Patent Description:
Energy storage is generally used to accommodate daily and seasonal imbalances in energy consumption and production. Power generation from renewable sources, such as concentrated solar power ("CSP"), solar photovoltaics ("PV"), and wind turbines is inherently variable. Accordingly, renewable energy sources are best used in conjunction with energy storage systems that store energy when production exceeds demand, and release energy when demand exceeds production.

Some renewable energy systems, such as solar PV and wind, use batteries to store electrical energy. Other storage systems include pumped hydro, compressed air, and flywheels, among others. Other renewable energy systems, such as CSP, incorporate thermal energy storage ("TES"). CSP plants typically incorporate sensible heat storage using materials such as molten salts, oil, sand, rock, or other particulate materials. Molten salt energy storage can have energy densities ranging from <NUM> to <NUM> MJm-<NUM>. TES systems typically operate at temperatures of less than <NUM>, limiting the exergy and thereby the thermal-to-electric conversion efficiency.

Some renewable energy systems incorporate thermochemical energy storage ("TCES"); however, many TCES systems have poor reactive stability (i.e., ability to be reused for thousands of cycles with negligible degradation in performance), moderate volumetric energy densities, and/or low energy discharge temperatures.

<CIT> discloses a dual-cavity method and device for collecting and storing solar energy with metal oxide particles.

<CIT> discloses a CSP system which couples a thermal and a chemical energy pathway.

In accordance with the present invention, which is defined by the features of the independent claims, a TCES device is provided. The TCES device includes a vessel, a porous bed, and a heater. The vessel defines an interior volume and includes a first opening and a second opening. The porous bed is disposed within the interior volume and is in fluid communication with the first and second openings. The porous bed comprises a reactive material. The reactive material is configured to release oxygen upon being heated to a reduction temperature, and generate heat when exposed to air or any oxygen-carrying gas and reacting with oxygen. The heater is configured to heat the reactive material, said heater is in contact with the reactive material or includes a heating element that is embedded in the reactive material. Moreover, the heater includes one or both of (A) a heating element embedded in the reactive material with ends electrically connected to an electricity source, and/or (B) a first pair of ceramic electrodes having the porous bed disposed therebetween and a second pair of ceramic electrodes having the first pair of ceramic electrodes and the porous bed disposed therebetween. A further aspect provides an electrical-to-electrical energy storage system. The electrical-to-electrical energy storage system includes the TCES device, a blower, a compressor, a turbine, and an electrical generator. The blower is configured to remove oxygen from the interior volume of the TCES device when the reactive material is heated. The compressor is configured to provide air or any oxygen-containing gas to the interior volume of the TCES device. The turbine is configured to receive a heated, oxygen-depleted gas from the interior volume of the TCES device. The generator is configured to be powered by the turbine to generate electricity. Yet another aspect provides a method of storing energy and releasing energy using the electrical-to-electrical energy storage system.

The present TCES device is advantageous in that it operates at high temperatures, such as at least about <NUM> in a preferred embodiment. Furthermore, the reactive material has a high reactive stability and a high volumetric energy density, such as at least about <NUM> MJm-<NUM> in a preferred embodiment. The reactive material may be cheap, abundant, and accepting of impurities, making it practical to use in large-scale operations. In a preferred embodiment, the reactive material comprises a magnesium-manganese oxide. The TCES device may be sized and shaped according to standard shipping container dimensions, thereby facilitating ease of transport. The present electrical-to-electrical energy storage system includes the TCES device. The system can include multiple TCES devices to achieve a desired capacity.

A preferred embodiment of a TCES device <NUM> can be observed in <FIG>. The TCES device <NUM> generally receives electricity, such as excess electricity generated by a renewable source, during an energy storage process and generates heat during an energy release process. The generated heat can be used together with high pressure air to generate electricity in a downstream component.

The TCES device <NUM> includes a bed <NUM> (also referred to as a "porous bed") comprising a reactive material <NUM>, a vessel <NUM>, a heater <NUM>, insulation <NUM>, and optionally a cooling system <NUM>. The TCES device <NUM> is configured to store and release energy. In an energy storage process, the TCES device <NUM> receives electricity. The electricity is converted to high-temperature heat by the heater <NUM>. The heater <NUM> heats the reactive material <NUM>, causing the reactive material to undergo chemical reduction, thereby releasing oxygen and being converted to a reduced state. In the energy recovery process, the TCES device <NUM> receives oxygen. The reactive material <NUM> is exposed to the oxygen to cause the reactive material <NUM> to be oxidized, thereby generating heat.

The vessel <NUM> includes a shell <NUM>, a first end cap <NUM>, and a second end cap <NUM>. The shell <NUM> and the first and second end caps <NUM>, <NUM> cooperate to at least partially define an interior volume <NUM>. The first end cap <NUM> defines a first opening <NUM>. The second end cap <NUM> defines a second opening <NUM>. The first opening <NUM> and the second opening <NUM> are fluidly connected to the interior volume <NUM>. The vessel <NUM> extends along a longitudinal axis <NUM>. The longitudinal axis <NUM> may extend through the first opening <NUM> and the second opening <NUM>. The shell <NUM> is substantially cylindrical. However, in alternative embodiments, the shell <NUM> may define other shapes.

The reactive material <NUM> is retained within the interior volume <NUM> of the vessel <NUM>. For example, the shell <NUM> may cooperate with a first support <NUM> and a second support <NUM> to retain the reactive material <NUM>. The first support <NUM> may be disposed adjacent to the first end cap <NUM> to form a physical barrier preventing the reactive material <NUM> from escaping through the first opening <NUM>. Similarly, the second support <NUM> may be disposed adjacent to the second end cap <NUM> to prevent the reactive material <NUM> from escaping through the second opening <NUM>. The first and second supports <NUM>, <NUM> are permeable to fluids, such as air. The first and second supports <NUM>, <NUM> may define substantially annular shapes. In one example, the first and second supports <NUM>, <NUM> comprise a ceramic grit.

The vessel <NUM> may receive high-pressure gases, such as high-pressure air. Accordingly the vessel <NUM> is preferably a pressure vessel. In a preferred embodiment, the pressure vessel is configured to contain a gas having a pressure of at least about <NUM> bar. The pressure vessel may preferably comprise carbon steel. However, the pressure vessel <NUM> may additionally or alternatively comprise one or more of another steel, a stainless steel, a nickel alloy, a steel super alloy, a titanium alloy, an oxide-dispersion-strengthened alloy, an Inconel alloy, and a HAYNES® alloy.

The heater <NUM> is configured to heat the reactive material <NUM>. Thus, the heater <NUM> is in thermal contact with the reactive material <NUM>, or is configured to be in thermal contact with the reactive material <NUM>. According to one option of the invention, the heater <NUM> is a heating element that is embedded in the reactive material <NUM>. The heating element is sized and shaped to heat substantially all of the reactive material <NUM>. Accordingly, the heating element spans an entire length of the interior volume <NUM>. The heating element spans at least about <NUM>% of a diameter of the interior volume <NUM>, optionally at least about <NUM>%, optionally at least about <NUM>%, optionally at least about <NUM>%, and optionally at least about <NUM>%.

In some embodiments, the heating element defines a serpentine shape between a first end <NUM> and a second end <NUM>. The heating element intersects a central longitudinal plane of the interior volume <NUM>. However, the heating element may optionally define alternative shapes and/or configurations within the interior volume <NUM>. The heating element may alternatively extend in multiple radial directions or define a coil, for example. The first end <NUM> extends through the first end cap <NUM> and the second end <NUM> extends through the second end cap <NUM>. The first and second ends <NUM>, <NUM> are configured to be electrically connected to an electricity source (see, e.g., electricity source <NUM> of <FIG> and <FIG>).

The heating element comprises a material that can withstand high temperatures, such as temperatures of at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, and preferably at least about <NUM>. In a preferred embodiment, the heating element is a resistive heating element. The resistive heating element may comprise a molybdenum disilicide. However, in alternative embodiments, the resistive heating element may comprise lanthanum chromite or zirconia.

In a preferred example embodiment, such as when the reactive material <NUM> is electrically conductive, the heater <NUM> is configured for bulk resistive heating of the bed <NUM>. Bulk resistive heating is preferable when the reactive material <NUM> is electrically conductive, at least at certain temperatures. With reference to <FIG>, an example embodiment of a heater <NUM>' is configured for bulk resistive heating of a porous bed <NUM>' including a reactive material <NUM>'. The heater <NUM>' and the reactive material <NUM>' may be included in a TCES device similar to the TCES device <NUM> of <FIG>.

The bed <NUM>' is electrically conductive at certain temperatures, such as at temperatures greater than or equal to about <NUM>. In addition to the reactive material <NUM>', the bed <NUM>' may optionally include one or more additional components <NUM> to reduce or prevent formation of instabilities due to preferential electrical pathways and localized hot spots. The additional components <NUM> may have different electrical properties than the reactive materials <NUM>'. More particularly, in a preferred embodiment, the additional components <NUM> may have a higher electrical conductivity than the reactive material <NUM>', a relatively higher electrical conductivity that decreases as temperature increases, and/or a less-temperature-dependent electrical conductivity than the reactive material <NUM>'. The additional components <NUM> may be in the form of pellets, rods, and/or one or more interlinking structures, for example.

According to another option of the invention, the heater <NUM>' comprises a pair of first or inner electrodes <NUM> and a pair of second or outer electrodes <NUM>. Each inner electrode <NUM> is disposed between the reactive material <NUM>' and a respective outer electrode <NUM>. Each outer electrode <NUM> may be disposed between a respective conductive component <NUM> and inner electrode <NUM>. The electrodes <NUM>, <NUM> are configured such that fluid can flow through the electrodes <NUM>, <NUM> and/or past the electrodes <NUM>, <NUM> within the TCES device. For example, the electrodes <NUM>, <NUM> may be porous, have apertures extending therethrough, and/or have dimensions smaller than an inner vessel dimension so that fluid may flow past an electrode periphery. In the embodiment shown in <FIG>, the inner electrodes <NUM> include pores or apertures that permit flow therethrough. The outer electrodes <NUM> are in the form of respective rods having reduced diameters such that fluid can flow around peripheries <NUM> of the outer electrodes <NUM>. In another embodiment, an outer electrode is in the form of pellets supported by a wire mesh conductive element.

The conductive component <NUM> may comprise wire mesh or electrical clamps, by way of example. Wire mesh may include a resistance wire, such as nichrome. Electrical clamps may be formed from a high-temperature alloy. The electrodes <NUM>, <NUM> may be electrically connected to an AC or DC voltage source via the conductive component <NUM>.

In some embodiments, each inner electrode <NUM> may comprise a plurality of inner electrodes <NUM>. Each outer electrode <NUM> may comprise a plurality of outer electrodes <NUM>. The pluralities of inner and outer electrodes <NUM>, <NUM> may be in a form of electrically-disconnected segments to facilitate changing electrical boundary conditions during heating of the bed <NUM>'. Electrical boundary conditions may be changed by switching voltages between segments, for example. Changing electrical boundary conditions during heating may reduce or avoid formation of instabilities due to preferential pathways with high temperature and high electrical conductivity.

The inner and outer electrodes <NUM>, <NUM> comprise ceramic materials. More particularly, the inner electrode <NUM> comprises a first ceramic material and the outer electrode <NUM> comprises a distinct second ceramic material. The use of ceramic materials in the heater <NUM>' provides advantages over the use of metal materials. For example, unlike many metals, the first and second ceramic materials are not subject to the formation of metal oxides, even in high-temperature (e.g., greater than or equal to about <NUM>) and high-oxygen-partial-pressure environments. Metal oxide formation is undesirable because it generally has a high resistance that inhibits electric current.

The first ceramic material of the inner electrode <NUM> generally has a high chemical stability and a low electrical resistivity at high temperatures. The first ceramic material is nonreactive with the reactive material <NUM>'. In a preferred embodiment, the first ceramic material has the chemical formula La<NUM>-xAxCrO<NUM>, where A is selected from the group consisting of Mg, Ca, Sr, Ba, or combinations thereof; and x ranges from <NUM>-<NUM>. In one example embodiment, x is <NUM> and the first ceramic material comprises LaCrO<NUM>. The first ceramic material may alternatively comprise a non-lanthanum oxide, such as ZrO<NUM>.

The first ceramic material may have a relatively higher electrical resistivity at low temperatures compared to high temperatures (e.g., on the order of <NUM>Ω-m). Accordingly, the outer electrode <NUM> is arranged on the colder side <NUM> (i.e., outer) of the inner electrode <NUM> to reduce or minimize heat loss on the colder side <NUM>. The second ceramic material of the outer electrode <NUM> therefore has a lower electrical resistivity (e.g., on the order of <NUM>-<NUM> Q-m) than the first ceramic material and a high electrical conductivity.

The second ceramic material may comprise a cathode material for solid oxide fuel cell applications. Examples of such cathode materials are described in <NPL>. In a preferred embodiment, the second ceramic material may have the chemical formula B<NUM>-yCyDO<NUM>, where B is selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Al, Sc, Ti, Y, Zr, Hf, or combinations thereof; C is selected from the group consisting of Sr, Ba, or a combination thereof; D is selected from the group consisting of Co, Mn, Ni, Fe, or combinations thereof; and y ranges from about <NUM>-<NUM>. In a preferred embodiment, B comprises La. In an example embodiment, the second ceramic material comprises lanthanum strontium cobaltite (LSC) having the chemical formula La<NUM>Sr<NUM>CoO<NUM>.

A TCES device may optionally include more than one type of heater. In at least one example embodiment, a TCES device includes first heater comprising a resistive coil and a second heater comprising ceramic electrodes (e.g., the heater <NUM>' of <FIG>). In one example, the resistive coil comprises nichrome and is wrapped around a periphery of the reactive material bed. The first heater may be used to preheat the bed (e.g., to a temperature at which the bed has a threshold electrical conductivity, such as about <NUM> for some reactive materials), and then the second heater may be used to further increase a temperature of the bed. Alternatively, the bed may be preheated by passing a hot gas stream over the bed, which has been previously heated with an external gas heater.

Returning to <FIG>, the heater <NUM> may be an alternative type of heater that is capable of receiving electricity and heating the reactive material. The heater <NUM> may comprise a plurality of heaters disposed to heat substantially all of the reactive material <NUM>. In one example, the heater <NUM> comprises one or more arc heaters.

The insulation <NUM> is disposed along an inside of the shell <NUM> to limit heat transfer from the reactive material <NUM> to the shell <NUM>. As described above, the vessel <NUM> preferably withstands high pressures. Accordingly, insulation <NUM> is provided to prevent the vessel <NUM>, and particularly the shell <NUM>, from becoming soft and having reduced structural integrity when the reactive material <NUM> is hot. The insulation <NUM> may facilitate a temperature drop between the reactive material <NUM> and the shell <NUM> when the reactive material is hot, thereby minimizing heat loss from the vessel <NUM>.

In a preferred embodiment, the insulation <NUM> includes a first or outer insulation layer <NUM> and a second or inner insulation layer <NUM>. The inner insulation layer <NUM> is disposed adjacent to the reactive material <NUM>. The outer insulation layer <NUM> is disposed circumferentially between the inner insulation layer <NUM> and the shell <NUM>. In alternative embodiments, the insulation <NUM> may comprise a single layer, or more than two layers (e.g., three layers or four layers).

The insulation <NUM> comprises a material having a low thermal conductivity. In a preferred embodiment, the outer insulation layer <NUM> includes refractory bricks, preferably comprising aluminum and/or calcium aluminate. The refractory bricks of the outer insulation layer <NUM> may additionally or alternatively comprise zirconia and/or magnesium aluminate. In other embodiments, the outer insulation layer <NUM> may comprise a non-refractory brick material. The inner insulation layer <NUM> preferably comprises a microporous insulating material. The microporous insulating material of the inner insulation layer <NUM> may preferably comprise microporous alumina and/or microporous silica. The inner insulation layer <NUM> may additionally or alternatively comprise alumina, fibrous zirconia, and/or microporous zirconia.

A thickness of the insulation <NUM> is dependent upon a size of the TCES device <NUM>, an operating temperature of the TCES device <NUM>, and characteristics of the shell <NUM> (e.g., melting point, thickness). In one example, the TCES device <NUM> is approximately the size of a standard shipping container (e.g., <NUM>'x8. <NUM>'x8'), is configured to operate at temperatures of at least about <NUM>, and has the shell <NUM> comprising carbon steel. The outer insulation layer <NUM> has a thickness of about <NUM> and the inner insulation layer <NUM> has a thickness of about <NUM>.

The cooling system <NUM> can be operated to reduce a temperature of the TCES device <NUM>, such as when the insulation alone is insufficient to maintain the shell <NUM> below its melting point. The cooling system <NUM> is disposed circumferentially between the outer insulation layer <NUM> and the shell <NUM>. In a preferred embodiment, the cooling system <NUM> includes one or more circumferentially disposed tubes for circulating a heat transfer fluid. In one example, the heat transfer fluid is air. However, other heat transfer fluids that are effective at an operating temperature of the TCES device <NUM> may alternatively be employed. The cooling system <NUM> may include other types of cooling systems that are capable of maintaining the shell <NUM> below its melting point. In some embodiments, the cooling system <NUM> is altogether omitted.

The TCES device <NUM> may be supported by and contained within a support frame <NUM>. The support frame <NUM> has a substantially rectangular prism shape. The support frame <NUM> includes bars along each edge and open faces between the bars. In a preferred embodiment, the support frame <NUM> defines standard shipping container outer dimensions. In an example, the support frame <NUM> may define a length <NUM> of about <NUM> feet, a width <NUM> of about <NUM> feet, and a height <NUM> of about <NUM> feet. Accordingly, the TCES device <NUM> may be readily transported.

The reactive material <NUM> of the bed <NUM> preferably has a high reactive stability (i.e., the ability to reuse the reactive material <NUM> for thousands of cycles with negligible degradation in performance), high discharge temperature, and high energy density. The reactive material <NUM> is configured to release oxygen upon being heated to a reduction temperature, and generate heat when exposed to oxygen. More particularly, the reactive material <NUM> is a redox material that undergoes oxidation and reduction reactions to change phase. During an energy storage process, the reactive material <NUM> consumes heat to undergo reduction and release oxygen. During an energy release process, the reactive material <NUM> consumes oxygen to undergo oxidation and generate heat. The reactive material <NUM> advantageously uses oxygen as a gaseous reactant, rather than CO<NUM>, H<NUM>, or CO, by way of example. The oxygen for the process may come from air.

The bed <NUM> of the reactive material <NUM> can be a packed bed or porous bed. In a preferred embodiment, the bed <NUM> is a packed bed including a plurality of granular particles. An average particle size ranges from about <NUM>-<NUM>. In one embodiment, particle sizes range from about <NUM>-<NUM>. The average particle size can be optimized to increase energy density.

The particles define an interparticle pore size between particles. The particles preferably also define an intraparticle pore size within the particles, so that the bed <NUM> has a dual porosity. The average interparticle pore size ranges from about <NUM>-<NUM>. In some embodiments, the average interparticle pore size is optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, or optionally about <NUM>-<NUM>. The average intraparticle pore size ranges from about <NUM>-<NUM>. In some embodiments, the average intraparticle pore size is optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, or optionally about <NUM>-<NUM>.

A total porosity of the bed <NUM> (both interparticle and intraparticle) is less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, optionally less than or equal to about <NUM>%, and optionally less than or equal to about <NUM>%. The total porosity may be optimized depending on acceptable pressure drop. In one embodiment, pressure drop is about <NUM> bar.

The porosity of the bed <NUM> can be decreased to increase energy density. A volumetric energy density of the bed <NUM> is at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, and optionally at least about <NUM> MJm-<NUM>. A specific energy density may be at least about <NUM> kJkg-<NUM>, and optionally at least about <NUM> kJkg-<NUM>.

The reactive material <NUM> may comprise a metal oxide, which may be a metal-metal oxide. In some embodiments, the reactive material <NUM> is formed from a transition metal oxide and an alkaline earth metal oxide that acts as a sintering inhibitor. In a preferred embodiment, the reactive material <NUM> comprises a magnesium-manganese oxide. In alternative embodiments, the reactive material <NUM> may comprise a perovskite such as doped calcium manganite or lanthanum strontium cobalt ferrite. The reactive material <NUM> may further comprise a dopant to increase energy density, such as cobalt, iron, chromium, molybdenum, vanadium, zinc, cerium, and/or nickel.

Magnesium oxide (MgO) and manganese oxide (MnO) react to form magnesium-manganate spinel (MgMn<NUM>O<NUM>) (both cubic and tetragonal) when heated in the presence of oxygen (e.g., from air). A molar ratio of manganese to magnesium can be adjusted for a specific operating temperature range to obtain high reactive stability. In general, increasing an amount of magnesium decreases slag formation (inhibiting undesirable sintering of the reactive material <NUM> when heated) and facilitates operation of the TCES device <NUM> at higher temperatures. The molar ratio ranges from about <NUM>:<NUM>-<NUM>:<NUM>, optionally about <NUM>:<NUM>-<NUM>:<NUM>, and optionally about <NUM>:<NUM>-<NUM>:<NUM>. The molar ratio is optionally about <NUM>:<NUM>, optionally about <NUM>:<NUM>, or optionally about <NUM>:<NUM>.

Reactive materials <NUM> comprising magnesium-manganese oxides have desirably high exergetic efficiencies via high operating temperatures, low cost, fast reaction kinetics, and the use of air as the reacting gas for discharging heat, thereby eliminating the need for gas storage-and-management systems. Magnesium-manganese oxides do not require very low partial pressures of oxygen to achieve high energy densities, making use of magnesium-manganese oxides in the TCES device <NUM> practical for large-scale operation.

Reactive materials <NUM> comprising magnesium-manganese oxides have a high degree of reactive stability under high-temperature cycling, such as between <NUM> and <NUM>, and optionally between <NUM> and <NUM>. Furthermore, magnesium-manganese oxide-containing reactive materials <NUM> undergo phase change reactions at high operating temperatures, such as at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, and preferably at least about <NUM>. The magnesium-manganese oxide reactive materials <NUM> may have volumetric energy densities of at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, optionally at least about <NUM> MJm-<NUM>, and optionally at least about <NUM> MJm-<NUM>. A specific energy density may be at least about <NUM> kJkg-<NUM>, and optionally at least about <NUM> kJkg-<NUM>.

As noted above, magnesium oxide and manganese oxide react to form magnesium-manganate spinel (both cubic and tetragonal) when heated in air or oxygen. The crystal structure of a spinel phase can be viewed as a face-centered cubic ("FCC") lattice of oxygen ions with cations at tetrahedral and octahedral sites. MgMn<NUM>O<NUM> is a tetragonal spinel at room temperature. At high temperatures (e.g., at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, and optionally at least about <NUM>), it undergoes an allotropic transformation to form a cubic spinel. The heat of formation of MgMn<NUM>O<NUM> from MgO and Mn<NUM>O<NUM> using high-temperature solution has been reported to be about -<NUM> kJmol-<NUM>. An enthalpy of transformation between the cubic spinel and tetragonal spinel is about <NUM> kJmol-<NUM> at <NUM>.

The chemical reactions of magnesium-manganese oxide as the reactive material <NUM> are described below. Magnesium-manganese oxide spinel is of the form (Mg<NUM>-xMnx)<NUM>-δO<NUM>. Here, δ is the cation vacancy concentration in the spinel and x is the manganese-to-magnesium molar ratio. The non-stoichiometric reaction for reduction of a magnesium-manganate spinel is given by <MAT>.

Here, the cation vacancy concentration in the spinel changes from δ<NUM> to δ<NUM> as the spinel loses oxygen.

The transformation of the non-stoichiometric spinel phase to non-stoichiometric monoxide phase (Mg<NUM>-yMny)<NUM>-δ*O is given by.

(Mg<NUM>-yMny)<NUM>-δ2O<NUM> → <NUM> (Mg<NUM>-yMny)<NUM>-δ1*O     (<NUM>).

Here, the spinel with a cation vacancy concentration of δ<NUM> transforms to a monoxide with a cation vacancy concentration of δ<NUM>* without losing oxygen. These phases are related by: δ<NUM>*= (<NUM>+δ<NUM>)/<NUM>.

A further source of energy storage is the decomposition of the monoxide phase, <MAT>.

Here, the cation vacancy concentration in the spinel phase changes from δ<NUM>* to δ<NUM>* as the spinel loses oxygen.

The amount of chemical energy storage increases with the amount of oxygen released from the non-stoichiometric monoxide. The defect reaction involves the reduction of Mn<NUM>+ to Mn<NUM>+ and the formation of charge-compensating cation vacancies according to the reaction, <MAT>.

The overall equation for the decomposition of two-phase spinel-monoxide solution to a monoxide phase in an Mg-Mn-O system is given by <MAT>.

Here nO<NUM> is the number of moles of oxygen released at thermodynamic equilibrium when a spinel -monoxide ((Mg<NUM>-yMny)<NUM>-δ<NUM>* O and (Mg<NUM>-zMnz)<NUM>- δ O<NUM>) two-phase solid solution is reduced to mole of (Mg<NUM>-xMnx) <NUM>-δ<NUM>*. The overall decomposition, simplifies into the reversible reaction given by <MAT>.

Here x represents the molar ratio of magnesium to manganese in the material and C1 denotes excess oxygen content (i.e. total oxygen atoms - oxygen atoms from x MgO + MnO). Equation (<NUM>) does not provide information about the phases present in the chemical reaction; however, if the value of C1 is known (at a given T and PO2), then the enthalpy of MgxMnO<NUM>+x+C1 can be measured using calorimetry. A gravimetric energy storage density and maximum achievable storage efficiency of magnesium-manganese oxides with various manganese-to-magnesium molar ratios can be calculated using the CALPHAD model described in <NPL>.

With reference to <FIG> and <FIG>, the TCES device <NUM> may be used in an electrical-to-electrical energy storage system <NUM> and operation. The system <NUM> generally includes the TCES device <NUM>, a blower <NUM>, and a turbo-generator set <NUM>. The turbo-generator set <NUM> includes a compressor <NUM>, a turbine <NUM>, and an electrical generator <NUM>.

A first valve <NUM>, which is a three-way valve, is fluidly connected to the TCES device <NUM>, the blower <NUM>, and the compressor <NUM>. In a first position, the first valve <NUM> fluidly connected the TCES device <NUM> and the blower <NUM>. In a second position, the first valve <NUM> fluidly connected the TCES device <NUM> and the compressor <NUM>. A first line <NUM> (e.g., pipe) is disposed between the first valve <NUM> and the TCES device <NUM>. A second line <NUM> is disposed between the first valve <NUM> and the blower <NUM>. A third line <NUM> is disposed between the first valve <NUM> and the compressor <NUM>.

A second valve <NUM> is disposed between the TCES device <NUM> and the turbine <NUM>. A fourth line <NUM> connects the TCES device <NUM> and the second valve <NUM>. A fifth line <NUM> connected the second valve <NUM> and the turbine <NUM>.

In a preferred embodiment, a sixth or bypass line is provided between the third line <NUM> and the fourth line <NUM>. A third valve <NUM>, which is preferably a variable control valve, is provided on the bypass line <NUM>. At a first junction <NUM>, an outlet gas discharged from the compressor <NUM> is split into a first portion and a second portion. The first portion is provided to the first valve <NUM>. The second portion is provided to the bypass line <NUM>. At a second junction <NUM>, the second portion is combined with an outlet gas discharged from the TCES device <NUM> and provided to the second valve <NUM>. An amount of the second portion is controlled by the third valve <NUM>.

The system <NUM> performs an energy storage operation and an energy recovery operation. In the energy storage operation, electricity is converted to heat to cause the reactive material <NUM> in the TCES device <NUM> to be reduced in an endothermic reaction. The reduction reaction generates oxygen, which is removed by the blower <NUM>. In the energy recovery operation, oxygen is provided to the TCES device <NUM> by the compressor <NUM> to react with the reactive material <NUM> in a highly exothermic manner. Accordingly, gas in the TCES device <NUM> is heated and discharged to the turbine <NUM>, which is used to power the generator <NUM>. In a preferred embodiment, the third valve <NUM> is operated to allow a portion of the oxygen to bypass the TCES device <NUM> in order to provide a consistent temperature to the turbine <NUM>.

In a preferred embodiment, the system <NUM> is modular such that it can be integrated with existing power grids and infrastructure. Furthermore, a quantity of TCES devices <NUM> can be increased to increase energy storage capacity. In one embodiment, <NUM>-<NUM> TCES devices <NUM> are stacked to achieve a desired storage capacity.

<FIG> depicts the system <NUM> during the energy storage operation. The system <NUM> stores energy, for example, when the grid produces excess power. During energy storage, a first valve <NUM> is in the first position to fluidly connect the blower <NUM> and the TCES device <NUM>. In a preferred embodiment, the blower <NUM> is an industrial suction blower. The second valve <NUM> is in a closed position. The TCES device <NUM> is fluidly isolated from the compressor <NUM> and the turbine <NUM>.

A method of storing energy using the system <NUM> includes electrically connecting the TCES device <NUM> to an electricity source <NUM> to heat the heater <NUM>, thereby heating the reactive material <NUM> (<FIG>). The reactive material <NUM> is heated to a reduction temperature of at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, optionally at least about <NUM>, and optionally at least about <NUM>. When the reactive material <NUM> is heated to at least the reduction temperature, the reactive material <NUM> is chemically reduced to generate oxygen. In a preferred embodiment, magnesium-manganate oxide spinel is reduced to magnesium oxide and manganese oxide, as described above.

The method further includes removing the evolved oxygen. The oxygen is removed by the blower <NUM>. In a preferred embodiment, the first opening <NUM> of the TCES device <NUM> is fluidly connected to a suction side of the blower <NUM>, with the first valve <NUM> being disposed therebetween. The blower <NUM> is operated at a constant oxygen partial pressure. The constant oxygen partial pressure ranges from about <NUM>-<NUM> atm, and preferably about <NUM>-<NUM> atm. The evolved oxygen may be collected, such as for sale or use in other processes. In some embodiments, an inert sweep gas may be circulated through the interior volume <NUM> (not shown). The use of an inert sweep gas may further improve energy density.

<FIG> depicts the system <NUM> during the energy recovery operation. The system <NUM> releases energy, for example, when energy demand exceeds supply. During energy release, the first valve <NUM> is in the second position to fluidly connect the compressor <NUM> and the TCES device <NUM>, while fluidly isolating the TCES device <NUM> from the blower <NUM>. The second valve <NUM> fluidly connects the turbine <NUM> and the TCES device <NUM>. The turbine <NUM> expands heated air/oxygen-rich gas that it receives from the TCES device <NUM> to power the generator <NUM>, which generates electricity.

A method of releasing energy includes providing oxygen to the interior volume <NUM> of the TCES device <NUM>. In a preferred embodiment, the compressor <NUM> is operated to flow pressurized oxygen into the second opening <NUM> and across and/or through the reactive material <NUM> (e.g., through the pores of the reactive material). In a preferred embodiment, the oxygen comes from pressurized air. An oxidation pressure of the inlet air ranges from about <NUM>-<NUM> bar. For example, the oxidation pressure ranges from optionally about <NUM>-<NUM> bar, optionally about <NUM>-<NUM> bar, optionally about <NUM>-<NUM> bar, optionally about <NUM>-<NUM> bar, or optionally about <NUM>-<NUM> bar. An oxidation temperature of the inlet air ranges from about <NUM>-<NUM>. For example, the oxidation temperature may range from optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, optionally about <NUM>-<NUM>, or optionally about <NUM>-<NUM>.

The oxygen in the air reacts with the reactive material <NUM> to chemically oxidize the reactive material <NUM>. In a preferred embodiment, magnesium oxide and manganese oxide react with the oxygen to form magnesium-manganate spinel. The reaction is highly exothermic, and therefore heat is released to the oxygen-depleted air. The reaction continues until substantially all of the reactive material <NUM> is oxidized.

The amount of heat released to the oxygen-depleted air varies as the oxidation reaction progresses. For example, a first temperature near the beginning of the energy recovery operation may be greater than a second temperature near an end of the energy recovery operation. However, in a preferred embodiment, a turbine temperature at the turbine inlet <NUM> is substantially constant.

In the preferred embodiment, the system <NUM> further includes a control unit (not shown). To control the turbine temperature, the third valve <NUM> allows some of the air leaving the compressor <NUM> to bypass the TCES device <NUM> through the bypass line <NUM>. The air in the bypass line <NUM> admixes with heated, oxygen-depleted air discharged from the second opening <NUM> of the TCES device <NUM> at the second junction <NUM>. An admixture air temperature is controlled to meet inlet specifications of the turbine <NUM>. A temperature sensor <NUM> in the mixture air feeds the control unit, which controls the third valve <NUM> to moderate the amount of bypass air so that the predetermined turbine inlet temperature is maintained.

The mixed air is received by the turbine <NUM> at the turbine inlet <NUM>. The mixed air expands across the turbine <NUM> to power the generator <NUM>. The generator <NUM> delivers electricity back to the power grid as needed.

Claim 1:
A thermochemical energy storage device (<NUM>) comprising:
a vessel (<NUM>) defining an interior volume, the vessel including a first opening and a second opening;
a porous bed (<NUM>, <NUM>') disposed within the interior volume and being in fluid communication with the first opening and the second opening, the porous bed comprising a reactive material (<NUM>, <NUM>'), the reactive material being configured to release oxygen upon being heated to a reduction temperature, and generate heat when exposed to oxygen; and
a heater (<NUM>, <NUM>') configured to heat the reactive material, the heater being in contact with the reactive material or including a heating element that is embedded in the reactive material (<NUM>), characterized in that the heater includes one or both of
(A) A heating element (<NUM>) embedded in the reactive material with ends electrically connected to an electricity source, and/or
(B) A first pair of ceramic electrodes (<NUM>, <NUM>) having the porous bed disposed therebetween and a second pair of ceramic electrodes having the first pair of ceramic electrodes and the porous bed disposed therebetween.