THERMAL ENERGY STORAGE SYSTEM AND MEDIA

Abstract

The present disclosure is directed to materials that can be used in a heat storage and transfer, and an improved method for storing thermal energy which includes a high heat capacity thermal energy storage system using pumped or flowing metallic phase change materials (MPCs). Heat is added by pumping a cold fluid of MPCs mixed with a fluid media such as a molten glass and/or salt from a tank through a heat exchanger, solar receiver, or electrical heater cell and returning the heated fluid to a tank, or solid MPCs can be transported physically, or via gas transport such as entrained flow or a circulating fluid bed. In the heat exchanger, heat can optionally be transferred directly to a counterflowing gas or other fluid, or indirectly through heat exchanger walls to a working fluid, which can be steam, CO.sub.2 or sCO.sub.2, He, H.sub.2, process gas, and/or heat transfer fluid. The MPCs (encapsulated MPCs, non-coated MPCs) are solid-liquid and/or solid-solid phase change particles, salts, metals, or other compounds with a melting point between the hot and cold fluid temperatures, and can optionally include high heat capacity, and/or energy absorbing (IR and divisible) nanoparticles.

Claims

1. An improved thermal energy storage system which includes: a. a storage tank; b. a thermal fluid, said thermal fluid includes i) glass and/or salt; and ii) a metallic phase change material (MPC), said MPC is mixed with said thermal fluid to form a thermal mixture, said MPC includes an encapsulated metallic mixture having a melting temperature or range that falls within a temperature range of 400-1500° C., said MPC is formulated to be reusable in a temperature environment of at least 400° C.

2. The thermal energy storage system as defined in claim 1, wherein the MPC has a particle size of 20-2000 microns.

3. The thermal energy storage system as defined in claim 1, wherein the MPC includes a core and coating, said core formulated to have a liquidus temperature of 550-750° C., said MPC formulated to be reusable to a temperature of at least 700° C.

4. The thermal energy storage system as defined in claim 1, wherein the MPC includes a core and coating, said core formulated to have a liquidus temperature above 400° C., said MPC formulated to be reusable in a temperature environment of at least 800° C., said MPC includes at least 50 wt. % Si metal.

5. The thermal energy storage system as defined in claim 1, wherein the MPC includes a core and coating, said MPC formulated to be reusable to a temperature of 800-1200° C.

6. The thermal energy storage system as defined in claim 1, wherein said thermal fluid includes a ternary salt, a quaternary salt, and/or a quinary salt.

7. The thermal energy storage system as defined in claim 1, wherein said thermal fluid includes a eutectic salt or near eutectic salt, or a eutectic glass or near eutectic glass.

8. The thermal energy storage system as defined in claim 1, wherein the molten salt includes one or more of a) sodium nitrate/potassium nitrate, b) a ternary salt mixture of NaNO.sub.3/KNO.sub.3/NaNO.sub.2, c) carbonate and fluoride salts, d) LiNaK fluorides, e) ZnNaK chlorides, f) MgNaK chlorides, g) AlCl.sub.3—NaCl—KCl, h) ZnCl2—NaCl—KCl, i) FeCl.sub.3—NaCl—KCl, j) NaCl—CaCl.sub.2—MgCl.sub.2, and/or k) KCl—NaCl—CaCl.sub.2—MgCl.sub.2 or the molten glass is a phosphate or borate low melting point glass.

9. The thermal energy storage system as defined in claim 1, wherein said thermal mixture includes 5-80 vol. % MPC and 20-95 vol. % salt and/or glass.

10. The thermal energy storage system as defined in claim 1, wherein said thermal mixture further includes 0.5-10 vol. % colloidal nanoparticles.

11. The thermal energy storage system as defined in claim 10, wherein said colloidal nanoparticles include one or more metals, metal oxides, and/or non-metal oxides; said colloidal nanoparticles have at least one dimension that is no more than 200 nm.

12. The thermal energy storage system as defined in claim 10, wherein said colloidal nanoparticles include one or more materials selected from the group consisting of SiO.sub.2, ZnO, Al.sub.2O.sub.3, TiO.sub.2, MgO, Fe.sub.2O.sub.3, BaTiO.sub.3, Ce.sub.2O.sub.3, ZrO.sub.2, CaO, Ni, Mo, Si, Re, Nb, Ta, W, Au, and Ag.

13. The thermal energy storage system as defined in claim 1, wherein said MPC has a latent heat of at least 300 J/g.

14. The thermal energy storage system as defined in claim 1, wherein said MPC has a latent heat that falls within the range of 300-2000 J/g.

15. The thermal energy storage system as defined in claim 1, wherein said MPC includes a) Si-(A and/or B) MPC; b) Si-(A and/or B)-(X) MPC; and/or c) Si-(A and/or B)-(X and Y); said A, B, X and Y are elements; said elements for A and B are selected from the group consisting of aluminum, copper, boron, germanium, and magnesium; said elements for X and Y are selected from the group consisting of aluminum, calcium, chromium, cobalt, copper, iron, magnesium, manganese, and nickel.

16. The thermal energy storage system as defined in claim 1, wherein said MPC includes one or more materials selected from the group consisting of Si—Al, Si—Mg—Cu, Si—Mg, S—Al—Ca, Si—Cu—Ca, Si—Ge, Si—B, Si—Ge—B, Si—Ge—Fe, Si—Ge—Mn, Si—Ge—Fe—Mn, Si—Ge—Cu, Si—Ge—Ni, Si—Ge—Cr, Si—Ge—Cu—Fe, Si—Ge—Ni—Fe, and Si—Ge—Cr—Fe.

17. The thermal energy storage system as defined in claim 1, wherein said MPC includes at least 10 wt. % Si; and one or more of a) at least 10 wt. % Al, b) at least 10 wt. % B; c) at least 10 wt. % Cu, d) at least 10 wt. % Ge, and e) at least 10 wt. % Mg.

18. The thermal energy storage system as defined in claim 1, wherein said MPC includes an outer coating; said outer coating includes one or more materials selected from the group consisting of SiC, SiOCN, SiCN, Si.sub.3N.sub.4, SiB.sub.6, TiO.sub.2, and an organic polysilizane preceramic polymer.

19. The thermal energy storage system as defined in claim 18, wherein said coating includes filler, said filler including one or more materials selected from the group consisting of carbon fibers, boron fibers, ceramic spheres, carbon powders, TiO.sub.2, SiC powder, Boron nitride, and B.sub.4C.

20. The thermal energy storage system as defined in claim 18, wherein said coating has a thickness of 1 to 500 microns.

21. The thermal energy storage system as defined in claim 1, wherein said MPC has a size having at least one dimension that is from 20-5000 microns.

22. A method for storing energy in a thermal energy storage system comprising: providing a thermal fluid, said thermal fluid includes i) one or more of glass, a ternary salt, a quaternary salt, and/or a quinary salt; and ii) a metallic phase change material (MPC); said MPC including a metallic component having a melting temperature that falls within a temperature range of 400-1500° C.; said MPC is formulated to be reusable in a temperature environment of at least 400° C.; and heating said thermal fluid.

23. A metallic phase change material (MPC), said MPC including a metallic mixture having a melting temperature that falls within a temperature range of 400-1500° C., said MPC is formulated to be reusable in a temperature environment of at least 400° C.; said metallic mixture of said MPC includes a) Si-(A and/or B) MPC; b) Si-(A and/or B)-(X) MPC; and/or c) Si-(A and/or B)-(X and Y); said A, B, X and Y are elements; said elements for A and B are selected from the group consisting of aluminum, copper, boron, germanium, and magnesium; said elements for X and Y are selected from the group selected from the group consisting of aluminum, calcium, chromium, cobalt, copper, iron, magnesium, manganese, and nickel.

24. The MPC as defined in claim 23, wherein said MPC includes a mixture of silicon metal and alloying elements to control a liquidus and solidus of said MPC to a temperature from 550° C. to 1250° C.; said MPC including a ceramic or ceramic composite coating.

25. The MPC as defined in claim 23, wherein said MPC includes at least 20 wt. % Si and one or more alloying elements selected from the group consisting of Al, Mg, Ge, Fe, Ni, Cu, Mn, and Cr.

26. The MPC as defined in claim 23, wherein said MPC has a particle size is of 20-2000 microns.

27. The MPC as defined in claim 23, wherein said MPC has a coating thickness of 1-200 microns.

28. The MPC as defined in claim 23, wherein said MPC has a latent heat of at least 300 J/g.

29. The MPC as defined in claim 23, wherein said MPC has a latent heat is at least 800 J/g.

30. The MPC as defined in claim 23, wherein said MPC has a latent heat is at least 1000 J/g.

31. The MPC as defined in claim 23, wherein said metallic mixture is not an eutectic mixture.

32. The MPC as defined in claim 23, wherein said metallic mixture includes 20-80 wt. % Si, and 20-80 wt. % of one or more of Mg, Al, Mn, Cr, Fe, Ge, and/or Cu.

33. The MPC as defined in claim 23, wherein said metallic mixture includes Si and two or more of Mn, Fe, Cr, Cu, Al, Mg, and/or Ge.

34. The MPC as defined in claim 23, wherein said metallic mixture includes Si and at least 1-25 wt. % of B.

35. The MPC as defined in claim 23, wherein said MPC includes a coating to modify its properties, including reactivity, emissivity, and/or adsorptivity.

36. The MPC as defined in claim 23, wherein said MPC has a shape that is selected from the group consisting of a sphere, an elliptical spheroid, a rounded cube, a flake, a prism, a cylinder, a rod, a cuboid, or a fiber.

37. The MPC as defined in claim 23, wherein said MPC is embedded and/or contained in or attached to a structure, alloy, and/or cavity for the function of controlling transient thermal response to a thermal load in such structure, alloy, and/or cavity.

38. The MPC as defined in claim 23, wherein said MPC is part of a thermal fluid that is used in a thermal energy storage system, and wherein said MPC is transported in said thermal fluid by gravity, mechanical means, gas entrainment, or fluid transport to a heat consuming or transfer device such as a heat exchanger, process vessel, reactor, or fluidized bed.

39. The energy storage system as defined in claim 38, wherein said thermal fluid includes i) a molten salt, ii) a molten glass, or iii) a process gas which includes hydrogen, steam, CO.sub.2, methane, hydrocarbon, Helium, Xenon, argon, and/or ammonia.

40. The energy storage system as defined in claim 38, wherein heat is to said thermal fluid supplied from i) a resistance heating element, ii) process heat from an industrial operation, iii) solar energy, or iv) nuclear energy.

41. The energy storage system as defined in claim 38, wherein said MPC in said thermal fluid is heated by direct radiation/absorption.

42. The energy storage system as defined in claim 38, wherein said MPC in said thermal fluid is used to heat a liquid or gaseous working fluid by direct or indirect contact.

43. The energy storage system as defined in claim 38, which is designed and sized for long term energy storage of at least 4 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0137] These and other features of the disclosure can be best understood from the following specification and one or more drawings, the following of which is a brief description:

[0138] FIG. 1 is a graph that illustrates the improvement in thermal-electric conversion efficiencies with increasing temperature.

[0139] FIG. 2 illustrates a non-limiting Concentrating Solar Power (CSP) plant that use mirrors to concentrate the sun's energy to drive traditional steam turbines or engines that create electricity.

DETAILED DESCRIPTION OF NON-LIMITING EMBODIMENTS

[0140] A more complete understanding of the articles/devices, processes and components disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and are, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments.

[0141] Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

[0142] The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.

[0143] As used in the specification and in the claims, the term “comprising” may include the embodiments “consisting of” and “consisting essentially of.” The terms “comprise(s),” “include(s),” “having,” “has,” “can,” “contain(s),” and variants thereof, as used herein, are intended to be open-ended transitional phrases, terms, or words that require the presence of the named ingredients/steps and permit the presence of other ingredients/steps. However, such description should be construed as also describing compositions or processes as “consisting of” and “consisting essentially of” the enumerated ingredients/steps, which allows the presence of only the named ingredients/steps, along with any unavoidable impurities that might result therefrom, and excludes other ingredients/steps.

[0144] Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value.

[0145] All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 grams to 10 grams” is inclusive of the endpoints, 2 grams and 10 grams, all the intermediate values and all intermediate ranges).

[0146] The terms “about” and “approximately” can be used to include any numerical value that can vary without changing the basic function of that value. When used with a range, “about” and “approximately” also disclose the range defined by the absolute values of the two endpoints, e.g., “about 2 to about 4” also discloses the range “from 2 to 4.” Generally, the terms “about” and “approximately” may refer to plus or minus 10% of the indicated number.

[0147] Percentages of elements should be assumed to be percent by weight of the stated element, unless expressly stated otherwise.

[0148] In one non-limiting configuration, the improved thermal energy storage includes one or more thermocline molten salt storage tanks, a thermal fluid that includes a ternary, quaternary, or quinary eutectic or near eutectic salt, and WPCs in slurry or suspension which circulates with the molten salt. As illustrated in FIG. 2, the thermal fluid is communicated to and from two thermal storage tanks or thermocline storage tanks. One of the thermocline storage tanks is a cold storage tank and the other storage tank is a hot storage tank. As can be appreciated, a single tank can be used to store both the hot and cold thermal fluid. “Cold” thermal fluid, which is around 550-780° F. (288-450° C.) in one example, is communicated from the cold storage tank through the central receiver system where it is heated. The “hot” thermal fluid, in the example, is around 1050-2000° F. (566-1100° C.), is then communicated to the hot storage tank. When power or thermal energy is required, the hot thermal fluid is pumped to a steam, CO.sub.2, air, or other working fluid generator system that produces steam or high-pressure working fluid. The working fluid drives a turbine/generator that creates electricity for communication to a power grid or other use. The thermal fluid is returned to the cold storage tank where it is stored and eventually reheated in the central receiver system. It should be understood that although a particular component arrangement is disclosed in the illustrated embodiment, any arrangement that utilizes a thermal storage tank system would also benefit from the disclosed examples.

[0149] When the thermal storage tank system is only formed of a single thermocline storage tank, such design is based on the principle that hot thermal fluid in a quiescent environment tends to rise and stay above colder thermal fluid in the same tank. This phenomenon is also known as thermal stratification as temperature versus distance from the bottom of the storage tank. A thermocline or thermogradient region is the layer in which temperature changes more rapidly with change in depth than do the temperatures in the layers above or below. As the temperature of the hot thermal fluid decreases in the hot storage region, its density increases and it tends to “fall” unless the thermal fluid below it is also cooling at an equal or greater rate. The relative densities of the thermal fluid determine its elevational position within the tank. The cold thermal fluid in the cold storage region is denser than the hot thermal fluid in the hot storage region. The fact that molten salt has a relatively low thermal conductivity tends to aid in this stratification effect. Molten salt can be thought of as liquid insulation. A thermocline region naturally develops and separates the cold and hot storage regions. However, this natural separation does not provide efficient enough storage for use in solar power systems.

[0150] The single thermocline storage tank, when used, can include a wall operatively secured to a bottom wall to provide a cavity. The wall includes a portion having a geometry or shape interiorly arranged within the storage tank. A baffle assembly can be arranged in the thermocline region. A baffle is an obstruction used to substantially separate the cold and hot storage regions.

[0151] The flows in and out of the hot and cold pools in the single thermocline storage tank can disturb the inherent quiescent nature of the thermal stratification. The baffle assembly in the single thermocline storage tank limits mixing of fluid between the cold and hot storage regions by substantially physically separating the cavity into the cold and hot storage regions, as well as reducing thermal transfer and leakage between the thermal fluids. If the baffle is sealed to the tank wall, such as through flexible metal bellows with or without anti-wear coatings, the baffle may be driven mechanically (using cables, screws, or hydraulics) to drive thermal fluid from the hot to cold tank. A dual baffle system (with one baffle at the hot/cold interface and one at the hot/head space interface) can be used to drive the thermal fluid in both directions. In one embodiment, the intermediate baffle is driven from a drive system in the low-temperature thermal fluid (which can be made from standard materials), and the hot side baffle is driven by connections in the head space (which can be cooled, and which are not exposed to the corrosive molten salt). Alternatively, conventional pumps can be used at the tank base (cold fluid) and at the top of the tank (hot thermal fluid). Due to the near constant volume of the tank (e.g., allowing for the expansion as the salt is heated), pumps can be mounted directly to the tank wall, instead of using large shaft pumps suspended from the ceiling, reducing pump cost and complexity, and simplifying freeze protection.

[0152] In one non-limiting example of a single thermocline storage tank, one or more immersion heaters may be used to heat the hot storage region thermal fluid, and trace heaters may be used to heat the cold storage region thermal fluid.

[0153] In another non-limiting examples, there is provided a high heat capacity thermal energy storage system using pumped or flowing MPCs. Heat is added by pumping a cold thermal fluid that includes MPCs through a heat exchanger, solar receiver, or electrical heater cell. The thermal fluid can include a molten salt and/or other fluid such as molten glass. The MPCs in the thermal fluid can be encapsulated MPCs, and/or non-coated MPCs. The MPCs can be used in temperature environments of at least 400° C. The use of higher storage temperatures of the MPCs enable much higher thermal-electrical efficiencies (up to 80% combined heat and power, >50% in direct power cycles), and operate in temperature regimes suitable for decarbonizing industrial processes, including steel, ammonia, hydrogen production, glass, and concrete manufacture. The use of MPCs based on Si in composition with Al, B, Ge, and/or Mg can provide for 100X-150X the energy density of lithium-ion batteries, scaling potentially to 50-100 MW-hrs of energy storage in the size of a 40 ft. container. This thermal energy storage that includes the use of MPCs can be generated using grid- or nuclear-generated power (100% conversion to heat), concentrating solar, or renewable and intermittent (PV, wind) sources. Such thermal energy storage can be a) paired with a turbogenerator (supercritical steam or CO.sub.2) to generate power, or b) coupled with thermochemical reactors to produce chemicals such as hydrogen and ammonia, and/or energy intense commodities such as steel and concrete.

[0154] The MPCs when used with molten glass (e.g., a quaternary molten glass, etc.) can be used to reduce the size and/or cost of thermal energy storage (and transport) systems compared to current solar salt systems. Such glass systems can optionally also contain 1-15% SiO.sub.2. In one non-limiting embodiment, the glass can have a viscosity of 80-120 pa-s at 700-800° C., and a glass transition temperature (Tg) of 400-450° C.; however, it will be appreciated that other glasses can be used.

[0155] The MPCs can be combined with a glass and/or salt for use in a thermal energy storage (and transport) systems or the like, the content of the MPCs in the thermal fluid is at least 5 vol. %.

[0156] In addition to the addition of MPCs and the salt and/or glass in the thermal fluid, the thermal fluid can optionally include colloidal nanoparticles to enhance heat capacity, absorptivity and/or thermal conductivity.

[0157] The MPCs include Si-(A and/or B) MPC; Si-(A and/or B)-(X) MPC; and/or Si-(A and/or B)-(X and Y). The elements for A and B are selected from aluminum (Al), copper (Cu), boron (B), germanium (Ge), and magnesium (Mg). When A and B are used in the MPCs, A and B are different elements. The elements for X and Y are selected from Al, Ca, Cr, Co, Cu, Fe, Mg, Mn, and Ni. When X and/or Y are used, X and/or Y are different from A and/or B. When X and Y are used in the MPCs, X and Y are different elements. Non-limiting MPCs include Si—Al, Si—Mg—Cu, Si—Mg, S—Al—Ca, Si—Cu—Ca, Si—Ge, Si—B, Si—Ge—B, Si—Ge—B—X, and/or Si—Ge—B—X—Y.

[0158] In one non-limiting formulation, the MPCs have a Si content of at least 10 wt. % (e.g., 10-90 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include Ge, the Ge content in the MPCs is at least 10 wt. % (e.g., 10-60 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include B, the B content in the MPCs is at least 10 wt. % (e.g., 10-60 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include Al, the Al content in the MPCs is at least 10 wt. % (e.g., 10-60 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include Cu, the Cu content in the MPCs is at least 10 wt. % (e.g., 10-60 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include Mg, the Mg content in the MPCs is at least 10 wt. % (e.g., 10-60 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include only X (e.g., Al, Ca, Cr, Co, Cu, Fe, Mg, Mn, Ni), the X content in the MPCs is at least 1 wt. % (e.g., 1-40 wt. % and all values and ranges therebetween). In another non-limiting aspect of the present disclosure, when the MPCs include both X (e.g., Al, Ca, Cr, Co, Cu, Fe, Mg, Mn, Ni) and Y (e.g., Al, Ca, Cr, Co, Cu, Fe, Mg, Mn, Ni), the X content in the MPCs is at least 1 wt. % (e.g., 1-30 wt. % and all values and ranges therebetween) and the Y content in the MPCs is at least 1 wt. % (e.g., 1-30 wt. % and all values and ranges therebetween).

[0159] The MPCs can optionally be formed by mixing together metal powders of Si and (A and/or B) and optionally (X and/or Y). During the mixing process, the metal powders can be optionally ground. The metals powders typically are pressed together and optionally sintered. Other forming process prior to sintering (e.g., spraying drying, etc.) can be used to form certain shapes (e.g., spherical, etc.). During the sintering process, the metal powders can optionally be heated above the lowest solidus temperature of the components of the MPCs to cause about 0.5-10% (and all values and ranges therebetween) of the components to liquify during the sintering process.

[0160] The MPCs can optionally be partially (e.g., 5-99.9% and all values and ranges therebetween of the outer surface of the MPC is coated) or fully coated particles. Various coating techniques can be used (e.g., vapor deposition [e.g., chemical vapor deposition, physical vapor deposition, etc.]; plasma spraying; coating with a ceramic precursor and then subsequent curing to form a ceramic coating; spray coating; dipping; brushing; rolling; etc.). Non-limiting coatings include SiC, SiOCN, SiCN, Si.sub.3N.sub.4, SiB.sub.6, TiO.sub.2, or an organic polysilizane (PSZ) preceramic polymer. The coating materials can optionally contain a filler (e.g., carbon fibers, boron fibers, ceramic spheres, carbon powders, TiO.sub.2, B.sub.4C, etc.). The coating thickness of the coating generally is at least 0.1 nm.

[0161] In another non-limiting aspect of the present disclosure, the MPCs can have various shapes and sizes. Non-limiting shapes include spheres, flakes, particles, beads, ribbons, etc. The size of the MPCs have at least one dimension that is from 20-5000 microns (and all values and ranges therebetween).

[0162] The following properties are desirable, but not are required, for a MPC: [0163] High latent heat of fusion −300+J/g. [0164] A melting point range is generally from 450-1500° C. (and all values and ranges therebetween). For chloride molten salt systems, the melting point range is generally from 500-750° C. For industrial processing applications, the melting point range is generally 600-1000° C. Non-limiting examples of melting point ranges for the molten salt and turbine cycle includes 850-1000° C., 600-720° C., 550-650° C., and 450-550° C.] [0165] Particle density is optionally selected to be between the densities of the hot and cold fluids, matching the cold fluid when solid, and the hot fluid when molten. However, a value independent between the hot and cold fluids is achievable, and is generally from 1.05 to 2.2 g/cc (and all values and ranges therebetween). In one non-limiting example, the particle density is from 1.8-2.2 g/cc, and typically 1.9 g/cc±0.1. For molten chloride salts, the particle density can be as low as 1.05 g/cc. [0166] The MPCs can optionally be mixed in the molten salt and/or glass and pumped as a slurry through the receiver and heat exchangers. To allow for flow, in addition to density, a fine-sized particle is used which is generally no greater than 500 microns (e.g., 10-500 microns and all values and ranges therebetween), typically 25 microns (325 mesh) to 200 microns (100 mesh), and more typically 50-125 microns. Too small of a particle can lead to higher viscosity, and excess loss to the inert encapsulation material. Too large of a particle has excessive tendencies to settle or float, and cannot melt or solidify quickly enough in the heat exchangers, leading to excessively large heat exchangers. The particle size is typically selected for a heat exchanger or receiver such that the exposure time of the particle based on a melting time under heat exchanger thermal flux is from 5-200 seconds residence time (and all values and ranges therebetween), and typically 10 and 60 seconds residence time. For some heat exchangers, residence times can be minutes or longer, [0167] The particle thermal conductivity is typically high to meet melting time requirements. Thermal conductivity of the MPCs is generally greater than 50 W/m-K (e.g., 50-350 W/m-K and all values and ranges therebetween), typically greater than 100 W/m-K, more typically greater than 150 W/m-K, and even more typically greater than 200 W/m-K. The optional encapsulation or coating material for the MPCs (when used) is generally selected to be compatible with the molten salt and/or glass, have good thermal conductivity, have a fairly low density, and/or form a thin coating on the MPC alloy. The desirable coating material should be easily applied, such as by plating, spray-coating, CVD, or solution deposition. Suitable techniques include sol-gel synthesis (oxides), plating (molten salt or electroless (metals), CVD, or via polymer chemistry. In one non-limiting embodiment, the coating or encapsulation includes polysilazane preceramic polymer, where the polysilazane can optionally include reactive fillers to offset shrinkage, fillers such as, but not limited to, boron nitride, boron carbide, boron nitride nanosheets, titanium oxide, metal particles, alumina, and/or other oxide particles to modify the thermal properties of the coating and the residual stress after pyrolysis and thermal cycling. The filler content in the coating material is generally 0-60 wt. % (and all values and ranges therebetween), and typically 0-40 wt. %. When the coating material includes filler, the filler content is generally at least 0.5 wt. %. The thickness of the coating layer can be 1-25% (and all values and ranges therebetween) of the metal alloy diameter of the MPC. For higher temperature MPC's with melting points above 850° C., CVD coating, such as in a fluidized or rotary bed, using the decomposition of chlorosilanes such as MTS (mthyltricholorsilane) or DDS (dimethyldichlorosilane). The use of preceramic polymers, such as polysilizanes and polycarbosilizanes is preferred when the MPC melting point is below 850° C.

Examples

[0168] Example 1: Coated MPCs are prepared by melt atomizing a mixture of 79 wt. % aluminum and 21 wt. % silicon metal into spherical powders to form core particles. The core particles were then sieved to −150 to +325 mesh (45-100 microns). The core SiAl particles were then coated with a thermosetting phenolic resin, and then further coated with a preceramic polymer coating formulation that includes polysilizane with 5 wt. % boron carbide and 5 wt. % fumed silica. The coated MPCs were initially cured at a temperature of 240° C. for about hour, and then slowly heating at 1C° /min to a temperature of 350° C-800° C. Thereafter, the coated MPCs were cooled. 30 vol. % MPC's are then added to a eutectic mixture of CaCl.sub.2, MgCl.sub.2, NaCl, and KCl to form a thermal fluid. The thermal fluid had the effective heat capacities and properties as shown in Table 1:

TABLE-US-00001 TABLE 1 PCM salt fluid Al Cp Si Cp densit density densit Cp salt ave delta H Cp PCM effective Cp temperatu wt % salt wt % PCM J/g .Math. K J/g .Math. K g/cc g/cc g/cc J/g .Math. K J/g .Math. K J/g .Math. K J/g .Math. K RT 0.9 0.71 2.35 500 63 37 1.15 0.88 2.29 1.67 1.855916 1.05 0 1.096 1.06702 520 63 37 1.17 0.88 2.28 1.66 1.845704 1.05 0 1.112 1.07294 540 63 37 1.195 0.88 2.27 1.65 1.83549 1.04 0 1.132 1.07404 560 63 37 1.219 0.89 2.26 1.64 1.825273 1.04 0 1.1532 1.081834 580 63 37 1.243 0.89 2.25 1.63 1.815055 1.03 28.8013 1.1724 11.739243 600 63 37 1.27 0.89 2.24 1.62 1.804834 1.03 1.08214286 1.194 1.491072857 620 63 37 1.29 0.9 2.23 1.61 1.794612 1.02 1.0824 1.212 1.491528 640 63 37 1.32 0.9 2.22 1.6 1.784387 1.02 1.08214286 1.236 1.500312857 660 63 37 1.35 0.9 2 1.59 1.7205 1.01 1.08214286 1.26 1.502892857 680 63 37 1.37 0.91 1.99 1.58 1.710385 1.01 1.08214286 1.278 1.509552857 700 63 37 1.4 0.91 1.98 1.57 1.700268 1 1.08214286 1.302 1.512132857 720 63 37 1.42 0.91 1.97 1.55 1.68274 1 1.08214286 1.318 1.518052857 total joules per gram total joules/cc all numbers in J/g .Math. K indicates data missing or illegible when filed

[0169] Example 2: Coated MPCs are prepared by melt atomizing a mixture of 57 wt. % Si and 43 wt. % Mg by gas atomization to form core particles, and then the core particles where screened to −150 to +325 mesh (45-100 micron). The core particles wherein then coated with SiC using the decomposition of DDS (dimethyledichlorosilane) at 875° C. in a fluidized bed to a thickness of 7-10 microns. The properties of the MPCs are shown in Table 2.

TABLE-US-00002 TABLE 2 PCM Mg Cp Si Cp density ave delta H Cp PCM effective Cp total energy temperature wt % PCM J/g .Math. K J/g .Math. K g/cc J/g .Math. K J/g .Math. K J/g .Math. K J/g RT 1.02 0.71 2.15 900 0 100 1.15 0.91 1.9 0 0.982 0.982 19.64 920 0 100 1.17 0.93 1.9 0 1.002 1.002 20.04 940 0 100 1.195 0.93 1.9 39.43 1.0095 0.4395 1208.79 960 0 100 1.219 0.94 1.91 1.37071429 1.0237 2.394414286 47.88828571 980 0 100 1.243 0.94 1.92 1.37071429 1.0309 2.401614286 48.03228571 1000 0 100 1.27 0.95 1.93 1.37071429 1.046 2.416714286 47.33428571 1020 0 100 1.29 0.95 1.94 1.37071429 1.052 2.422714286 48.45428571 1040 0 100 1.29 0.96 1.95 1.37071429 1.059 2.429714286 48.59428571 1060 0 100 1.3 0.97 1.98 1.37071429 1.069 2.439714286 48.79428571 1080 0 100 1.3 0.98 2.2 1.37071429 1.076 2.446714286 48.93428571 1100 0 100 1.31 0.98 2.21 1.37071429 1.079 2.449714286 48.99428571 1120 0 100 1.32 0.99 2.21 1.37071429 1.089 2.459714286 49.19428571 total joules per gram 1666.050571 total joules/cc % packing density 1832.655629 indicates data missing or illegible when filed

[0170] Example 3: Coated MPCs are prepared as in Example 2, but by melting and atomizing a mixture of 18 wt. % Si and 82 wt. % Ge to form core particles, and then the core particles wherein screened to −150 to +325 mesh. The core particles were then coated with SiC using CVD to a thickness of 7-10 microns. The MPCs had a latent heat above 1500J/g and the core of the MPCs has a melting range of 945° C-1210° C.

[0171] Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.

[0172] It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the constructions set forth without departing from the spirit and scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. The disclosure has been described with reference to preferred and alternate embodiments. Modifications and alterations will become apparent to those skilled in the art upon reading and understanding the detailed discussion of the disclosure provided herein. This disclosure is intended to include all such modifications and alterations insofar as they come within the scope of the present disclosure. It is also to be understood that the following claims are intended to cover all of the generic and specific features of the disclosure herein described and all statements of the scope of the disclosure which, as a matter of language, might be said to fall therebetween. The disclosure has been described with reference to the preferred embodiments. These and other modifications of the preferred embodiments, as well as other embodiments of the disclosure, will be obvious from the disclosure herein, whereby the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims.