CROSS-REFERENCE

[0001] This application claims priority to U.S. Provisional Patent Application Serial No.

61/489,605, filed May 24, 2011 ("NOVEL RENEWABLE ENERGY STORAGE SYSTEM"), which is entirely incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] There are various types of approaches for thermal energy storage, which may be broadly classified under sensible heat storage, latent heat storage and chemical energy storage. Storage and removal of energy using sensible heat storage involves a temperature change of the storage medium in solid, liquid or gaseous form. Storage and removal of energy using latent heat storage involves a state change of the storage medium, e.g., liquid to gas. Storage and removal of energy using chemical energy storage involves a chemical change in the storage medium, e.g., burning hydrogen.

[0003] Some thermal energy technologies rely on sensible heat energy. Phase change materials, in contrast, rely on the latent heat energy and can therefore store substantially large amounts of heat. Thermal energy technologies having phase change materials may require large heat transfer areas.

[0004] There are limitations associated with current thermal energy storage systems. Chemical reactions may offer interesting tunable systems with key challenges around kinetics and mass transfer for the reversible reactions. Sensible heat storage materials (e.g. molten salt) may have narrow temperature range limitations, but are widely used in concentrating solar. Phase change materials may be limited to low temperatures (<200 °C).

[0005] As system temperatures increase, costs associated with containment materials may also increase. A key challenge is to minimize containment material cost.

SUMMARY OF THE INVENTION

[0006] In view of the limitations associated with current thermal energy storage systems recognized herein, there is a need in the art for improved apparatuses and methods for energy storage.

[0007] The invention provides energy storage systems that benefit from improved heat capacities in relation to other systems, while minimizing materials and operating costs and expenses. Some embodiments provide a supercritical fluid mixture that comprises carbon dioxide. The supercritical fluid mixture can include other components, such as hydrocarbons. Provided herein are efficient, high energy density, cost effective, reliable and maintainable thermal energy storage systems.

[0008] In some embodiments, supercritical fluid mixtures are provided having substantial increase in heat capacity in relation to other systems. The substantial increase in heat capacity may be on the order of at least 1.1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, or 10 over other fluid mixtures. In some cases, supercritical fluid mixtures provided herein comprising C0 ₂ and one or more secondary components may have heat capacities on the order of at least 1.1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, or 10 over a supercritical fluid that only includes C0 ₂ .

[0009] In some embodiments, supercritical fluids mixtures include two or more components. In some situations, a supercritical fluid includes carbon dioxide and other fluids.

[0010] In some embodiments, supercritical fluid mixtures are provided with heat capacities that are targeted to increase over broad temperature ranges. Such supercritical fluid mixtures may be tailored for particular applications— for example, for concentrating solar power, the mixtures may be able to store significant energy up to a temperature of about 600 °C. For geothermal processes, lower temperatures may be acceptable.

[0011] In some embodiments, supercritical fluid mixtures are provided with substantially high heat capacities, but configured to operate in a manner that does not substantially increase system pressure. This advantageously reduces containment costs, which may decrease overall system and operating costs.

[0012] In some embodiments, supercritical fluid mixtures are provided for use as working fluids and thermal energy storage mediums in energy storage systems that operate per a Brayton cycle.

[0013] In some embodiments, supercritical fluid mixtures are provided for use as thermal energy storage mediums in energy storage systems that operate per a Rankine cycle.

[0014] An aspect of the invention provides a system, comprising a circulatory fluid flow path having (a) a compressor configured to accept a working fluid and increase the pressure of the working fluid, wherein the working fluid is a supercritical fluid mixture comprising carbon dioxide and at least one disorder-inducing species; (b) a heat exchanger downstream of the compressor, the heat exchanger adapted to accept the working fluid from the compressor and provide heat to the working fluid; and (c) a power generator downstream of the heat exchanger, the power generator adapted to generate power upon the flow of the working fluid through the power generator, and direct the working fluid to the compressor. The system (i) circulates the working fluid along the circulatory fluid flow path, and (ii) increases the pressure and/or temperature of the working fluid to above the critical pressure and critical temperature of the working fluid. In an embodiment, the system further comprises an another circulatory fluid flow path for circulating an another working fluid, the another fluid flow path comprising: (a) an another heat exchanger for providing energy to the another working fiuid; (b) a pump downstream of the another heat exchanger, the pump adapted to accept an another working fluid and increase the pressure of the another working fluid; and (c) the heat exchanger. The heat exchanger accepts the another working fluid and transfers energy from the another working fluid to the working fiuid. In another embodiment, the system further comprises an energy source in thermal communication with the another heat exchanger. The energy source provides energy to the another heat exchanger. In another embodiment, the energy source is a combustion system, nuclear reactor or solar concentrator. In another embodiment, the another working fluid comprises a supercritical fluid mixture comprising carbon dioxide and at least one disorder- inducing species. In another embodiment, the system further comprises (d) an another heat exchanger downstream of the power generator, the another heat exchanger adapted to (i) accept the working fluid from the power generator, (ii) remove heat from the working fluid and (iii) direct the working fluid to the compressor. In another embodiment, the at least one disorder- inducing species is selected from the group consisting of an alkane, alkene, alcohol, aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons and carboxylic acid. In another embodiment, the compressor increases the pressure of the working fluid at or above a critical pressure of the working fluid. In another embodiment, the heat exchanger maintains or increases the temperature of the working fluid at or above a critical temperature of the working fluid.

[0015] Another aspect of the invention provides a system comprising a circulatory fluid flow path having: (a) a first heat exchanger adapted to accept a working fluid and provide energy to the working fluid. The working fluid is a supercritical fluid mixture comprising carbon dioxide and at least one disorder-inducing species; (b) a pump downstream of the first heat exchanger. The pump accepts the working fluid from the first heat exchanger and increases the pressure of the working fluid; and (c) a second heat exchanger downstream of the pump, the second heat exchanger adapted to accept the working fluid from the pump and transfer energy from the working fluid to a secondary fluid. The system circulates the working fluid along the circulatory fluid flow path. In an embodiment, the system further comprises an energy source in thermal communication with the first heat exchanger. The energy source provides energy to the first heat exchanger. In another embodiment, the energy source is a combustion system, nuclear reactor of solar concentrator. In another embodiment, the system maintains the pressure and temperature of the working fluid at a supercritical pressure and supercritical temperature, respectively, of the working fluid. In another embodiment, the at least one disorder-inducing species is selected from the group consisting of an alkane, alkene, alcohol, aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons and carboxylic acid. [0016] Another aspect of the invention provides a thermal energy storage fluid, comprising carbon dioxide and at least one disorder-inducing species. The proportion of carbon dioxide to the at least one disorder-inducing species in the thermal energy storage fluid is sufficient to induce disorder in the thermal energy storage fluid at or above a critical point of the thermal energy storage fluid. In an embodiment, the at least one disorder-inducing species is selected from the group consisting of an alkane, alkene, alcohol, aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons and carboxylic acid. In another embodiment, the thermal energy storage fluid has a heat capacity of at least about 1 Kilojoule

(KJ)/Kg*K, 2 KJ/Kg*K, 3 KJ/Kg*K, 4 KJ/Kg*K, 5 KJ/Kg*K, 6 KJ/Kg*K, 7 KJ/Kg*K, 8

KJ/Kg*K, 9 KJ/Kg*K, 10 KJ/Kg*K, 15 KJ/Kg*K, 20 KJ/Kg*K, 25 KJ/Kg*K, 30 KJ/Kg*K, 40

KJ/Kg*K, or 50 KJ/Kg*K. In another embodiment, the proportion is at least about 0.5: 1. In another embodiment, the proportion is at least about 1 : 1. In another embodiment, the proportion is at least about 2: 1.

[0017] Another aspect of the invention provides a method for forming a thermal energy storage fluid, comprising providing a fluid mixture having carbon dioxide and at least one disorder- inducing species in a vessel at a proportion of carbon dioxide to the at least one disorder- inducing species that is selected to induce disorder in the thermal energy storage fluid at or above a critical point of the thermal energy storage fluid. In an embodiment, the at least one disorder-inducing species is selected from the group consisting of an alkane, alkene, alcohol, aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons and carboxylic acid. In another embodiment, the thermal energy storage fluid has a heat capacity of at least about 1 KJ/Kg*K, 2 KJ/Kg*K, 3 KJ/Kg*K, 4 KJ/Kg*K, 5 KJ/Kg*K, 6 KJ/Kg*K, 7 KJ/Kg*K, 8 KJ/Kg*K, 9 KJ/Kg*K, 10 KJ/Kg*K, 15 KJ/Kg*K, 20 KJ/Kg*K, 25 KJ/Kg*K, 30 KJ/Kg*K, 40 KJ/Kg*K, or 50 KJ/Kg*K. In another embodiment, the proportion is at least about 0.5: 1. In another embodiment, the proportion is at least about 1 : 1. In another embodiment, the proportion is at least about 2: 1.

[0018] Additional aspects and advantages of the present disclosure will become readily apparent to those skilled in this art from the following detailed description, wherein only illustrative embodiments of the present disclosure are shown and described. As will be realized, the present disclosure is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the disclosure.

Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. INCORPORATION BY REFERENCE

[0019] All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference in their entireties to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The novel features of the invention are set forth with particularity in the appended claims. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which:

[0021] FIG. 1 shows a phase diagram of carbon dioxide;

[0022] FIG. 2 shows a plot of the heat capacity of carbon dioxide along specified isotherms (Tr);

[0023] FIG. 3 shows a phase diagram of a fluid;

[0024] FIG. 4 is a phase diagram of a fluid mixture, showing the critical line;

[0025] FIG. 5 is an exemplary plot of heat capacity (Cp) as a function of temperature (T) and component composition (X) for a fluid mixture comprising nitromethane and isobutanol;

[0026] FIG. 6 is a cross-section of the plot of FIG. 5;

[0027] FIG. 7 is a system configured to generate power, in accordance with an embodiment of the invention;

[0028] FIGs. 8A and 8B show pressure- volume and temperature-entropy diagrams for a system operating under a Brayton cycle;

[0029] FIG. 9 shows a system configured to generate power, in accordance with an embodiment of the invention;

[0030] FIG. 10 shows a system configured to store energy and generate power, in accordance with an embodiment of the invention; and

[0031] FIG. 11 shows a system for storing thermal energy, in accordance with an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0032] While various embodiments of the invention have been shown and described herein, it will be obvious to those skilled in the art that such embodiments are provided by way of example only. Numerous variations, changes, and substitutions may occur to those skilled in the art without departing from the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. [0033] The term "fluid" generally refers to a substance that continually deforms or flows under an applied shear stress. A fluid can have various phases, such as solid phase, liquid phase, gas phase, or supercritical phase. A fluid can include a liquid, gas, plasma, or semi-solid, such as a gel-like substance, or a mixture of fluids, such as a gas-liquid mixture. In some embodiments, a fluid can be selected from an organic (e.g., carbon-containing species) and/or inorganic substances, such as a substance including one or more -OH groups, =0 groups, carbon-to-carbon double bonds, and/or carbon-to-carbon triple bonds. In an example, a fluid can be selected from water, alcohols (e.g., methanol, ethanol), aldehydes, ketones, carboxylic acids, and combinations thereof, such as a water-alcohol mixture (e.g., water-methanol mixture). A fluid can be a fluid mixture having two or more components.

[0034] The term "supercritical fluid" generally refers to a substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. A

supercritical fluid can effuse through solids like a gas, and dissolve materials in a manner similar to a liquid. In addition, close to the critical point, small changes in pressure or temperature can result in large changes in fluid density. Examples of supercritical fluids include carbon dioxide (C0 ₂ ) and water.

[0035] The term "secondary fluid," as used herein, refers to a fluid for use in removing heat from, or adding heat to, another fluid. A secondary fluid can be a liquid, gas, gas-solid or gas- liquid mixture. In some cases, a secondary fluid is air.

[0036] The term "disorder-inducing species" generally refers to an atom or molecule, or a mixture of atoms and/or molecules, which is adapted to generate disorder in a fluid at or above the critical point of the fluid. For example, in a fluid mixture having C0 ₂ and a disorder- inducing species, the disorder-inducing species can generate disorder in the fluid mixture at or above the critical point of the fluid mixture. This may be effected, for example, by reduced (or disrupted) intermolecular interactions between atoms or molecules of the fluid, such as between CO ₂ molecules, and in some cases increased attractive interactions between C0 ₂ or disorder- inducing species, which may result in local inhomogeneities and extended disorder at or above the critical point. For example, a disorder-inducing species in a fluid mixture comprising CO ₂ can disrupt the interaction between CO ₂ molecules in the fluid mixture. In some embodiments, a disorder-inducing species introduces disorder in a fluid mixture comprising supercritical CO ₂ . In some embodiments, the disorder-inducing species is an organic species, such as an alkane, alkene, alcohol, aldehyde, ketone, ether, ester, water, fluorinated hydrocarbons, nitromethane, aromatic hydrocarbons and carboxylic acid.

[0037] The term "cycle," as used herein, refers to a system having one or more components (or unit operations, also "units" herein) for facilitating fluid flow and/or fluid phase change, such as pumps, compressors, fluid separators, heat exchangers and reservoirs (or vessels). A cycle can be a circulatory flow system. In the context of such circulatory flow systems, the terms

"downstream" and "upstream" are used to indicate the location of one component in relation to another component along a fluid flow path that brings the components in fluid communication with one another. Components can be interconnected with the aid of fluid flow paths (or fluid streams, also "streams" herein), which can include channels, fluid passages or conduits for aiding in fluid flow from one unit to another.

[0038] FIG. 1 is a phase diagram of carbon dioxide as a function of pressure (bar) and temperature (Kelvin, K). Below the critical temperature, e.g., 280K, C0 ₂ exists as a gas, liquid or solid, depending on the pressure. As the pressure increases, the gas compresses and eventually (at just over 40 bar) condenses into a much denser liquid. The system of FIG. 1 includes two phases in equilibrium, a dense liquid and a low density gas. As the critical temperature is approached (300K), the density of the gas at equilibrium becomes denser, and that of the liquid lower. At the critical point, (304.1 K and 7.38 MPa (73.8 bar)), there is no difference in density, and the two phases of C0 ₂ become one fluid phase, namely a supercritical fluid. In some situations, for carbon dioxide at 400 K, the density increases almost linearly with pressure.

[0039] FIG. 2 is a plot of the heat capacity of carbon dioxide along specified isotherms (Tr). The invention provides supercritical fluid mixtures that can have both broadened and increased heat capacities with minimal system pressure increases.

[0040] A supercritical fluid may be generated by increasing the pressure of a fluid at a constant temperature above the critical temperature; increasing the temperature of the fluid at a constant pressure above the critical pressure; or increasing the temperature and the pressure of the fluid to a point above the critical temperature and critical pressure. The pressure of the fluid can be increased at constant (or substantially constant) volume.

[0041] A fluid may be pressurized with the aid of a compressor or pump. As an example, at a temperature above the critical temperature, C0 ₂ may be compressed by a single or multi-stage compressor to a pressure above the critical pressure of C0 ₂ to generate supercritical C0 ₂ .

[0042] The invention provides fluid mixtures that are based on the unexpected realization that by introducing disorder in a supercritical fluid mixture, the thermal energy properties, such as heat capacity, of the fluid mixture can be tailored for use in a given application, such as a thermal energy storage application. Such fluid mixtures can be used in energy storage systems, such as systems adapted for use in a Brayton or Rankine Cycle. In some embodiments, an ultra- thermally dense thermal energy storage fluid is provided by enhancing critical fluctuations and introducing disorder to tailor the properties of supercritical fluids over a wide range in pressure (P), temperature (T) and composition (X). Fluid mixtures provided herein can demonstrate extended thermal capacity along the critical line of the fluid mixtures and throughout the adjacent critical surface.

[0043] In some embodiments, a fluid mixture comprises a primary component (atom or molecule) and one or more secondary components. The primary component can be a non- carcinogenic and/or an environmentally friendly species, and the one or more secondary components can be disorder-inducing species. The secondary components may be non- carcinogenic and/or environmentally friendly. In some embodiments, the primary component is selected from C0 ₂ and H ₂ 0. In an example, the primary component is C0 ₂ . In another example, the primary component is H ₂ 0. In another example, the primary component is a mixture of C0 ₂ and H ₂ 0. The temperature, pressure and composition of the fluid are selected such that the fluid mixture is supercritical.

Thermal energy storage fluids

[0044] An aspect of the invention provides a thermal energy storage fluid comprising carbon dioxide and at least one disorder-inducing species. The proportion of carbon dioxide to the at least one disorder-inducing species in the thermal energy storage fluid is selected such that disorder is induced in the thermal energy storage fluid at or above a critical point of the thermal energy storage fluid. In some cases, disorder is induced at or above a critical point of C0 ₂ . The fluid with the disorder-inducing species can be at a pressure and temperature such that C0 ₂ is in a supercritical state.

[0045] In some embodiments, the thermal energy storage fluid has a heat capacity of at least about 0.1 Kilojoules (KJ)/Kg*K, 1 KJ/Kg*K, 2 KJ/Kg*K, 3 KJ/Kg*K, 4 KJ/Kg*K, 5 KJ/Kg*K, 6 KJ/Kg*K, 7 KJ/Kg*K, 8 KJ/Kg*K, 9 KJ/Kg*K, 10 KJ/Kg*K, 15 KJ/Kg*K, 20 KJ/Kg*K, 25 KJ/Kg*K, 30 KJ/Kg*K, 40 KJ/Kg*K, or 50 KJ/Kg*K. The thermal energy storage fluid may have a heat capacity from about 1 KJ/Kg*K and 100 KJ/Kg*K, or 3 KJ/Kg*K and 50 KJ/Kg*K.

[0046] In some embodiments, the proportion of carbon dioxide to the at least one disorder- inducing species is at least about 0.1 : 1, or at least about 0.5: 1, or at least about 1 : 1, or at least about 2: 1, or at least about 3: 1, or at least about 4:1, or at least about 5: 1, or at least about 10: 1, or at least about 20: 1, or at least about 30: 1, or at least about 40: 1, or at least about 50: 1, or at least about 100: 1.

[0047] Supercritical fluid mixtures of the invention are based on the unexpected realization that heat storage properties of fluids, such as heat capacities, may be increased by inducing inhomogeneities at or above the critical point (see FIG. 3). With reference to the phase diagram of FIG. 3, a supercritical fluid may experience an improved heat capacity in the supercritical portion of the phase diagram of the fluid along, for example, a "ridge" extending from the critical point, separating "liquid-like" and "vapor-like" region. Such improved heat capacity may be experienced along other paths in the critical region. With reference to FIG. 4, for fluid mixtures the properties of the fluid mixture extending away from a critical line to a plane may be adjusted to enhance the heat capacity of the fluid mixture. In some embodiments, the composition of a fluid mixture comprising carbon dioxide and one or more disorder-inducing species may be selected to enhance the heat capacity of the fluid mixture. This may provide increases in intrinsic heat capacity of the fluid mixture in the vicinity of the critical point (line), and provide extended enhancement of the heat capacity into the supercritical region.

[0048] In some situations, carbon dioxide is mixed with a disorder-inducing species to form a fluid mixture, and the pressure and/or temperature of the fluid mixture is increased to a point such that the fluid mixture is in a supercritical regime. In some cases, this is achieved via a trans-critical process with fluid properties (e.g., pressure, temperature, composition) selected to bring the fluid to supercritical conditions. In some cases, this is above a critical point

(temperature, pressure) of the fluid mixture.

[0049] In some embodiments, the thermal energy storage fluid has a disorder-inducing species at a composition (mole %) between 0.1% and 70%>, or between about 1% and 60%>, or between about 5%> and 50%>. In some situations, the thermal energy storage fluid has a disorder-inducing species at a composition of at least about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%), 30%), 40%), 50%o, or 60%>. The balance of the fluid mixture is C0 ₂ . In an example, a supercritical fluid mixture comprises about 60%> C0 ₂ and 40%> ethanol.

[0050] As an example, a fluid mixture comprises 80% C0 ₂ and 20% ethanol. As another example, a fluid mixture comprises 90% C0 ₂ and 10% nitromethane. As another example, a fluid mixture comprises 95% C0 ₂ and 5% acetic acid.

[0051] In some embodiments, the thermal energy storage fluid has two or more disorder- inducing species at a composition (mole %) between 0.1% and 70%, or between about 1% and 60%), or between about 5% and 50%. In some situations, the thermal energy storage fluid has two or more disorder-inducing species at a composition of at least about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 30%, 40%, 50%, or 60%. The balance of the fluid mixture is C0 ₂ . In an example, a supercritical fluid mixture comprises about 60% C0 ₂ and 20% methanol and 20% ethanol.

[0052] In some embodiments, the disorder-inducing species comprises one or more organic compounds. An organic compound can be selected from alkanes, alkenes, alkynes, alcohols, carboxylic acids, ketones, aldehydes, or combinations thereof. In some situations, the disorder- inducing species comprises one or more compounds selected from Table 1. Table 1: disorder-inducing species. Pc = critical pressure; Tc = critical temperature.

Compound Pc (M Pa) Tc (dcg C ^' )

Nitromethane 5.9 315

Acetic acid 5.8 320

Acetone 4.8 325

Methanol 8.1 240

Ethanol 6.1 241

Isopropanol 4.9 235

Glycerol 7.5 577

Ethyl acetate 3.8 257

Isobutyl acetate 3 287

Dimethyl carbonate 4.8 284

Dimethyl ether 5.3 127

Perfluorobutane 2.3 113

R32 (difluoromethane) 5.8 78

Methane 4.6 -82

Cyclohexane 4.1 281

Toluene 4.1 319

2-Methyl Propylcyclohexane 3.1 386

Dodecane 1.8 385

Tetra hydrofuran 5.2 267

Water 22 374

[0053] A fluid mixture comprising carbon dioxide and one or more disorder-inducing species can include one or more, two or more, three or more, four or more, five or more, or six or more disorder-inducing species. For instance, a fluid mixture can comprise carbon dioxide and ethanol, or carbon dioxide, ethanol and propanol (e.g., isopropanol).

[0054] In some embodiments, anomalously high heat capacities are achieved at or above the critical point, which may arise from extended density inhomogeneities as may be produced with the aid of disorder-inducing species. The length scale is characterized by a correlation length (ξ) much greater than the molecular scale and is due to enhanced attractive interactions

("clustering") between molecules near the critical point. Maximization of heat capacity and its persistence in some cases is achieved by maximizing disorder in fluid systems.

[0055] The heat capacity of a fluid mixture comprising C02 and one or more other components may be continuously tuned by varying the fluid mixture composition. In some cases, this may provide at least a 2, 3, 4, 5, 6, 7, 8, 9 or 10-fold increase in heat capacity at the critical point with respect to the pure components, and a significant enhancement of heat capacity in the

supercritical regime and away from the critical point.

[0056] Such increases in heat capacity have been achieved in a fluid mixture having

nitromethane and isobutanol, as discussed in Losada-Perez, P, G. Perez-Sanchez, J. Troncoso, and C.A. Cerdeirina, J. Chem. Phys, 132, 154509 (2010), which is entirely incorporated herein by reference. FIG. 5 is an exemplary plot of heat capacity (Cp) as a function of temperature (T) and component composition (X) for a fluid mixture comprising nitromethane and isobutanol. FIG. 6 is a cross-section of the plot of FIG. 5 taken at 291 K. FIGs. 5 and 6 show a substantial increase in Cp at a temperature of about 291 Kelvin (K) and a fluid mixture composition of about 45 mole % nitromethane.

Power generation systems

[0057] Another aspect of the invention provides a circulatory fluid flow system configured to store energy and/or generate power. In some embodiments, the system employs a fluid mixture comprising a supercritical fluid and a disorder-inducing species. The fluid mixture is both a thermal energy storage fluid and a working fluid. The supercritical fluid in some cases is C0 ₂ . In other embodiments, the system employs a fluid mixture comprising a supercritical fluid and a disorder-inducing species. The fluid mixture in such a case is the thermal energy storage material. The system employs a separate working fluid, which may be a supercritical fluid.

[0058] In some cases, a supercritical fluid is the working fluid of a power generation system. In other cases, the supercritical fluid is the thermal energy storage medium of a power generation system.

[0059] FIG. 7 shows a system 700 configured to generate power, in accordance with an embodiment of the invention. The system 700 comprises a compressor 701, heat exchanger 702 and power generator 703. The power generator 702 may be a turbine adapted to generate electricity, such as, for example, via electromagnetic induction. In some situations, the system 700 includes an energy source 704, such as a renewable energy source. The system 700 is a circulatory fluid flow system in which a working fluid comprising a fluid mixture having a supercritical fluid, such as C0 ₂ , and one or more secondary components, such as a disorder- inducing species, is directed through a fluid flow path having the compressor 701, heat exchanger 702 and power generator 703. The direction of fluid flow in the circulatory fluid flow system is indicated by arrows between the unit operations of the system 700.

[0060] The renewable energy source can be configured to direct thermal energy to the heat exchanger 702 or a fluid in fluid communication with the heat exchange 702. For example, the renewable energy source 704 can be a solar concentrator that collects solar radiation and directs thermal energy collected from collected solar radiation to the heat exchanger 702, either directly, such as with the aid of optics, or with the aid of a secondary fluid or thermal energy storage material, such as, for example, molten salt or a supercritical fluid adapted to store thermal energy. Solar concentrators are described in, for example, WO/2011/116141 to Joseph et al, which is entirely incorporated herein by reference.

[0061] In an exemplary operation of the system 700, the working fluid is compressed by the compressor 701 and directed into the heat exchanger 702, where heat is applied to the working fluid. The compressor 701 compresses the working fluid to a pressure at or above the critical pressure of the working fluid, and the heat exchanger 702 supplies heat to the working fluid, which may increase the temperature of the working fluid to a temperature at or above the critical temperature of the working fluid. Next, the working fluid is directed to the power generator 703, which generates electricity. In some situations, heat is removed from the working fluid, such as in the power generator 703 and/or another heat exchanger between the power generator 703 and the compressor 701.

[0062] In some embodiments, the working fluid is operated at or above a critical point of the working fluid from the point the system 700 is initiated. In such a case, the system 700 can initiate with the working fluid in a supercritical regime of the working fluid. The compressor and/or heat exchanger may be used to increase the pressure and temperature of the working fluid from a first point (Pi, Ti) in the supercritical regime to a second point (P ₂ , T ₂ ) in the supercritical regime.

[0063] The system 700 may be adapted to operate under the Brayton cycle. FIGs. 8A and 8B show pressure- volume and temperature-entropy diagrams for a system operating under a Brayton cycle, in accordance with an embodiment of the invention. The system of FIGs. 8A and 8B operates with the aid of a working fluid comprising a supercritical fluid mixture. The

supercritical fluid mixture in some cases includes C0 ₂ and one or more secondary fluids, such as one or more disorder-inducing species.

[0064] From step 1 to step 2, the working fluid is compressed, such as with the aid of a compressor. Next, from step 2 to step 3, heat is added to the compressed working fluid. In some cases, heat is added to the working fluid with the aid of a heat exchanger in thermal

communication with the working fluid. Next, from step 3 to step 4, the working fluid is directed to a power generator, which is used to generate electricity from the working fluid. From step 3 to step 4, the working fluid undergoes expansion and a reduction in pressure. From step 4 to step 1, heat is rejected from the working fluid. Heat can be removed from the working fluid with the aid of a heat exchanger. Heat rejected from the working fluid can be directed to a secondary fluid, such as water to generate steam. The secondary fluid may be a supercritical fluid, such as supercrirical C0 ₂ and, in some cases, a disorder-inducing species.

[0065] In some embodiments, from step 2 to step 3, heat can be added to the working fluid (e.g., supercritical fluid) from an energy source, such as heat generated from combustion (e.g., fuel or coal combustion), a nuclear reactor or a solar concentrator. In some situations, heat from the energy source may be stored in a thermal energy storage material, which may be brought in thermal communication with the working fluid to transfer heat from the thermal energy storage material to the working fluid. In some cases, the thermal energy storage material is brought in direct contact with the working fluid to transfer energy.

[0066] With continued reference to FIGs. 8A and 8B, energy is introduced to the working fluid in the form of heat from step 2 to step 3. In addition, energy may be introduced to the working fluid in the form of work upon compression from step 1 to step 2.

[0067] Heat (energy) introduced into the system is represented as Q ₂ _ ₃ = C _P (T ₃ -T ₂ ), where system enthalpy is typically increased via temperature increase. An increase in C _p by a factor of 10 can effectuate an increase in the amount of energy stored in the system by about the same factor without an increase in temperature. This avoids operation at higher temperature and some of the associated materials and maintenance issues. Heat loss may decline as the thermal gradient to the ambient is significantly reduced. Efficiency may be improved with reduced containment insulation. Overall cycle efficiency can also directly improve as input enthalpy is increased.

[0068] In an exemplary implementation of the cycle of FIGs. 8A and 8B, a working fluid comprising a supercritical fluid mixture having C0 ₂ and one or more secondary species (or components), such as one or more disorder-inducing species, is directed through a closed-loop cycle comprising a compressor, heat exchanger in thermal communication with a source of energy, and a turbine. From step 1 to step 2, the pressure of the working fluid is increased with the aid of the compressor. Next, from step 2 to step 3, the working fluid is directed to the heat exchanger that is in thermal communication with the source of energy, such as a solar concentrator for collecting and concentrating solar radiation, a source of combustion, or nuclear reactor. In the heat exchanger, heat is added to the working fluid. Next, from step 3 to step 4, the working fluid is directed to the power generator, such as a turbine, to generate electricity. From step 4 to step 1, the working fluid is directed from the power generator to the compressor, and heat is removed from the working fluid.

[0069] Supercritical fluid mixtures provided herein can be tailored to decrease the cost of systems operating under the Brayton cycle. For example, a supercritical working fluid with a high heat capacity can decrease the operating pressure and/or temperature of the system, leading to savings in materials and maintenance costs and expenses.

[0070] FIG. 9 shows a system 900 comprising a renewable energy source 901, a compressor 902, a first fluid storage vessel 903, a second fluid storage vessel 904, a power generator 905, an electricity (or power) grid 906 and an electrical load 907, in accordance with an embodiment of the invention. The renewable energy source 901 can include one or more solar concentrators, wind turbines, wave generators, or geothermal energy sources.

[0071] In some cases, the system 900 can include a heat exchanger between the compressor 902 and the second fluid storage vessel 904. The heat exchanger can be configured to direct energy to the working fluid. The energy can be provided by way of an energy source, such as a renewable energy source (e.g., solar concentrator).

[0072] In some embodiments, the compressor 902, first fluid storage vessel 903, second fluid storage vessel 904 and power generator 905 define a close-loop power generation cycle. The cycle in some cases can operate under the Brayton cycle (see below). The cycle can operate with the aid of a working fluid that comprises a primary fluid and one or more secondary fluids. In some embodiments, the working fluid is a supercritical fluid mixture that comprises C0 ₂ as the primary fluid and one or more secondary fluids, such as one or more disorder-inducing species (e.g., ethanol, isopropanol), as described elsewhere herein.

[0073] In an exemplary implementation of the system 900, the renewable energy source 901 generates electricity, which may be directed to the electricity grid 906 and subsequently directed to the electrical load 907. In some cases, electricity from the renewable energy source is used to power the compressor 902, which compresses a fluid from the first fluid storage vessel 903 and directs the compressed fluid to the second fluid storage vessel 904. In an example, the renewable energy source 901 is used to compress the fluid from the first storage vessel 903 to the second storage vessel 904 in off-peak (or low electricity demand) conditions. In some embodiments, when energy is required, such as in on-peak (or high electricity demand) conditions, the compressed fluid from the second fluid storage vessel 904 is directed to the power generator 905, which generates electricity. Electricity generated by the power generator 905 may be directed to the power grid 906 and subsequently directed to the electrical load 907.

[0074] In some embodiments, a circulatory fluid flow system is provided having a fluid mixture comprising a supercritical fluid and a disorder-inducing species. The fluid mixture in such a case is the thermal energy storage material. The system employs a separate working fluid, which may be a supercritical fluid.

[0075] FIG. 10 shows a system 1000 configured to generate electricity, in accordance with an embodiment of the invention. The system 1000 includes a first circulatory fluid flow cycle comprising a compressor 1001, first heat exchanger 1002 and power generator 1003. A fluid flow path directs a first working fluid from the compressor 1001 to the first heat exchanger, from the first heat exchanger 1002 to the power generator 1003, and subsequently from the power generator 1003 to the compressor 1001. The first circulatory fluid flow cycle can include another heat exchanger between the power generator 1003 and the compressor 1001, which is configured to remove heat from the first working fluid of the first circulatory fluid flow cycle. In some situations, the first circulatory fluid flow cycle may operate under the Brayton cycle.

[0076] The system 1000 also includes a second circulatory fluid flow cycle that comprises the first heat exchanger 1002, a second heat exchanger 1004 and pump 1005. A fluid flow path directs a second working fluid of the second circulatory fluid flow cycle from the first heat exchanger 1002 to the second heat exchanger 1004, and subsequently from the second heat exchanger 1004 to the pump 1005. Flow of the second working fluid through the second circulatory fluid flow path can be facilitated with the aid of the pump 1005. Together, the first circulatory fluid flow cycle and the second circulatory fluid flow cycle can operate under the

Rankine cycle.

[0077] The system 1000 in some cases includes an energy source 1006 configured to supply energy to the second heat exchanger 1006. The energy source 1006 can be a combustion furnace, nuclear reactor or solar concentrator or other source of energy. In some embodiments, the energy source 1006 is configured to supply thermal energy to the second heat exchanger 1002, which is subsequently directed to the second working fluid of the second circulatory fluid flow cycle.

[0078] In some embodiments, the first working fluid is water, a hydrocarbon or other organic species. In some situations, the first working fluid is an alkane or alkene, such as n-pentane or toluene. In some embodiments, the first working fluid is a supercritical fluid.

[0079] In some embodiments, the second working fluid is a supercritical fluid mixture. In an example, the supercritical fluid mixture includes supercritical C0 ₂ and one or more secondary components, such as one or more disorder-inducing species. The properties of the second working fluid in some cases are selected to optimize the heat capacity of the second working fluid.

[0080] In an exemplary operation of the system 1000, the second working fluid of the second circulatory fluid flow cycle is heated in the second heat exchanger 1004 with the aid of thermal or electrical energy provided by the energy source 1006 and directed to the first heat exchanger 1002 with the aid of the pump 1005. In the first heat exchanger 1002, energy is transferred from the second working fluid to the first working fluid of the first circulatory fluid flow cycle. The first working fluid is then directed to the power generator 1003, which generates power from the first working fluid. The first working fluid is then directed to the compressor 1001, which compresses the first working fluid and directs the first working fluid to the first heat exchanger 1002.

Thermal energy storage systems

[0081] Another aspect of the invention provides a system for storing thermal energy. The system comprises a supercritical fluid. In some embodiments, the supercritical fluid is a fluid mixture comprising carbon dioxide and one or more disorder-inducing species. The disorder- inducing species can be as described elsewhere herein.

[0082] FIG. 11 shows a system 1100 for storing thermal energy, in accordance with an embodiment of the invention. The system 1100 includes a first vessel 1101 in thermal communication with a source of energy 1102. The source of energy 1102 may be a combustion system, nuclear reactor or renewable energy source (e.g., solar concentrator). A pump 1103 directs a working fluid from the first vessel 1101 to a second vessel 1104. In some

embodiments, the working fluid is a supercritical fluid comprising carbon dioxide and one or more disorder-inducing species. The disorder-inducing species can be as described elsewhere herein. The pump can select the pressure of the working fluid to be at or above the critical pressure of the working fluid. The temperature of the working fluid can be selected to be at or above the critical temperature with the aid of the source of energy 1102.

[0083] The first vessel 1101 and/or the second vessel 1104 can be a storage vessel for storing the working fluid for later use. In some cases, the first vessel 1101 and/or the second vessel 1104 can be detachable from a fluid flow path having the first vessel 1101, the pump 1103 and the second vessel 1104. This may permit energy to be transferred to the working fluid, which may be transported to another location (e.g., remote location) for use.

[0084] In some situations, one or both of the first vessel 1101 and the second vessel 1104 is a heat exchanger adapted to facilitate the transfer of energy to the working fluid or from the working fluid to another fluid, such as a secondary fluid (e.g., water).

[0085] Systems and methods provided herein can be implemented with the aid of a controller having a processor (e.g., central processing unit) that executes machine-executable instructions, which may be located on a memory location (e.g., read-only memory, random-access memory, flash memory) of the controller. The controller can be a feedback controller. The controller can regulate various system parameters, such as system pressure, temperature, and flow rate. The controller may be programmed to monitor fluid pressure and/or temperature and maintain system and/or fluid parameters to achieve a given system performance. For instance, the controller can be programmed to maintain system pressure and/or temperature to maximize the heat capacity of the thermal energy storage medium or working fluid. Example 1

[0086] A supercritical fluid mixture comprises carbon dioxide C0 ₂ and ethanol in a composition (mole %) of about 90% C0 ₂ and 10% ethanol. The composition is selected to provide a heat capacity of the fluid mixture that may be suitable for use with thermal energy storage.

Example 2

[0087] A system, such as the system 700 of FIG. 7, is used to generate power. The working fluid is a fluid mixture comprising Supercritical C0 ₂ and ethanol at a composition (mole %) of about 90% C0 ₂ and 10% ethanol. The temperature of the working fluid is about 305 K. In a first step, the supercritical working fluid is directed to a heat exchanger, which supplies heat to the working fluid. In a second step, the working fluid is directed to a turbine to generate power. Next, in a third step, heat is removed from the working fluid, in some cases with the aid of another heat exchanger. The working fluid is then returned to the compressor.

[0088] It should be understood from the foregoing that, while particular implementations have been illustrated and described, various modifications may be made thereto and are contemplated herein. It is also not intended that the invention be limited by the specific examples provided within the specification. While the invention has been described with reference to the aforementioned specification, the descriptions and illustrations of embodiments of the invention herein are not meant to be construed in a limiting sense. Furthermore, it shall be understood that all aspects of the invention are not limited to the specific depictions, configurations or relative proportions set forth herein which depend upon a variety of conditions and variables. Various modifications in form and detail of the embodiments of the invention will be apparent to a person skilled in the art. It is therefore contemplated that the invention shall also cover any such modifications, variations and equivalents.