 50 GW, at least about 75 GW, at least about 100 GW, or more. 
     Pumped thermal systems can have a given energy storage capacity. In one example, a pumped thermal system is configured as a 100 MW unit operating for 10 hours. In another example, a pumped thermal system is configured as a 1 GW plant operating for 12 hours. In some instances, the energy storage capacity can be less than about 1 megawatt hour (MWh), at least about 1 megawatt hour, at least about 10 MWh, at least about 100 MWh, at least about 1 gigawatt hour (GWh), at least about 5 GWh, at least about 10 GWh, at least about 20 GWh, at least 50 GWh, at least about 100 GWh, at least about 200 GWh, at least about 500 GWh, at least about 700 GWh, at least about 1000 GWh, or more. 
     In some cases, a given power capacity may be achieved with a given size, configuration and/or operating conditions of the heat engine/heat pump cycle. For example, size of turbomachinery, ducts, heat exchangers, or other system components may correspond to a given power capacity. 
     In some implementations, a given energy storage capacity may be achieved with a given size and/or number of hot side thermal storage tanks and/or cold side thermal storage tanks. For example, the heat engine/heat pump cycle can operate at a given power capacity for a given amount of time set by the heat storage capacity of the system or plant. The number and/or heat storage capacity of the hot side thermal storage tanks may be different from the number and/or heat storage capacity of the cold side thermal storage tanks. The number of tanks may depend on the size of individual tanks. The size of hot side storage tanks may differ from the size of cold side thermal storage tanks. In some cases, the hot side thermal storage tanks, the hot side heat exchanger and the hot side thermal storage medium may be referred to as a hot side heat (thermal) storage unit. In some cases, the cold side thermal storage tanks, the cold side heat exchanger and the cold side thermal storage medium may be referred to as a cold side heat (thermal) storage unit. 
     A pumped thermal storage facility can include any suitable number of hot side storage tanks, such as at least about 2, at least about 4, at least about 10, at least about 50, at least about 100, at least about 500, at least about 1,000, at least about 5,000, at least about 10,000, and the like. In some examples, a pumped thermal storage facility includes 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1,000 or more hot side tanks. 
     A pumped thermal storage facility can also include any suitable number of cold side storage tanks, such as at least about 2, at least about 4, at least about 10, at least about 50, at least about 100, at least about 500, at least about 1,000, at least about 5,000, at least about 10,000, and the like. In some examples, a pumped thermal storage facility includes 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900, 1,000 or more cold side tanks. 
     Pumped Thermal Storage Cycles 
     An aspect of the disclosure relates to pumped thermal systems operating on pumped thermal storage cycles. In some examples, the cycles allow electricity to be stored as heat (e.g., in the form of a temperature differential) and then converted back to electricity through the use of at least two pieces of turbomachinery, a compressor and a turbine. The compressor consumes work and raises the temperature and pressure of a working fluid (WF). The turbine produces work and lowers the temperature and pressure of the working fluid. In some examples, more than one compressor and more than one turbine is used. In some cases, the system can include at least 1, at least 2, at least 3, at least 4, or at least 5 compressors. In some cases, the system can include at least 1, at least 2, at least 3, at least 4, or at least 5 turbines. The compressors may be arranged in series or in parallel. The turbines may be arranged in series or in parallel. 
       FIGS.  2  and  3    are schematic flow diagrams of working fluid and heat storage media of an exemplary pumped thermal system in a charge/heat pump mode and in a discharge/heat engine mode, respectively. The system may be idealized for simplicity of explanation so that there are no losses (i.e., entropy generation) in either the turbomachinery or heat exchangers. The system can include a working fluid  20  (e.g., argon gas) flowing in a closed cycle between a compressor  1 , a hot side heat exchanger  2 , a turbine  3  and a cold side heat exchanger  4 . Fluid flow paths/directions for the working fluid  20  (e.g., a gas), a hot side thermal storage (HTS) medium  21  (e.g., a low viscosity liquid) and a cold side thermal storage (CTS) medium  22  (e.g., a low viscosity liquid) are indicated by arrows. 
       FIGS.  4  and  5    are schematic pressure and temperature diagrams of the working fluid  20  as it undergoes the charge cycles in  FIGS.  2  and  3   , respectively, once again simplified in the approximation of no entropy generation. Normalized pressure is shown on the y-axes and temperature is shown on the x-axes. The direction of processes taking place during the cycles is indicated with arrows, and the individual processes taking place in the compressor  1 , the hot side CFX  2 , the turbine  3  and the cold side CFX  4  are indicated on the diagram with their respective numerals. 
     The heat exchangers  2  and  4  can be configured as counter-flow heat exchangers (CFXs), where the working fluid flows in one direction and the substance it is exchanging heat with is flowing in the opposite direction. In an ideal counter-flow heat exchanger with correctly matched flows (i.e., balanced capacities or capacity flow rates), the temperatures of the working fluid and thermal storage medium flip (i.e., the counter-flow heat exchanger can have unity effectiveness). 
     The counter-flow heat exchangers  2  and  4  can be designed and/or operated to reduce entropy generation in the heat exchangers to negligible levels compared to entropy generation associated with other system components and/or processes (e.g., compressor and/or turbine entropy generation). In some cases, the system may be operated such that entropy generation in the system is minimized. For example, the system may be operated such that entropy generation associated with heat storage units is minimized. In some cases, a temperature difference between fluid elements exchanging heat can be controlled during operation such that entropy generation in hot side and cold side heat storage units is minimized. In some instances, the entropy generated in the hot side and cold side heat storage units is negligible when compared to the entropy generated by the compressor, the turbine, or both the compressor and the turbine. In some instances, entropy generation associated with heat transfer in the heat exchangers  2  and  4  and/or entropy generation associated with operation of the hot side storage unit, the cold side storage unit or both the hot side and cold side storage units can be less than about 50%, less than about 25%, less than about 20%, less than about 15%, less than about 10%, less than about 5%, less than about 4%, less than about 3%, less than about 2%, or less than about 1% of the total entropy generated within the system (e.g., entropy generated by the compressor  1 , the hot side heat exchanger  2 , the turbine  3 , the cold side heat exchanger  4  and/or other components described herein, such as, for example, a recuperator). For example, entropy generation can be reduced or minimized if the two substances exchanging heat do so at a local temperature differential ΔT→0 (i.e., when the temperature difference between any two fluid elements that are in close thermal contact in the heat exchanger is small). In some examples, the temperature differential ΔT between any two fluid elements that are in close thermal contact may be less than about 300 Kelvin (K), less than about 200 K, less than about 100 K, less than about 75 K, less than about 50 K, less than about 40 K, less than about 30 K, less than about 20 K, less than about 10 K, less than about 5 K, less than about 3 K, less than about 2 K, or less than about 1 K. In another example, entropy generation associated with pressure drop can be reduced or minimized by suitable design. In some examples, the heat exchange process can take place at a constant or near-constant pressure. Alternatively, a non-negligible pressure drop may be experienced by the working fluid and/or one or more thermal storage media during passage through a heat exchanger. Pressure drop in heat exchangers may be controlled (e.g., reduced or minimized) through suitable heat exchanger design. In some examples, the pressure drop across each heat exchanger may be less than about 20% of inlet pressure, less than about 10% of inlet pressure, less than about 5% of inlet pressure, less than about 3% of inlet pressure, less than about 2% of inlet pressure, less than about 1% of inlet pressure, less than about 0.5% of inlet pressure, less than about 0.25% of inlet pressure, or less than about 0.1% of inlet pressure. 
     Upon entering the heat exchanger  2 , the temperature of the working fluid can either increase (taking heat from the HTS medium  21 , corresponding to the discharge mode in  FIGS.  3  and  5   ) or decrease (giving heat to the HTS medium  21 , corresponding to the charge mode in  FIGS.  2  and  4   ), depending on the temperature of the HTS medium in the heat exchanger relative the temperature of the working fluid. Similarly, upon entering the heat exchanger  4 , the temperature of the working fluid can either increase (taking heat from the CTS medium  22 , corresponding to the charge mode in  FIGS.  2  and  4   ) or decrease (giving heat to the CTS medium  22 , corresponding to the discharge mode in  FIGS.  3  and  5   ), depending on the temperature of the CTS medium in the heat exchanger relative the temperature of the working fluid. 
     As described in more detail with reference to the charge mode in  FIGS.  2  and  4   , the heat addition process in the cold side CFX  4  can take place over a different range of temperatures than the heat removal process in the hot side CFX  2 . Similarly, in the discharge mode in  FIGS.  3  and  5   , the heat rejection process in the cold side CFX  4  can take place over a different range of temperatures than the heat addition process in the hot side CFX  2 . At least a portion of the temperature ranges of the hot side and cold side heat exchange processes may overlap during charge, during discharge, or during both charge and discharge. 
     As used herein, the temperatures T 0 , T 1 , T 0   +  and T 1   +  are so named because T 0   + , T 1   +  are the temperatures achieved at the exit of a compressor with a given compression ratio r, adiabatic efficiency η c  and inlet temperatures of T 0 , T 1  respectively. The examples in  FIGS.  2 ,  3 ,  4  and  5    can be idealized examples where η c =1 and where adiabatic efficiency of the turbine η t  also has the value η t =1. 
     With reference to the charge mode shown in  FIGS.  2  and  4   , the working fluid  20  can enter the compressor  1  at position  30  at a pressure P and a temperature T (e.g., at T 1 , P 2 ). As the working fluid passes through the compressor, work W 1  is consumed by the compressor to increase the pressure and temperature of the working fluid (e.g., to T 1   +  P 1 ), as indicated by P⬆ and T⬆ at position  31 . In the charge mode, the temperature T 1   +  of the working fluid exiting the compressor and entering the hot side CFX  2  at position  31  is higher than the temperature of the HTS medium  21  entering the hot side CFX  2  at position  32  from a second hot side thermal storage tank  7  at a temperature T 0   +  (i.e., T 0   + &lt;T 1   + ). As these two fluids pass in thermal contact with each other in the heat exchanger, the working fluid&#39;s temperature decreases as it moves from position  31  position  34 , giving off heat Q 1  to the HTS medium, while the temperature of the HTS medium in turn increases as it moves from position  32  to position  33 , absorbing heat Q 1  from the working fluid. In an example, the working fluid exits the hot side CFX  2  at position  34  at the temperature T 0   +  and the HTS medium exits the hot side CFX  2  at position  33  into a first hot side thermal storage tank  6  at the temperature T 1   + . The heat exchange process can take place at a constant or near-constant pressure such that the working fluid exits the hot side CFX  2  at position  34  at a lower temperature but same pressure P 1 , as indicated by P and T⬇ at position  34 . Similarly, the temperature of the HTS medium  21  increases in the hot side CFX  2 , while its pressure can remain constant or near-constant. 
     Upon exiting the hot side CFX  2  at position  34  (e.g., at T 0   + , P 1 ), the working fluid  20  undergoes expansion in the turbine  3  before exiting the turbine at position  35 . During the expansion, the pressure and temperature of the working fluid decrease (e.g., to T 0 , P 2 ), as indicated by P⬇ and T⬇ at position  35 . The magnitude of work W 2  generated by the turbine depends on the enthalpy of the working fluid entering the turbine and the degree of expansion. In the charge mode, heat is removed from the working fluid between positions  31  and  34  (in the hot side CFX  2 ) and the working fluid is expanded back to the pressure at which it initially entered the compressor at position  30  (e.g., P 2 ). The compression ratio (e.g., P 1 /P 2 ) in the compressor  1  being equal to the expansion ratio in the turbine  3 , and the enthalpy of the gas entering the turbine being lower than the enthalpy of the gas exiting the compressor, the work W 2  generated by the turbine  3  is smaller than the work W 1  consumed by the compressor  1  (i.e., W 2 &lt;W 1 ). 
     Because heat was taken out of the working fluid in the hot side CFX  2 , the temperature T 0  at which the working fluid exits the turbine at position  35  is lower than the temperature T 1  at which the working fluid initially entered the compressor at position  30 . To close the cycle (i.e., to return the pressure and temperature of the working fluid to their initial values T 1 , P 2  at position  30 ), heat Q 2  is added to the working fluid from the CTS medium  22  in the cold side CFX  4  between positions  35  and  30  (i.e., between the turbine  3  and the compressor  1 ). In an example, the CTS medium  22  enters the cold side CFX  4  at position  36  from a first cold side thermal storage tank  8  at the temperature T 1  and exits the cold side CFX  4  at position  37  into a second cold side thermal storage tank  9  at the temperature T 0 , while the working fluid  20  enters the cold side CFX  4  at position  35  at the temperature T 0  and exits the cold side CFX  4  at position  30  at the temperature T 1 . Again, the heat exchange process can take place at a constant or near-constant pressure such that the working fluid exits the cold side CFX  2  at position  30  at a higher temperature but same pressure P 2 , as indicated by P and T⬆ at position  30 . Similarly, the temperature of the CTS medium  22  decreases in the cold side CFX  2 , while its pressure can remain constant or near-constant. 
     During charge, the heat Q 2  is removed from the CTS medium and the heat Q 1  is added to the HTS medium, wherein Q 1 &gt;Q 2 . A net amount of work W 1 -W 2  is consumed, since the work W 1  used by the compressor is greater than the work W 2  generated by the turbine. A device that consumes work while moving heat from a cold body or thermal storage medium to a hot body or thermal storage medium is a heat pump; thus, the pumped thermal system in the charge mode operates as a heat pump. 
     In an example, the discharge mode shown in  FIGS.  3  and  5    can differ from the charge mode shown in  FIGS.  2  and  4    in the temperatures of the thermal storage media being introduced into the heat exchangers. The temperature at which the HTS medium enters the hot side CFX  2  at position  32  is T 1   +  instead of T 0   + , and the temperature of the CTS medium entering the cold side CFX  4  at position  36  is T 0  instead of T 1 . During discharge, the working fluid enters the compressor at position  30  at T 0  and P 2 , exits the compressor at position  31  at T 0   + &lt;T 1   +  and P 1 , absorbs heat from the HTS medium in the hot side CFX  2 , enters the turbine  3  at position  34  at T 1   +  and P 1 , exits the turbine at position  35  at T 1 &gt;T 0  and P 2 , and finally rejects heat to the CTS medium in the cold side CFX  4 , returning to its initial state at position  30  at T 0  and P 2 . 
     The HTS medium at temperature T 1   +  can be stored in a first hot side thermal storage tank  6 , the HTS medium at temperature T 0   +  can be stored in a second hot side thermal storage tank  7 , the CTS medium at temperature T 1  can be stored in a first cold side thermal storage tank  8 , and the CTS medium at temperature T 0  can be stored in a second cold side thermal storage tank  9  during both charge and discharge modes. In one implementation, the inlet temperature of the HTS medium at position  32  can be switched between T 1   +  and T 0   +  by switching between tanks  6  and  7 , respectively. Similarly, the inlet temperature of the CTS medium at position  36  can be switched between T 1  and T 0  by switching between tanks  8  and  9 , respectively. Switching between tanks can be achieved by including a valve or a system of valves (e.g., valve systems  12  and  13  in  FIG.  7   ) for switching connections between the hot side heat exchanger  2  and the hot side tanks  6  and  7 , and/or between the cold side heat exchanger  4  and the cold side tanks  8  and  9  as needed for the charge and discharge modes. In some implementations, connections may be switched on the working fluid side instead, while the connections of storage tanks  6 ,  7 ,  8  and  9  to the heat exchangers  2  and  4  remain static. In some examples, flow paths and connections to the heat exchangers may depend on the design (e.g., shell-and-tube) of each heat exchanger. In some implementations, one or more valves can be used to switch the direction of both the working fluid and the heat storage medium through the counter-flow heat exchanger on charge and discharge. Such configurations may be used, for example, due to high thermal storage capacities of the heat exchanger component, to decrease or eliminate temperature transients, or a combination thereof. In some implementations, one or more valves can be used to switch the direction of only the working fluid, while the direction of the HTS or CTS can be changed by changing the direction of pumping, thereby maintaining the counter-flow configuration. In some implementations, different valve configurations may be used for the HTS and the CTS. Further, any combination of the valve configurations herein may be used. For example, the system may be configured to operate using different valve configurations in different situations (e.g., depending on system operating conditions). 
     In the discharge mode shown in  FIGS.  3  and  5   , the working fluid  20  can enter the compressor  1  at position  30  at a pressure P and a temperature T (e.g., at T 0 , P 2 ). As the working fluid passes through the compressor, work W 1  is consumed by the compressor to increase the pressure and temperature of the working fluid (e.g., to T 0   + , P 1 ), as indicated by P⬆ and T⬆ at position  31 . In the discharge mode, the temperature T 0   +  of the working fluid exiting the compressor and entering the hot side CFX  2  at position  31  is lower than the temperature of the HTS medium  21  entering the hot side CFX  2  at position  32  from a first hot side thermal storage tank  6  at a temperature T 1   +  (i.e., T 0   + &lt;T 1   + ). As these two fluids pass in thermal contact with each other in the heat exchanger, the working fluid&#39;s temperature increases as it moves from position  31  position  34 , absorbing heat Q 1  from the HTS medium, while the temperature of the HTS medium in turn decreases as it moves from position  32  to position  33 , giving off heat Q 1  to the working fluid. In an example, the working fluid exits the hot side CFX  2  at position  34  at the temperature T 1   +  and the HTS medium exits the hot side CFX  2  at position  33  into the second hot side thermal storage tank  7  at the temperature T 0   +  The heat exchange process can take place at a constant or near-constant pressure such that the working fluid exits the hot side CFX  2  at position  34  at a higher temperature but same pressure P 1 , as indicated by P and T⬆ at position  34 . Similarly, the temperature of the HTS medium  21  decreases in the hot side CFX  2 , while its pressure can remain constant or near-constant. 
     Upon exiting the hot side CFX  2  at position  34  (e.g., at T 1   + , P 1 ), the working fluid  20  undergoes expansion in the turbine  3  before exiting the turbine at position  35 . During the expansion, the pressure and temperature of the working fluid decrease (e.g., to T 1 , P 2 ), as indicated by P⬇, and T⬇, at position  35 . The magnitude of work W 2  generated by the turbine depends on the enthalpy of the working fluid entering the turbine and the degree of expansion. In the discharge mode, heat is added to the working fluid between positions  31  and  34  (in the hot side CFX  2 ) and the working fluid is expanded back to the pressure at which it initially entered the compressor at position  30  (e.g., P 2 ). The compression ratio (e.g., P 1 /P 2 ) in the compressor  1  being equal to the expansion ratio in the turbine  3 , and the enthalpy of the gas entering the turbine being higher than the enthalpy of the gas exiting the compressor, the work W 2  generated by the turbine  3  is greater than the work W 1  consumed by the compressor  1  (i.e., W 2 &gt;W 1 ). 
     Because heat was added to the working fluid in the hot side CFX  2 , the temperature T 1  at which the working fluid exits the turbine at position  35  is higher than the temperature T 0  at which the working fluid initially entered the compressor at position  30 . To close the cycle (i.e., to return the pressure and temperature of the working fluid to their initial values T 0 , P 2  at position  30 ), heat Q 2  is rejected by the working fluid to the CTS medium  22  in the cold side CFX  4  between positions  35  and  30  (i.e., between the turbine  3  and the compressor  1 ). The CTS medium  22  enters the cold side CFX  4  at position  36  from a second cold side thermal storage tank  9  at the temperature T 0  and exits the cold side CFX  4  at position  37  into a first cold side thermal storage tank  8  at the temperature T 1 , while the working fluid  20  enters the cold side CFX  4  at position  35  at the temperature T 1  and exits the cold side CFX  4  at position  30  at the temperature T 0 . Again, the heat exchange process can take place at a constant or near-constant pressure such that the working fluid exits the cold side CFX  2  at position  30  at a higher temperature but same pressure P 2 , as indicated by P and T⬇ at position  30 . Similarly, the temperature of the CTS medium  22  increases in the cold side CFX  2 , while its pressure can remain constant or near-constant. 
     During discharge, the heat Q 2  is added to the CTS medium and the heat Q 1  is removed from the HTS medium, wherein Q 1 &gt;Q 2 . A net amount of work W 2 -W 1  is generated, since the work W 1  used by the compressor is smaller than the work W 2  generated by the turbine. A device that generates work while moving heat from a hot body or thermal storage medium to a cold body or thermal storage medium is a heat engine; thus, the pumped thermal system in the discharge mode operates as a heat engine. 
       FIG.  6    is a simplified schematic perspective view of a closed working fluid system in the pumped thermal system in  FIGS.  2 - 3   . As indicated, the working fluid  20  (contained inside tubing) circulates clockwise between the compressor  1 , the hot side heat exchanger  2 , the turbine  3 , and the cold side heat exchanger  4 . The compressor  1  and the turbine  3  can be ganged on a common mechanical shaft  10  such that they rotate together. In some implementations, the compressor  1  and the turbine  3  can have separate mechanical shafts. A motor/generator  11  (e.g., including a synchronous motor-synchronous generator converter on a single common shaft) provides power to and from the turbomachinery. In this example, the compressor, the turbine and the motor/generator are all located on a common shaft. Pipes at positions  32  and  33  transfer hot side thermal storage fluid to and from the hot side heat exchanger  2 , respectively. Pipes at positions  36  and  37  transfer cold side thermal storage fluid to and from the cold side heat exchanger  4 , respectively. 
     Although the system of  FIG.  6    is illustrated as comprising a compressor  1  and turbine  3 , the system can include one or more compressors and one or more turbines, which may operate, for example, in a parallel configuration, or alternatively in a series configuration or in a combination of parallel and series configurations. In some examples, a system of compressors or turbines may be assembled such that a given compression ratio is achieved. In some cases, different compression ratios (e.g., on charge and discharge) can be used (e.g., by connecting or disconnecting, in a parallel and/or series configuration, one or more compressors or turbines from the system of compressors or turbines). In some examples, the working fluid is directed to a plurality of compressors and/or a plurality of turbines. In some examples, the compressor and/or turbine may have temperature dependent compression ratios. Arrangement and/or operation of the turbomachinery and/or other elements of the system may be adjusted in accordance with the temperature dependence (e.g., to optimize performance). 
       FIG.  7    is a simplified schematic perspective view of the pumped thermal system in  FIGS.  2 - 3    with hot side and cold side storage tanks and a closed cycle working fluid system. In this example, the HTS medium is a molten salt and the CTS medium is a low temperature liquid. One, two or more first hot side tanks  6  (at the temperature T 1   + ) and one, two or more second hot side tanks  7  (at the temperature T 0   + ), both for holding the HTS medium, are in fluid communication with a valve  13  configured to transfer the HTS medium to and from the hot side heat exchanger  2 . One, two or more first cold side tanks  8  (at the temperature T 1 ) and one, two or more second cold side tanks  9  (at the temperature T 0 ), both for holding the CTS medium, are in fluid communication with a valve  12  configured to transfer the CTS medium to and from the cold side heat exchanger  4 . 
     The thermal energy reservoirs or storage tanks may be thermally insulated tanks that can hold a suitable quantity of the relevant thermal storage medium (e.g., heat storage fluid). The storage tanks may allow for relatively compact storage of large amounts of thermal energy. In an example, the hot side tanks  6  and/or  7  can have a diameter of about 80 meters, while the cold side tanks  8  and/or  9  can have a diameter of about 60 meters. In another example, the size of each (i.e., hot side or cold side) thermal storage for a 1 GW plant operating for 12 hours can be about 20 medium-sized oil refinery tanks. 
     In some implementations, a third set of tanks containing storage media at intermediate temperatures between the other tanks may be included on the hot side and/or the cold side. In an example, a third storage or transfer tank (or set of tanks) at a temperature intermediate to the temperatures of a first tank (or set of tanks) and a second tank (or set of tanks) may be provided. A set of valves may be provided for switching the storage media between the different tanks and heat exchangers. For example, thermal media may be directed to different sets of tanks after exiting the heat exchangers depending on operating conditions and/or cycle being used. In some implementations, one or more additional sets of storage tanks at different temperatures may be added on the hot side and/or the cold side. 
     The storage tanks (e.g., hot side tanks comprising hot side thermal storage medium and/or cold side tanks comprising cold side thermal storage medium) may operate at ambient pressure. In some implementations, thermal energy storage at ambient pressure can provide safety benefits. Alternatively, the storage tanks may operate at elevated pressures, such as, for example, at a pressure of at least about 2 atm, at least about 5 atm, at least about 10 atm, at least about 20 atm, or more. Alternatively, the storage tanks may operate at reduced pressures, such as, for example, at a pressure of at most about 0.9 atm, at most about 0.7 atm, at most about 0.5 atm, at most about 0.3 atm, at most about 0.1 atm, at most about 0.01 atm, at most about 0.001 atm, or less. In some cases (e.g., when operating at higher/elevated or lower pressures or to avoid contamination of the thermal storage media), the storage tanks can be sealed from the surrounding atmosphere. Alternatively, in some cases, the storage tanks may not be sealed. In some implementations, the tanks may include one or more pressure regulation or relief systems (e.g., a valve for safety or system optimization). 
     As used herein, the first hot side tank(s)  6  (at the temperature T 1   + ) can contain HTS medium at a higher temperature than the second hot side tank(s)  7  (at the temperature T 0   + ), and the first cold side tank(s)  8  (at the temperature T 1 ) can contain CTS medium at a higher temperature than the second cold side tank(s)  9  (at the temperature T 0 ). During charge, HTS medium in the first (higher temperature) hot side tank(s)  6  and/or CTS medium in the second (lower temperature) cold side tank(s)  9  can be replenished. During discharge, HTS medium in the first (higher temperature) hot side tank(s)  6  and/or CTS medium in the second (lower temperature) cold side tank(s)  9  can be consumed. 
     In the foregoing examples, in either mode of operation, two of the four storage tanks  6 ,  7 ,  8  and  9  are feeding thermal storage medium to the heat exchangers  2  and  4  at the inlets  32  and  36 , respectively, and the other two tanks are receiving thermal storage medium from the heat exchangers  2  and  4  from the exits  33  and  37 , respectively. In this configuration, the feed tanks can contain a storage medium at a given temperature due to prior operating conditions, while the receiving tanks&#39; temperatures can depend on current system operation (e.g., operating parameters, loads and/or power input). The receiving tank temperatures may be set by the Brayton cycle conditions. In some cases, the receiving tank temperatures may deviate from desired values due to deviations from predetermined cycle conditions (e.g., variation of absolute pressure in response to system demand) and/or due to entropy generation within the system. In some cases (e.g., due to entropy generation), at least one of the four tank temperatures can be higher than desired. In some implementations, a radiator can be used to reject or dissipate this waste heat to the environment. In some cases, heat rejection to the environment may be enhanced (e.g., using evaporative cooling etc.). The waste heat generated during operation of the pumped thermal systems herein can also be utilized for other purposes. For example, waste heat from one part of the system may be used elsewhere in the system. In another example, waste heat may be provided to an external process or system, such as, for example, a manufacturing process requiring low grade heat, commercial or residential heating, thermal desalination, commercial drying operations etc. 
     Components of pumped thermal systems of the disclosure may exhibit non-ideal performance, leading to losses and/or inefficiencies. The major losses in the system may occur due to inefficiencies of the turbomachinery (e.g., compressor and turbine) and the heat exchangers. The losses due to the heat exchangers may be small compared to the losses due to the turbomachinery. In some implementations, the losses due to the heat exchangers can be reduced to near zero with suitable design and expense. Therefore, in some analytical examples, losses due to the heat exchangers and other possible small losses due to pumps, the motor/generator and/or other factors may be neglected. 
     Losses due to turbomachinery can be quantified in terms of adiabatic efficiencies η c  and η t  (also known as isentropic efficiencies) for compressors and turbines, respectively. For large turbomachinery, typical values may range between η c =0.85-0.9 for compressors and η t =0.9-0.95 for turbines. The actual amount of work produced or consumed by a cycle can then be expressed as 
                 Δ   ⁢           ⁢   W     =         W   actual     (   out   )       -     W   actual     (   in   )         =         η   t     ⁢     W   ideal     (   out   )         -       1     η   c       ⁢     W   ideal     (   in   )               ,         
where, in an example assuming constant specific heats of the working fluid, W ideal   (in) =c p  T inlet (ψ−1), W ideal   (out) =c p T inlet (1−ψ −1 ), where
 
               ψ   =     r       γ   -   1     γ         ,         
r is the compression ratio (i.e., ratio of the higher pressure to the lower pressure), and γ=c p /c v  is the ratio of specific heats of the working fluid. Due to compressor and turbine inefficiencies, more work is required to achieve a given compression ratio during compression, and less work is generated during expansion for a given compression ratio. Losses can also be quantified in terms of the polytropic, or single stage, efficiencies, η cp  and η tp , for compressors and turbines, respectively. The polytropic efficiencies are related to the adiabatic efficiencies η c  and η t  by the equations
 
     
       
         
           
             
               η 
               c 
             
             = 
             
               
                 
                   
                     ψ 
                     - 
                     1 
                   
                   
                     ψ 
                     
                       1 
                       / 
                       
                         η 
                         
                           cp 
                           - 
                           1 
                         
                       
                     
                   
                 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 and 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   η 
                   t 
                 
               
               = 
               
                 
                   
                     1 
                     - 
                     
                       ψ 
                       
                         - 
                         
                           η 
                           tp 
                         
                       
                     
                   
                   
                     1 
                     - 
                     
                       ψ 
                       
                         - 
                         1 
                       
                     
                   
                 
                 . 
               
             
           
         
       
     
     In examples where η c =η t =1, pumped thermal cycles of the disclosure can follow identical paths in both charge and discharge cycles (e.g., as shown in  FIGS.  4  and  5   ). In examples where η c &lt;1 and/or η t &lt;1, compression in the compressor can lead to a greater temperature increase than in the ideal case for the same compression ratio, and expansion in the turbine can lead to a smaller temperature decrease than in the ideal case. 
     In some implementations, the polytropic efficiency of the compressor η cp  may be at least about 0.3, at least about 0.5, at least about 0.6, at least about 0.7, at least about 0.75, at least about 0.8, at least about 0.85, at least about 0.9, at least about 0.91, at least about 0.92, at least about 0.93, at least about 0.96, or more. In some implementations, the polytropic efficiency of the compressor η tp  may be at least about 0.3, at least about 0.5, at least about 0.6, at least about 0.7, at least about 0.75, at least about 0.8, at least about 0.85, at least about 0.9, at least about 0.91, at least about 0.92, at least about 0.93, at least about 0.96, at least about 0.97 or more. 
     T 0   + , T 1   +  were previously defined as the temperatures achieved at the exit of a compressor with a given compression ratio r, adiabatic efficiency η c  and inlet temperatures of T 0 , T 1  respectively. In some examples, these four temperatures are related by the equation 
     
       
         
           
             
               
                 T 
                 0 
                 + 
               
               
                 T 
                 0 
               
             
             = 
             
               
                 
                   T 
                   1 
                   + 
                 
                 
                   T 
                   1 
                 
               
               = 
               
                 
                   ψ 
                   
                     1 
                     / 
                     
                       η 
                       cp 
                     
                   
                 
                 . 
               
             
           
         
       
     
       FIG.  8    shows an exemplary heat storage charge cycle for a water (CTS)/molten salt (HTS) system with η c =0.9 and η t =0.95. The dashed lines correspond to η c =η t =1 and the solid lines show the charge cycle with η t =0.95 and η c =0.9. In this example, the CTS medium on the cold side is water, and the HTS medium on the hot side is molten salt. In some cases, the system can include 4 heat storage tanks. In the charge cycle, the working fluid at T 0  and P 2  can exchange heat with a CTS medium in the cold side heat exchanger  4 , whereby its temperature can increase to T 1  (assuming negligible pressure drop, its pressure can remain P2). In the compressor  1  with η c =0.9, the temperature and pressure of the working fluid can increase from T 1 , P 2  to T 1   + , P 1 . The working fluid can then exchange heat with an HTS medium in the hot side heat exchanger  2 , such that its temperature can decrease (at constant pressure P 1 , assuming negligible pressure drop). If the working fluid enters the turbine  3  with η t =0.95 at the temperature T 0   +  and expands back to its original pressure P 2 , its temperature when exiting the turbine may not be T 0 . Instead, the working fluid may enter the turbine at a temperature {tilde over (T)} 0   +  and exit the turbine at the temperature T 0  and pressure P 2 . In some examples, the temperatures are related by the relation 
                   T   ~     0   +       T   0       =       ψ     η   tp       .           
In some examples, {tilde over (T)} 0   +  is the temperature at which the working fluid enters the inlet of a turbine with adiabatic efficiency η t  and compression ratio r in order to exit at the temperature T 0 .
 
     In some implementations, the temperature {tilde over (T)} 0   +  may be incorporated into charge cycles of the disclosure by first heat exchanging the working fluid with the HTS medium from T 1   +  to T 0   + , followed by further cooling the working fluid from T 0   +  to {tilde over (T)} 0   + , as illustrated by section  38  of the cycle in  FIG.  8   . 
       FIG.  9    shows an exemplary heat storage discharge (extraction) cycle for the water/molten salt system in  FIG.  8    with η c =0.9 and η t =0.95. The dashed lines correspond to η c =η t =1 and the solid lines show the charge cycle with η t =0.95 and η c =0.9. In the discharge cycle, the working fluid at T 1  and P 2  can exchange heat with a CTS medium in the cold side heat exchanger  4 , whereby its temperature can decrease to T 0  (assuming negligible pressure drop, its pressure can remain P 2 ). In the compressor  1  with η c =0.9, the temperature and pressure of the working fluid can increase from T 0 , P 2  to T 0   + , P 1 . The working fluid can then exchange heat with an HTS medium in the hot side heat exchanger  2 , such that its temperature can increase (at constant pressure P 1 , assuming negligible pressure drop). Working fluid entering the turbine  3  at T 1   +  may not exit the turbine at the temperature T 1  as in the charge cycle, but may instead exit at a temperature {tilde over (T)} 1 , where, in some examples, {tilde over (T)} 1 =T 1   + ψ −ηtp . In some examples, {tilde over (T)} 1  is the temperature at which the working fluid exits the outlet of a turbine with adiabatic efficiency η t  and compression ratio r after entering the inlet of the turbine at the temperature T 1   + . 
     In some implementations, the temperature {tilde over (T)} 1  may be incorporated into the discharge cycles of the disclosure by first cooling the working fluid exiting the turbine at {tilde over (T)} 1  to T 1 , as illustrated by section  39  of the cycle in  FIG.  9   , followed by heat exchanging the working fluid with the CTS medium from T 1  to T 0 . 
     The charge and discharge cycles may be closed by additional heat rejection operations in sections  38  (between T 0   +  and {tilde over (T)} 0   + ) and  39  (between {acute over (T)} 1  and T 1 ), respectively. In some cases, closing the cycles through heat rejection in sections of the cycles where the working fluid can reject heat to ambient at low cost may eliminate the need for additional heat input into the system. The sections of the cycles where the working fluid can reject heat to ambient may be limited to sections where the temperature of the working fluid is high enough above ambient temperature for ambient cooling to be feasible. In some examples, heat may be rejected to the environment in sections  38  and/or  39 . For example, heat may be rejected using one or more working fluid to air radiators, intermediate water cooling, or a number of other methods. In some cases, heat rejected in sections  38  and/or  39  may be used for another useful purpose, such as, for example, cogeneration, thermal desalination and/or other examples described herein. 
     In some implementations, the cycles may be closed by varying the compression ratios between the charge and discharge cycles, as shown, for example, in  FIG.  10   . The ability to vary compression ratio on charge and discharge may be implemented, for example, by varying the rotation speed of the compressor and/or turbine, by variable stator pressure control, by bypassing a subset of the compression or expansion stages on charge or discharge by the use of valves, or by using dedicated compressor/turbine pairs for charge and discharge mode. In one example, the compression ratio in the discharge cycle in  FIG.  9    can be changed such that heat rejection in section is 39 is not used, and only heat rejection in section  38  in the charge cycle is used. Varying the compression ratio may allow heat (i.e., entropy) to be rejected at a lower temperature, thereby increasing overall roundtrip efficiency. In some examples of this configuration, the compression ratio on charge, r C , can be set such that 
                   T   1   +       T   1       =     ψ   C     1   /     η   cp           ,         
and on discharge, the compression ratio r D  can be set such that
 
                 T   1   +       T   1       =       ψ   D     η   tp       .           
In some cases, the upper temperatures T 1   +  and T 1  can be identical on charge and discharge and no heat removal may be needed in this portion (also “leg” herein) of the cycle. In such cases, the temperature T 0   +  on charge (e.g., T 0   +(c) =T 0 ψ C   ηtp ) and the temperature T 0   +  on discharge (e.g., T 0   +(d) =T 0 ψ D   1/ηcp ) can be dissimilar and heat may be rejected (also “dissipated” or “dumped” herein) to the environment between the temperatures T 0   +(c)  and T 0   +(d) . In an implementation where only the storage media exchange heat with the environment, a heat rejection device (e.g., devices  55  and  56  shown in  FIG.  16   ) can be used to lower the temperature of the CTS from T 0   +(d)  to T 0   +(c)  between discharge and charge.
 
       FIG.  10    shows an example of a cycle with variable compression ratios. The compression ratio can be higher on discharge (when work is produced by the system) than on charge (when work is consumed by the system), which may increase an overall round trip efficiency of the system. For example, during a charge cycle  80  with T 0   +(c) , a lower compression ratio of &lt;3 can be used; during a discharge cycle  81  with T 0   +(d) , a compression ratio of &gt;3 can be used. The upper temperatures reached in both cycles  80  and  81  can be T 1  and T 1   + , and no excess heat may be rejected. 
     The compression ratio may be varied between charge and discharge such that the heat dissipation to the environment needed for closing the cycle on both charge and discharge occurs between the temperatures T 0   +(C)  (the temperature of the working fluid before it enters the turbine during the charge cycle) and T 0   +(D)  (the temperature of the working fluid as it exits the compressor on discharge) and not above the temperature T 1  (the temperature of the working fluid before it enters the compressor on charge and/or exits the turbine on discharge). In some examples, none of the heat is rejected at a temperature above the lowest temperature of the HTS medium. 
     In the absence of system losses and/or inefficiencies, such as, for example, in the case of pumped thermal systems comprising heat pump(s) and heat engine(s) operating at the zero entropy creation/isentropic limit, a given amount of heat Q H  can be transferred using a given quantity of work Win heat pump (charge) mode, and the same Q H  can be used in heat engine (discharge) mode to produce the same work W, leading to a unity (i.e., 100%) roundtrip efficiency. In the presence of system losses and/or inefficiencies, roundtrip efficiencies of pumped thermal systems may be limited by how much the components deviate from ideal performance. 
     The roundtrip efficiency of a pumped thermal system may be defined as η store =|W cv   extract |/|W cv   charge |. In some examples, with an approximation of ideal heat exchange, the roundtrip efficiency can be derived by considering the net work output during the discharge cycle, 
                      W   cv   extract          =         η   t     ⁢     W   ideal   out       -       W   ideal   in       η   c           ,         
and the net work input during the charge cycle,
 
                    W     c   ⁢   v     charge          =         W   ideal   in       η   c       -       η   t     ⁢     W   ideal   out               
using the equations for work and temperature given above.
 
     Roundtrip efficiencies may be calculated for different configurations of pumped thermal systems (e.g., for different classes of thermal storage media) based on turbomachinery component efficiencies, η c  and η t . 
     In one example,  FIG.  11    shows roundtrip efficiency contours for a water/salt system, such as, for example, the water/salt system in  FIGS.  8  and  9    with T 0 =273 K (0° C.), T 1 =373 K (100° C.) and a compression ratio of r=5.65 chosen to achieve compatibility with the salt(s) on the hot side. Exemplary roundtrip efficiency contours at values of η store  of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% are shown as a function of component efficiencies η c  and η t  on the x- and y-axes, respectively. The symbols ⊕ and ⊗ represent the approximate range of present large turbomachinery adiabatic efficiency values. The dashed arrows represent the direction of increasing efficiency. 
       FIG.  12    shows roundtrip efficiency contours for a colder storage/salt system, such as, for example a hexane/salt system with a gas-gas heat exchanger in  FIGS.  13 ,  14 ,  17  and  18    with T 0 =194 K (−79° C.), T 1 =494 K (221° C.) and a compression ratio of r=3.28. Exemplary roundtrip efficiency contours at values of η store  of 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% and 90% are shown as a function of component efficiencies η c  and η t  on the x- and y-axes, respectively. The symbols ⊕ and ⊗ represent the approximate range of present large turbomachinery adiabatic efficiency values. As discussed in detail elsewhere herein, using hexane, heptane and/or another CTS medium capable of low temperature operation may result in significant improvements of system efficiency. 
     Pumped Thermal Storage Cycles with Recuperation 
     Another aspect of the disclosure is directed to pumped thermal systems with recuperation. In some situations, the terms regeneration and recuperation can be used interchangeably, although they may have different meanings. As used herein, the terms “recuperation” and “recuperator” generally refer to the presence of one or more additional heat exchangers where the working fluid exchanges heat with itself during different segments of a thermodynamic cycle through continuous heat exchange without intermediate thermal storage. The roundtrip efficiency of pumped thermal systems may be substantially improved if the allowable temperature ranges of the storage materials can be extended. In some implementations, this may be accomplished by choosing a material or medium on the cold side that can go to temperatures below 273 K (0° C.). For example, a CTS medium (e.g., hexane) with a low temperature limit of approximately T 0 =179 K (−94° C.) may be used in a system with a molten salt HTS medium. However, T 0   +  (i.e., the lowest temperature of the working fluid in the hot side heat exchanger) at some (e.g., modest) compression ratios may be below the freezing point of the molten salt, making the molten salt unviable as the HTS medium. In some implementations, this can be resolved by including a working fluid to working fluid (e.g., gas-gas) heat exchanger (also “recuperator” herein) in the cycle. 
       FIG.  13    is a schematic flow diagram of working fluid and heat storage media of a pumped thermal system in a charge/heat pump mode with a gas-gas heat exchanger  5  for the working fluid. The use of the gas-gas heat exchanger can enable use of colder heat storage medium on the cold side of the system. The working fluid can be air. The working fluid can be dry air. The working fluid can be nitrogen. The working fluid can be argon. The working fluid can be a mixture of primarily argon mixed with another gas such as helium. For example, the working fluid may comprise at least about 50% argon, at least about 60% argon, at least about 70% argon, at least about 80% argon, at least about 90% argon, or about 100% argon, with balance helium. 
       FIG.  17    shows a heat storage charge cycle for the storage system in  FIG.  13    with a cold side storage medium (e.g., liquid hexane) capable of going down to approximately to 179 K (−94° C.) and a molten salt as the hot side storage, and η c =0.9 and η t =0.95. The CTS medium can be hexane or heptane and the HTS medium can be molten salt. In some cases, the system can include four heat storage tanks. 
     In one implementation, during charge in  FIGS.  13  and  17   , the working fluid enters the compressor at T 1  and P 2 , exits the compressor at T 1   +  and P 1 , rejects heat Q 1  to the HTS medium  21  in the hot side CFX  2 , exiting the hot side CFX  2  at T 1  and P 1 , rejects heat Q recup  (also “Q regen ” herein, as shown, for example, in the accompanying drawings) to the cold (low pressure) side working fluid in the heat exchanger or recuperator  5 , exits the recuperator  5  at T 0   +  and P 1 , rejects heat to the environment (or other heat sink) in section  38  (e.g., a radiator), enters the turbine  3  at {tilde over (T)} 0   +  and P 1 , exits the turbine at T 0  and P 2 , absorbs heat Q 2  from the CTS medium  22  in the cold side CFX  4 , exiting the cold side CFX  4  at T 0   +  and P 2 , absorbs heat Q recup  from the hot (high pressure) side working fluid in the heat exchanger or recuperator  5 , and finally exits the recuperator  5  at T 1  and P 2 , returning to its initial state before entering the compressor. 
       FIG.  14    is a schematic flow diagram of working fluid and heat storage media of the pumped thermal system in  FIG.  13    in a discharge/heat engine mode. Again, the use of the gas-gas heat exchanger can enable use of colder heat storage fluid (CTS) and/or colder working fluid on the cold side of the system. 
       FIG.  18    shows a heat storage discharge cycle for the storage system for the storage system in  FIG.  14    with a cold side storage medium (e.g., liquid hexane) capable of going down to 179 K (−94° C.) and a molten salt as the hot side storage, and η x =0.9 and η t =0.95. Again, the CTS medium can be hexane or heptane and the HTS medium can be molten salt, and the system may include 4 heat storage tanks. 
     During discharge in  FIGS.  14  and  18   , the working fluid enters the compressor at T 0  and P 2 , exits the compressor at T 0   +  and P 1 , absorbs heat Q recup  from the cold (low pressure) side working fluid in the heat exchanger or recuperator  5 , exits the recuperator  5  at T 1  and P 1 , absorbs heat Q 1  from the HTS medium  21  in the hot side CFX  2 , exiting the hot side CFX  2  at T 1   +  and P 1 , enters the turbine  3  at T 1   +  and P 1 , exits the turbine at {tilde over (T)} 1  and P 2 , rejects heat to the environment (or other heat sink) in section  39  (e.g., a radiator), rejects heat Q recup  to the hot (high pressure) side working fluid in the heat exchanger or recuperator  5 , enters the cold side CFX  4  at T 0   +  and P 2 , rejects heat Q 2  to the CTS medium  22  in the cold side CFX  4 , and finally exits the cold side CFX  4  at T 0  and P 2 , returning to its initial state before entering the compressor. 
     In another implementation, shown in  FIG.  15   , the charge cycle remains the same as in  FIGS.  13  and  17   , except that the working fluid exits the recuperator  5  at {tilde over (T)} 0   +  and P 1  (instead of at T 0   +  and P 1  as in  FIGS.  13  and  17   ), enters the turbine  3  at {tilde over (T)} 0   +  and P 1 , exits the turbine at T 0  and P 2 , absorbs heat Q 2  from the CTS medium  22  having a temperature {tilde over (T)} 0   +  (instead of at T 0   +  as in  FIGS.  13  and  17   ) in the cold side CFX  4 , and exits the cold side CFX  4  at {tilde over (T)} 0   +  and P 2  (instead of at T 0   +  and P 2  as in  FIG.  13   ) before reentering the recuperator  5 . The heat between temperatures T 0   +  and {tilde over (T)} 0   +  is no longer rejected from the working fluid to the environment directly (as in section  38  in  FIGS.  13  and  17   ). 
     During discharge in  FIG.  16   , the discharge cycle remains the same as in  FIGS.  14  and  8 B , except that the temperature of the HTS medium being deposited in tank  7  is changed. The working fluid exits the recuperator  5  at {tilde over (T)} 1  and P 1  (instead of at T 1  and P 1  as in  FIGS.  14  and  8 B ) and absorbs heat Q 1  from the HTS medium  21  in the hot side CFX  2 . The HTS medium exits the hot side CFX  2  having a temperature {tilde over (T)} 1  (instead of at T 1  as in  FIGS.  14  and  18   ). The working fluid then exits the hot side CFX  2  at T 1   +  and P 1 , enters the turbine  3  at T 1   +  and P 1 , and exits the turbine at {tilde over (T)} 1  and P 2  before reentering the recuperator  5 . The heat between temperatures {tilde over (T)} 1  and T 1  is no longer rejected from the working fluid to the environment directly (as in section  39  in  FIGS.  14  and  18   ). As in  FIG.  14   , the CTS medium enters the tank  8  at temperature T 0   + . 
     After the discharge in  FIG.  16   , in preparation for the charge in  FIG.  15   , heat exchange with ambient may be used to cool the HTS medium  21  from the temperature {tilde over (T)} 1  used in the discharge cycle to the temperature T 1  used in the charge cycle. Similarly, heat exchange with ambient may be used to cool the CTS medium  22  from the temperature T 0   +  used in the discharge cycle to the temperature {tilde over (T)} 0   +  used in the charge cycle. Unlike in the configuration in  FIGS.  13  and  14   , where the working fluid may need to reject a substantial amount of heat (in sections  38  and  39 , respectively) at a fast rate, in this configuration, the hot side and cold side storage media may be cooled at an arbitrarily slow rate (e.g., by radiating away or by other means of giving off the heat to the environment). 
     As shown in  FIG.  16   , in some implementations, heat can be rejected from the CTS medium to the environment by circulating the CTS medium in the tank  8  in a heat rejection device  55  that can absorb heat from the CTS medium and reject heat to the environment until the CTS medium cools from the temperature T 0   +  to the temperature {tilde over (T)} 0   + . In some examples, the heat rejection device  55  can be, for example, a radiator, a thermal bath containing a substance such as water or salt water, or a device immersed in a natural body of water such as a lake, river or ocean. In some examples, the heat rejection device  55  can also be an air cooling apparatus, or a series of pipes which are thermally connected to a solid reservoir (e.g., pipes embedded in the ground). 
     Similarly, in some implementations, heat can be rejected from the HTS medium to the environment by circulating the HTS in the tank  7  in a heat rejection device  56  that can absorb heat from the HTS medium and reject heat to the environment until the HTS medium cools from the temperature {tilde over (T)} 1  to the temperature T 1 . In some examples, the heat rejection device  56  can be, for example, a radiator, a thermal bath containing a substance such as water or salt water, or a device immersed in a natural body of water such as a lake, river or ocean. In some examples, the heat rejection device  56  can also be an air cooling apparatus or a series of pipes which are thermally connected to a solid reservoir (e.g., pipes embedded in the ground). 
     In some implementations, rejecting heat to ambient through the use of the thermal storage media may be used in conjunction with the variable compression ratio charge and/or discharge cycles described, for example, in  FIG.  10   . In this system, only the CTS medium may exchange heat with ambient. Such a system can also be implemented with a recuperator to extend the temperature ranges of the HTS and CTS media in the cycles. 
     In some implementations, three separate cold side storage tanks at respective temperatures T 0 , {tilde over (T)} 1   +  and T 0   +  may be used (e.g., an extra tank may be used in addition to the tanks  8  and  9 ). During heat exchange in the cold side CFX  4  in the discharge cycle, heat from the working fluid exiting the recuperator  5  may be transferred to the CTS medium in the T 0   + -tank. The CTS medium may be cooled in/by, for example, the heat rejection device  55  prior to entering the {tilde over (T)} 0   + -tank. In some implementations, three separate hot side storage tanks at respective temperatures T 1 , {tilde over (T)} 1  and T 1   +  may be used (e.g., an extra tank may be used in addition to the tanks  6  and  7 ). During heat exchange in the hot side CFX  2  in the discharge cycle, heat from the working fluid exiting the recuperator  5  may be transferred to the HTS medium in the {tilde over (T)} 1 -tank. The HTS medium may be cooled in/by, for example, the heat rejection device  56  prior to entering the T 1 -tank. Heat rejection to the environment in such a manner may present several advantages. In a first example, it may eliminate the need for a potentially expensive working fluid to ambient heat exchanger that is capable of absorbing heat from the working fluid at a rate proportional to the power input/output of the system. The HTS and CTS media may instead reject heat over extended time periods, thus reducing the cost of the cooling infrastructure. In a second example, it may allow the decision regarding when heat is rejected to the environment to be delayed such that heat exchange to ambient may be performed when temperature (e.g., the ambient temperature) is most favorable. 
     In the charge and discharge cycles of  FIGS.  13  and  17   , and  FIGS.  14  and  18   , respectively, the same compression ratios and temperature values are used for both charge and discharge. In this configuration, the roundtrip efficiency can be about η store =74%, as given by T 0 =194 K (−79° C.), T 1 =494 K (221° C.). η t t=0.95, η c =0.9 and r=3.3. 
     Thus, in some examples involving working fluid to working fluid recuperation, heat rejection on the hot side (high pressure) side of the closed charge cycle can take place in three operations (heat exchange with the HTS medium, followed by recuperation, followed by heat rejection to the environment), and heat rejection on the cold side (low pressure) side of the closed discharge cycle can take place in three operations (heat rejection to the environment, followed by recuperation, followed by heat exchange with the CTS medium). As a result of recuperation, the higher temperature HTS tank(s)  6  can remain at T 1   +  while the lower temperature HTS tank(s)  7  can now be at the temperature T 1 &gt;T 0   + , and the lower temperature CTS tank(s)  9  can remain at T 0  while the higher temperature CTS tank(s)  8  can now be at the temperature T 0   + &lt;T 1 . 
     In some cases, recuperation may be implemented using the heat exchanger  5  for direct transfer of heat between the working fluid on the high pressure side and the working fluid on the low pressure side. In an alternative configuration, an additional pair (or plurality) of heat exchangers together with an additional heat transfer medium or fluid (e.g., a dedicated thermal heat transfer fluid that is liquid in an appropriate temperature range, such as, for example, Therminol®) may be used to achieve recuperation. For example, an additional heat exchanger may be added in series with the cold side heat exchanger and an additional heat exchanger may be added in series with the hot side heat exchanger. The additional heat transfer medium may circulate between the two additional heat exchangers in a closed loop. In other examples, one or more additional heat exchangers may be placed elsewhere in the system to facilitate recuperation. Further, one or more additional heat transfer media or mixtures thereof may be used. The one or more additional heat transfer media fluids may be in fluid or thermal communication with one or more other components, such as, for example, a cooling tower or a radiator. 
     In one example, hexane or heptane can be used as a CTS medium, and nitrate salt can be used as an HTS medium. On the low pressure side of the cycle, the operating temperatures of the pumped thermal storage cycles may be limited by the melting point of hexane (178 K or −95° C.) at T 0  and by the melting point of the nitrate (494 K or 221° C.) at T 1 . On the high pressure side of the cycle, the operating temperatures may be limited by the boiling point of hexane (341 K or 68° C.) at T 0   +  and by the decomposition of nitrate (873 K or 600° C.) at T 1   + . At these conditions, the high pressure and low pressure temperature ranges can overlap such that recuperation can be implemented. The actual temperatures T 0 , T 1 , T 0   +  and T 1   +  and pressure ratios implemented in hexane/nitrate systems may differ from the limits above. 
     In some examples, recuperation may enable the compression ratio to be reduced. In some cases, reducing the compression ratio may result in reduced compressor and turbine losses. In some cases, the compression ratio may be at least about 1.2, at least about 1.5, at least about 2, at least about 2.5, at least about 3, at least about 3.5, at least about 4, at least about 4.5, at least about 5, at least about 6, at least about 8, at least about 10, at least about 15, at least about 20, at least about 30, or more. 
     In some cases, T 0  may be at least about 30 K, at least about 50 K, at least about 80 K, at least about 100 K, at least about 120 K, at least about 140 K, at least about 160 K, at least about 180 K, at least about 200 K, at least about 220 K, at least about 240 K, at least about 260 K, or at least about 280 K. In some cases, T 0   +  may be at least about 220 K, at least about 240 K, at least about 260 K, at least about 280 K, at least about 300 K, at least about 320 K, at least about 340 K, at least about 360 K, at least about 380 K, at least about 400 K, or more. In some cases, the temperatures T 0  and T 0   +  can be constrained by the ability to reject excess heat to the environment at ambient temperature. In some cases, the temperatures T 0  and T 0   +  can be constrained by the operating temperatures of the CTS (e.g., a phase transition temperature). In some cases, the temperatures T 0  and T 0   +  can be constrained by the compression ratio being used. Any description of the temperatures T 0  and/or T 0   +  herein may apply to any system or method of the disclosure. 
     In some cases, T 1  may be at least about 350K, at least about 400 K, at least about 440 K, at least about 480 K, at least about 520 K, at least about 560 K, at least about 600 K, at least about 640 K, at least about 680 K, at least about 720 K, at least about 760 K, at least about 800 K, at least about 840 K, at least about 880 K, at least about 920 K, at least about 960 K, at least about 1000 K, at least about 1100 K, at least about 1200 K, at least about 1300 K, at least about 1400 K, or more. In some cases, T 1   +  may be at least about 480 K, at least about 520 K, at least about 560 K, at least about 600 K, at least about 640 K, at least about 680 K, at least about 720 K, at least about 760 K, at least about 800 K, at least about 840 K, at least about 880 K, at least about 920 K, at least about 960 K, at least about 1000 K, at least about 1100 K, at least about 1200 K, at least about 1300 K, at least about 1400 K, at least about 1500 K, at least about 1600 K, at least about 1700 K, or more. In some cases, the temperatures T 1  and T 1   +  can be constrained by the operating temperatures of the HTS. In some cases, the temperatures T 1  and T 1   +  can be constrained by the thermal limits of the metals and materials being used in the system. For example, a conventional solar salt can have a recommended temperature range of approximately 560-840 K. Various system improvements, such as, for example, increased roundtrip efficiency, increased power and increased storage capacity may be realized as available materials, metallurgy and storage materials improve over time and enable different temperature ranges to be achieved. Any description of the temperatures T 1  and/or T 1   +  herein may apply to any system or method of the disclosure. 
     In some cases, the roundtrip efficiency η store  (e.g., electricity storage efficiency) with and/or without recuperation can be at least about 5%, at least about 10%, at least about 15%, at least about 20%, at least about 25%, at least about 30%, at least about 35%, at least about 40%, at least about 45%, at least about 50%, at least about 55%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, or at least about 95%. 
     In some implementations, at least a portion of heat transfer in the system (e.g., heat transfer to and from the working fluid) during a charge and/or discharge cycle includes heat transfer with the environment (e.g., heat transfer in sections  38  and  39 ). The remainder of the heat transfer in the system can occur through thermal communication with thermal storage media (e.g., thermal storage media  21  and  22 ), through heat transfer in the recuperator  5  and/or through various heat transfer processes within system boundaries (i.e., not with the surrounding environment). In some examples, the environment may refer to gaseous or liquid reservoirs surrounding the system (e.g., air, water), any system or medium capable of exchanging thermal energy with the system (e.g., another thermodynamic cycle or system, heating/cooling systems, etc.), or any combination thereof. In some examples, heat transferred through thermal communication with the heat storage media can be at least about 25%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, or at least about 90% of all heat transferred in the system. In some examples, heat transferred through heat transfer in the recuperator can be at least about 5%, at least about 10%, at least about 15%, at least about 20%, at least about 25%, at least about 50%, or at least about 75% of all heat transferred in the system. In some examples, heat transferred through thermal communication with the heat storage media and through heat transfer in the recuperator can be at least about 25%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, or even about 100% of all heat transferred in the system. In some examples, heat transferred through heat transfer with the environment can be less than about 5%, less than about 10%, less than about 15%, less than about 20%, less than about 30%, less than about 40%, less than about 50%, less than about 60%, less than about 70%, less than about 80%, less than about 90%, less than about 100%, or even 100% of all heat transferred in the system. In some implementations, all heat transfer in the system may be with the thermal storage media (e.g., the CTS and HTS media), and only the thermal storage media may conduct heat transfer with the environment. 
     Pumped thermal cycles of the disclosure (e.g., the cycles in  FIGS.  13  and  14   ) may be implemented through various configurations of pipes and valves for transporting the working fluid between the turbomachinery and the heat exchangers. In some implementations, a valving system may be used such that the different cycles of the system can be interchanged while maintaining the same or nearly the same temperature profile across at least one, across a subset or across all of counter-flow heat exchangers in the system. For example, the valving may be configured such that the working fluid can pass through the heat exchangers in opposite flow directions on charge and discharge and flow directions of the HTS and CTS media are reversed by reversing the direction of the pumps. 
     In some implementations, the system with a recuperator may have a different compression and/or expansion ratio on charge and discharge. This may then involve heat rejection at only one or both of the heat rejection locations  38  and  39  as shown in  FIG.  5 C  along the lines described above. 
       FIG.  19    is a schematic flow diagram of hot side recharging in a pumped heat cycle in solar mode with heating of a solar salt solely by solar power. The system can comprise a solar heater for heating the hot side heat storage. The HTS medium  21  in the second hot thermal storage tank  7  of a discharge cycle, such as, for example, the HTS medium of the discharge cycle in  FIG.  14   , can be recharged within element  17  using heating provided by solar radiation. The HTS medium (e.g., molten salt) can be heated by solar heating from the temperature T 1  in the second hot thermal storage tank  7  to the temperature T 1   +  in the first hot thermal storage tank  6 . 
     In some implementations, such as, for example, for the systems in  FIG.  19    solar heat for heating the HTS medium (e.g., from T 1 =493 K (220° C.) to T 1   + =873 K (600° C.)) may be provided by a concentrating solar facility. In some examples, a small scale concentrating facility may be utilized for providing heat. In some cases, the concentrating solar facility may include one or more components for achieving high solar concentrating efficiency, including, for example, high-performance actuators (e.g., adaptive fluidic actuators manufactured from polymers), multiplexing control system, dense heliostat layout etc. In some examples, the heat provided for heating the HTS medium (e.g., in the element  17 ) may be a waste heat stream from the concentrating solar facility. 
       FIG.  20    is a schematic flow diagram of a pumped thermal system discharge cycle that can be coupled with external heat input (e.g., solar, combustion) with heat rejection to ambient. Such a discharge cycle may be used, for example, in situations where the capacity for hot side recharging (e.g., using solar heating, waste heat or combustion) is greater than the capacity for cold side recharging. Solar heat may be used to charge the HTS medium  21  in the hot side storage tanks from T 1  to T 1   + , as described elsewhere herein. The discharge cycle can operate similarly to the discharge cycle in  FIG.  3   , but after exiting the turbine  3 , the working fluid  20  can proceed to the cold side CFX  4  heat exchanger  4  where it exchanges heat with an intermediate thermal storage (ITS) medium  61  having a lower temperature T 0  at or near ambient temperature. The ITS medium  61  enters the cold side CFX  4  from a second intermediate thermal storage tank  59  at the temperature T 0  (e.g., ambient temperature) and exits the cold side CFX  4  into a first intermediate thermal storage tank  60  at the temperature {tilde over (T)} 1 , while the working fluid  20  enters the cold side CFX  4  at the temperature {tilde over (T)} 1  and exits the cold side CFX  4  at the temperature T 0 . The working fluid enters the compressor  1  at T 0  and P 2 , exits the compressor at T 0   +  and P 1 , absorbs heat Q 1  from the HTS medium  21  in the hot side CFX  2 , exits the hot side CFX  2  at T 1   +  and P 1 , enters the turbine  3  at T 1   +  and P 1 , exits the turbine at {tilde over (T)} 1  and P 2 , rejects heat Q 2  from the ITS medium  61  in the cold side CFX  4 , and exits the cold side CFX  4  at T 0  and P 2 , returning to its initial state before entering the compressor. 
     In some implementations, the ITS medium  61  may be a liquid over the entire range from T 0  to {tilde over (T)} 1 . In other implementations, the ITS medium  61  may not be a liquid over the entire range from T 0  to {tilde over (T)} 1 , but may be provided to the counter-flow heat exchanger  4  at a higher flow rate in order to achieve a lower temperature rise across the counter-flow heat exchanger (e.g., such that the temperature of the ITS medium at the exit of the counter-flow heat exchanger  4  is lower than {tilde over (T)} 1 ) while still cooling the working fluid from {tilde over (T)} 1  to T 0 . In this instance, the temperature of the ITS medium in the tank  60  can be lower than {tilde over (T)} 1 . The ITS medium in the tank  60  can exchange heat with ambient (e.g., through a radiator or other implementations described herein) in order to cool back to the temperature T 0 . In some cases, the ITS medium can then be returned to the tank  59 . In some cases, the heat deposited in the ITS medium may be used for various useful purposes such as, for example, residential or commercial heating, thermal desalination or other uses described elsewhere herein. 
       FIG.  21    is a schematic flow diagram of a pumped thermal system discharge cycle in solar mode or combustion heated mode with heat rejection to an intermediate fluid circulated in a thermal bath at ambient temperature. The discharge cycle can operate similarly to the discharge cycle in  FIG.  20   , but after exiting the turbine  3 , the working fluid  20  can proceed to the cold side CFX  4  where it exchanges heat with an intermediate medium or fluid  62  circulating through a thermal bath  63  at the temperature T 0  at or near ambient temperature. The intermediate medium or fluid  62  (e.g., Therminol®, or a heat transfer oil) may be used for exchanging heat between the working fluid  20  and a thermal bath  63  in the cold side CFX  4 . The use of the intermediate fluid  62  may provide an advantage over contacting an inexpensive thermal sink or medium (e.g., water) directly with the working fluid. For example, directly contacting such a thermal medium with the working fluid in the cold side CFX  4  may cause problems, such as, for example, evaporation or over-pressurization (e.g., explosion) of the thermal medium. The intermediate fluid  62  can remain in liquid phase throughout all, at least a portion of, or a significant portion of the operation in the cold side CFX  4 . As the intermediate fluid  62  passes through the thermal bath  58 , it can be sufficiently cooled to circulate back into the cold side CFX  4  for cooling the working fluid from {tilde over (T)} 1  to T 0 . The thermal bath  63  may contain a large amount of inexpensive heat sink material or medium, such as, for example, water. In some cases, the heat deposited in the heat sink material may be used for various useful purposes such as, for example, residential or commercial heating, thermal desalination or other uses described elsewhere herein. In some cases, the heat sink material may be re-equilibrated with ambient temperature (e.g., through a radiator or other implementations described herein). 
     In some implementations, the discharge cycles in  FIGS.  20  and/or  21    may include a recuperator, as described in greater detail in examples throughout the disclosure. Such systems may be implemented using the temperatures T 1   + , T 1 , T 0   +  and T 0  described in greater detail elsewhere herein. 
     Solar Assisted Pumped Thermal Storage Cycles with Intercooling 
     In some instances, the pumped thermal system may provide heat sources and/or cold sources to other facilities or systems such as, for example, through co-location with a gas to liquids (GTL) facility or a desalination facility. In one example, the GTL facilities may make use of one or more of the cold reservoirs in the system (e.g., the CTS medium in the tank  9  for use in oxygen separation in the GTL facility) and/or one or more hot reservoirs in the system (e.g., the HTS medium in the tank  6  for use in a Fischer-Tropsch process in the GTL facility). In another example, one or more hot reservoirs or one or more cold reservoirs in the pumped thermal system may be used for the operation of thermal desalination methods. Further examples of possible heat and cold uses include co-location or heat exchange with building/area heating and cooling systems. 
     Conversely, in some cases, the pumped thermal system may make use of waste heat sources and/or waste cold sources from other facilities or systems such as, for example, through co-location with a liquefied natural gas import or export terminal. For example, a waste cold source may be used for cooling the cold side thermal storage media  22 . In some implementations, recharging of the cold side using waste cold may be combined with recharging of the hot side thermal storage media  21  by external heat input (e.g., solar, combustion, waste heat, etc.). In some cases, the recharged storage media can then be used in a discharge cycle such as, for example, the discharge cycles in  FIG.  14  or  16   . In some cases, the pumped thermal system may be used as a heat engine with a waste heat source serving as the hot side heat input and a waste cold source serving as the cold side heat sink. In another implementation, the hot side storage media may be recharged using a modified version of the cycle shown in  FIG.  15   , where the temperature T 0  is about the ambient temperature and {tilde over (T)} 0   +  corresponds to a temperature above the ambient temperature. In some examples, a waste heat source can be used to provide the heat needed at a temperature of at least {tilde over (T)} 0   +  for heating the working fluid and/or the CTS medium to {tilde over (T)} 0   + . In another implementation, an intermediate fluid (e.g., Therminol®) which can remain liquid between the temperatures {acute over (T)} 0   +  and T 0  may be used to transfer the heat from the waste heat source to the working fluid. 
     Pumped Thermal Systems with Dedicated Compressor/Turbine Pairs 
     In a further aspect of the disclosure, pumped thermal systems comprising multiple working fluid systems, or working fluid flow paths are provided. In some cases, pumped thermal system components in the charge and discharge modes may be the same. For example, the same compressor/turbine pair may be used in charge and discharge cycles. Alternatively, one or more system components may differ between charge and discharge modes. For example, separate compressor/turbine pairs may be used in charge and discharge cycles. In one implementation, the system has one set of heat exchangers, and a common set of HTS and CTS tanks which are charged or discharged by two pairs or sets of compressors and turbines. In another implementation, the system has a common set of HTS and CTS tanks, but separate sets of heat exchangers and separate sets of compressors and turbines. 
     Pumped thermal systems with recuperation, utilization of external sources of heat, cold and/or waste heat/cold may benefit from having separate compressor/turbine pairs as a result of operation of turbomachinery over large and/or different temperature ranges in charge and discharge modes. For example, temperature changes between charge and discharge cycles may lead to a thermal adjustment period or other difficulties during transition between the cycles (e.g., issues or factor related to metallurgy, thermal expansion, Reynolds number, temperature dependent compression ratios, tip clearance and/or bearing friction etc.). In another example, turbomachinery (e.g., turbomachinery used in systems with recuperation) may operate over a relatively low pressure ratio (e.g., with relatively few compression stages) but over relatively large temperature during both compression and expansion. The temperature ranges may change (e.g., switch as in  FIGS.  17  and  18   ) between charge and discharge modes. In some cases, the operation over large temperature ranges during compression and/or expansion may complicate design of a combined compressor/turbine for both charge and discharge. Furthermore, recuperation, waste heat/cold incorporation and/or other pumped thermal system features may reduce the compression ratio of the compressor/turbine in the charge cycle and/or the discharge cycle, thereby reducing the cost associated with duplicating compressor/turbine sets. 
       FIGS.  22  and  23    show pumped thermal systems with separate compressor  1 /turbine  3  pairs for charge mode C and discharge mode D. The separate compressor/turbine pairs may or may not be ganged on a common mechanical shaft. In this example, the compressor/turbine pairs C and D can have separate shafts  10 . The shafts  10  may rotate at the same speed or at different speeds. The separate compressor/turbine pairs or working fluid systems may or may not share heat exchangers (e.g., the heat exchangers  2  and  4 ). 
     In the example in  FIG.  22   , the system has a common set of HTS tanks  6  and  7  and CTS tanks  8  and  9 . The system has separate pairs of heat exchangers  2  and  4  and separate compressor  1 /turbine  3  pairs for the charge mode C and the discharge mode D. The HTS and CTS storage media flow paths for the charging cycle are shown as solid black lines. The HTS and CTS storage media flow paths for the discharge cycle are shown as the dashed grey lines. 
     In the example in  FIG.  23   , the system, shown in a charge configuration, has one set of heat exchangers  2  and  4 , and a common set of HTS tanks  6  and  7  and CTS tanks  8  and  9 . The HTS and CTS tanks can be charged by a compressor/turbine set C, or discharged by a compressor/turbine set D, each set comprising a compressor  1  and a turbine  3 . The system may switch between the sets C and D using valves  83 . In the example in  FIG.  22   , the system, again shown in a charge configuration, has a common set of HTS tanks  6  and  7  and CTS tanks  8  and  9 . The HTS and CTS tanks can be charged by the charge set C that includes a first set of the heat exchangers  2  and  4 , the compressor  1  and the turbine  3 . The HTS and CTS tanks can be discharged by switching to a separate discharge set C that includes a second set of the heat exchangers  2  and  4 , the compressor  1  and the turbine  3 . 
     In one example, if the charge and discharge sets of compressors and turbines in  FIGS.  22  and  23    are not operated at the same time, the charge and discharge sets may share a common set of heat exchangers that are switched between the turbomachinery pairs using the valves  83 . In another example, if the charge and discharge turbomachinery sets or pairs in  FIGS.  22  and  23    are operated at the same time (e.g., in order for one set to charge, following intermittent generation, and the other set to discharge at the same time, following load), then each set of turbomachinery may have a dedicated set of heat exchangers. In this instance, the charge and discharge sets may or may not share a set of HTS and CTS tanks. 
     In some implementations, separate compressor/turbine sets or pairs may advantageously be used in pumped thermal systems used with intermittent and/or variable electric power inputs. For example, a first compressor/turbine set can be used in a charge cycle that follows wind and/or solar power (e.g., electric power input from wind and/or solar power systems) while a second compressor/turbine set can be used in a discharge cycle that follows load (e.g., electric power output to a power grid). In this configuration, pumped thermal systems placed between a power generation system and a load may aid in smoothing variations/fluctuations in input and/or output power requirements. 
     Hybrid Pumped Thermal Systems 
     In accordance with another aspect of the disclosure, pumped thermal systems can be augmented by additional energy conversion processes and/or be directly utilized as energy conversion systems without energy storage (i.e., as power generation systems). In some examples, pumped thermal systems herein can be modified to allow for direct power generation using natural gas, Diesel fuel, petroleum gas (e.g., propane/butane), dimethyl ether, fuel oil, wood chips, landfill gas, hexane, hydrocarbons or any other combustible substance (e.g., fossil fuel or biomass) for adding heat to the working fluid on a hot side of a working fluid cycle, and a cold side heat sink (e.g., water) for removing heat from the working fluid on a cold side of the working fluid cycle. 
       FIGS.  24  and  25    show pumped thermal systems configured in generation mode. In some examples, pumped thermal systems herein can be modified by adding two additional heat exchangers  40  and  41 , four additional valves  19   a ,  19   b ,  19   c  and  19   d , a heat sink (e.g., a water cooling system; water from a fresh water reservoir such as a river, a lake or a reservoir; salt water from a salt water reservoir such as a sea or an ocean; air cooling using radiators, fans/blowers, convection; or an environmental heat sink such as ground/soil, cold air etc.)  42 , and a heat source (e.g., a combustion chamber with a fuel-oxidant mixture)  43 . The heat source  43  can exchange heat with a first of the two additional heat exchangers  40 , and the heat sink  42  can exchange heat with a second of the two additional heat exchangers  41 . The heat source  43  may be used to for exchanging heat with the working fluid  20 . 
     The heat source  43  may be a combustion heat source. In some examples, the combustion heat source can comprise a combustion chamber for combusting a combustible substance (e.g., a fossil fuel, a synthetic fuel, municipal solid waste (MSW) or biomass). In some cases, the combustion chamber may be separate from the heat exchanger  40 . In some cases, the heat exchanger  40  may comprise the combustion chamber. The heat source  43  may be a waste heat source, such as, for example waste heat from a power plant, an industrial process (e.g., furnace exhaust). 
     In some examples, a solar heater, a combustion heat source, a waste heat source, or any combination thereof may be used for heating the hot side heat storage fluid and/or the working fluid. In an example, the working fluid can be heated directly using any of these heat sources. In another example, the hot side heat storage fluid (or HTS medium) can be heated using any of these heat sources. In another example, the hot side heat storage fluid (or HTS medium) can be heated in parallel with the working fluid using any of these heat sources. 
     The pumped thermal systems in  FIGS.  24  and  25    may be operated as hybrid systems. For example, the valves  19   a ,  19   b ,  19   c  and  19   d  can be used to switch between two modes. When the valves are in a first position, the system can operate as a pumped thermal storage system (e.g., closed system in charge/discharge mode). In this configuration, the working fluid  20  (e.g., argon or air) can exchange heat with an HTS medium (e.g., molten salt) in the hot side heat exchanger  2  and with a CTS medium (e.g., hexane) in the cold side heat exchanger  4 . When the valves are in a second position, the system can operate as a power generation system (e.g., open system in generation mode). In this configuration, the heat exchangers  2  and  4  may be bypassed, and the working fluid  20  can exchange heat with the combustion chamber  43  in the hot side heat exchanger  40  and with the heat sink  42  in the cold side heat exchanger  41 . Any description of configuration and/or design of heat transfer processes (e.g., heat transfer in heat exchangers) described herein in relation to pumped thermal systems may also be applied to hybrid pumped thermal systems, and vice versa. For example, the heat sink  42 , the heat source  43 , the heat exchangers  40  and  41 , and/or the quantity of heat transferred on the cold side and/or the hot side may be configured to decrease or minimize entropy generation associated with heat transfer processes and/or to maximize system efficiency. 
     In some implementations, the hybrid systems may operate in storage and generation modes simultaneously. For example, the valves  19   a ,  19   b ,  19   c  and  19   d  can be configured to allow a given split between a working fluid flow rate to the heat exchangers  40  and  41  and a working fluid flow rate to the heat exchangers  2  and  4 . Alternatively, the hybrid systems may operate exclusively in storage mode, or exclusively in generation mode (e.g., as a natural gas peaking plant). In some cases, the split between modes may be selected based on system efficiency, available electric power input (e.g., based on availability), desired electric power output (e.g., based on load demand) etc. For example, thermal efficiency of an ideal system (i.e., assuming isentropic compression and expansion processes, ideal heat transfer processes) operating exclusively in generation mode can be the thermal efficiency of a working fluid undergoing an ideal Brayton cycle. In some examples, thermal efficiencies in hybrid systems of the disclosure (e.g., exclusive and/or split mode operation) can be at least about 10%, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, or more. 
     The heat source  43  may be used for exchanging heat with an HTS medium (e.g., a molten salt). For example, the combustion heat source  43  may be used for heating the HTS medium  21 . In some instances, rather than using the combustion heat source  43  for exchanging heat in the heat exchanger  40  or for directly exchanging heat between flue gases from the combustion heat source and the working fluid, the combustion heat source  43  may be used to heat up the HTS medium  21  between the two HTS tanks  7  and  6 . 
       FIG.  26    is a schematic flow diagram of hot side recharging in a pumped heat cycle through heating by the heat source  43  (e.g., combustion heat source, waste heat source). In an example, the heat source  43  is a waste heat source, such as a waste heat source from a refinery or other processing plant. In an example, the heat source  43  is obtained from combusting natural gas in order to ensure the delivery of electricity even if the pumped thermal system runs out of charged storage media. For example, recharging of the hot side storage media using the heat source  43  may provide an advantage over recharging using electricity or other means (e.g., the price of electricity at the time may be too high). The heat source  43  can be used to heat up the HTS medium  21  from the temperature T 1  in the tank  7  to the temperature T 1   +  in the tank  6 . The HTS medium can then be used in the CFX  2  for exchanging heat with the working fluid to in a discharge cycle, such as, for example, the discharge cycles in  FIGS.  20  and  21   . 
     In some examples, such as, for example, when the CTS medium is a combustible substance such as a fossil fuel (e.g., hexane or heptanes), burning of the CTS medium stored in the CTS tanks (e.g., the tanks  8  and  9 ) may be used for providing thermal energy for heating the HTS medium as shown, for example, in  FIG.  26    or for operation of the cycles in the configurations shown, for example, in  FIGS.  24  and  25   . 
     The systems of the disclosure may be capable of operating both in an electricity only storage cycle (comprising heat transfer with an HTS medium and a CTS medium below ambient temperature) and in a heat engine to ambient cycle, where, in a discharge mode, heat is input from the HTS medium to the working fluid and rejected to the ambient environment rather than to the CTS medium. This capability may enable the use of heating of the HTS with combustible substances (e.g., as shown in  FIG.  26   ) or the use of solar heating of the HTS (e.g., as shown in  FIG.  19   ). Heat rejection to ambient may be implemented using, for example, the discharge cycles in  FIGS.  20  and  21   . In some cases, heat may be rejected to the environment with the aid of the ITS medium  61  or the intermediate medium  62 . 
     Aspects of the disclosure may be synergistically combined. For example, the systems capable of operating both in an electricity only storage cycle and in a heat engine to ambient cycle may comprise a recuperator. Any description in relation to such hybrid systems without a recuperator may readily be applied to hybrid systems with a recuperator at least in some configurations. In some instances, the hybrid systems may be implemented using, for example, the parallel, valved configuration in  FIG.  24   . For example, the counter-flow heat exchangers  4  in FIGS.  20  and  21  may be implemented as separate counter-flow heat exchangers  67  for exchanging heat with the ambient environment, and may be used in combination with cold side counter-flow heat exchangers  4  of the disclosure. 
     In some implementations, the systems herein may be configured to enable switching between different cycles of the disclosure using a shared set of valves and pipes. For example, the system may be configured to switch between the electricity only charge cycle (such as shown in, for example,  FIG.  15   ), the electricity only discharge cycle (such as shown in, for example,  FIG.  16   ), and the heat engine to ambient cycle (such as shown in  FIG.  21   ). 
     Pumped Thermal Systems with Pressure Regulation Power Control 
     In an aspect of the disclosure, the pressure of working fluids in pumped thermal systems can be controlled to achieve power control. In an example, the power provided to a closed system in charge mode and/or the power extracted from the closed system in discharge and/or generation mode (e.g., work input/output using the shaft  10 ) is proportional to the molar or mass flow rate of the circulating working fluid. The mass flow rate is proportional to density, area, and flow velocity. The flow velocity can be kept fixed in order to achieve a fixed shaft speed (e.g., 3600 rpm or 3000 rpm in accordance with power grid requirements of 60 and 50 Hz respectively). Thus, as the pressure of the working fluid changes, the mass flow rate and the power can change. In an example, as the mass flow rate increases in a discharge and/or generation mode, more load should be added to the system to maintain a constant speed of the rotating shaft, and vice versa. In another example, if load is reduced during operation in a discharge and/or generation mode, the reduced load can cause the shaft speed to increase, thus increasing the mass flow rate. For some period of time, before the heat stored in the thermal capacity of the heat exchangers themselves is dissipated, this increased mass flow rate can lead to an increase in the power delivered, in turn increasing the shaft speed. The shaft speed and the power can continue to increase uncontrollably, resulting in a runaway of the rotating shaft. In some examples, pressure regulation may enable control, and thus stabilization of runaway, through adjustment of the amount (e.g., density) of circulating working fluid in accordance with system requirements. In an example where shaft speed (and turbomachinery, such as a turbine, attached to the shaft) begins to run away, a controller can reduce the mass of circulating working fluid (e.g., mass flow rate) in order to decrease the power delivered, in turn decreasing the shaft speed. Pressure regulation may also allow for an increase in mass flow rate in response to an increase in load. In each of these instances, the flow rates of the HTS and CTS media through the heat exchangers can be matched to the heat capacity of the working fluid passing through the heat exchangers. 
     In some examples, the working fluid pressure in the closed system can be varied by using an auxiliary working fluid tank in fluid communication with the closed system. In this configuration, power input/output can be decreased by transferring the working fluid from the closed cycle loop to the tank, and power input/output can be increased by transferring the working fluid from the tank to the closed cycle loop. In an example, when the pressure of the working fluid is decreased, less heat can be transferred between the thermal storage tanks on the hot and cold sides of the system as a result of the decreased mass flow rate and less power can be input to/output by the system. 
     As the pressure of the working fluid is varied, the compression ratios of turbomachinery components may remain substantially unchanged. In some cases, one or more operating parameters and/or configuration (e.g., variable stators, shaft speed) of turbomachinery components can be adjusted in response to a change in working fluid pressure (e.g., to achieve a desired performance of the system). Alternatively, one or more pressure ratios may change in response to a change in working fluid pressure. 
     In some cases, reduced cost and/or reduced parasitic energy consumption may be achieved using the power control configuration relative to other configurations (e.g., using a choke valve for controlling the flow of the working fluid). In some examples, variation of working fluid pressure while keeping the temperature and flow velocity constant (or near-constant) may lead to negligible entropy generation. In some examples, an increase or decrease in system pressure may lead to changes in, for example, turbomachinery efficiencies. 
       FIG.  27    shows an example of a pumped thermal system with power control. The temperature of the working fluid on the hot and cold sides of the system may remain constant or near-constant for a given period of time regardless of working fluid mass flow rate due to large heat capacities of the heat exchangers  2  and  4  and/or the hot and cold side thermal storage media in the tanks  6 ,  7 ,  8  and  9 . In some examples, the flow rates of the HTS and CTS media through the heat exchangers  2  and  4  are varied in concert with a change in the pressure of the working fluid in order to keep the temperatures in the heat exchangers and working fluid optimized over longer time periods. Thus, pressure can be used to vary the mass flow rate in the system. One or more auxiliary tanks  44  filled with the working fluid  20  (e.g., air, argon or argon-helium mix) can be in fluid communication with a hot (e.g., high pressure) side of the pumped thermal system and/or a cold (e.g., low pressure) side of the pumped thermal system. In some examples, the auxiliary tank can be in fluid communication with the working fluid adjacent to an inlet of the compressor  1  and/or adjacent to an outlet of the compressor  1 . In some examples, the auxiliary tank can be in fluid communication with the working fluid adjacent to an inlet of the turbine  3  and/or adjacent to an outlet of the turbine  3 . In further examples, the auxiliary tank can be in fluid communication with the working fluid in one or more locations system (e.g., one or more locations on the high pressure side of the system, on the low pressure side of the system, or any combination thereof). For example, the auxiliary tank can be in fluid communication with the working fluid on a high pressure side and a low pressure side of the closed cycle. In some cases, the fluid communication on the high pressure side can be provided after the compressor and before the turbine. In some cases, the fluid communication on the low pressure side can be provided after the turbine and before the compressor. In some instances, the auxiliary tank can contain working fluid at a pressure intermediate to the high and low pressures of the system. The working fluid in the auxiliary tank can be used to increase or decrease the amount of working fluid  20  circulating in the closed cycle of the pumped thermal system. The amount of working fluid circulating in the closed cycle loop can be decreased by bleeding the working fluid from the high pressure side of the closed cycle loop into the tank through a fluid path containing a valve or mass flow controller  46 , thereby charging the tank  44 . The amount of working fluid circulating in the closed cycle loop can be increased by bleeding the working fluid from the tank into the low pressure side of the closed cycle loop through a fluid path containing a valve or mass flow controller  45 , thereby discharging the tank  44 . 
     Power control over longer timescales may be implemented by changing the pressure of the working fluid and by adjusting the flow rates of the hot side  21  and cold side  22  thermal storage fluids through the heat exchangers  2  and  4 , respectively. 
     In some examples, flow rates of the thermal storage media  21  and/or  22  may be controlled (e.g., by a controller) to maintain given heat exchanger inlet and outlet temperatures. In some examples, a first controller(s) may be provided for controlling the flow rates (e.g., mass flow rates) of thermal storage media, and a second controller may be provided for controlling the mass flow rate (e.g., by controlling mass, mass flow rate, pressure etc.) of the working fluid. 
     Pumped Thermal Systems with Pressure-Encased Motor/Generator 
     In another aspect of the disclosure, pumped thermal systems with a pressure-encased motor/generator are provided. The pressure-encased motor/generator may be provided as an alternative to configurations where a shaft (also “crankshaft” herein) penetrates through a working fluid containment wall (where it can be exposed to one or more relatively high pressure differentials) in order to connect to a motor/generator outside the working fluid containment wall. In some cases, the shaft may be exposed to pressures and temperatures of the working fluid in the low pressure portion of the working fluid cycle, in the high pressure portion of the working fluid cycle, or both. In some cases, crankshaft seal(s) capable of holding back the pressures which the crankshaft is exposed to inside the working fluid containment wall can be difficult to manufacture and/or difficult to maintain. In some cases, a rotating seal between high and low pressure environments may be difficult to achieve. Thus, coupling of the compressor and turbine to the motor/generator can be challenging. In some implementations, the motor/generator can therefore be placed entirely within the low pressure portion of the working fluid cycle, such that the exterior pressure vessel or working fluid containment wall may not need to be penetrated. 
       FIG.  28    shows an example of a pumped thermal system with a pressure encased generator  11 . The motor/generator is encased within the pressure vessel or working fluid containment wall (shown as dashed lines) and only feed-through electric leads  49  penetrate through the pressure vessel. A thermal insulation wall  48  is added between the motor/generator  11  and the working fluid in the low pressure portion of the cycle. The technical requirements for achieving an adequate seal through the thermal insulation wall can be less stringent due to the pressure being the same on both sides of the thermal insulation wall (e.g., both sides of the thermal insulation wall can be located in the low pressure portion of the cycle). In an example, the low pressure value can be about 10 atm. In some cases, the motor/generator may be adapted for operation at elevated surrounding pressures. An additional thermal insulation wall  50  can be used to create a seal between the outlet of the compressor  1  and the inlet of the turbine  3  in the high pressure portion of the cycle. In some examples, placing the motor/generator on the cold side of the pumped thermal systems may be beneficial to the operation of the motor/generator (e.g., cooling of a superconducting generator). 
     Pumped Thermal Systems with Variable Stator Pressure Ratio Control 
     A further aspect of the disclosure relates to control of pressure in working fluid cycles of pumped thermal systems by using variable stators. In some examples, use of variable stators in turbomachinery components can allow pressure ratios in working fluid cycles to be varied. The variable compression ratio can be accomplished by having movable stators in the turbomachinery. 
     In some cases, pumped thermal systems (e.g., the systems in  FIGS.  17  and  18   ) can operate at the same compression ratio in both the charge and the discharge cycles. In this configuration, heat can be rejected (e.g., to the environment) in section  38  in the charge cycle and in section  39  in the discharge cycle, wherein the heat in section  38  can be transferred at a lower temperature than the heat in section  39 . In alternative configurations, the compression ratio can be varied when switching between the charge cycle and the discharge cycle. In an example, variable stators can be added to both the compressor and the turbine, thus allowing the compression ratio to be tuned. The ability to vary compression ratio between charge and discharge modes may enable heat to be rejected at the lower temperature only (e.g., heat may be rejected in section  38  in the charge cycle but not in section  39  in the discharge cycle). In some examples, a greater portion (or all) of the heat rejected to the environment is transferred at a lower temperature, which may increase the roundtrip efficiency of the system. 
       FIG.  29    is an example of variable stators in a compressor/turbine pair. The compressor  1  and the turbine  3  can both have variable stators, so that the compression ratio for each can be tuned. Such tuning may increase roundtrip efficiency. 
     The compressor and/or the turbine can (each) include one or more compression stages. For example, the compressor and/or the turbine can have multiple rows of repeating features distributed along its circumference. Each compression stage can comprise one or more rows of features. The rows may be arranged in a given order. In one example, the compressor  1  and the turbine  3  each comprise a sequence of a plurality of inlet guide vanes  51 , a first plurality of rotors  52 , a plurality of stators  53 , a second plurality of rotors  52  and a plurality of outlet guide vanes  54 . Each plurality of features can be arranged in a row along the circumference of the compressor/turbine. The configuration (e.g., direction or angle) of the stators  53  can be varied, as indicated in  FIG.  29   . 
     The compressor/turbine pair can be matched. In some cases, an outlet pressure of the compressor can be about the same as an inlet pressure of the turbine, and an inlet pressure of the compressor can be about the same as the outlet pressure of the turbine; thus, the pressure ratio across the turbine can be the same as the pressure ratio across the compressor. In some cases, the inlet/outlet pressures and/or the pressure ratios may differ by a given amount (e.g., to account for pressure drop in the system). The use of variable stators on both the compressor and the turbine can allow the compressor and the turbine to remain matched as the compression ratio is varied. For example, using the variable stators, operation of the compressor and the turbine can remain within suitable operating conditions (e.g. within a given range or at a given point on their respective operating maps) as the compression ratio is varied. Operation within given ranges or at given points on turbomachinery operating maps may allow turbomachinery efficiencies (e.g., isentropic efficiencies) and resulting roundtrip storage efficiency to be maintained within a desired range. In some implementations, the use of variable stators can be combined with other methods for varying the compression ratios (e.g. variable shaft rotation speed, bypassing of turbomachinery stages, gears, power electronics, etc.). 
     Pumped Thermal System Units Comprising Pumped Thermal System Subunits 
     A further aspect of the disclosure relates to control of charging and discharging rate over a full range from maximum charging/power input to maximum discharging/power output by building composite pumped thermal system units comprised of pumped thermal system subunits. In some examples, pumped thermal systems may have a minimum power input and/or output (e.g., minimum power input and/or minimum power output) above 0% of maximum power input and/or output (e.g., maximum power input and/or maximum power output), respectively. In such cases, a single unit by itself may be able to continuously ramp from the minimum power input to the maximum power input and from the minimum power output to the maximum power output, but may not be able to continuously ramp from the minimum power input to the minimum power output (i.e., from the minimum power input to zero power input/output, and from zero power input/output to the minimum power output). An ability to continuously ramp from the minimum power input to the minimum power output may enable the system to continuously ramp from the maximum power input to the maximum power output. For example, if both the output power and the input power may be turned down all the way to zero during operation, the system may be able to continuously vary the power consumed or supplied across a range from the maximum input (e.g., acting as a load on the grid) to the maximum output (e.g., acting as a generator on the grid). Such functionality may increase (e.g., more than double) the continuously rampable range of the pumped thermal system. Increasing the continuously rampable range of the pumped thermal system may be advantageous, for example, when continuously rampable power range is used as a metric for determining the value of grid assets. Further, such functionality may enable the systems of the disclosure to follow variable load, variable generation, intermittent generation, or any combination thereof. 
     In some implementations, composite pumped thermal system units comprised of multiple pumped thermal system subunits may be used. In some cases, each subunit may have a minimum power input and/or output above 0%. The continuous ramping of the power from the maximum power input to the maximum power output may include combining a given quantity of the subunits. For example, a suitable (e.g., sufficiently large) number of subunits may be needed to achieve continuous ramping. In some examples, the number of subunits can be at least about 2, 5, 10, 20, 30, 40, 50, 100, 200, 500, 750, 1000, and the like. In some examples, the number of subunits is 2, 5, 10, 20, 30, 40, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000 or more. Each subunit may have a given power capacity. For example, each subunit can have a power capacity that is less than about 0.1%, less than about 0.5%, less than about 1%, less than about 5%, less than about 10%, less than about 25%, less than about 50%, or less than about 90% of the total power capacity of the composite pumped thermal system. In some cases, different subunits may have different power capacities. In some examples, a subunit has a power capacity of about 10 kW, 100 kW, 500 kW, 1 MW, 2 MW, 5 MW, 10 MW, 20 MW, 50 MW, 100 MW, or more. The continuous ramping of the power from the maximum power input to the maximum power output may include controlling each subunit&#39;s power input and/or output (e.g., power input and/or power output) separately. In some cases, the subunits may be operated in opposing directions (e.g., one or more subunits may operate in power input mode while one or more subunits may operate in power output mode). In one example, if each pumped thermal system subunit can be continuously ramped between a maximum power input and/or output down to about 50% of the maximum power input and/or output, respectively, three or more such pumped thermal system subunits may be combined into a composite pumped thermal system unit that can be continuously ramped from the maximum input power to the maximum output power. In some implementations, the composite pumped thermal system may not have a fully continuous range between the maximum input power and the maximum output power, but may have an increased number of operating points in this range compared to a non-composite system. 
     Energy Storage System Units Comprising Energy Storage System Subunits 
     A further aspect of the disclosure relates to control of charging and discharging rate over a full range from maximum charging/power input to maximum discharging/power output by building composite energy storage system units comprised of energy storage system subunits. In some examples, energy storage systems may have a minimum power input and/or output (e.g., minimum power input and/or minimum power output) above 0% of maximum power input and/or output (e.g., maximum power input and/or maximum power output), respectively. In such cases, a single unit by itself may be able to continuously ramp from the minimum power input to the maximum power input and from the minimum power output to the maximum power output, but may not be able to continuously ramp from the minimum power input to the minimum power output (i.e., from the minimum power input to zero power input/output, and from zero power input/output to the minimum power output). An ability to continuously ramp from the minimum power input to the minimum power output may enable the system to continuously ramp from the maximum power input to the maximum power output. For example, if both the output power and the input power may be turned down all the way to zero during operation, the system may be able to continuously vary the power consumed or supplied across a range from the maximum input (e.g., acting as a load on the grid) to the maximum output (e.g., acting as a generator on the grid). Such functionality may increase (e.g., more than double) the continuously rampable range of the energy storage system. Increasing the continuously rampable range of the energy storage system may be advantageous, for example, when continuously rampable power range is used as a metric for determining the value of grid assets. Further, such functionality may enable the systems of the disclosure to follow variable load, variable generation, intermittent generation, or any combination thereof. 
     In some implementations, composite energy storage system units comprised of multiple energy storage system subunits may be used. In some examples, any energy storage system having power input/output characteristics that may benefit from a composite configuration may be used. In some examples, systems having power input and/or power output characteristics that may benefit from a composite configuration may include various power storage and/or generation systems such as, for example, natural gas or combined cycle power plants, fuel cell systems, battery systems, compressed air energy storage systems, pumped hydroelectric systems, etc. In some cases, each subunit may have a minimum power input and/or output above 0%. The continuous ramping of the power from the maximum power input to the maximum power output may include combining a given quantity of the subunits. For example, a suitable (e.g., sufficiently large) number of subunits may be needed to achieve continuous ramping. In some examples, the number of subunits can be at least about 2, 5, 10, 20, 30, 40, 50, 100, 200, 500, 750, 1000, and the like. In some examples, the number of subunits is 2, 5, 10, 20, 30, 40, 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000 or more. Each subunit may have a given power capacity. For example, each subunit can have a power capacity that is less than about 0.1%, less than about 0.5%, less than about 1%, less than about 5%, less than about 10%, less than about 25%, less than about 50%, or less than about 90% of the total power capacity of the composite energy storage system. In some cases, different subunits may have different power capacities. In some examples, a subunit has a power capacity of about 10 kW, 100 kW, 500 kW, 1 MW, 2 MW, 5 MW, 10 MW, 20 MW, 50 MW, 100 MW, or more. The continuous ramping of the power from the maximum power input to the maximum power output may include controlling each subunit&#39;s power input and/or output (e.g., power input and/or power output) separately. In some cases, the subunits may be operated in opposing directions (e.g., one or more subunits may operate in power input mode while one or more subunits may operate in power output mode). In one example, if each energy storage system subunit can be continuously ramped between a maximum power input and/or output down to about 50% of the maximum power input and/or output, respectively, three or more such energy storage system subunits may be combined into a composite energy storage system unit that can be continuously ramped from the maximum input power to the maximum output power. In some implementations, the composite energy storage system may not have a fully continuous range between the maximum input power and the maximum output power, but may have an increased number of operating points in this range compared to a non-composite system. 
     Control Systems 
     The present disclosure provides computer control systems (or controllers) that are programmed to implement methods of the disclosure.  FIG.  30    shows a computer system  1901  (or controller) that is programmed or otherwise configured to regulate various process parameters of energy storage and/or retrieval systems disclosed herein. Such process parameters can include temperatures, flow rates, pressures and entropy changes. 
     The computer system  1901  includes a central processing unit (CPU, also “processor” and “computer processor” herein)  1905 , which can be a single core or multi core processor, or a plurality of processors for parallel processing. The computer system  1901  also includes memory or memory location  1910  (e.g., random-access memory, read-only memory, flash memory), electronic storage unit  1915  (e.g., hard disk), communication interface  1920  (e.g., network adapter) for communicating with one or more other systems, and peripheral devices  1925 , such as cache, other memory, data storage and/or electronic display adapters. The memory  1910 , storage unit  1915 , interface  1920  and peripheral devices  1925  are in communication with the CPU  1905  through a communication bus (solid lines), such as a motherboard. The storage unit  1915  can be a data storage unit (or data repository) for storing data. The computer system  1901  can be operatively coupled to a computer network (“network”)  1930  with the aid of the communication interface  1920 . The network  1930  can be the Internet, an internet and/or extranet, or an intranet and/or extranet that is in communication with the Internet. The network  1930  in some cases is a telecommunication and/or data network. The network  1930  can include one or more computer servers, which can enable distributed computing, such as cloud computing. The network  1930 , in some cases with the aid of the computer system  1901 , can implement a peer-to-peer network, which may enable devices coupled to the computer system  1901  to behave as a client or a server. 
     The computer system  1901  is coupled to an energy storage and/or retrieval system  1935 , which can be as described above or elsewhere herein. The computer system  1901  can be coupled to various unit operations of the system  1935 , such as flow regulators (e.g., valves), temperature sensors, pressure sensors, compressor(s), turbine(s), electrical switches, and photovoltaic modules. The system  1901  can be directly coupled to, or be a part of, the system  1935 , or be in communication with the system  1935  through the network  1930 . 
     The CPU  1905  can execute a sequence of machine-readable instructions, which can be embodied in a program or software. The instructions may be stored in a memory location, such as the memory  1910 . Examples of operations performed by the CPU  1905  can include fetch, decode, execute, and writeback. 
     With continued reference to  FIG.  30   , the storage unit  1915  can store files, such as drivers, libraries and saved programs. The storage unit  1915  can store programs generated by users and recorded sessions, as well as output(s) associated with the programs. The storage unit  1915  can store user data, e.g., user preferences and user programs. The computer system  1901  in some cases can include one or more additional data storage units that are external to the computer system  1901 , such as located on a remote server that is in communication with the computer system  1901  through an intranet or the Internet. 
     The computer system  1901  can communicate with one or more remote computer systems through the network  1930 . For instance, the computer system  1901  can communicate with a remote computer system of a user (e.g., operator). Examples of remote computer systems include personal computers, slate or tablet PC&#39;s, telephones, Smart phones, or personal digital assistants. The user can access the computer system  1901  via the network  1930 . 
     Methods as described herein can be implemented by way of machine (e.g., computer processor) executable code stored on an electronic storage location of the computer system  1901 , such as, for example, on the memory  1910  or electronic storage unit  1915 . The machine executable or machine readable code can be provided in the form of software. During use, the code can be executed by the processor  1905 . In some cases, the code can be retrieved from the storage unit  1915  and stored on the memory  1910  for ready access by the processor  1905 . In some situations, the electronic storage unit  1915  can be precluded, and machine-executable instructions are stored on memory  1910 . 
     The code can be pre-compiled and configured for use with a machine have a processor adapted to execute the code, or can be compiled during runtime. The code can be supplied in a programming language that can be selected to enable the code to execute in a pre-compiled or as-compiled fashion. 
     Aspects of the systems and methods provided herein, such as the computer system  1901 , can be embodied in programming. Various aspects of the technology may be thought of as “products” or “articles of manufacture” typically in the form of machine (or processor) executable code and/or associated data that is carried on or embodied in a type of machine readable medium. Machine-executable code can be stored on an electronic storage unit, such memory (e.g., read-only memory, random-access memory, flash memory) or a hard disk. “Storage” type media can include any or all of the tangible memory of the computers, processors or the like, or associated modules thereof, such as various semiconductor memories, tape drives, disk drives and the like, which may provide non-transitory storage at any time for the software programming. All or portions of the software may at times be communicated through the Internet or various other telecommunication networks. Such communications, for example, may enable loading of the software from one computer or processor into another, for example, from a management server or host computer into the computer platform of an application server. Thus, another type of media that may bear the software elements includes optical, electrical and electromagnetic waves, such as used across physical interfaces between local devices, through wired and optical landline networks and over various air-links. The physical elements that carry such waves, such as wired or wireless links, optical links or the like, also may be considered as media bearing the software. As used herein, unless restricted to non-transitory, tangible “storage” media, terms such as computer or machine “readable medium” refer to any medium that participates in providing instructions to a processor for execution. 
     Hence, a machine readable medium, such as computer-executable code, may take many forms, including but not limited to, a tangible storage medium, a carrier wave medium or physical transmission medium. Non-volatile storage media include, for example, optical or magnetic disks, such as any of the storage devices in any computer(s) or the like, such as may be used to implement the databases, etc. shown in the drawings. Volatile storage media include dynamic memory, such as main memory of such a computer platform. Tangible transmission media include coaxial cables; copper wire and fiber optics, including the wires that comprise a bus within a computer system. Carrier-wave transmission media may take the form of electric or electromagnetic signals, or acoustic or light waves such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media therefore include for example: a floppy disk, a flexible disk, hard disk, magnetic tape, any other magnetic medium, a CD-ROM, DVD or DVD-ROM, any other optical medium, punch cards paper tape, any other physical storage medium with patterns of holes, a RAM, a ROM, a PROM and EPROM, a FLASH-EPROM, any other memory chip or cartridge, a carrier wave transporting data or instructions, cables or links transporting such a carrier wave, or any other medium from which a computer may read programming code and/or data. Many of these forms of computer readable media may be involved in carrying one or more sequences of one or more instructions to a processor for execution. 
     III. Illustrative Power Generation System 
       FIG.  31    illustrates a power generation system that includes a shared hot side thermal store  160  and a shared cold side thermal store  161 . The shared hot side thermal store  160  may include a hot thermal storage (“HTS”) tank  106  and an HTS tank  107 . HTS tanks  106  and  107  may be configured to hold, dispense, and/or receive an HTS medium  121 . The shared hot side thermal store  160  may also include an HTS supply stream  132  configured to receive a flow of HTS medium  121  from HTS tank  106  and an HTS return stream  133  configured to send a flow of HTS medium to HTS tank  107 . The shared cold side thermal store  161  may comprise a cold thermal storage (“CTS”) tank  108  and a CTS tank  109 . CTS tanks  108  and  109  may be configured to hold, dispense, and/or receive a CTS medium  122 . The shared cold side thermal store  161  may also include a CTS supply stream  137  configured to receive a flow of CTS medium  122  from CTS tank  109  and a CTS return stream  136  configured to send a flow of CTS medium  122  to CTS tank  108 . 
     The power generation system may also include a plurality of power subunits  140 . Each power subunit may include a Brayton system as described with reference to other Figures and description here, except that thermal media stores may be outside of the individual power subunits  140  and shared across the plurality of power subunits  140 . For example, a power subunit  140  may include a generator  11  configured to generate electrical power, a compressor  1 , a hot side heat exchanger  2 , a turbine  3 , a cold side heat exchanger  4 , and a working fluid  20  circulating in a closed cycle path. The closed cycle path may include a working fluid circulating, in sequence, through the compressor, the hot side heat exchanger, the turbine, and the cold side heat exchanger. A recuperator  5  may also be included in the closed cycle path. Examples and further detail of power subunits are shown in at least of  FIGS.  2 ,  3 ,  6 ,  7 ,  13 ,  14 ,  15 ,  16 ,  20 ,  21 ,  22 ,  23 ,  24 ,  25 ,  27   , and  28 , and described herein with respect to each of the afore identified Figures.  FIGS.  2 ,  3 ,  6 ,  7 ,  13 ,  14 ,  15 ,  16 ,  20 ,  21 ,  22 ,  23 ,  24 ,  25 ,  27 , and  28    are illustrative only and other fluids, components and/or fluid paths may be present. Some components, such as a hot side or cold side heat exchanger, may be replaced with other components that serve a similar thermal purpose. Each of the above identified fluids, components, and/or fluid paths may be the same as or similar to previously described closed cycle (e.g., Brayton cycle) elements, such as working fluid  20 , compressor  1 , hot side heat exchanger  2 , turbine  3 , cold side heat exchanger  4 , and recuperator  5 . 
     Each power subunit  140  may also include a valve arrangement configurable to be in a connected state or an isolated state. The valve arrangement may include valves  142   a ,  142   b ,  142   c , and  142   d . In the connected state the valve arrangement may be configured to connect the power subunit  140  to the HTS supply stream  132 , the HTS return stream  133 , the CTS supply stream  137 , and the CTS return stream  136 . For example, the valve arrangement may be configured to open the respective valves  142   a - d  for a given power subunit and connect the hot side heat exchanger  2  to the HTS supply stream  132  and the HTS return stream  133  and to connect the cold side heat exchanger  4  to the CTS supply stream  137  and the CTS return stream  136 . Hot side valves  142   a  and  142   b  be connected to a hot side heat exchanger  2 , and each hot side valve  142   a  and  142   b  may be connected or switchable to allow the HTS medium  121  to flow in counterflow for a given operation mode (i.e., charge or discharge). Cold side valves  142   c  and  142   d  be connected to a cold side heat exchanger  4 , and each cold side valve  142   c  and  142   d  may be connected or switchable to allow the CTS medium  122  to flow in counterflow for a given operation mode (i.e., charge or discharge). 
     In the isolated state, the valve arrangement may be configured to isolate the power subunit  140  from the HTS supply stream  132 , the HTS return stream  133 , the CTS supply stream  137 , and the CTS return stream  136 . For example, the valve arrangement may be configured to close the valves  142   a - d  for a respective power subunit and isolate the hot side heat exchanger  2  from the HTS supply stream  132  and the HTS return stream  133  and to isolate the cold side heat exchanger  4  from the CTS supply stream  137  and the CTS return stream  136 . 
     Each power subunit  140  may also include an electrical bus  150  electrically coupled to each generator  11  of each power subunit  140  of the plurality of power subunits. The electrical bus  150  may be configured to carry electrical power generated by each generator  11  to an electrical node  152 . For example, the electrical bus  150  may carry DC power, AC power, or preferably three-phase AC power at 60-600 Hz. Electrical node  152  may comprise a frequency converter and the frequency converter may be configured to convert electrical power carried by the electrical bus  150  to electrical power for distribution to an electrical grid system, for example, converting the power to 60 Hz. 
     The power generation system may further comprise one or more CTS heat exchangers  155  configured to remove excess heat from the CTS medium  122 . The CTS heat exchangers  155  may be, for example, a cooling tower or a radiator. In one embodiment, a CTS heat exchanger  155  may be coupled to CTS tank  108 . In another embodiment, a CTS heat exchanger  155  may be coupled to the CTS return stream  136 . 
     Advantages of the disclosed power generation system using power subunits versus a large power generation system include less time needed to power up the system, continued power generation in the event that one power subunit fails, and out-of-phase transient cancelling. In addition, the power subunits be used in parallel or in sequence (or in some combination), thus allowing for varying power levels. Output power can be at low or high frequency. 
     IV. Conclusion 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.