WO2011149791A1

WO2011149791A1 - High-density energy storage and retrieval

Info

Publication number: WO2011149791A1
Application number: PCT/US2011/037437
Authority: WO
Inventors: Anton Pietsch
Original assignee: 5Th Generation Energy, Inc.
Priority date: 2010-05-28
Filing date: 2011-05-20
Publication date: 2011-12-01
Also published as: CN103154657A; US20110289924A1; EP2577211A1

Abstract

High temperature energy can be provided using a containment vessel (204), a heat retention matrix (202) contained within the containment vessel (204), a volume of a working fluid (206) contained within the containment vessel (204) and in contact with the heat retention matrix (202), and, optionally, a reactive compound removal system (220) that removes reactive compounds from the working fluid. The heat retention matrix (202) includes an allotropic form of carbon. The working fluid (206) includes nitrogen gas and one or more noble gases. Related systems and methods are also described.

Description

HIGH-DENSITY ENERGY STORAGE AND RETRIEVAL

CROSS-REFERENCE TO RELATED APPLICATION

[001] The current application claims priority under 35 U.S.C. § 1 19(e) to U.S. Provisional patent application serial number 61/396,523, filed on May 28, 2010, entitled "Compositions and Methods for High-Energy-Density Storage and Retrieval," which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

[002] The subject matter described herein relates to storage and subsequent retrieval of energy, for example storage of energy as heat or thermal energy.

BACKGROUND

[003] Existing renewable energy sources (e.g. solar, wind, tidal, or the like) often produce energy in an episodic fashion, for example on a diurnal cycle or with a dependency on weather patterns or other non-constant factors. For example, many renewable sources are tied either directly (e.g. solar energy) or indirectly (e.g. wind energy) to daily or longer cycles of solar energy flux. Non-renewable energy sources, such as for example fossil fuel fired plants and nuclear generation facilities, are typically operated at a less than peak load late at night due to the light demand and need to keep the system "hot," for example to avoid startup delays and inefficiencies when demand increases. In many cases, running a generation facility at less than peak load results in lowered generation efficiencies. In some areas, additional "peaker" plants may be maintained by local utility providers for activation when peak loads occur on the electrical grid, for example during the warmer portions of the day when air conditioning loads are highest. Such additional plants can be expensive to build, particularly in light of their episodic usage. These and other factors contribute to situations in which electrical generation capacity requirements are dictated by the load occurring at peak periods rather than according to consideration of providing a total integrated energy output throughout a daily or longer cycle at the highest possible efficiency in terms of usage of fuel and capital expenditures.

[004] Storage of at least some of the generated energy at the time of generation can enable distribution of the collected energy to match a temporal energy demand pattern that may not otherwise coincide with a temporal energy generation pattern, especially to meet demanding power quality requirements. Location-independent bulk storage of energy that is size and cost effective can be helpful in resolving this temporal dislocation of demand and generation capacity, for example by providing load leveling capabilities that can enhance energy generation efficiency, for example for fossil fuel fired generation facilities, by allowing operation of generators at peak efficiency independent of time of need or time of usage. For example, energy generated in excess of current demand can be stored for use in augmenting the generation capacity at times of peak demand to smooth out the demand curve.

SUMMARY

[005] In one aspect, a system includes a containment vessel that contains a heat retention matrix contained within the containment vessel for storing input energy as thermal energy. The heat retention matrix includes an allotropic form of carbon. A volume of a working fluid that includes nitrogen gas and a noble gas is contained within the containment vessel and in contact with the heat retention matrix. The working fluid is generally inert to reaction with the allotropic form of carbon. The system can optionally include a reactive compound removal system that removes reactive compounds from the working fluid. [006] In an interrelated aspect, a method of storing energy in an energy storage system that includes a containment vessel containing a volume of a working fluid and a heat retention matrix is described. The method includes receiving input energy at the energy storage system and retaining the input energy as thermal energy by the heat retention matrix and the working fluid. The heat retention matrix includes an allotropic form of carbon, and the working fluid includes nitrogen gas and a noble gas and is generally inert to reaction with the allotropic form of carbon at an operating temperature of the energy storage system. The method includes delivering output energy from the energy storage system, and the method can optionally further include treating the working fluid using a reactive compound removal system to remove reactive compounds from the working fluid.

[007] In another interrelated aspect, a system includes a containment vessel and a means for retaining received input energy as thermal energy. The means for retaining the thermal energy, which is contained within the containment vessel, includes an allotropic form of carbon. The system also includes a volume of a working fluid contained within the containment vessel and in contact with the heat retention matrix. The working fluid includes nitrogen gas and at least one noble gas and is generally inert to reaction with the allotropic form of carbon at an operating temperature of the energy storage system. The system can also optionally include means for removing one or more reactive compounds from the working fluid.

[008] In some variations one or more of the features discussed in the following paragraphs can optionally be included in a system or a method, either individually, or in combinations of two or more of such features that are not otherwise incompatible or mutually exclusive. [009] A reactive compound removal system or a means for removing one or more reactive compounds from a working fluid can optionally include a filtration medium that can optionally include activated carbon.

[0010] An energy input system can optionally be provided for delivering input energy to a heat retention matrix from an energy source. An energy output system for retrieving output energy from a heat retention matrix for delivery to an energy demand can also optionally be provided. An energy input system and an energy output system can optionally be independently operable such that receiving of input energy by an energy input system does not interfere with or preclude delivery of output energy by an energy output system.

[0011] A working fluid can optionally include an energy input fluid. An energy input system can optionally include an energy input working fluid circulator that passes a working fluid through at least one of an input energy heat exchanger and a first electrical energy conversion system that converts input energy from electrical energy to thermal energy that is transferred to a working fluid. An energy output system can optionally include an energy output working fluid loop circulator that passes an energy output fluid through at least one of an output energy heat exchanger and a second electrical energy conversion system that converts thermal energy in an energy output fluid to electrical energy for delivery to an energy demand.

[0012] An energy input system can optionally deliver input energy to a heat retention matrix or a means for retaining received input energy from an energy source. Input energy can optionally include electrical input energy. An energy input system can optionally include an input energy conversion system that converts electrical input energy to thermal energy. An input energy conversion system that can optionally include at least one of an electrical induction system can optionally include an inductive heating element disposed inside a containment vessel and that is inductively heated using a magnetic field generated using electrical input energy, a resistive heating system that can optionally include a resistive heating element disposed inside a containment vessel and that is resistively heated by passing the electrical input energy through the resistive heating element, a plasma heating system can optionally include a plasma generating system powered by the electrical input energy for creating a plasma within at least one of a containment vessel and a plasma chamber through which a working fluid passes, and an energy input working fluid circulator that passes a working fluid through a first electrical energy conversion system that converts input energy from electrical energy to thermal energy that is transferred to a working fluid.

[0013] Input energy can optionally include heat and an energy input system can optionally include an input energy heat transfer system that transfers heat to at least one of a working fluid and a heat retention matrix or a means for retaining received input energy. An energy output system can be included for retrieving output energy from a heat retention matrix or from a means for storing thermal energy for delivery to an energy demand.

[0014] An energy demand can optionally be for electrical output energy, and an energy output system can optionally include an output energy conversion system that converts stored thermal energy to electrical output energy. An output energy conversion system can optionally include at least one of a Stirling engine, a Brayton engine, a Rankine engine, an Otto engine, a boiler, a fluidic heat pump, a solid state heat pump, a turbine-based generator, a piston-based generator, a thermionic device, and a thermo-photovoltaic device. An energy output system can optionally include a heat engine that can optionally be operated with a Carnot efficiency greater than 30% or a Carnot efficiency greater than 45% or a Carnot efficiency greater than 55%.

[0015] An energy demand can optionally be for thermal output energy, and an energy output system can optionally include at least one of a heat exchanger manifold and a circulation pump for transferring stored thermal energy to an energy demand. A heat exchanger manifold can optionally include a coil containing a heat transfer fluid. A coil or other heat exchanger type can optionally be embedded in or between one or more thermal insulation layers of a containment vessel to absorb heat into a heat transfer fluid for extraction to at least one of an electrical generation system that converts the absorbed heat to electricity and a thermal energy utilization system that uses the absorbed heat to perform useful work.

[0016] A containment vessel can optionally include an inner layer comprising a refractory material, an outer layer comprising a structural material, and an insulation layer interposed between an inner layer and an outer layer. An inner layer can optionally include at least one of glassy carbon and silicon carbide. An insulation layer can optionally include a porous material that constricts the mean free path of molecules of a working fluid. A porous form of amorphous carbon can optionally include at least one of a carbon fiber matrix, a carbon felt, and a carbon foam.

[0017] A working fluid can optionally include nitrogen gas (N₂) at a nitrogen mole fraction of approximately 35% or greater, argon gas (Ar) at an argon mole fraction of approximately 35% or greater, and neon gas (Ne) at a neon mole fraction of approximately 2% or greater. Alternatively a working fluid can optionally include nitrogen gas (N₂) at a nitrogen mole fraction of approximately 50%, argon gas (Ar) at an argon mole fraction of approximately 45%, neon gas (Ne) at a neon mole fraction of approximately 4%, and helium gas (He) at a helium mole fraction of approximately 1%. A working fluid can optionally further include at least one of krypton gas (Kr) at a krypton mole fraction that is greater than zero and less than approximately 1% and xenon gas (Xe) at a xenon mole fraction that is greater than zero and less than approximately 1%. A working fluid substantially can optionally consist substantially of non-toxic compounds and elemental gases found in ambient air. [0018] A control system can optionally maintain a temperature of a heat retention matrix or a means for storing thermal energy in a range of approximately 900 K to approximately 1500 K during energy storage operations. A control system can additionally or alternatively optionally maintain a temperature of a heat retention matrix or a means for storing thermal energy below approximately 2500 K during energy storage operations.

[0019] Received input energy can include electrical input energy and an energy input system can include an input energy conversion system that converts the electrical input energy to the thermal energy. A means for storing thermal energy or a heat retention matrix can optionally have a maximum operating temperature of less than approximately 1800 K. An energy output system can optionally be included for retrieving output energy from a means for retaining thermal energy or a heat retention matrix at a rate of up to approximately 50 kilowatts per hour for a duration of at least 12 hours. A means for storing thermal energy or a heat retention matrix that performs thusly can optionally include a graphite core occupying a volume of less than approximately 2.5 cubic meters. Input energy can optionally include electrical input energy, and an energy input system can optionally include an electrical resistance type conversion system that converts the electrical input energy to thermal energy. A means for storing thermal energy or a heat retention matrix can optionally include a graphite core that, when heated to a temperature of approximately 1200 , can cooperate with an energy output system to retrieve output energy from the graphite core at a rate up to approximately 5 kilowatts per hour for a duration of at least 12 hours. A means for storing thermal energy or a heat retention matrix that performs thusly can optionally include a graphite core occupying a volume of less than approximately 0.30 cubic meters.

[0020] Implementations of the current subject matter can provide one or more advantages. For example, an incoming source of energy can be readily converted or transferred from the form that the entering energy exists into a chosen media for storage of the energy and later retrieval. Storage operations can be performed at least at the maximum rate of power generation and/or the maximum gradient (e.g. rate of change) of the power entry or retrieval rate. Energy can be retained within a storage system with reduced, minimized, or in some cases even substantially eliminated rates of leakage or other losses. As a consequence, longer storage times can be achieved in some implementations, thereby providing greater flexibility of utilization of the stored energy. Implementations of the current subject matter can be capable of converting or transferring stored energy into a form that is readily utilized, transferred, or transmitted to the point of practical utilization at the maximum rate and rate gradient of desired power delivery, for example, as is limited by the physics of the conversion or transfer device chosen. Mechanisms for installation, service, operation, repair, removal and disposal of implementations of the current subject matter can be practical, cost efficient, and associated with relatively low risks to personnel or the environment.

[0021] Some implementations can include the ability to renew, restore, recycle, etc. one or more components of a storage system, potentially including but not limited to the storage media, the working fluid, and containment vessel and reactive compound removal system components and mechanisms. An additional possible advantage of implementations of the current subject matter is the ability to locate an energy storage system in close proximity to any of a generating source, a distribution system or network, and an energy usage location. Location of the stored energy can affect the overall efficiency with which it is generated, transmitted, and/or used. Also, with ever-growing social, political, etc. limitations on siting for energy-related projects, maximizing siting flexibility for an energy storage system can also be beneficial.

[0022] The details of one or more variations of the subject matter described herein are set forth in the accompanying drawings and the description below. Other features and advantages of the subject matter described herein will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

[0023] The accompanying drawings, which are incorporated in and constitute a part of this specification, show certain aspects of the subject matter disclosed herein and, together with the description, help explain some of the principles associated with the disclosed implementations. In the drawings,

[0024] FIG. 1 shows a diagram illustrating features that can be present in a power grid;

[0025] FIG. 2 shows a diagram illustrating features of an energy storage system;

[0026] FIG. 3 shows a process flow diagram illustrating features of an energy storage method;

[0027] FIG. 4 shows a diagram of an energy storage system incorporating illustrative examples of energy input and energy output systems;

[0028] FIG. 5 shows a diagram of an energy storage system incorporating illustrative examples of alternative energy input and energy output systems;

[0029] FIG. 6 shows a layer of a heat retention matrix material that includes pass- through holes for heat transfer rods or pipes;

[0030] FIG. 7 shows a three-dimensional detail of a manifold assembly;

[0031] FIG. 8 shows a cross-sectional detail view of a manifold assembly;

[0032] FIG. 9 shows a cross-sectional view of a heat retention matrix and heat transfer pipes or rods passing therethrough; [0033] FIG. 10 shows an example of an energy storage system including multiple discrete heat retention matrix cores;

[0034] FIG. 1 1 shows an example of an energy conversion system including a compressor pump and magneto-plasma dynamic thermal generator;

[0035] FIG. 12 shows an example of an energy storage system including a compressor pump and magneto-plasma dynamic thermal generator as the energy input system;

[0036] FIG. 13 shows a diagram of an energy generation system incorporating an energy storage system;

[0037] FIG. 14 shows an example of an energy storage system incorporating a single- ended interconnection manifold;

[0038] FIG. 15A and FIG. 15B show detail views of a single-ended interconnection manifold;

[0039] FIG. 16 shows an energy storage system incorporating integrated power fill and drain mechanisms;

[0040] FIG. 17 shows an energy storage system incorporating aspects of combined heating and cooling approaches; and

[0041] FIG. 18 is a chart showing relationships between insulation thicknesses and theoretical operating temperatures.

[0042] When practical, similar reference numbers denote similar structures, features, or elements. DETAILED DESCRIPTION

[0043] FIG. 1 shows illustrative features of an electrical grid system 100 in which an implementation of the current subject matter can be employed to beneficial effect. A power generation source 102, which can be any kind of power generation station, such as for example a solar, wind, or other renewable energy installation of any size; a fossil fuel, nuclear, or other type of power plant of any size, or any other type of electrical energy or other power source, can be connected via a first transmission substation 104, which can in some cases include step-up transformer to convert a generation voltage to a transmission voltage, to one or more transmission lines 106, which can be part of a transmission grid. The transmission grid can also include a second transmission substation 110, for example to provide voltage step-down from the transmission voltage. Power can be delivered to industrial customers 112 that can typically use power at the stepped down voltage and/or to commercial customers 1 14 and residential customers 1 16 after further stepping down the voltage via transformers at a distribution substation 120.

[0044] In various implementations of the current subject matter, energy storage approaches incorporating one or more features described herein can be used at one or more locations within an electrical grid system 100 such as that of FIG. 1. For example, the first transmission substation 104 associated with the generation source 102 can include such an energy storage system to provide a load-leveling mechanism for a large section of the grid, which can allow operation of the generating station at a higher efficiency peak or near peak output level. In this manner, a power plant can be designed with a generation capacity dictated by a total time-integrated energy output rather than as a function of a peak demand that may occur for only a short time during a daily, weekly, monthly, seasonal, annual, or other cycle. Excess heat can thereby be stored at periods of energy demand that are less than the peak energy generation rate of the source 102 and later retrieved during periods of energy demand that exceed the peak energy generation of the source 102.

[0045] In other implementations, the second transmission substation 110 or one or more distribution substations 120 can include an energy storage system that can provide the capability to receive energy inputs from one or more episodic sources and to convert such energy inputs into dispatchable power, for example to provide benefits that can include but are not limited to load-leveling, improved power quality, baseload generation, power backup capabilities (e.g. for commercial, industrial, or even residential applications). Positioning a system having one or more of the features described herein closer to the location of energy usage provides a variety of options for advantageous use, including but not limited to making local use of "waste heat" derived from the storage system for other beneficial uses, and the possibility of a combined heat and power system or the ability for a power consumer to incorporate on-site power co-generation (e.g. using a renewable energy source, local generation capabilities, or the like).

[0046] As illustrated in FIG. 2, an energy storage system 200 consistent with one or more implementations of the current subject matter can generally include a heat retention matrix 202 contained within a containment vessel 204 that can include one or more layers of insulation to reduce the amount of heat that escapes from the energy storage system to the environment through the walls or other containment surfaces of the containment vessel 204. A working fluid 206 is also present within the containment vessel and at least some of the time directly contacts the heat retention matrix 202. The energy storage system 200 can include one or more energy input systems or devices 210 to deliver energy from an energy source 212 for retention as heat by at least one of the heat retention matrix 202 and the working fluid 206. The energy storage system 200 can also include one or more energy retrieval systems or devices 214 to extract thermal energy from at least one of the heat retention matrix 202 and the working fluid 206 and to either deliver that thermal energy as heat energy or to convert the retrieved thermal energy to electrical energy to meet one or more energy demands 216. To facilitate the use of a working fluid 206 that includes one or more components that may not be perfectly inert at the elevated temperatures expected to be achieved within the containment vessel 204, the energy storage system 200 can also include a reactive compound removal system 220 that removes or otherwise reduces concentrations of oxidizing compounds and/or other reactive compounds or atoms that exist or are formed within the working fluid. In some implementations the reactive compound removal system 220 can include a filtration medium 222 capable of removing reactive components from the working fluid 206.

[0047] In the example shown in FIG. 2, the energy storage system can optionally include one or more reactive compound removal systems 220 positioned externally to the containment vessel 204, for example to receive working fluid 206 from the interior of the containment vessel 204 via a conduit loop that includes an output conduit 224 and a return conduit 226. The output conduit can in some implementations include or otherwise pass through a heat exchanger system 230 to extract heat from the working fluid 206 output through the output conduit 224 to ensure that the working fluid 206 is at an appropriate temperature for effective removal of reactive components from the working fluid 206, for example by filtration with a filtration medium 222. Alternative approaches to cooling the working fluid (if necessary for effective removal of reactive entities by the reactive compound removal system 220) can also be used, including but not limited to those discussed in greater detail below. For example, the reactive compound removal system 220 can be incorporated into or otherwise receive lower temperature working fluid 206 from the one or more energy retrieval systems or devices 214 after useful energy has been extracted from the heat exchange fluid 206 by the one or more energy retrieval systems or devices 214. [0048] FIG. 3 shows a process flow chart 300 illustrating method features, one or more of which can be present in implementations of the current subject matter. At 302, an energy storage system 200 can receive input energy. At 304, a heat retention matrix 202 and a working fluid 206 of the energy storage system 200 retain the input energy as thermal energy. The heat retention matrix 202 and at least some of the working fluid 206 can be contained within a containment vessel 204 of the energy storage system 200. The heat retention matrix 202 can in some implementations include an allotropic form of carbon that contacts the working fluid 206 as discussed in greater detail below. At 306, the working fluid 206 can optionally be treated using a reactive compound removal system 220 to remove reactive compounds from the working fluid 206. At 310, output energy can be delivered from the energy storage system 200, for example in at least partial satisfaction of an energy demand.

[0049] In some implementations, the heat retention matrix 202 can include an allotropic form of carbon, for example graphite, carbon nanotubes, Buckminsterfullerene, doped diamond, compounds incorporating silicon or nitrogen (e.g. nitrides) and the like, and other carbon and non-carbon materials with high-temperature, high stability structures. Pure or substantially pure graphite, along with many other allotropic forms of carbon, has both an extremely high melting, or triple (gas, liquid and solid) point temperature as well as being exceptionally stable (e.g. relatively chemically non-reactive) at high temperatures, for example in excess of 4000 K. Most forms of carbon including graphite also exhibit above average constant pressure and constant volume heat capacities (Cp and Cy) that each increase in value with temperature, which in turn increases the total available energy storage capacity of a mass of graphite as its temperature increases with storage of additional heat energy. Graphite additionally has a very low thermal coefficient of expansion making it an ideal working substance for operating over a very large temperature range. These features, as well as the relative abundance of graphite and its correspondingly low cost, makes graphite a desirable material for use in a sensible heat storage system such as those described herein. Industrial grade graphite is inexpensive and can be readily purified of ash, thereby creating a porous structure that can be utilized to advantage in varying some heat transfer characteristics of the bulk graphite. Graphite can be mined or manufactured into blocks of desired shape and structure such that the graphene planes are perpendicular to the input and output gas flow using known methods and those described herein. Multiple blocks can be assembled with small care for fit into larger layers.

[0050] The high available heat storage temperature of graphite and other carbon allotropes can enable a very high Carnot efficiency to be achieved for use with heat engines. Graphite has been little used in sensible heat storage applications on a commercially viable scale because of the perceived need for graphite at high temperature to be held in a helium or helium-xenon gas environment to provide efficient heat transfer without undesirable degradation of the graphite or chemical reaction with other materials used in heat transfer systems and processes over long periods of time. This concern has been especially true at the extremely high temperatures at which carbon is capable of storing the greatest amount of energy. Gas atmospheres comprised entirely or substantially of helium or a helium xenon mixture can be quite expensive to generate and maintain.

[0051] At extremely high temperatures, chemical reactions between various containment materials, impure graphite, and even exceptionally stable gases such as the noble gases and nitrogen can possibly produce potentially toxic and/or reactive or chemically unstable compounds. For example, nitrogen gas begins to dissociate into individual atoms from the diatomic N₂ molecule at approximately 3000 K and can consequently generate a wide variety of charged ions and free radicals as well as a variety of cyano-type compounds when in the presence of graphite or other carbon materials at high temperature. Such compounds can be toxic and potentially explosive when they come in contact with an oxidation agent. Accordingly, in some implementations, an energy storage system can advantageously be operated in a temperature range at or below approximately 2500 K to reduce the formation of the above-noted and other reactive compounds. An acceptably high Carnot efficiency can be achieved using such temperatures according to one or more implementations.

[0052] Some implementations of the current subject matter include the use of a multi- component working fluid 206. Such a working fluid 206 can include mixtures based on one or more of nitrogen and argon and optionally including one or more other common noble gases found in the air (e.g. neon, helium, krypton, xenon, and the like) and may be produced at greatly reduced cost compared with previously used helium-xenon mixtures. The addition of noble gases has been found to enhance processes for the intake and output of heat energy via Rayleigh-Benard convection at heat exchanger walls by lowering the exchange temperature drop significantly. Nitrogen, argon and some amount of trace noble gases can be extracted efficiently from air by removing or substantially removing water, carbon dioxide, oxygen, and other undesirable gases through a series of phase separations at various temperatures followed by scrubbing and filtering. Gas extraction equipment can be utilized in association with implementations of the current subject matter to provide on-site generation and replenishment of the working fluid 206, which can be an advantageous approach that enables compensation for the inevitable loss of working fluid components through imperfections in working fluid loop construction as well as natural diffusion processes, particularly for low molecular or atomic weight gases such as helium, neon, and the like.

[0053] Consistent with some implementations of the current subject matter, the addition of small amounts of the non-radioactive noble gases to the working fluid 206 can provide a variety of advantages. For example, a working fluid 206 having a wide variety of elemental and molecular gases having both a wide spectrum of atomic masses and also of available quantum electronic orbital excitation states (heat is held in the translational, rotational and vibrational quanta for ideal gases) and energies can facilitate a nearly continuous range of energy transfer states when the molecules or atoms collide with solid material, such as for example the heat retention matrix 202, other metallic surfaces, hybrid plasmon surfaces, graphite diffusion spheres, or the like. A working fluid 206 containing a variety of gases undergoing a Penning ionization process can experience very efficient and rapid thermal transfer among the various gases, particularly when applied via a plasma heating technique such as is described in greater detail below. The presence of one or more noble gases as described herein can present a wide variety of quantum excitation states in translational, rotational and vibrational modes that can facilitate both the transfer of heat energy as well as facilitate inter-gas stimulation and heating similar to that found in the Penning Ionization effect.

[0054] In one implementation, a working fluid can include the following components: nitrogen gas (N₂) at a mole fraction of approximately 50%, argon gas (Ar) at a mole fraction of approximately 45%, neon gas (Ne) at a mole fraction of approximately 4%, and helium gas (He) at a mole fraction of approximately 1%. In another implementation, the working fluid 206 can include the following components: nitrogen gas (N₂) at a nitrogen mole fraction of approximately 35% or greater, argon gas (Ar) at an argon mole fraction of approximately 35% or greater, and neon gas (Ne) at a neon mole fraction of approximately 2% or greater. Beneficial effects have been found from including at least trace amounts of krypton gas (Kr) and xenon gas (Xe). Thus, each implementation of the working fluid described herein can optionally include a non-zero mole fraction of less than approximately 1% of either or both of krypton gas and xenon gas. Inclusion of even small amounts of the high atomic weight noble gases can have significant effects on the average molecular weight of the working fluid, which can assist in providing a wide range of heat transfer capabilities.

[0055] The reactive compound removal system 220, which can in some implementations be a filtration system that brings the working fluid 206 into contact with a filtration medium 222 such as activated charcoal, activated carbon, and the like, can remove or substantially reduce concentrations of reactive compounds, toxic compounds, and other contaminants in the working fluid 206. In an implementation, the reactive compound removal system 220 can dynamically scavenge reactive or otherwise unwanted compounds during routine operation of the energy storage system 200 to trap or otherwise render inert impurities released or generated by the high heat of operation of the energy storage system 200. Throughout this disclosure, the term "reactive compound" is intended to refer generally to compounds or single atoms having reactive, oxidizing, toxic, explosive, or otherwise undesirable characteristics unless otherwise specifically noted or made clear by the context within which the term is used. Hydrogen atoms or molecules, for example, tend to react with graphite to form hydrocarbons that can diminish the heat transfer characteristics of a carbon- containing heat retention matrix such as graphite or the like and that can pollute the working fluid 206.

[0056] In some implementations, activated charcoal, activated carbon, or other absorptive materials can act as a useful filter medium 222 for use in a reactive compound removal system 220 to adsorb and/or absorb a variety of chemical moieties. While activated charcoal or carbon can usefully remove certain contaminants, even at operating temperatures that are elevated relative to commonly encountered in the ambient environment, reactive compound removal can in some implementations be improved by lowering the temperature of the fluid being treated by the reactive compound removal system 220. Activated charcoal and activated carbon are characterized by high surface area and van der Waals potential for binding. Highly amorphous carbon is also typically characterized by its low conductivity of heat and electricity. These two properties can be modulated by compression or changes in pressure, for example to affect the absorptive characteristics of the compound removal system. Other options for a reactive compound removal system 2220 can include, but are not limited to, chemical scrubbing systems, for example those using a reactive or catalytic adsorbent or injection of a gas or liquid phase reactant designed to react with and either convert or enable removal of unwanted reactive compounds from the working fluid 206.

[0057] A reactive compound removal system 220 can be incorporated into an energy storage system 200 such that scrubbing or other removal or reduction in concentration of reactive compounds from the working fluid 206 is not performed on an entire volume of the working fluid simultaneously. In some implementations, the reactive compound removal system 220 can treat a relatively small portion of the working fluid 206 at a reduced temperature. The working fluid 206 can thereby be continuously cleaned in small successively treated volumes without interfering with the main functions or creating additional impedance in heat transfer loops or other optional aspects of an energy storage system 200 such as are described in greater detail below.

[0058] Implementations of energy storage systems consistent with one or more features described herein can involve potentially extreme amounts of energy in thermal storage. It is therefore important that the method of containment be capable of safely restricting the transport of that high thermal energy load to the environment. As such, it can be advantageous to insulate and/or separate the heat retention matrix 202 and/or the working fluid 206 from the external environment by a redundant set of materials. In some implementations, a containment vessel 204 can include an inner layer formed at least partially of a refractory material, an outer layer formed at least partially of a structural material, and an insulation layer interposed between the inner layer and the outer layer. [0059] The inner layer can form a direct enclosure of each of one or more cores of the heat retention matrix material (e.g. graphite) and can, in some implementations, include a material such as glassy carbon, silicon carbide, structurally strong amorphous or amorphous- like carbon structures, boron nitride, high-temperature aero gels made from carbon and related complexes, and similar materials with one or more characteristics such as resistance to elevated temperatures, high strength, and low porosity to act as an effective retention barrier for the working fluid 206 and also as a thermal barrier to reduce heat transfer from the working fluid 206 and the heat retention matrix to other layers of the containment vessel 204 and to the outside environment. The inner layer can constrict an input flow of the working fluid through the heat retention matrix 202 and is therefore advantageously relatively non- porous in the radial direction (e.g. perpendicular to the direction of working fluid flow across the heat retention matrix 202.

[0060] The use of glassy carbon in some implementations can permit cores of a heat retention matrix 202 to be lined with relatively inexpensive heat shield raw material. In the case of a graphite or similar heat retention matrix 202, glassy carbon can provide similar operational characteristics of high strength, low thermal expansion coefficient and some thermal conductivity as it ages that closely mimics those of the heat retention matrix material itself. Silicon carbide can form a high-impedance barrier to the free flow of the working fluid 206 as part of a heat input loop (described in more detail below) and can be used in some implementations to enable the uniform heating of the heat retention matrix 202. In some implementations, the structural strength of a boron nitride inner layer can be improved by combination of boron nitride with another material, such as for example a carbon fiber support matrix.

[0061] Suitable materials for the insulation layer can include, a relatively thick micro- porous carbon felt, carbon foam, graphite, or other high strength, refractory material that is resistance to elevated temperatures and that permits the core energy in the form of heat flux to exit as slowly as is economically feasible. Due to the bulk nature of this barrier it is desired to be light and inexpensive to manufacture and install. Carbon-based substances such as graphite and carbon foam or carbon felt typically have a triple point near 4000 K at a wide range of pressures, and so remain solid at practical temperatures that storage container materials can withstand without extensive, expensive vessel cooling systems that are subject to a non-zero chance of failure. Insulation materials such as those described herein and structural or functional equivalents thereto can be used to hold the working fluid 206 at a variety of pressures that can facilitate efficient heat transfer, both to and from the heat retention matrix 202 and, during energy retrieval processes consistent with implementations of the current subject matter, to one or more heat engines or generators. Operational pressures can, in some implementations, be in a range of approximately 0.1 bar to approximately 20 bar absolute pressure.

[0062] The insulation layer can optionally include more than one layer, optionally of different materials. For example, the insulation layer can also include a secondary refractory thermal barrier made of standard furnace material such as "kaowool" that is able to withstand all but the highest of core temperatures. For additional safety, a low temperature glass wool or similar flexible matting can be added outside of the inner layers of insulation to fill in any gaps between the inner layers of insulation and the outer layer of the containment vessel 204. In addition to an insulation layer interposed between the inner layer and the outer layer, additional insulating and/or chemically inert material can be layered on the outside of the containment vessel 204 to reduce the cost and toxicity of the structural containment material of the outer layer and/or to protect it from environmental damage (e.g. corrosion or other chemical attack, physical damage, or the like). [0063] In an alternative implementation, one or both of the inner layer and the insulation layer can include both containment and insulation capabilities. Structurally strong amorphous or amorphous-like carbon structures can provide a porous refractory insulation material. Examples of such materials include, but are not limited to, carbon felt, carbon foam, or other like materials such as carbon fibers mixed with sawdust or other appropriate lightweight starting material for the formation of amorphous carbon by anaerobic heating to high temperature on the order of approximately 1200 K to fully carbonize the insulator. A material can be formed in this manner that has high strength due to the presence of the carbon fibers and micro-porous cells due to the presence of amorphous carbon, which can itself be a highly insulating material. The micro-pores (for example, pores with diameters on the order of 1 to 3 microns or less) can restrict the mean free path of the working fluid 206 such that it becomes a poorer thermal conductor and one that progressively loses conductivity as its temperature drops. In general, smaller pores result in shorter mean free path of gas molecules and thereby improve insulation properties.

[0064] The use of refractory insulating materials can not only limit any damage that might be done by utilizing high temperatures, but can also limit the normal escape of heat to the environment (as required by thermodynamics). In effect, the insulating material can serve as a high value thermal resistor that also limits the rate of self-discharge for high temperature energy storage.

[0065] The utilization of amorphous carbon or related structural materials, such as for example amorphous boron nitride, can significantly reduce the cost of the necessary refractory insulating material, while at the same time being able to withstand the high operating temperatures on the same scale as with graphite or other allotropic forms of carbon used in the heat retention matrix 202. Amorphous carbon can have a similar coefficient of expansion as silicon carbide, glassy carbon, graphite, and the like, making it ideal for use in the wide range of temperatures to which the insulating material can be subjected.

[0066] The outer layer can include a final barrier of a structural and environmentally robust material, such as for example stainless steel or the like. The outer layer can ensure the physical integrity of the inner layers and also provide a barrier to leakage of the working fluid 206. Any sensors, controls, external plumbing, and other items that need to penetrate into the containment vessel 202 can be made from materials that are resistant to high temperatures and capable of forming gas impervious seals. Other materials that can be used in the outer layer can include, but are not limited to, for example, standard steel with a baked on insulating "paint." Such paint can in some implementations be covered with insulating foam that also serves as a shock protector for shipping and handling.

[0067] An outer layer of a containment vessel 204 consistent with implementations of the current subject matter can be formed of one or more metals or alloys. For example, available alloys of steel, nickel, vanadium, chromium, titanium, tungsten, and the like can exhibit suitable strengths and melting points to withstand full exposure to temperatures at the lower end of a working temperature range of a heat retention matrix 202 as described herein. If the outer layer of the containment vessel 204 formed of such an alloy is further inside lined with a low heat flux insulator and convectively cooled with a non-reactive gas such as argon, the containment vessel 204 is expected to have an operational lifetime of between approximately 20 and approximately 40 years, making it practical for medium to utility scale storage of large amounts of energy economically with minimal servicing. Similar measures can be taken for interconnection piping, pumps, compressors, valves and heat exchangers to provide exceptional operational life spans and operational safety.

[0068] An energy storage system including a layered containment vessel 204 providing sequential stages of passive thermal insulation such as is described above can provide an effective fail safe mechanism for preventing dangerous conditions from arising, for example during an emergency shutdown. Implementations of the current subject matter can be designed and operated such that no active cooling is necessary for normal operation. Furthermore, as a working fluid 206 can be used that includes only compounds that are typically found in ambient air, a breach in the containment vessel 204 is likely to result in dangerous conditions only from asphyxiation (e.g. due to preclusion of oxygen) or from a substantial breach of the containment vessel 204 or working fluid circulation system that results in oxygenated air being brought into contact a very high temperature heat retention matrix 202. Such a situation could result in ignition of graphite and/or other carbonaceous allotropes of the heat retention matrix. However, an energy storage system such as described herein is generally operated at an elevated positive internal pressure that is expected to minimize intrusion of large amounts of oxygenated air except in the case of a full structural breach of the containment vessel 204. In the event of ignition of a carbonaceous heat retention matrix material, most of the combustion by-products (e.g. carbon dioxide, carbon monoxide, soot, etc.) are likely to be relatively benign or at least not acutely or chronically toxic except in very high concentrations in enclosed spaces. The layered construction of the containment vessel 204 using a series of refractory or other high heat resistance materials can minimize such escapes of heated working fluid and intrusions of oxygenated air. Such features can provide passive protection against such breaches and can permit the system to passively shut down in the case of an emergency, including complete loss of power to control the system.

[0069] During normal operation, an energy system consistent with an implementation of the current subject matter may need to be started from a completely de-energized state (black start) or shut down to a completely de-energized state for servicing. Control systems for the energy storage system can be provided to ensure that start up and shut down processes do not occur so rapidly that thermal shocks to the device and system components result. Such thermal shocks may lead to premature failure of one or more system components. In ordinary operation of some implementations, heat exchanger features of the energy input system 210 can be utilized for both start up heating and shut down cooling. In the case of a heat exchanger being fed by working fluid 206 from within the containment vessel 204 by one or more fluid delivery loops or circuits, the working fluid 206 in the one or more fluid delivery loops directly contacts the heat retention matrix 202 where the greatest energy is stored and temperatures exist. The draining of energy to the de-energized state can in some implementations be accomplished over a short time by utilizing a cooling heat exchanger, or alternatively the system can be allowed to passively self-discharge over a much longer period of time.

[0070] A cyclic system, such as an energy storage system consistent with one or more of the implementations of the current subject matter, will generally experience wear in the form of thermal shock, vibration, friction, diffusion, chemical reaction, and many other related forces that degrade the materials as they were originally fabricated until the entire cyclic system or components of it need servicing or replacement. When it is no longer cost effective to maintain the system in a safe and effective manner, it is said to have reached end- of-life and will need to be replaced or recycled. A common goal of most modern machinery is to extend the expected life cycle of that machinery for as long a period as is practical based upon cost and risk analysis. When such a system is finally retired, however, the bulk of the mass requiring disposal will be carbon in one or more of its allotropic forms. Such materials are generally quite safe for normal disposal (e.g. not as hazardous waste). Similarly, other components of such a system can be manufactured from non-toxic metals, ceramic materials, and the like. There is therefore little danger in normal disposal of such materials, many of which may also be amendable to recycling, either in part or in whole. [0071] Referring again to FIG. 2, an energy storage system 200 can include one or more power input systems 210. A heat retention matrix 202 of an energy storage system 200 generally retains energy in the form of heat. As such, delivery of input energy to be stored by the energy storage system 200 can in some implementations be possible by directly supplying heat energy to either or both of the heat retention matrix 202 and the working fluid 206. For example, one or more heat transfer rods or pipes can be directed through the volume contained within the containment vessel 204 to serve as heat exchangers that deliver heat from a high temperature heat source to the working fluid 206 and/or the heat retention matrix 202. Alternatively or in addition, heat exchange can occur outside of the containment vessel 204, for example via a fluid circuit that circulates working fluid 206 form within the containment vessel 204 to the outside heat exchanger. For example, the introduction of input energy to heat retention matrix 202 can be performed indirectly through the working fluid gas to utilize any desired heat source including geothermal, solar, or combustion without first converting that energy to electrical form.

[0072] If an existing or new heat sourcing and work generating system is not integrated with an energy storage system, input energy in substantially any form can be converted with high efficiency to heat and transferred to the heat retention matrix 202. One approach to providing input energy is "stepping up" a thermal energy source to a high temperature through one or more heat exchangers and heat pumps. Another example of an approach consistent with the current subject matter is the conversion of electrical energy to high temperature thermal energy through use of an electrical to thermal conversion system, such as for example a resistive heating element or other heat conduction or radiation element or an inductive heating system in which one or more inductive elements are heated via generation of a magnetic field. For example, if the heat retention matrix 202 includes graphite, graphene platelets in the graphite can be heated via inductive energy transfer using an oscillating magnetic field.

[0073] One approach to adding input energy to the heat retention matrix 202 can be direct heating of the heat retention matrix 202 by transferring incoming electrical energy to the heat retention matrix 202. For systems using graphite as the heat retention matrix 202, one energy input approach can utilize the anisotropic properties of the graphite to heat the material through electrical resistance by attaching contacts on opposite sides or ends of a graphite core. Other heat retention matrix materials having similar electrical conduction properties can be heated in a similar manner.

[0074] An alternative approach to adding input energy can make use of the rapid and facile heat transfer between plasma and gas. When gas and plasma are both present in a volume, they quickly reach equilibrium at a uniform temperature. By comparison, heat transfer between fluids and solids, including fluids such as gases and their mixtures can be relatively slow. Thus it can be thermodynamically advantageous to excite the working fluid 206 into a plasma state, either within the containment vessel or in an external plasma chamber and to allow the plasma-gas heat transfer on the working fluid 206 to rapidly produce a high temperature uniform mixture with extremely high efficiency. If the containing vessel 204 or other containment features (e.g. a tube, pipe, etc.) is a solid that has a very low heat transfer rate, nearly all of the entering plasma generating energy will therefore be retained heat in the working fluid 206, leading to an extremely effective and high power flux (on the order of kilowatts) transformation method over a relatively short distance, particularly from electrical to thermal forms.

[0075] When the plasma ions generated collide with uncharged gas, there can be a net force applied upon the gas as a whole, leading to an electro-magneto-fluidic pump effect. Thus, the working fluid 206 can be both uniformly heated and simultaneously pumped in one direction in an open-ended (but still closed loop) containment vessel 204, external plasma chamber, or other containment feature with the pumping direction following the ion cyclotron element. Nitrogen gas has been found to be effective in quenching the plasma "flame" and rapidly spreading the transferred heat to the remaining gases.

[0076] Referring again to FIG. 2, an energy storage system 200 can include one or more power output systems 214. For distribution of thermal energy stored within an energy storage system as electricity, the thermal energy must be converted to electricity using one or more approaches. Alternatively, if the energy demand is for thermal energy, such energy can be delivered directly or indirectly, for example via heat exchangers that can involve a flow of the working fluid 206 or a static container into which a volume of the working fluid 206 is contained. To retrieve the thermal energy in the form of heat from storage, a fluidic heat exchanger can be utilized to transport the energy out of the heat retention matrix 202 for conversion into an appropriate useful energy form. In some non-limiting implementations a mechanical to electrical generator, such as a steam powered turbine, Stirling cycle engine, Brayton cycle engine, or the like can be employed. A Brayton cycle turbine and related heat engines can be configured and operated in "closed loop" cycles that permit the working fluid 206 to be isolated from the ambient environment and can be pressurized at any desired level, including less than atmospheric.

[0077] As a consequence of ability to achieve high operating temperatures (e.g. of approximately 2500 K), energy storage systems such as those described herein can be utilized in conjunction with the electrical generating machinery of nearly all existing conventional power plants, thereby converting or augmenting them into renewable energy storage facilities by reusing their most expensive components including their existing connection to the electrical distribution grid. A high Carnot efficiency heat engine can be coupled to an energy storage system to produce useful work or energy, particularly where pre-existing generating equipment is not already present or incorporated.

[0078] An energy storage system can be quite cost effective when it is used in conjunction with an existing thermal cycle conversion system to produce useful energy. If such a system is already in use, then the thermal energy stored within the containment vessel 204 can mimic the hot end temperature of the thermal cycle and extend the operational time period of the generator by acting as a simple thermal capacitor to augment that thermal cycle system. If the thermal cycle uses a working fluid that is incompatible with graphite material or other materials used as the heat retention matrix 202, then a set of highly efficient heat exchangers can be used to couple the heat retention matrix 202 and working fluid 206 as described herein to the thermal cycle system, with only a very small loss in efficiency from the round-trip transfer of heat into and out of the heat retention matrix 202.

[0079] If there is a difference between the hot source temperature of an existing thermal system and the optimal generating temperature of that system, then the energy can be stored at the hotter of the two points and the required heat exchanger or economizer (for hotter source than generation temperature) or heat pump or super-heater (for colder source temperature than generation temperature) can be utilized to reach the generation entry temperature. In an optional implementation, part of the "exhaust" cold working fluid can be cycled back to be mixed, for example using a variable control coupling valve or the like, with the hot working fluid to obtain a constant hot source temperature at the input to the heat engine or other heat to electrical conversion technology.

[0080] Thermal energy storage such as is described herein can also be applied at generally any or each thermal level of a cogeneration or integrated combined cycle generation system for conversion of thermal to useful electrical energy. Waste heat (e.g. the exit temperature energy that exceeds the ambient or lowest operational cold temperature of such a system) can also be utilized for other derivative purposes such as heating potable water or an inhabitable or working enclosure.

[0081] One of the most significant challenges in any thermal cycle based system is to be found in the thermal energy transfer between different phase materials, such as solid-gas, solid-liquid, liquid-gas or triple-point coexistence. It is generally advantageous, for example, if a heat exchanger manifold is facile in its transfer of heat between the fluids entering at different temperatures on either side of the manifold, while at the same time not permitting or substantially permitting physical transfer of either fluid to mix with the other. Such features and components also advantageously restrict buildup of substantial solid or viscous material that might block the flow of the fluid cycle, either from phase change or a chemical reaction between or within the working fluid and the walls of the heat exchanger.

[0082] Conversely, it can be advantageous for pipes, containers, etc. of materials at specific temperatures and pressures that relocate the heat held by the fluid to have low heat flux solid liner inside them to avoid transferring accumulated heat to their surrounding environment. Such features can prevent or reduce thermal losses and also prevent chemical or physical changes to the working containment materials. Such pipes, containers, etc. can also advantageously restrict physical transfer of fluid or other material into or out of their walls at any appreciable rate.

[0083] Evaporative, condensing, and adiabatic cycles that involve phase change of a working fluid or isothermal compression or expansion are also devices that accommodate changes in pressure and/or volume while containing the working fluid 206 in physical isolation from the environment. Heat can be transferred or retained by the containing walls independent of the process being performed on the working fluid 206, thereby leading to the appropriate choice of container that will remain intact and function correctly through startup, operation and shutdown of the desired working thermal cycle. The container can also advantageously isolate physical transfer of environmental contaminants to or from the working fluid 206 to be safe and effective.

[0084] Heat and work have the same units of measure, promoting the use of heat engines and pumps to transform one form of energy into the other. Thermal efficiency is limited by the difference between the engine's or pump's entry and exit temperatures as limited by the second law of thermodynamics. To achieve high thermal efficiency it can be advantageous to have as great a difference between effective inlet and outlet temperatures as possible, and to utilize a heat conversion device that makes optimal use of the isothermal and isentropic (reversible adiabatic process) portions of its heat to work cycle.

[0085] The efficiency of a Carnot cycle engine is, as noted above, a function of the difference between the high temperature reference (also referred to as the "hot point") and the low temperature reference (also referred to as the "cold point"). Theoretical Carnot efficiencies for various hot point temperatures and typically available cold point temperatures are summarized in Table 1.

[0086] Table 1. Theoretical Carnot Efficiencies

[0087] As shown, the thermal conversion efficiency can change significantly for both hot entry point temperatures below approximately 1000 K and cold exit point temperature above room temperature where thermal storage temperatures would be considered safe in residential or high population density areas, or for use in directly storing solar thermal heat energy (approx. between approximately 450 and approximately 820 K). [0088] Thermal energy step-up and step-down techniques similar to those utilized to change voltage in electronic transformers can be accomplished using heat pumps and similar variants. For example, two separate heat exchangers and a working fluid that can optionally include a condensing/evaporating or adsorbing (or absorbing)/evaporating gas. By way of illustration, via a continuous (turbine, for example) or periodic (piston, for example) pumping mechanism, the gas can be compressed into one (Hot) heat exchanger where it transfers heat to a media at a lower temperature than the compressed gas. When the compressed gas, which may have condensed, enters the second (Cold) heat exchanger, it passes through a narrow or capillary tube and expands, cooling the fluid which then receives heat the heat expelled at the hot exchanger from a second media. Any mechanical work done by the heat pump itself can become heat through friction or other means and applied to the first or "hot" heat exchanger so that the heat pump is made more efficient by expelling heat from work done into the first media. Many variants of this principle are available and consistent with implementations of the current subject matter, including but not limited to Peltier stacks, Hilsch vortex tubes, and other techniques that make use of other physical properties of matter to transfer heat in one direction preferentially. When the "hot" media and the "cold" media are swapped, the pump becomes a generator and work may be produced.

[0089] Approaches and systems as described herein do not preclude the utilization of "waste heat" at the cold end of the thermal to electrical generator sub-system. Any and possibly all of the options traditionally associated with combined heat and power (CHP), co- generation using a low grade heat engine, or any of the myriad options for utilizing this otherwise wasted heat to cool, heat, or otherwise process this energy to useful purpose is intended. This is true of any residential, commercial and industrial augmentation to increase the overall efficiency of the storage invention. As a non-limiting example, waste heat can drive a adsorption chiller to reduce the HVAC electrical load for a building and hence reduce the overall need for electrical energy for the building. Waste heat can also be incorporated into utility generation in multiple stages to likewise increase the overall efficiency of the heat to electrical energy process.

[0090] In general, practical limits exist to the magnitude of temperature difference that can be achieved using heat pumps, because the hot media should generally be colder than the temperature of the first exchanger, and the cold media should be hotter than the temperature the second heat exchanger. The two heat exchanger temperatures that can be achieved are therefore generally limited by the pressure difference possible and the nature of the exchange fluid. However, the choice of pressure differential and exchange fluid can vary for each step-up or step-down temperature differential desired, and thus a plurality of heat pumps in series can achieve a very wide range of total temperature differentials. The use of such heat pumps/generators can enhance the efficiency of energy storage systems, particular those using allotropic forms of carbon as described herein, because the heat storage capacity of such materials increases with temperature. If the incoming source of energy is not already at the desired storage temperature, then a heat pump can be utilized to raise the source media to a temperature above the target storage temperature to reach that target.

[0091] FIG. 4 through FIG. 17 show features of illustrative examples of energy input systems 210 and energy output systems 214 that can be used in conjunction with energy storage systems consistent with implementations of the current subject matter. Except where the features discussed below in reference to the various illustrative examples are mutually exclusive or otherwise incompatible, implementations of the current subject matter can include one or more of such features in any combination.

[0092] As shown in FIG. 4, implementations of the current subject matter can include dual independently operating working counter-flow fluid thermal transfer loops, each of which may be pressurized and circulated at different mass transfer rates. Dual fluid thermal transfer loops have generally not been applied to bulk high-temperature storage and have typically made specific use of driving the heat transfer process indirectly by varying a working fluid density and speed as the primary control mechanism for the rate of heat energy transfer. Advantages of utilizing independent loops can include the ability to both fill and draw from an energy storage system 200 at substantially different rates and operating pressures, thereby allowing each loop to be optimized for efficiency separately. An energy source 402 provides input energy in the form of either heat or electrical energy to be stored by the energy storage system 400. An energy input system 210, which can include one or more of a heat absorption or exchange system, a heat generator (e.g. for converting electrical energy to heat), or the like, can receive the input energy from energy source 402. In one implementation, the energy input system 210 can be linked to a heat retention matrix 202 retained within a containment vessel 204 by an energy input heat transfer loop 404 that transports an energy input fluid from a input fluid expansion unit 406 that the input fluid from within the containment vessel 204. The energy input fluid can have a composition consistent with those discussed above in regards to the working fluid.

[0093] The energy input heat transfer loop 404 can include one or more of piping, conduits, valves, connectors, pressure vessels, and the like necessary to convey the energy input fluid in a closed circuit between the heat retention matrix 202 stored within the containment vessel 204 and the energy input system 210. As shown in FIG. 4, the energy input heat transfer loop 404 brings the expanded energy input fluid into contact with the heat absorption or exchange system, heat generator, etc. of the energy input system 210 where the energy input fluid absorbs heat before returning to a input fluid compression unit 408 that re- compresses the energy input fluid for return to the containment vessel 204. The energy input heat transfer loop 404 can also include connectors, valves, piping, conduit, etc. to enable exchange of the energy input fluid with a first adaptive pressure reservoir 410 that can provide pressure equilibration in the energy input heat transfer loop 404 as needed to ensure optimal energy transfer. For example, the first adaptive pressure reservoir 410 can include a variable pressure vessel that permits modulation of heat transfer at variable temperatures. Higher pressures can be utilized at lower temperatures to achieve the required higher heat transfer characteristics. While FIG. 4 shows the first adaptive pressure reservoir 410 situated between the energy input system 210 and the input fluid compression unit 408 that returns the energy input fluid to the containment vessel 204, it can be advantageous in some implementations to site the first adaptive pressure reservoir 410 at a location on the energy input heat transfer loop 404 where the temperature of the energy input fluid is lower, for example prior to energy being added to the energy input fluid by the energy input system 210.

[0094] A first fluid cleaning system 412, which can for example include one or more activated charcoal filters or other features consistent with a reactive compound removal system 220 as is discussed elsewhere herein, can be positioned on the energy input heat transfer loop 404, for example near or adjacent to the first adaptive pressure reservoir 410, to enable continuous or periodic removal of undesirable reactive compounds from the working fluid. Because one or more implementations of a reactive compound removal system may perform more efficiently at lower temperatures, it can be advantageous to site the first fluid cleaning system 412 along the energy input heat transfer loop 404 where it experiences as low an energy input fluid temperature as possible. In some implementations, a cooling system can be included to further reduce the temperature of the energy input fluid to which the first fluid cleaning system 412 is exposed.

[0095] An energy output heat transfer loop 414 can bring an energy output fluid, which can in some implementations also include a fluid composition consistent with those described above for the working fluid 206, to a power generation system 416, which can be an electrical energy generation system, a heat engine, etc. that extracts heat from the energy output fluid for conversion to electricity and/or for performing other useful functions, work, etc. The power generation system 416 can optionally include a heat exchanger 420 to isolate the energy output fluid from one or more components (e.g. another heat transfer fluid) of the power generation system 416. The energy output heat transfer loop 414 can include one or more of piping, conduits, valves, connectors, pressure vessels, and the like necessary to convey the energy output fluid in a closed circuit between the heat retention matrix 202 stored within the containment vessel 204 and the heat exchanger 420 or other components of the power generation system 416.

[0096] The energy output heat transfer loop 414 can receive compressed energy output fluid from an output fluid compression unit 422, for example after the energy output fluid has absorbed heat within one or more Stirling regenerator tubes that pass through, near, around, etc. the heat material of the heat retention matrix 202 within the containment vessel 204, and can return the energy output fluid via a generation fluid expansion unit 424. The energy output heat transfer loop 414 can include connectors, valves, piping, conduit, etc. to enable exchange of the energy output fluid with a second adaptive pressure reservoir 426 that can provide pressure equilibration in the energy output heat transfer loop 414 as needed to ensure optimal energy transfer. For example, the second adaptive pressure reservoir 426 can include a variable pressure vessel that permits modulation of heat transfer at variable temperatures. Higher pressures can be utilized at lower temperatures to achieve the required higher heat transfer characteristics. While FIG. 4 shows the second adaptive pressure reservoir situated between the output fluid compression unit 422 and the heat exchanger 420 or other components of the power generation system 416, it can be advantageous in some implementations to site the second adaptive pressure reservoir 426 at a location on the energy output heat transfer loop 414 where the temperature of the energy output fluid is lower, for example after exchange or other removal of energy from the energy output fluid by the heat exchanger 420 or other components of the power generation system 416.

[0097] A second fluid cleaning system 428, which can for example include one or more activated charcoal filters or other features consistent with a reactive compound removal system 220 as is discussed elsewhere herein, can be positioned on the energy output heat transfer loop 414, for example near or adjacent to the second adaptive pressure reservoir 426, to enable continuous or periodic removal of undesirable reactive compounds from the working fluid. As noted above, it can be advantageous to site the second fluid cleaning system 428 along the energy output heat transfer loop 414 where it experiences as low an energy output fluid temperature as possible. In some implementations, a cooling system can be included to further reduce the temperature of the energy input fluid to which the second fluid cleaning system 428 is exposed.

[0098] The energy output heat transfer loop 414 can exchange the energy output fluid with a second adaptive pressure reservoir 426 on the way to the heat exchanger 420 or other components of the power generation system 416. The heat exchanger 420 can receive the energy output fluid (e.g. a generation gas) gas that is collected in an upper manifold (e.g. the generation fluid compression unit 422) and pressurized using the second adaptive reservoir 426 to match the current operating heat transfer rate. The pressurized heat transfer gas can enter the optional heat exchanger 420 to transfer the heat to another heat transfer fluid, such as saturated steam depending on the type of generating engine 212 being used, which can in some implementations be a boiler.

[0099] The power generation system 416 can provide energy to satisfy all or part of an energy demand 430 and can also be coupled to a heat recovery system, cooling, heat sink, or the like 432, which can act as the reference low operating temperature for a heat engine that performs the electrical power generation 416. The power generation system 416 can be a conventional gas single or combined cycle power generation system. Use of an isolated energy output fluid that is separate from the energy input fluid permits the removal of heat energy in any amount from zero to the maximum heat engine operating capacity permits, and is independent of the rate of energy addition to the heat retention matrix 202 from the energy input heat transfer loop 404. As noted above, in some implementations, the energy output fluid heat transfer loop and the energy input heat transfer loop can be completely discrete with no mixing between the energy output fluid and the energy input fluid. In alternative implementations, it is possible to use the same working fluid to perform both energy input and energy output functions.

[00100] Different reactive compound removal systems can be employed in the first fluid cleaning system 412 and the second cleaning system 428. For example, in the system 400 of FIG. 4, the energy input fluid that the first fluid cleaning system 412 is tasked to treat can include reactive compounds resulting from degradation or other reactions of the energy input fluid with the carbonaceous material of the heat transfer matrix 202 at high temperatures (e.g. above 900 K, in a range of 900 K to 1500 K, between 900 , and 2500 K, greater than 2500 K, etc.). The contaminants resulting from such processes may differ in composition, reactivity, stability, etc. from contaminants resulting from high temperature interactions of the energy output fluid with heat exchanger structures and/or other components of a power generation system 416.

[00101] FIG. 5 shows an energy storage system 500 illustrating features consistent with one or more implementations of the current subject matter. The energy storage system 500 includes an energy input fluid in direct contact with the heat retention matrix 202 within a containment vessel 204. The containment vessel 204 as shown in FIG. 5 includes an outer layer 502, an insulation layer 504, and a refractory inner layer 506, and can in some implementations be cylindrical in shape with hemispherical or otherwise curved end caps 508. A cylindrical configuration, which is not intended to be a limiting example, can provide a beneficial internal volume to containment vessel surface area ratio, which can help in limiting points of entry or exit of heat energy form the energy storage system 502.

[00102] An upper manifold 510 and a lower manifold 512 that respectively deliver the energy input fluid to the containment vessel 204 and remove the energy input fluid from the containment vessel 204 can serve an energy input heat transfer loop 514. The heat retention matrix can be provided as a series of stacked layers of the heat retention matrix material (e.g graphite or some other carbonaceous allotrope). The containment vessel 204 and the components of the energy input heat transfer loop 514 can form a closed loop that isolates the energy input fluid from the environment surrounding the containment vessel 204 and other components of the system 500. One or more safety pressure relief valves (not shown) can be included on the containment vessel 204 and/or the energy input heat transfer loop 514.

[00103] An energy output heat transfer loop 516 can convey an energy output fluid through an energy output system 214. The energy output fluid can be separate from the energy input fluid 206, and either or both can have a similar composition to one or more of the implementations of working fluids discussed above. The lower manifold 512 can also be connected via a separate flow path to the energy output heat transfer loop 516 to return the energy output fluid to within the containment vessel 204 and can be connected to a series of heat transfer pipes or rods 520 that pass through the heat retention matrix 202 and direct the energy output fluid to the upper manifold 510, which likewise includes a flow path to the energy output heat transfer loop 516 that is separate from the energy input heat transfer loop 514. The energy output fluid can absorb heat from the heat retention matrix 202 as it passes through the heat transfer pipes or rods 520 between the lower manifold 512 and the upper manifold 510 and convey this absorbed heat to the energy output system 214. The heat transfer pipes or rods 520 can have relatively thin walls without need for substantial structural strength if the differential pressures between the energy input fluid and the energy output fluid are relatively small. One or more safety pressure relief valves (not shown) can be included on the energy output heat transfer loop 516

[00104] FIG. 6 shows an example of a layer 600 of the heat retention matrix material having pass-through holes 602 for the heat transfer pipes or rods 520. A layer 600 can include multiple blocks or other pieces of the heat retention matrix material that are assembled, optionally within a tray or container. Multiple containers can be designed to be self-aligning with adjacent containers to permit easy fabrication of a multi-layer heat retention matrix 202. In the case of graphite or some other anisotropic material, the blocks can be oriented with the axes of individual graphene planes in a largely co-planar arrangement, for example with the higher heat conductivity axis oriented perpendicularly to the axis that is parallel with the of heat transfer pipes or rods 520. The low thermal conductivity axis of the graphene platelets, e.g. the direction orthogonal to the plane of the graphene molecule, can be oriented in parallel to the axis that is parallel with the heat transfer pipes or rods 520. The energy input fluid can fill gaps between the blocks or pieces of the heat retention matrix material. Each layer 600 forming the heat retention matrix 202 can be assembled into a structure formed of a metal or non-oxidizing ceramic that is compatible with elevated temperatures. Alternatively or in addition, such structures can be formed of or include one or more carbonaceous materials or other materials capable of withstanding the high temperature conditions of the heat retention matrix 202.

[00105] The volume within the pass-through holes 602 around the heat transfer pipes or rods 520 can optionally be at least partially filled with smaller particles of the heat retention matrix material, for example to mechanically stabilize the heat transfer pipes or rods 520 and/or to allow for expansion and contraction between the different materials (e.g. the heat retention matrix material and the heat transfer pipes or rods 520) during startup, shutdown and routine operation of the energy storage system 500. Such particles can advantageously be spherical or nearly spherical in shape and can in some implementations have diameters in a range of approximately 1 to 3 mm. Such particles can efficiently conduct heat from the blocks or other bulk material of the heat retention matrix 202 to the heat transfer pipes or rods 520. Another potential advantage of such spheres includes providing sufficient clearance between the rods and storage blocks to allow the primary path of input energy fluid flow that can either directly heat the heat transfer pipes or rods 520 or have their heat transferred to the blocks or bulk material of the heat retention matrix. Such spheres will naturally pack via gravity into hexagonal matrices and provide short mean free paths for the input energy fluid. This fluidic friction can cause all vertical input fluid flow paths to present nearly identical resistance that balances the input energy fluid flows surrounding each heat transfer pipe or rod 520.

[00106] In an implementation, the energy input fluid can flow downward through the blocks or particles of the heat retention matrix 202 surrounding each heat transfer pipes or rods 520 to more effectively transmit heat to the energy output fluid moving in the opposite direction in the heat transfer pipes or rods 520. It should be noted that a configuration in which flow is reversed (e.g. the energy output fluid enters through the upper manifold 510 and exits the containment vessel 204 through the lower manifold 512 is also contemplated and included within the scope of the current subject matter.

[00107] FIG. 7 shows a detail of an assembly 700 that can be used as an upper manifold 510 or lower manifold 512 in an energy storage system 500 similar to that shown in FIG 5. A series of pass-through holes 702 can be dispersed across the face of the manifold assembly 700 to allow the energy input fluid to pass through the manifold and into direct contact with the material (e.g. graphite, etc.) of the heat retention matrix 202. The manifold assembly 700 can also include a series of ports 704 that each connect to an end of a heat transfer pipes or rod 520 and are connected by an inner fluid flow passage to a connector 706 that can be connected to conduits or pipes of the energy output heat transfer loop 516 to isolate the energy output fluid from the energy input fluid.

[00108] FIG. 8 shows a cross sectional detail view 800 of the manifold assembly 700 shown in FIG. 7. A layer of thermal insulation or other refractory material 802 can be applied to the upper surface 804 of the upper manifold 510 (or conversely to the lower surface of the lower manifold 512) with holes 806 provided therein to allow for the passage of the energy input fluid into and through the pass-through holes 702 to allow access of the energy input fluid to the heat retention matrix located within the containment vessel 204 between the upper manifold 510 and the lower manifold 512. The barrier of insulation or other refractory material 802 can both protect the manifold assembly 700 from uneven energy input fluid inlet temperatures that might lead to erosion or melting of the manifold assembly 700 and also isolate the energy input fluid passages 806 within the manifold assembly from the energy input fluid, thereby enhancing the thermal transfer efficiency to the heat retention matrix 202.

[00109] To facilitate the transfer of heat energy between the heat transfer pipes or rods 520 and the energy output fluid, a Stirling type heat transfer mesh can be located inside one or more of the heat transfer pipes or rods 520. Such mesh layers or windings can be placed at relatively high density within the heat transfer pipes or rods 520 and can contact the inside material of the heat transfer pipes or rods 520 to create a very large surface area for heat transfer, as well as a short mean free path for the energy output fluid, both of which improve the heat transfer characteristics of the energy output fluid /solid interface, and evenly distribute the fluid flow between heat transfer pipes or rods 520. FIG. 9 shows a cross- section view 900 illustrating multiple layers 600 of a heat retention matrix 202 through which several heat transfer pipes or rods 520 are located. A packing 902 of smaller particles of the heat retention matrix material are placed around the heat transfer pipes or rods 520.

[00110] FIG. 10 shows an illustrative implementation of the current subject matter characterized by a system 1000 in which multiple cylindrical heat retention matrix cores 1002 are buried or partially buried within an earthen or concrete containment area 1004. In FIG. 10, the system 1000 includes multiple cylindrical modular sub-arrays 1006, each containing seven cylindrical heat retention matrix cores 1002. Each modular sub-array 1006 can include a containment wall 1010 in addition to the containment vessels of each of the individual cylindrical heat retention matrix cores 1002. Burying the cylindrical heat retention matrix cores 1002 within their containment vessels (either individually or in groups) provides the opportunity to flood the remaining volume of the containment area 1004 with either a non- reactive gas or working fluid (for example consistent with implementations discussed previously herein) to assisting in cooling and to potentially serve as a control mechanism in case of primary cylinder containment failure. In the example shown, input energy can be converted in an input energy conversion area 1012, and harvested for delivery in response to an energy demand in an output energy conversion area 1014. The interconnecting plumbing or electrical connections to accomplish these functions are not shown to reduce the complexity of the diagram. The interconnections between the cylindrical heat retention matrix cores 1002 and the input energy converter 1012 and output energy converter need not be thermal, but may instead have an associated electrical input and output conversion module attached to each cylindrical heat retention matrix core 1002, for example to its individual containment vessel, leading to redundancy to increase total operational time by limiting points of failure. In this case the input energy converter 1012 and output energy converter 1014 can include electrical switching banks and transformers to convert the input and output power requirements of each cylindrical heat retention matrix core 1002 to that of the electrical grid or other electrical power demand to which they are attached.

[00111] FIG. 11 shows a diagram of an electrical to thermal conversion system 1100 that includes a combination compressor pump and magneto-plasma dynamic thermal generator. It should be noted that the implementation shown in FIG. 11 is merely an example of one possible type of electrical to thermal conversion system consistent with the current subject matter. Resistive elements (for instance Ni-chrome wire or ceramics) can be used in implementations with lower power requirements. Other electrical to thermal energy conversion approaches are within the scope of the current subject matter, including but not limited to those discussed below.

[00112] The entering inlet gas 1 102 can be compressed first using an integral conformal flow restrictor 1104 and fan 1 106 supported at startup by ring bearings 11 10, which becomes a balanced air bearing using the compressor core 1112 as the fan reaches operating speed, offering low friction and wear. The fan 1106 can be driven electro- mechanically using electric three phase inputs 1 1 14 and coils 11 16 to inductively drive magnetic fan blades 1120 which also function as stators for the three phase motor/fan.

[00113] The partially compressed gas 1 122 can be initially excited into ions using one or more ultraviolet light lasers 1 124 aimed at the compressor core 1 1 12. Ionization can thereby be ensured even at working gas pressures in excess of approximately 0.1 atmospheres (typically between approximately 1.0 and approximately 10.0 atmospheres at startup). The UV ionized gas 1 126 can be further excited using one or more microwave induction coils 1130, which can be driven by an electrical circuit, for example to ground 1132 from a high- power electricity source 1 134 into one or more ionization coils 1 136, which can be embedded in an insulation layer 1 140. [00114] The resulting ionized gas 1142 can enters a strong magnetic field 1 144, which can be generated by one or more 1 146 permanent magnets 1150 embedded in the insulation layer 1 140. The ions and electrons counter-spin driven by ion cyclotron resonance (ICR) semi-circular plates 1152 also embedded in the insulation layer 1140 and driven by an alternating current field, which can be driven through two or more electrical entry points 1 154. The plasma generator/driver can be isolated from the surrounding environment by the containment tube wall 1 156, which can also acts as the electrical ground 1132. The resulting plasma rapidly dissipates its energy into the remaining inlet gas 1 102 to produce hot outlet gas 1 160.

[00115] FIG. 12 shows a diagram of an energy storage system 1200 that integrates an electrical ICR plasma gas heater driven by the energy input system, which in this case takes input electrical energy to drive induction coils, etc. similar to those discussed above in relation to FIG. 11. The energy input fluid heat transfer loop 514 delivers the energy input fluid via a plenum or manifold 1202 to the plasma generator 1 100. A fan 1 106 can provide propulsive forces to assist in circulation of the working fluid 206. The working fluid then passes through one or more energized coils 1 130 as well as one or more other plasma generating features such as are described above in relation to FIG. 11. The energized plasma can rapidly mix and attain thermal equilibrium with non-energized components of the energy input fluid, thereby promoting rapid and highly efficient delivery of the electrical energy from the energy input source 510 for storage as thermal energy by the heat retention matrix 202 within the containment vessel 204.

[00116] Conversion of electrical to thermal energy using electrical resistance can be used to a solid or other medium, which in turn may heat a working fluid, energy input fluid, transfer gas, or the like through a solid/gas interface. This transfer of heat can be accomplished with efficiencies in excess of approximately 99%, leading to a near-perfect one-way transfer of energy. The primary limitations to heat transfer in such a system are the rate at which energy will effectively transfer through the solid/gas or solid/solid interface and the delay rate of heating of the solid when different levels of electrical energy are applied.

[00117] Electrical energy can create ions and voltage potential can accelerate those ions to produce plasma, and this has been used in tests as a low specific impulse rocket engine. Plasma can also react with any surrounding gas to heat that gas and cool the plasma. Additional techniques are also available to create ions and electrons using microwave excitation such that the generated ions and electrons counter-spin in an ion cyclotron resonance generator to become accelerated in a fixed magnetic field. Significant (Mega- Watt) levels of electrical energy can thus be converted to a plasma at temperatures approaching approximately 7000 K which in turn will heat surrounding gas, including but not limited to argon, to temperatures in excess of approximately 2000 K with efficiencies in excess of approximately 90% and directionality better than approximately 3%. This type of rocket propulsion generally generates excess heat, which in the case of the current subject matter can be used to pre-heat some of the input gas such that this otherwise waste heat is also retained within the energy storage system for later useful use. The power levels and temperature levels produced are preferable for this application, and a preferred embodiment of this methodology is shown in FIG. 1 1.

[00118] FIG. 13 shows a diagram of a utility-scale electrical power generation system 1300 integrating an energy storage system 1302 consistent with features described herein. Use of the energy storage system 1302 can enhance a traditional simple cycle steam generation plant by enabling reductions in C0₂ emissions and maintenance required relative to a conventional fossil fuel fired generating plant. FIG. 13 also shows a purely thermal energy input fluid loop. A "boiler" 1304 burns a fossil fuel (or alternatively creates heat via nuclear reactions or other means) that is used to provide thermal energy conveyed to the heat retention matrix within the containment vessel of an energy storage system 1302. The energy input fluid can be kept circulating while the boiler 1304, which need not be operated continuously, is active. Operating the boiler only periodically permits its operation only a peak efficiency (e.g. at an optimum stoichiometric fuel/air balance for a fossil fuel fired plant) to thereby produce maximum heat, which can in some examples provide a temperature of approximately 2300 K. The production of nitrogen oxides (e.g. ΝΟχ) can be minimized under such conditions by operating the boiler 1304 at atmospheric pressure and utilizing the flue heat to preheat the incoming air for burning, thereby resulting in the shortest possible boiler operation duration to satisfy an energy demand on a given cycle (e.g. one day, one week, etc.) the needed energy.

[00119] From the energy storage system 1302, a heated energy output fluid, which can be separate from the working fluid used as the energy input fluid or the same working fluid in a different loop, enters a heat exchanger 1306 to superheat steam for use with, for example, a single shaft steam generator 1308 that can include a high pressure turbine 1310 and low pressure turbine 1312 in series. The high pressure turbine 1310 receives the fully pressurized steam and expands it to produce energy in the form of rotating the generation shaft 1314. Part of the remaining high pressure steam can be re-circulated via a feed pump 1316 to a preheater 1320 where it can be preheated for the next cycle using waste heat from the gas/steam heat exchanger 1306.

[00120] The remaining low pressure steam can enters the low pressure turbine 1312 coaxially situated on the generation shaft 1314 to produce additional power. The output of the expander in the low pressure turbine 1312 can be passed to a condenser 1322 to minimize the exit temperature and obtain the highest overall engine (Carnot) efficiency. The condensed steam can pass through a second feed pump 1324 to a feed water heat exchanger 1326 that makes final use of the heated energy output fluid from the energy storage system W

1302 to partially return the feed water to a hot state with some steam produced. The energy output fluid from the energy storage system 1302 can then be returned to be heated again by the heat retention matrix within the energy storage system 1302 via a third feed pump 1330. By regulating the flow rate of the energy output fluid in the feed pump 1330, the amount of heat energy utilized to produce electricity at the generator 1308 can be modulated to serve, for example, a 3 -phase AC electrical output 1332. In this fashion the boiler 1304 can be operated at constant output for a fixed amount of time to load the energy storage system 1302 with heat that can then be delivered to the customer load as needed by drawing heat from the energy storage system 1302 via the energy output fluid. The rate of power entry into the heat retention matrix of the energy storage system 1302 from the boiler 1304 can thereby be completely independent of other power losses or usage and can allow the heat generation process at the boiler 1304 to be operated its own closed loop to add heat energy to the energy storage system 1302 in any amount from zero to the maximum amount of energy that the energy storage system 1302 can safely retain.

[00121] The final output of the system 1300 is useful electrical power. Operation of the electrical power generation sub-system with its own energy output fluid in a separate fluid circulation loop from the energy input fluid used for heat energy delivery to the energy storage system 1302 can permit the removal of heat energy from the energy storage system 1302 in any amount from zero to the maximum operating capacity of the generator 1308 and turbines 1310 and 1312 independent of the rate of energy addition to the energy storage system 1302 from the boiler via the energy input fluid loop.

[00122] FIGS. 14 through 17 show additional examples of energy storage systems consistent with implementations of the current subject matter. FIG. 14 shows an energy storage system 1400 that includes a "pipe matrix" manifold 1402 with output fluid heat transfer tubes that have inner 1404, and outer 1406 piping to send the energy output fluid to the bottom of the heat retention matrix 202 and return it to the top via a metallic mesh wrapping between the tubes to further enhance the heat transfer. An energy input system 210 is fed via an energy input fluid loop 1410. The containment vessel 204 and end caps 508 can cooperate with the energy input fluid loop 1410 to maintain the energy input fluid in a closed loop. A heat retention matrix 202 can optionally include layers of graphite. An energy output fluid loop can feed an energy output system 214, such as a generation unit.

[00123] As illustrated in the two views 1500 and 1501 of pipe matrix manifold 1402 of the energy output fluid loop, heat collection rods 1502 can be fed via a heat extraction tubular matrix 1504 and combined for distribution via a cold return tubular matrix 1506. The diagram 1501 in FIG. 15B Figure 1 IB shows further detail via a cut away view in which the heat collection rods 1502 further contain feeder lines 1510. The heat collection rods 1502 can be fluidically connected to heat extraction tubular matrix 1506, and the inner feed rods 1510 can be fluidically connected to the cold return tubular matrix 1504. The energy output fluid entering via the feeder lines 1510 can exit at the bottom to turn around and be fluidically connected to the closed ends of heat collection rods 1502. The feeder rods 1510 and heat collection rods 1502 are separated by spirally rolled metallic gauze, for example to encourage rapid heat transfer.

[00124] FIG. 16 shows an example of an energy storage system 1600 that includes energy input and energy output systems integrated within the containment vessel 204. A resistive heating element 1602 placed inside the containment vessel 204 can be driven by electrical power inputs from an energy source (not shown). A pressurized or pressurizable source of a heat control gas or other working fluid (e.g. one having a composition similar to those described herein) can be connected to the containment vessel 204 to adaptively maintain a pressure of the working fluid within the containment vessel 204. An energy output fluid can be circulated in an output energy loop 1606, which can be entirely contained within the containment vessel 204, and which can include one or more features as described herein (e.g. a series of heat transfer pipes or rods 520 that pass through the heat retention matrix 202) for absorbing heat from the heat retention matrix 202 into the energy output fluid. The heated energy output fluid can feed a closed loop heat engine, for example a Brayton cycle microturbine, a Stirling engine, or the like. The closed loop heat engine can be provided with a cold reference temperature by an energy recovery fluid return loop 1612 that passes through a radiator 1614, cooling tower, heat sink, or other heat exchange mechanism to withdraw heat. An adaptive pressure controller 1616 and/or electronics controller 1620 can be provided to enable maximum efficiency of the system under safe operating conditions.

[00125] FIG. 17 shows an energy storage system 1700 incorporating aspects of combined heating and cooling approaches along with features discussed above in reference to FIG. 16. Fluid can be diverted from the energy recovery fluid return loop 1612 to pass through one or more heat exchangers 1702 for use in water heating 1704, for example to approximately 130 ^°F (approximately 54 °C). The fluid can then pass to a space heating system 1706, for example to provide heated air at approximately 80 °F (approximately 27 ^°C), and from there to the radiator 1614 for further cooling. Prior to return to the containment vessel, the energy recovery fluid return loop 1612 can pass to an adsorption chiller 1710 to provide refrigeration or a source of cooling for use in air conditioning. In this manner, the amount of energy retrieved for useful work from the heat extracted by the energy output fluid from the heat retention matrix 202 can be maximized.

[00126] As noted in reference to FIG. 5 and elsewhere in this disclosure, a layer of refractory insulation material 504 can be located between the inner layer 506 of a containment vessel 204 and the outer layer 502 of the containment vessel 204. This insulation layer can in some implementations include amorphous carbon with fiber reinforcement. The use of amorphous carbon as a micro-porous material can limit the mean free path of the working gas below the Knudsen number limit and hence limit the thermal conductivity as well as the radiation component of heat transfer, making it a better insulator. Other allotropes of carbon can also be used in the insulating layer 504. Such materials can have similar properties in the areas of chemical reactivity, melting or triple point, and cross contamination.

[00127] The chart 1800 of FIG. 18 shows data regarding the thermal conductivity of a carbonaceous material as well as a working fluid held in 10 micron or smaller pores. As noted above, many carbonaceous materials consistent with implementations of the current subject matter have thermal conductivities that vary substantially with temperature. The chart 1800 shows the relative insulation thickness in centimeters as a function of the heat retention matrix temperature such that the outer layer 502 of containment vessel 204 (which can among other possible materials be formed of stainless steel) does not exceed an exposed temperature of 40°C, which can be considered quite safe for use in open environments.

[00128] Despite the presence of insulation, however, in some implementations it can be desirable to actively draw heat that passes through the inner layer and insulation layers to prevent excess heat flux across the outer layer. The drawn heat can in some implementations be "recycled" using economizers and other intermediate temperature heating cycles for power generation. The lowest level of heat radiated can determine the Carnot efficiency that can be achieved by any heat engine driven from an energy storage system according to this and other implementations .

[00129] Table 2 lists illustrative dimensional and operational parameters for examples of energy storage systems consistent with implementations of the current subject matter. As noted elsewhere, a heat retention matrix 202 having one or more features as described herein can be retained in contact with a working fluid substantially consisting of a nitrogen and noble gas mixture and that is generally inert to reactions with the material (e.g. one or more allotropes of carbons as in some implementations). Such an energy storage system can be practical and generally safe for residential, industrial, and utility applications and can be readily scalable according to the energy needs of a specific application. As examples, Table 2 includes non-limiting examples of a power demand (kW), an operating temperature (K) range, an expected period of use, and dimensions for the heat retention matrix core of an energy storage system that can be used in conjunction with residential, commercial, wind and solar (or other cyclic renewable energy sources), or utility scale applications.

[00130] Table 2. Illustrative examples of energy storage system parameters.

[00131] A system for use in a residential application, for example, can provide approximately 5 kW of power for delivery during approximately all 24 hours of a diurnal cycle and can have an operating temperature range between approximately 900-1200 K, or in some implementations approximately 400-1200 K. A system for use in a commercial application, for example, can provide approximately 50 kW of power for delivery during approximately 36 hours and can have an operating temperature range between approximately 1200-1800 K, or in some implementations approximately 400-1800 K. A system for use in conjunction with a renewable power generation (e.g. wind, solar, etc. or a combination thereof) application, for example, can provide approximately 500 kW of power for delivery during approximately 24 hours of a diurnal cycle and can have an operating temperature range between approximately 1200-1800 K, or in some implementations approximately 400- 1800 K. A system for use in conjunction with a utility-scale power generation (e.g. nuclear, natural gas combustion, fossil fuel combustion, or the like) application, for example, can provide approximately 5000 kW of power for delivery during approximately 8 hours of a diurnal cycle and can have an operating temperature range between 1500-2500 K, or in some implementations approximately 400-2500 K.

[00132] The dimensional parameters shown in Table 2 apply to a cylindrical and elongate core, which can be an advantageous implementation of the current subject matter, for example because such a configuration is generally a highly efficient use of space. Other configurations are also within the scope of the current disclosure. A residential core can in some implementations use a resistive type energy input and can requires a space similar to existing central air conditioner compressors that typically occupy a volume of less than approximately 0.3 cubic meters (e.g. approximately 10 or 11 cubic feet). A segmented layered core, for example such as is shown in FIG. 6 and the ready availability of industrial grade graphite or other heat retention matrix material can provide advantages in ease of manufacture and marketing, sales, etc. The small space requirement and significant power output of the system for both residential and commercial applications can provide an advantageous and practical solution to peak demand shifting capabilities.

[00133] Energy storage systems consistent with the current subject matter can also advantageously allow increases in energy storage capacity if and when required. Additional units that include one or more additional heat retention matrix cores can be added without rendering the original units obsolete. The small core size can typically be easily accommodated and can provide a rapid return on the capital expenditure due to the ability to reduce and/or smooth peak demand requirements and to thereby use energy more efficiently.

[00134] As the capacity of the containment vessel 204 within which the heat retention matrix 202 is contained increases, it is possible to use more efficient arrangements for the components power input and power output systems 210, 214. The use of a working fluid as described herein, for example one having two dominant components (e.g. nitrogen and a noble gas mixture, advantageously with argon included in the noble gas mixture such that nitrogen and argon approximately 95% or more of the working fluid by mole fraction) that are generally inert with respect to the materials used in the heat retention matrix as well as any support structures used to provide containment, shaping, orientation, support, or the like to the heat retention matrix can enable ready and effective commercialization of systems including such features as well as other described herein. A reactive compound removal system can be included as discussed above, for example to advantageously increase the useful life of the system as some reactive compounds may be initially present or produced from time to time particularly at high or maximum operating temperatures.

[00135] Technologies such as or similar to those described herein can be readily modified or otherwise varied to take advantage of alternative or additional low cost energy inputs that may be site or region specific. Conventional energy inputs, such as for example electrical or natural gas, can be used, as can direct or indirect solar input. The disclosed working fluids are significantly more cost effective, safe and practical compared to previously used helium/xenon mixtures.

[00136] The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail herein, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and sub-combinations of the disclosed features and/or combinations and sub-combinations of one or more features further to those disclosed herein. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. The scope of the following claims may include other implementations or embodiments.

[00137] Where a term is provided in the singular, the plural of that term is contemplated and included. Likewise, where the term is provided in the plural, the singular of that term is contemplated and included. Where the term "electrical grid" or simply "grid" is utilized, it is intended to be inclusive of all electrical grids and grids, inclusive of large, medium, small and micro-grids, as well as other composite electrical systems for the generation, transmission and distribution of electrical energy.

Claims

CLAIMS What is claimed is:

1. A system comprising:

a containment vessel;

a heat retention matrix contained within the containment vessel, the heat retention matrix comprising an allotropic form of carbon that stores input energy as thermal energy; and

a volume of a working fluid contained within the containment vessel and in contact with the heat retention matrix; the working fluid comprising nitrogen gas and a noble gas and being generally inert to reaction with the allotropic form of carbon.

2. A system as in claim 1, further comprising a reactive compound removal system that removes reactive compounds from the working fluid.

3. A system as in claim 2, wherein the reactive compound removal system comprises a filtration medium.

4. A system as in claim 3, wherein the filtration medium comprises activated carbon.

5. A system as in any of claims 1-4, further comprising an energy input system for delivering the input energy to the heat retention matrix from an energy source and an energy output system for retrieving output energy from the heat retention matrix for delivery to an energy demand, the energy input system and the energy output system being independently operable such that receiving of the input energy by the energy input system does not interfere with or preclude delivery of the output energy by the energy output system.

6. A system as in claim 5, wherein the working fluid comprises an energy input fluid;

the energy input system comprises an energy input working fluid circulator that passes the working fluid through at least one of an input energy heat exchanger and a first electrical energy conversion system that converts input energy from electrical energy to thermal energy that is transferred to the working fluid; and

the energy output system comprises an energy output working fluid loop circulator that passes an energy output fluid through at least one of an output energy heat exchanger and a second electrical energy conversion system that converts thermal energy in the energy output fluid to electrical energy for delivery to the energy demand.

7. A system as in any of claims 1-4, further comprising an energy input system for delivering the input energy to the heat retention matrix from an energy source.

8. A system as in any of claims 5-7, wherein the input energy comprises electrical input energy and the energy input system comprises an input energy conversion system that converts the electrical input energy to the thermal energy.

9. A system as in claim 8, wherein the input energy conversion system conversion system comprises at least one of an electrical induction system comprising an inductive heating element inside the containment vessel that is inductively heated using a magnetic field generated using the electrical input energy, a resistive heating system comprising a resistive heating element inside the containment vessel that is resistively heated by passing the electrical input energy through the resistive heating element, a plasma heating system comprising a plasma generating system powered by the electrical input energy for creating a plasma within at least one of the containment vessel and a plasma chamber through which the working fluid passes, and an energy input working fluid circulator that passes the working fluid through a first electrical energy conversion system that converts input energy from electrical energy to thermal energy that is transferred to the working fluid.

10. A system as in any of claims 5-7, wherein the input energy comprises heat and the energy input system comprises an input energy heat transfer system that transfers the heat to at least one of the working fluid and the heat retention matrix.

11. A system as in any of claims 1-4 and 7-10, further comprising an energy output system for retrieving output energy from the heat retention matrix for delivery to an energy demand.

12. A system as in any of claims 5-6 or 1 1, wherein the energy demand is for electrical output energy, and wherein the energy output system comprises an output energy conversion system that converts the stored thermal energy to the electrical output energy.

13. A system as in claim 12, wherein the output energy conversion system comprises at least one of a Stirling engine, a Brayton engine, a Rankine engine, an Otto engine, a boiler, a fluidic heat pump, a solid state heat pump, a turbine-based generator, a piston-based generator, a thermionic device, and a thermo-photovoltaic device.

14. A system as in any of claims 5-6 or 1 1, wherein the energy demand is for thermal output energy, and wherein the energy output system comprises at least one of a heat exchanger manifold and a circulation pump for transferring the stored thermal energy to the energy demand.

15. A system as in claim 14, wherein the heat exchanger manifold comprises a coil containing a heat transfer fluid, the coil being embedded in or between one or more thermal insulation layers of the containment vessel to absorb heat into the heat transfer fluid for extraction to at least one of an electrical generation system that converts the absorbed heat to electricity and a thermal energy utilization system that uses the absorbed heat to perform useful work.

16. A system as in any of claims 1-15, wherein the containment vessel comprises an inner layer comprising a refractory material, an outer layer comprising a structural material, and an insulation layer interposed between the inner layer and the outer layer.

17. A system as in claim 16, wherein the inner layer comprises at least one of glassy carbon and silicon carbide.

18. A system as in any of claims 16-17, wherein the insulation layer comprises a porous material that constricts a mean free path of molecules of the working fluid.

19. A system as in claim 18, wherein the porous form of amorphous carbon comprises at least one of a carbon fiber matrix, a carbon felt, and a carbon foam.

20. A system as in any of claims 1-19, wherein the working fluid comprises nitrogen gas (N₂) at a nitrogen mole fraction of approximately 35% or greater, argon gas (Ar) at an argon mole fraction of approximately 35% or greater, and neon gas (Ne) at a neon mole fraction of approximately 2% or greater.

21. A system as in any of claims 1-19, wherein the working fluid comprises nitrogen gas (N₂) at a nitrogen mole fraction of approximately 50%, argon gas (Ar) at an argon mole fraction of approximately 45%, neon gas (Ne) at a neon mole fraction of approximately 4%, and helium gas (He) at a helium mole fraction of approximately 1%.

22. A system as in any of claims 20-21, wherein the working fluid further comprises at least one of krypton gas (Kr) at a krypton mole fraction that is greater than zero and less than approximately 1% and xenon gas (Xe) at a xenon mole fraction that is greater than zero and less than approximately 1%.

23. A system as in any of claims 1-22, wherein the working fluid substantially consists of non-toxic compounds or atoms found in ambient air.

24. A system as in any of claims 1-23, further comprising a control system that maintains a temperature of the heat retention matrix in a range of approximately 900 K to approximately 1500 K during energy storage operations.

25. A system as in any of claims 1-23, further comprising a control system that maintains a temperature of the heat retention matrix below approximately 2500 K during energy storage operations.

26. A method of storing energy in an energy storage system comprising a containment vessel containing a volume of a working fluid and a heat retention matrix, the method comprising:

receiving input energy at the energy storage system;

retaining the input energy as thermal energy by the heat retention matrix and the working fluid, the heat retention matrix comprising an allotropic form of carbon, and the working fluid comprising nitrogen gas and a noble gas;

treating the working fluid using a reactive compound removal system to remove reactive compounds from the working fluid; and

delivering output energy from the energy storage system.

27. A method in claim 26, further comprising delivering the input energy to the heat retention matrix from an energy source via an energy input system and retrieving output energy from the heat retention matrix for delivery to an energy demand via an energy output system, the energy input system and the energy output system being independently operable such that receiving of the input energy by the energy input system does not interfere with or preclude delivery of the output energy by the energy output system.

28. A method as in claim 27, wherein the energy output system comprises a heat engine, and the method further comprises operating the heat engine with a Carnot efficiency greater than 30% or a Carnot efficiency greater than 45% or a Carnot efficiency greater than 55%.

29. A system comprising:

a containment vessel;

means for retaining received input energy as thermal energy, the means for retaining the thermal energy comprising an allotropic form of carbon and being contained within the containment vessel;

a volume of a working fluid contained within the containment vessel and in contact with the heat retention matrix; the working fluid comprising nitrogen gas and a noble gas and being generally inert to reactions with the allotropic form of carbon at an operating temperature range within which the system is operated; and

means for removing one or more reactive compounds from the working fluid.

30. A system as in claim 29, wherein the received input energy comprises electrical input energy and the energy input system comprises an input energy conversion system that converts the electrical input energy to the thermal energy, and wherein the means for storing thermal energy has a maximum operating temperature of less than approximately 1500 K; the system further comprising: an energy output system for retrieving output energy from the means for storing thermal energy at a rate of up to approximately 50 kilowatts per hour for a duration of at least 12 hours.

31. A system as claimed in claim 30, wherein the means for storing thermal energy comprises a graphite core occupying a volume of less than approximately 2.5 cubic meters.

32. A system as claimed in claim 29, wherein the input energy comprises electrical input energy, the energy input system comprises an electrical resistance type conversion system that converts the electrical input energy to thermal energy, and the means for storing thermal energy, when heated to a temperature of approximately 1500 K, cooperates with an energy output system to retrieve energy from the means for storing thermal energy at a rate up to approximately 5 kilowatts per hour for a duration of at least 12 hours.

33. A system as claimed in claim 32, wherein the means for storing thermal energy comprises a graphite core occupying a volume of less than approximately 0.30 cubic meters.