Heat recovery system

Abstract

An electrical energy storage system configured to store electrical energy in the form of heat comprising: a plurality of electric heating elements (1); a plurality of thermal energy storage elements (2), wherein the thermal energy storage elements (2) are arranged such that between the various thermal energy storage elements (2) there are: first cavities (5), wherein each first cavity (5) is configured to house at least one electric heating element (1), and; second cavities (6), wherein each second cavity (6) is configured to house an energy converter element (7); and wherein the system comprises a thermal insulation element (10) configured to cover and thermally insulate the set of thermal energy storage elements (2).

Description

Heat recovery system Object of the invention The present invention relates to an electrical energy storage system that allows electrical energy to be stored in the form of heat and delivered when needed, either in the form of heat or in the form of electrical energy or in a combination of heat and electricity. The electrical energy storage system, which is the subject of this invention, allows for the efficient storage of a huge amount of energy by reducing thermal gradients between internal elements of the system, using a simple system with low-cost elements that are easy to manufacture and easy to recycle, without resorting to the use of electrolytes or other highly polluting substances that are difficult to manage environmentally. The electrical energy storage system, the subject of this invention, is especially applicable in any industry that requires the storage of a surplus of electrical energy generation. Background of the invention and technical problem to be solved High-temperature thermal energy storage systems are useful for managing the intermittency of renewable energy sources and providing a reliable energy source. However, existing systems face challenges related to scalability, efficiency, and safety. For example, US patents 11,994,347 B2 and 11,159,119 B2 describe systems for managing thermal energy within a sensible heat-based thermal storage solution, but they do not specifically address the optimization of thermal storage material packing, safety aspects, or the minimization of internal thermal gradients, which are especially relevant in latent heat storage systems. On the other hand, US patent 2015256 119 A1 discloses an electrical energy storage system that uses phase change materials in a refractory material container, but it does not solve the problem of container packing for a larger-scale system, and it may also pose safety problems due to the circulation of heat transfer fluids in the vicinity of very high temperature materials. Description of the invention In order to solve the aforementioned problems, the present invention relates to an electrical energy storage system. The electrical energy storage system, the subject of the present invention, is configured to store electrical energy in the form of heat. The electrical energy storage system, the subject of this invention, novelly comprises: - a plurality of electric heating elements, wherein each electric heating element is configured to transform an electric current into heat; - a plurality of thermal energy storage elements, where each thermal energy storage element comprises an irregular prismatic geometry with at least two of its faces in contact with an adjacent thermal energy storage element, where the thermal energy storage elements are arranged such that between the different thermal energy storage elements there exist: • first cavities, where each first cavity is configured to house at least one electric heating element, and; • second cavities, where each second cavity is configured to house an energy converter element, where each energy converter element is configured to convert the heat stored in the adjacent thermal energy storage elements into energy usable by an external system; - a thermal insulation element configured to cover and thermally insulate the set of thermal energy storage elements. The first cavities are preferably smaller than the second cavities. Thanks to the electrical energy storage system described above, it is possible to store electrical energy in the form of heat, minimizing losses due to thermal gradients and improving (compared to other conventional storage systems) scalability, efficiency and safety in high energy density thermal storage applications. The thermal energy storage elements form a compact structure that maximizes space utilization and minimizes heat losses. The described arrangement allows for a high packing factor of the thermal energy storage elements and a reduced distance between the thermal energy storage element and the electric heating and energy conversion elements, thus reducing thermal gradients during charging and discharging processes. Furthermore, by means of the electric heating elements inserted in the first (smaller volume) cavities formed between the thermal energy storage elements, when an electric current is applied, these heating elements generate heat that is transferred uniformly to the surrounding thermal energy storage elements, ensuring a homogeneous temperature distribution and preventing the formation of significant thermal gradients. The thermal energy storage unit is surrounded by thermal insulation, leaving only minimal openings (e.g., at the top) for the insertion of the electric heating elements and energy conversion components. This design minimizes heat loss and maintains high system efficiency. Preferably, each thermal energy storage element comprises a refractory material vessel (e.g., ceramic material) filled with at least one heat storage element. The at least one heat storage element preferably comprises a melting point equal to or greater than 1,000 °C. In this case, the utilization of the latent heat of phase change to store energy is optimized, taking advantage of the large energy storage capacity provided by these materials (for example, silicon, ferrosilicon), whose latent heat of phase change is very high. Preferably, at least one heat storage element is selected from silicon, iron, or a ferrosilicon alloy. Also, preferably, each vessel is made of a material selected from graphite or silicon carbide. According to a first embodiment of the invention, each thermal energy storage element comprises a prismatic geometry with an irregular hexagonal section. According to a second embodiment of the invention, each thermal energy storage element comprises a prismatic geometry with an irregular octagonal section. For example, each electric heating element can be made of a material selected from either silicon carbide or molybdenum disilicide. However, other materials can also be used in electric heating elements. Preferably, each electric heating element comprises an electrical connection at at least one of its ends (e.g., its upper and / or lower end in use) configured to allow connection in series with another electric heating element configured to heat thermal energy storage elements located at a higher or lower level within the electrical energy storage system. Preferably, each energy conversion element comprises a thermosiphon. One or more of the thermosiphons may be made of copper and configured to hold water at a maximum temperature of 300 °C. Additionally, one or more of the thermosiphons may be made of steel and configured to contain at least one alkali metal at a temperature between 350 °C and 1,000 °C. Preferably, each thermosiphon comprises a widening corresponding to its upper part, in use. Some of the energy conversion elements may include, in addition to the thermosiphon, a photovoltaic module. In this way, the system can release the stored energy as heat, or through combined heat and power (CHP). According to one aspect of the present invention, the electrical energy storage system can be configured so that a first set of energy converter elements are arranged in their corresponding second cavities while a second set of energy converter elements are outside their corresponding second cavities (e.g., located at a higher level or above the second cavities). In this way, by selecting or configuring the number of power converters that are introduced into the second cavities and those that are not, it is possible to modulate and manage the total power delivered by the system. Depending on the energy demand, the system can operate in thermal conversion mode or hybrid (thermal and electric): - Thermal Conversion: This system utilizes thermosiphons inserted into the cavities. Each thermosiphon is a closed tube containing a fluid in liquid-vapor equilibrium. The lower end of the thermosiphon absorbs heat by radiation from the vessel walls, causing the internal fluid to evaporate. The vapor rises, condenses at the top, and releases heat to an external heat exchanger through which a heat transfer fluid circulates. After condensation at the upper end, the fluid returns to the lower end by gravity or capillary action. This design eliminates the need for fluid circulation near high-temperature materials, improving safety by reducing the risk of leaks and potential explosions or fires. - Hybrid conversion (thermal and electrical): This uses thermophotovoltaic modules (coupled to the external surface of the lower end of a thermosiphon). These modules convert the thermal radiation emitted by the vessel walls into electricity. To optimize heat transfer and electrically insulate the thermophotovoltaic cells, electrically insulating and thermally conductive substrates and thermal interface materials such as heat-conducting greases or epoxies are used. Preferably, the thermal insulation element is composed of several layers of porous ceramic materials. Thus, the thermal insulation element may comprise mullite fibers or bricks in at least one inner layer and silica fibers in at least one outer layer. Brief description of the figures A series of figures, non-limiting examples, are briefly described here to help to better understand the invention: Figure 1 shows a schematic cross-sectional view of a possible configuration of the electrical energy storage system, where the thermal energy storage elements are shown in section. Figure 2 shows a cross-sectional view of the thermal energy storage elements, according to a first embodiment of the invention. Figure 3 shows a detailed schematic view of the thermal energy storage elements, according to the first embodiment of the invention. Figure 4 shows a detailed schematic view of the thermal energy storage elements, according to a second embodiment of the invention. Figure 5 shows a possible configuration of the electrical energy storage system, where a plurality of energy converter elements are observed, composed of thermophotovoltaic generators mounted on thermosiphons. Figure 6 shows the same possible system configuration as Figure 5, in which a first set of energy-converting elements is introduced into the second cavities, and a second set of energy-converting elements is arranged at a higher level, outside the second cavities. Detailed description A description of possible ways of implementing the electrical energy storage system, the subject of the present invention, is then made. The electrical energy storage system is configured to store electrical energy in the form of thermal energy in thermal energy storage elements (2). Figure 1 shows a cross-sectional view of a possible configuration of the electrical energy storage system, where a plurality of thermal energy storage elements (2) surrounded by a thermal insulation element (10) are observed in section. Each thermal energy storage element (2) comprises a ceramic vessel (3) filled with a heat storage element (4). The vessels (3) comprise a prismatic geometry with an irregular polygonal section. According to a first embodiment of the invention, shown in Figure 2 and Figure 3, the vessels (3) comprise an irregular octagonal prismatic geometry. According to a second embodiment of the invention, shown in Figure 4, the vessels (3) comprise an irregular hexagonal prismatic geometry. Four of the faces of each vessel (3) are arranged to face the faces of adjacent vessels (3). At least two other faces of each vessel (3) create quadrangular prismatic cavities (5, 6): first cavities (5), of smaller volume, configured to house an electric heating element (1) and second cavities (6), of larger volume, configured to house an energy converter element (7) configured to convert the thermal energy stored in the thermal energy storage elements (2) into energy usable by systems or elements external to the electrical energy storage system. Each electric heating element (1) is configured to transform the electrical energy arriving at the storage system of the invention into thermal energy that is stored (in the form of heat) in the thermal energy storage elements (2). The vessels (3) are preferably made of ceramic materials such as graphite or silicon carbide, where each vessel (3) houses at least one heat storage element (4) made of a thermal storage material such as silicon (with a melting point of 1,414 °C), iron (with a melting point of 1,538 °C), or ferrosilicon alloys with melting points between 1,207 °C and 1,538 °C. These vessels (2), which are more than 20 centimeters long, can be stacked to increase storage capacity. The electric heating elements (1) used can be made of silicon carbide (SiC) or molybdenum disilicide (MoSi2), both with connections at the top end, allowing their interconnection in series. As shown in Figure 5, each energy conversion element (7) comprises a thermosiphon (8). The thermosiphon (8) may be made of copper configured to hold water at a temperature of up to 300°C. Alternatively, the thermosiphon (8) may be made of steel configured to hold one or more alkali metals (cesium, sodium, potassium, or lithium) at temperatures between 350°C and 1,000°C. Some thermosiphons (8) may have a flare (11) at the top to improve heat transfer and reduce thermal gradients. The thermosiphons (8) may include safety valves for releasing the working fluid in case of overheating or overpressure. Each energy converter element (7) may comprise, in addition to the corresponding thermosiphon (8), at least one thermophotovoltaic module (9) (or thermophotovoltaic converter). Each thermophotovoltaic module (9) is mounted on a corresponding thermosiphon (8). As shown in Figure 6, a first set (12) of energy conversion elements (7) are inserted into the second cavities (6) to convert the energy stored in the thermal energy storage elements (2) into energy usable by an external system. Simultaneously, a second set (13) of energy conversion elements (7) may be located at a higher level, outside the second cavities (6). By selecting the number of energy converters inserted into the second cavities (6) and those excluded, it is possible to modulate and manage the total power delivered by the system. Each thermophotovoltaic module (9) is made up of semiconductor cells such as germanium (bandgap of 0.67 eV) or indium gallium arsenide (bandgap of 0.74 eV), mounted on DBC substrates (from the English "Direct Bonding Copper" or direct copper connection) or IMS (from the English "Insulated Metal Substrate" or insulated metal substrate), with materials of high thermal conductivity such as tin / silver solders or pyrolytic graphite. The thermal insulation element (10) is composed of several layers of porous ceramic materials, with mullite fibers or bricks in the inner layers and silica fibers in the outer layers, maximizing thermal resistance and minimizing energy losses.

Claims

1. An electrical energy storage system configured to store electrical energy in the form of heat, characterized in that it comprises: - a plurality of electric heating elements (1), wherein each electric heating element (1) is configured to transform an electric current into heat; - a plurality of thermal energy storage elements (2), wherein each thermal energy storage element (2) comprises an irregular prismatic geometry with at least two of its faces in contact with an adjacent thermal energy storage element (2), wherein the thermal energy storage elements (2) are arranged such that between the different thermal energy storage elements (2) there exist: • first cavities (5), wherein each first cavity (5) is configured to house at least one electric heating element (1); and • second cavities (6),wherein each second cavity (6) is configured to house an energy converter element (7), wherein each energy converter element (7) is configured to convert the heat stored in the adjacent thermal energy storage elements (2) into energy usable by an external system; - a thermal insulation element (10) configured to cover and thermally insulate the assembly of thermal energy storage elements (1).

2. An electrical energy storage system according to claim 1, characterized in that each thermal energy storage element (2) comprises a vessel (3) of refractory material filled with at least one heat storage element (4).

3. An electrical energy storage system according to claim 2,characterized in that the at least one heat storage element (4) comprises a melting point equal to or greater than 1,000 °C.

4. An electrical energy storage system according to claim 3, characterized in that the at least one heat storage element (4) is selected from silicon, iron, or a ferrosilicon alloy.

5. An electrical energy storage system according to any of claims 2 to 4, characterized in that each vessel (3) is made of a material selected from graphite or silicon carbide.

6. An electrical energy storage system according to any of the preceding claims, characterized in that each thermal energy storage element (2) comprises a prismatic geometry with an irregular octagonal cross-section.

7. An electrical energy storage system according to any of claims 1 to 5,characterized in that each thermal energy storage element (2) comprises a prismatic geometry with an irregular hexagonal cross-section.

8. An electrical energy storage system according to any of the preceding claims, characterized in that each electric heating element (1) is made of a material selected from silicon carbide or molybdenum disilicide.

9. An electrical energy storage system according to any of the preceding claims,characterized in that each electric heating element (1) comprises an electrical connection at at least one of its ends configured to permit connection in series with another electric heating element (1) configured to heat thermal energy storage elements (2) located at a higher or lower level within the electrical energy storage system.

10. Electrical energy storage system according to any of the preceding claims, characterized in that each energy converter element (7) comprises a thermosiphon (8).

11. Electrical energy storage system according to claim 10, characterized in that at least one thermosiphon (8) is made of copper and is configured to contain water at a maximum temperature of 300 °C.

12. Electrical energy storage system according to claim 10 or 11,characterized in that at least one thermosiphon (8) is made of steel and is configured to contain at least one alkali metal at a temperature between 350 °C and 1,000 °C.

13. An electrical energy storage system according to any of claims 10 to 12, characterized in that each thermosiphon (8) comprises a widening (11) corresponding to its upper portion in use.

14. An electrical energy storage system according to any of claims 10 to 13, characterized in that at least one energy converter element (7) comprises a thermophotovoltaic module (9).

15. An electrical energy storage system according to any of the preceding claims, characterized in that the thermal insulation element (10) is composed of several layers of porous ceramic materials.

16. An electrical energy storage system according to claim 15,characterized in that the thermal insulation element (10) comprises mullite fibers or bricks in at least one inner layer and silica fibers in at least one outer layer.

17. An electrical energy storage system according to any of the preceding claims, characterized in that it is configured so that a first set (12) of energy conversion elements (7) are arranged in corresponding second cavities (6) while a second set (13) of energy conversion elements (7) are outside their corresponding second cavities (6).

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