ENERGY STORAGE DEVICE AND METHOD FOR HEATING A HEAT TRANSFER FLUID - Patent application

Abstract

The present invention provides a thermal energy storage device (100) comprising a powder bed (110), at least two electrodes (301, 302, 303), and at least one heat transfer tube (200). The powder bed (110) has an electrical resistivity in the range of 500 to 50,000 Ωm. The at least two electrodes (301, 302, 303) are embedded in the powder bed (110) and arranged to heat the powder bed (110) by applying a voltage between the electrodes (301, 302, 303). The at least one heat transfer tube (200) is arranged to contain a heat transfer fluid and has an inlet (210) and an outlet (220) connectable to a thermal energy consumer (30). The heat transfer tube (200) and the powder bed (110) are thermally coupled via an electrically insulating material.

Description

[Technical field]

[0001] The present invention relates to a thermal energy storage device. [Background technology]

[0002] In recent decades, environmental concerns and the desire to reduce manmade CO2 have stimulated the development of technologies to generate electricity from renewable sources such as wind and solar power. One problem associated with the use of such renewable energy sources is their intermittent and sometimes unpredictable availability. Wind speeds vary throughout the day and can be very low for longer periods of time. While the position of the sun relative to solar panels can be highly predictable, local weather variations cause large variations in the amount of solar energy captured and converted to electricity.

[0003] Not only energy production, but also energy demand varies over time. Since the variability of energy production and energy demand does not match, technical solutions are needed to store energy when it is abundantly available and allow consumers to retrieve the stored energy when needed. As the use of renewable energy sources to generate electricity increases, a wide variety of new energy storage solutions are currently being developed. Different solutions are needed for different usage scenarios. When deciding how to temporarily store available energy for later use, storage capacity, spatial constraints, portability, cost, charging speed, durability, and many other relevant factors are considered.

[0004] In large-scale industrial applications, thermal energy storage can be used to balance energy demand over time. In thermal energy storage, when there is a surplus of available electrical energy, some medium is heated electrically. When demand exceeds supply, the stored heat can be used to generate steam, which can be used as a power source itself or converted to electricity using a steam turbine generator. The materials used to store thermal energy are preferably cheap and safe. One of the cheapest and most commonly used options is a water tank, but materials such as molten salt, sand, or metal can be heated to higher temperatures, thus offering higher storage capacity or higher levels of useful energy.

[0005] A common drawback of many currently available thermal energy storage systems is that some form of system is required to convert electrical energy produced, for example, by a wind turbine or solar panel, into thermal energy that is stored in the storage medium. For example, International Patent Application No. 2020 / 254001(A1) describes the use of an electric heating device to heat a thermal storage medium. The storage medium may be 10- 4 The conductive medium has a low electrical resistivity of Ωm to 1 Ωm. According to one embodiment, the electric heating device uses an induction coil to store the electric energy in the form of heat. In another embodiment, the electric heating device uses contact electrodes to generate an electric current in the material, thereby heating the thermal energy storage medium. Even when heating the thermal storage medium by direct contact with the contact electrodes, the heating device of International Patent Application No. 2020 / 254001(A1) still requires a large and expensive transformer to first convert the high voltage of the power grid, supplied by electricity from a renewable energy source, to a much lower voltage that can be used to heat the conductive medium with low electrical resistivity.

[0006] It is an object of the present invention to overcome at least some of the disadvantages of known thermal energy storage systems. Summary of the Invention

[0007] According to one aspect of the present invention, there is provided a thermal energy storage device, comprising: a powder bed having an electrical resistivity in the range of -500 Ωm to 50,000 Ωm; at least two electrodes embedded in the powder bed and arranged to heat the powder bed by providing an electric current between the at least two electrodes; - at least one heat transfer tube arranged to contain a heat transfer fluid, the at least one heat transfer tube having an inlet and an outlet connectable to a thermal energy consumer, wherein the heat transfer tube and the powder bed are thermally coupled via an electrically insulating material.

[0008] According to another aspect of the present invention, there is provided a method of heating a heat transfer fluid, comprising the steps of: providing a thermal energy storage device as described above, comprising at least a powder bed, at least two electrodes, an electrically insulating material, and at least one heat transfer tube thermally coupled to the powder bed via the electrically insulating material; - passing an electric current between at least two electrodes, thereby generating heat in the powder bed, thereby heating said electrically insulating material; - passing a heat transfer fluid through at least one heat transfer tube, thereby heating the heat transfer fluid with heat from the electrically insulating material. [Brief description of the drawings]

[0009] [Figure 1] 1 is a schematic diagram showing a wind farm, a solar farm, and a thermal energy storage device. [Diagram 2] FIG. 2 is a schematic diagram illustrating a perspective view of an embodiment of the thermal energy storage device of FIG. [Diagram 3] FIG. 3 is a schematic diagram illustrating an example of a heat transfer tube for use in the thermal energy storage device of FIG. [Figure 4] FIG. 3 is a schematic diagram showing the thermal energy storage device of FIG. 2 with added insulation. [Diagram 5] 2 is a schematic diagram showing a perspective view of a different embodiment of the thermal energy storage device of FIG. 1. [Figure 6] FIG. 6 is a schematic diagram showing the thermal energy storage device of FIG. 5 with additional insulation. [Figure 7] FIG. 2 is a schematic diagram illustrating a perspective view of an alternative embodiment of the thermal energy storage device of FIG. [Figure 8] FIG. 8 is a schematic diagram illustrating an expanded view of a portion of the thermal energy storage device of FIG. 7 with the external insulation layer removed.

[0010] The drawings illustrate one or more implementations according to the present teachings, by way of example only, and not by way of limitation.In the drawings, like reference numbers refer to the same or similar elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Described herein is a thermal energy storage device comprising a powder bed, at least two electrodes, and at least one heat transfer tube. The powder bed has an electrical resistivity in the range of 500 Ωm to 50,000 Ωm. The at least two electrodes are embedded in the powder bed and are arranged to heat the powder bed by providing an electrical current between the electrodes. The at least one heat transfer tube is arranged to contain a heat transfer fluid and has an inlet and an outlet connectable to a thermal energy consumer. The heat transfer tube and the powder bed are thermally coupled via an electrically insulating material.

[0012] The electrical resistivity of the powder bed can be advantageously selected so that the thermal energy storage device can be directly connected to the electrical energy source without requiring the use of any transformer to first convert the high voltage of the electrical energy source to a much lower voltage that can be used to heat the conductive medium between the electrodes. Such a direct connection to the electrical energy source allows the powder bed to simultaneously perform the functions of energy conversion and energy storage. This not only results in significant cost reduction, but also better control over local peak temperatures, thereby increasing the durability and life of the system.

[0013] The heating power of the electrodes is proportional to the resistance of the powder bed and the square of the current. According to Ohm's law, this can also be expressed as proportional to the square of the voltage divided by the electrical resistance. The electrical resistance provided by the powder bed is proportional to the resistivity of the powder bed material and the electrode spacing. With the above-mentioned preferred resistivities in the range of 500 Ωm to 50,000 Ωm, a practical electrode spacing of, for example, 50 cm to 200 cm can be used to achieve a suitable heating power and heat the powder bed to a suitable temperature.

[0014] Preferably, semiconductor particles are used in the powder bed. The use of the semiconductor material in powder form allows free thermal expansion of the electrode and the powder bed during the heating process. If the electrode shrinks during cooling, the powder bed can self-recover under the influence of its own weight. Thus, good contact between the electrode and the semiconductor material is guaranteed for many repeated energy storage and release cycles.

[0015] In an exemplary embodiment of the present invention, the semiconductor material of the powder bed comprises a silicon carbide matrix. The silicon carbide matrix may be undoped or doped. Preferably, the silicon carbide matrix may be doped with nitrogen, phosphorus, beryllium, boron, aluminum, or gallium, or a combination thereof. The advantage of using silicon carbide particles is that it is a readily available bulk material and can be used in powder form without the need for any post-processing steps such as sintering. Doped silicon carbide can have a very suitable electrical resistivity, for example, about 1,000 Ωm. It should be noted that the resistivity of the powder bed depends not only on the material of the particles used, but also on, for example, the particle size, particle shape, and interparticle spacing.

[0016] Preferably, the electrodes are in direct contact with the powder bed to ensure efficient and effective heat transfer.

[0017] In an exemplary embodiment, the electrodes comprise graphite or sintered silicon carbide, which provide good conductivity and long life.

[0018] The electrical resistivity of the electrical insulating material is higher than that of the powder bed. In a preferred embodiment, the electrical insulating material is an electrically insulating layer of a bulk material. Suitable bulk materials include silicon carbide (preferably undoped), sand, quartz, and selected grades of iron ore. The heat transfer tube may be embedded in the bulk material. In addition to being readily available and relatively inexpensive, such bulk materials provide the advantage of allowing the heat transfer tube to expand and contract freely if its temperature changes without the insulating material losing direct contact with the heat transfer tube. Therewith, efficient and effective heat transfer between the electrical insulating layer and the heat transfer tube is ensured for repeated long-term use. Although the use of particulate bulk materials is preferred, alternative solid materials, possibly porous materials, may be used as alternatives. For example, concrete may be a suitable material in terms of cost, electrical insulation, and thermal conductivity.

[0019] In an exemplary embodiment, a thermal energy storage device comprises a plurality of thermally coupled modules, each module having: a heating layer comprising a powder bed and at least two electrodes; a heat transfer tube and a heat dissipation layer comprising an electrically insulating material.

[0020] Some of the exemplary thermal energy storage devices described above may further comprise a buffer material thermally coupled to the powder bed and separated from the heat transfer tube by at least the powder bed. Such a buffer material may further increase the total storage capacity of the thermal energy storage device and may help control the maximum temperature of the heat generating powder bed. The buffer material may include a material that stores energy in the form of sensible heat, or in the form of latent heat, or a combination of both.

[0021] For example, a thermal energy storage device may include a plurality of thermally coupled modules, each module having: a first heating layer comprising a powder bed and at least two electrodes; a heat dissipation layer comprising a heat transfer tube and an electrical insulating material; a second heating layer comprising a powder bed and at least two electrodes; a buffer layer.

[0022] Other configurations are also envisioned, such as thermally coupled modules each having only one heat generating layer.

[0023] A preferred thermal energy storage device may further comprise an insulating lower layer supporting at least the powder bed, the heat transfer tube and the electrical insulating material. The lower insulating layer serves to prevent heat loss to the soil on which the thermal energy storage device is placed. Similarly, an upper insulating layer may be added to prevent heat loss to the air above the thermal energy storage device. Additional insulation may be provided on one or more sides of the thermal energy storage device. All insulating layers may be made of any suitable insulating material. Preferably, mineral wool, an inexpensive bulk material, such as one of the bulk materials already discussed above, is used. Possible materials include, but are not limited to, mineral wool, ceramic foam, vacuum panels, or beds of granular insulating materials such as sand, quartz, pumice, or volcanic ash.

[0024] In a preferred embodiment, the upper and / or lower insulation layers may further include cooling tubes embedded in the insulating lower layer. The cooling tubes may be filled with a cooling fluid, such as water, to absorb some of the heat that would otherwise warm the soil below the thermal energy storage device. The cooling tubes may be connected to a pump to provide a continuous supply of cold cooling fluid. The cooling tubes may further be connected to the inlets of the heat transfer tubes so that they may be used to preheat the heat transfer fluid.

[0025] The thermal energy storage device described herein above may be used in a method of heating a heat transfer fluid, comprising passing an electric current between at least two electrodes, thereby generating heat in the powder bed, thereby heating said electrically insulating material, and passing a heat transfer fluid through at least one heat transfer tube, thereby heating the heat transfer fluid with heat from the electrically insulating material. The electric current may vary over time, as is often the case when the current is derived from a renewable source, such as solar and / or wind power. The electrically insulating material and optional buffer layer act as a thermal buffer that continues to heat the heat transfer fluid for a certain amount of time during interruption of the electric current, when power is not available or is insufficient to replenish the heat extracted from the device.

[0026] FIG. 1 shows a schematic diagram of a wind farm 10, a solar power farm 20, and a thermal energy storage device 100. One or more detailed examples of the thermal energy storage device 100 are described in more detail below. When a suitable amount of wind is present, the rotating rotor blades of the wind turbines 11 in the wind farm 10 drive a generator that produces electricity. Similarly, during the day, the solar panels 21 in the wind farm 20 absorb light from the sun 50 and also generate electricity. Both the wind farm 10 and the solar power farm 20 are electrically connected to the thermal energy storage device 100. The purpose of the thermal energy storage device 100 is to store the electrical energy generated by the wind farm 10 and the solar power farm 20 by heating the storage medium. The stored energy is released by heating a heat transfer fluid when required. The heated heat transfer fluid, typically water in the form of steam, can then be led through one or more pipes 35 to a thermal energy consumer, here symbolically represented by a factory 30. The consumer may, for example, directly use the steam in some industrial process or alternatively convert the steam to electricity using a steam turbine generator before feeding the steam to an industrial process or a steam condenser. A return flow of the heat transfer fluid (at least a portion thereof) may be circulated back to the thermal energy storage device 100 for further extraction of heat from the thermal energy storage device 100.

[0027] 2 shows a perspective view of an embodiment of a thermal energy storage device 100. The thermal energy storage device 100 is made from a series of alternating layers 110, 120, 130 having different functions, materials, and other features. The layers may extend vertically and / or form columns. Before describing in more detail some of the materials and features that may be used in the different layers, we first provide a brief description of how a thermal storage device may be used to store and release energy.

[0028] The current passing through the electrode layer 110 is converted into thermal energy, warming the material in this layer 110 to a temperature that may exceed, for example, 800° C. In the adjacent heat dissipation layer 120, a tube structure 200 is provided through which a heat transfer liquid, for example water, may be guided to receive a portion of the stored energy and become a heated fluid, for example in the form of steam. Although the combination of a single electrode layer 110 and a single heat dissipation layer 120 is sufficient to obtain a working thermal energy device 100, by providing multiple such layers in an alternating pattern, a larger storage capacity and improved control over the local peak temperatures and the energy storage and release process may be obtained. To further increase the total storage capacity of the thermal energy storage device 100 and to control maximum temperature and release duty fluctuations, an optional buffer layer 130 may be added.

[0029] In this embodiment, the electrode layer 110 includes a powder bed of a semiconductor material that provides the powder bed with an electrical resistivity in the range of 500-50,000 Ωm. At least two electrodes are embedded in the powder bed and arranged to heat the powder bed by providing a voltage between the at least two electrodes. The semiconductor material may include, for example, silicon carbide (SiC), optionally doped with a suitable amount of nitrogen, phosphorus, beryllium, boron, aluminum, or gallium to obtain the desired electrical resistivity. Doped silicon carbide has excellent electrical and thermal properties (in terms of conductivity and storage capacity) for use in the electrode layer 110 of the thermal energy storage device 100. Such doped silicon carbide may have an electrical resistivity of about 1,000 Ωm, for example, for use at intermediate grid supply voltages. Due to impurities in the bulk production of silicon carbide, undoped silicon carbide may also be suitable for use as the main component of the powder bed. Undoped silicon carbide, with a resistivity of up to 50,000 Ωm, for example, may be used with high power grid supply voltages.

[0030] The resistivity of the powder bed depends not only on the material of the powder bed particles used, but also, for example, on the particle size, particle shape, and interparticle spacing. The electrical resistivity of the powder bed is preferably selected in such a way that the thermal energy storage device 100 can be directly connected to an electrical energy source, such as a wind power plant 10 or a solar power plant 20, without the need to use any transformer to first convert the high voltage of the electrical energy source to a much lower voltage that can be used to heat the conductive medium between the electrodes. Such a direct connection to the electrical energy source allows the selected semiconductor material to simultaneously perform the functions of energy conversion and energy storage. This results in significant cost reduction.

[0031] A further advantage of using silicon carbide is that it is a readily available bulk material and can be used in powder form without the need for any post-processing steps such as sintering. The use of the semiconductor material in powder form also allows free thermal expansion of the electrodes during the heating process. If the electrodes shrink during cooling, the powder bed can self-heal under the influence of its own weight. Thus, good contact between the electrodes and the semiconductor material is guaranteed for many repeated energy storage and release cycles. The degradation of the electrical properties of silicon carbide over time due to prolonged exposure to high temperatures can be minimized by controlling the peak temperature in the heating layer, for example by limiting the peak temperature to about 800°C. Other methods of limiting the degradation of the powder bed material include the selection of particle size and the periodic injection of a protective purge gas (e.g., nitrogen, argon, or carbon dioxide).

[0032] The preferred choice of material for the electrodes 301, 302, 303 is graphite or sintered silicon carbide. Both graphite and sintered silicon carbide electrodes have good electrical conductivity and long life. As discussed above, the electrodes 301, 302, 303 may be directly connected to a high voltage power supply. The voltage of such a high voltage power supply may be greater than 1 kV (1,000 volts), 5 kV, or 10 kV. For example, the wind power farm 10 and / or the solar power farm 20 may provide a voltage of 33 kV using three-phase alternating current. In this example, the first line of the interconnected first electrode 301 may be connected to a first phase, the second line of the interconnected second electrode 302 may be connected to a second phase, and the third line of the interconnected third electrode 303 may be connected to a third phase. This pattern may be repeated for the fourth, fifth, sixth, and subsequent lines of the interconnected electrodes. In other embodiments, lower or higher voltages (eg, 6 kV, 11 kV, 22 kV, 66 kV), two-phase AC, or even DC may be applied.

[0033] By applying a voltage to the electrodes 301, 302, 303, a current flows through the semiconductor material between two adjacent electrodes that are not directly connected. Due to the electrical resistivity of the semiconductor material, the electrode layer 110 warms up as a result of its ohmic resistance. The highest heating (and thus temperatures, possibly up to more than 800° C.) is expected to occur in the vicinity of the electrodes 301, 302, 303. The electrical and thermal conductivity properties and heat storage capacity of the semiconductor material determine the further distribution of the generated heat through the electrode layer 110.

[0034] The heat dissipation layer 120 is provided adjacent to the electrode layer 110, either in direct contact with the electrode layer 110 or in contact with an intermediate buffer layer that may comprise a material different from the electrode layer 110 and the heat dissipation layer 120. The thermal conduction and storage properties of the material used for this heat dissipation layer are such that the heat generated in the electrode layer 110 is effectively transferred to the heat transfer tube 200. One function of the heat dissipation layer 120 is to dampen the daily or hourly intermittency of the renewable energy source in order to provide the consumer with an acceptable heat supply fluctuation. For the thermal energy storage device 100 to function, it is important that the heat transfer tube 200 is in intimate contact with the heat dissipation layer 120 so that the heat transfer tube 200 can efficiently exchange heat with the heat dissipation layer 120. If the heat transfer tube 200 is made of a material that contracts and expands under the effect of temperature changes, it is preferable that the material used for the heat dissipation layer 120 can accommodate these changes. Therefore, for the electrode layer 110, the use of a bulk material consisting of loose particles such as a powder is preferred.

[0035] Suitable materials that may be used for the heat dissipation layer 120 include bulk materials such as (non-conductive) silicon carbide, sand, quartz, or iron ore. If the heat transfer tube 200 is made of an electrically conductive material such as metal, the material used for the heat dissipation layer 120 is preferably not electrically conductive, for example to electrically insulate the heat transfer tube 200 and to avoid undesired currents flowing through it. Alternatively, the heat dissipation layer 120 may comprise the same semiconductor material as used for the electrode layer 110, or another conductive or semiconductive material. In that case, an insulating layer may be applied to the heat transfer tube 200 to avoid undesired currents flowing through it. However, it is important that such an electrical insulating layer does not significantly impede the heat exchange between the thermal energy storage device 100 and the heat transfer fluid flowing through the tube 200. In other embodiments, an electrical insulating layer may be provided between the electrode layer 110 and the heat dissipation layer 120, thereby creating the opportunity to use an electrically conductive material for the heat dissipation layer 120.

[0036] Optionally, an additional buffer layer 130 is provided to further increase the total storage capacity of the thermal energy storage device 100 and control the maximum temperature. To this end, materials are selected based on, for example, cost, thermal conductivity, and thermal storage capacity. Suitable materials for use in this buffer layer include bulk materials such as (non-conductive) silicon carbide, sand, quartz, iron ore, or materials capable of storing latent heat, possibly in combination with sensible heat, such as miscibility gap alloys (MGA), solar salt, or low melting point metals, which would be mixed with non-molten porous or bulk solids. The buffer layer 130 may be sandwiched between two electrode layers 110, as in FIG. 2, although alternative arrangements are possible, such as omitting one or two of the adjacent electrode layers 110 or adding more electrical insulation layers.

[0037] All layers may be purged periodically or continuously using, for example, nitrogen, argon, or carbon dioxide to prevent degradation of desired properties or moisture accumulation within the layers and / or around the electrodes 301, 302, 303, or heat transfer tubes 200.

[0038] FIG. 3 shows an example of a heat transfer tube 200 for use in the thermal energy storage device of FIG. 2. The heat transfer tube 200 includes a number of substantially parallel straight sections 230 connected to each other by interconnecting sections 240. The interconnecting sections 240 may, for example, be curved, flanged, or implemented as cross joints. In use, for example, water (W) enters the heat transfer tube 200 at the inlet 210, flows down the tube 200 while removing heat from the heat exchange layer 130 (see FIG. 1), and finally exits the tube 200 as steam (S) at the outlet 220. Although the use of water is preferred, alternative heat transfer fluids may be used. A pump (not shown) may be provided to pump the water into the inlet 210. After leaving the outlet 220, the steam, typically at a temperature of about 350° C., may be directed to one or more consumers. The consumers may, for example, use the steam directly in some industrial processes or first convert the steam into electricity using a steam turbine generator. Optionally, the heat transfer tube 200 and the consumer form a closed circuit, and the spent steam is condensed to water and pumped back to the inlet 210 of the heat transfer tube 200 .

[0039] In a thermal energy storage device 100 with two or more heat transfer tubes 200, each inlet 210 may be connected to its own source of heat transfer fluid, and the heat transfer fluid may have a similar temperature at all inlets 210. Alternatively, two or more heat transfer tubes 200 may be interconnected such that the outlet 220 of a first heat transfer tube 200 is connected to the inlet 210 of a second heat transfer tube 200, and the heat transfer fluid passes through two or more heat transfer tubes before exiting the thermal energy storage device 100 and being sent to a consumer.

[0040] The heat transfer tube 200 may be made of a metal such as stainless steel. An important property of the material used for the heat transfer tube 200 is that it allows efficient heat exchange between the outer heat dissipation layer 120 and the inner heat transfer fluid. Permanent contact between the heat transfer tube 200 and the heat dissipation layer 120 may be promoted by using a granular material that expands or contracts freely relative to the tube. As already explained above, if the tube material is electrically conductive, it is important that some form of electrical insulation is provided to insulate the heat transfer tube 200 from the electrodes 301, 302, 303. In addition, grounding and bonding of the heat transfer tube may be provided.

[0041] Preferably, one or more of the interconnecting portions 240 of the heat transfer tubes 200 can be easily removed from the straight portions 230 and reattached to the straight portions 210. Similar to the embodiment of FIG. 2, when the interconnecting portions 240 protrude from the body of the thermal energy storage device 100, removing the interconnecting portions 240 allows maintenance and inspection access to the horizontally oriented straight portions 230 of the heat transfer tubes 200.

[0042] FIG. 4 shows the thermal energy storage device 100 of FIG. 2 with added insulation. The lower insulation layer 410 helps to prevent loss of heat to the soil in which the thermal energy storage device 100 is placed. The lower insulation layer 410 can be made of any suitable insulating material. Preferably, an inexpensive bulk material with good insulating and load-bearing properties is used. Possible materials include, but are not limited to, sand, quartz, pumice, or volcanic ash. The lower insulation layer 410 can include a cooling tube 412 that can be filled with a cooling fluid, such as water, to absorb some of the heat that would otherwise warm the soil below the thermal energy storage device 100. The cooling tube 412 can be made of a similar material as the heat transfer tube 200. Preferably, the cooling tube 412 is connected to a pump to provide a continuous supply of cold cooling fluid. The cooling tube 412 can be connected to the inlet 210 of the heat transfer tube 200 so that it can be used to preheat the heat transfer fluid.

[0043] To prevent excessive heat loss to the immediate environment, a similar upper insulation layer 420 may be provided on top of the thermal energy storage device 100. The upper insulation layer 420 may comprise the same or similar materials as the lower insulation layer 410.

[0044] Figure 5 shows a perspective view of a different embodiment of the thermal energy storage device 100. The main difference from the thermal energy storage device 100 shown in Figure 4 is that it does not include a buffer layer 130 and includes an alternating pattern of electrode layers 110 and heat dissipation layers 120.

[0045] FIG. 6 shows the thermal energy storage device 100 of FIG. 5 with further added insulation. An insulating sleeve 500 is applied to all side walls of the thermal energy storage device 100. Many readily available insulating materials can be used for this insulating sleeve 500. By way of example only, perlite can be used for its suitability for high temperatures. The possibility of adding such an insulating sleeve 500 is not limited to the embodiment of FIG. 5, but can also be beneficial in other embodiments, such as the embodiments of FIG. 2 and FIG. 4. The sides of the thermal energy storage device 100 can have vertical walls or no walls according to the natural angle of repose of the bulk material.

[0046] Figure 7 shows a perspective view of an alternative embodiment of thermal energy storage device 100. Figure 8 shows an expanded view of a portion of thermal energy storage device 100 of Figure 7 with the outer insulation layers 410, 420, 430 removed. Many features of this embodiment are similar to, and perform the same functions as, corresponding features in the embodiments described above with reference to Figures 2-6. Accordingly, the same reference numbers are used for such corresponding features.

[0047] As can be clearly observed in Fig. 8, one important difference with the previously described embodiment is that the different layers 110, 120, 130, 140 have a substantially horizontal orientation. It should be noted that the use of such a layered configuration may be advantageous for reasons of performance and / or ease of construction, but the invention is not limited to such a configuration. Also, if layers are used, their orientation may differ from the substantially vertical or horizontal orientation shown in the drawings. Similarly, the layers may have various thicknesses and different shapes. For example, a cylindrical thermal energy storage device may be provided in which the different layers are provided as concentric rings.

[0048] In the embodiment of Figures 7 and 8, the first, second and third electrodes 301, 302, 303 are aligned parallel in a substantially horizontal electrode layer 110 that is sandwiched between two substantially horizontal electrical insulating layers 140, for example filled with one of the electrical insulating bulk materials described above. The important function of these electrical insulating layers 140 is to enable the use of conductive materials for the subsequent buffer layer 130 and heat dissipation layer 120. The buffer layer 130 and / or the heat dissipation layer 120 can be made of the same or different materials. All materials described above for the corresponding layers 120, 130 in the embodiments described in Figures 2 to 6 can also be used in this embodiment.

[0049] A high heat flux per cubic meter is desirable to reduce the footprint of the thermal energy storage device. In a typical, non-limiting example, the maximum temperature fluctuation between day and night in the heat generating electrode layer 110 is about 100°C (e.g., cycling between 700°C and 800°C). The main function of the heat dissipation layer 120 is to dampen the day-night fluctuation of the duty output to the consumer. This can be achieved by reducing the temperature swing in the heat generating electrode layer 110.

[0050] To further reduce this power duty swing or to further increase the maximum temperature variation in the heating electrode layer 110 between day and night, for example to 400° C., one or both of the layers 120, 130 may contain a phase change material that can store energy in the form of latent heat. A more constant temperature buffer is obtained if the composition of the layer is such that a part of it changes phase, for example by melting during the heating cycle and solidifying when less renewable energy is produced and the layer cools. This helps to prevent overheating of the steam pipe during periods of low consumer demand.

[0051] When using a phase change material, it is important that it remains stationary within layer 120 when in the molten state. This can be achieved, for example, by mixing low melting point granules with a non-melting bulk material (i.e., a bulk material with a melting temperature high enough to avoid melting during normal use of the thermal energy storage device 100). Possible combinations are a 50%-50% mixture of either magnesium (phase change at 650° C.) and iron (phase change at 1538° C.), or zinc (phase change at 420° C.) and graphite (phase change at 3600° C.), or an aluminum magnesium eutectic with sand, silicon carbide, iron ore, or graphite, either pure or mixed. Leakage into other layers is prevented if complete encapsulation by the non-melting components is achieved, or if the non-melting particles remain in load-bearing contact with each other throughout the melting and solidification stages of the meltable material, and the low melting point phase is surrounded on all sides by an impermeable layer. Although the phase change material, including metal, is electrically conductive, an electrical insulating layer 140 is provided for electrical insulation between the electrode layer 110 and the vapor conduit 200 .

[0052] Those skilled in the art will readily appreciate that while the detailed description of the invention has been presented with reference to one or more embodiments each having a particular combination of features and means, many of those features and means may equally or similarly be applied independently in other embodiments or combinations. Moreover, those skilled in the art will appreciate that the invention and its teachings may be implemented in many different ways without departing from the scope of the appended claims.

Claims

1. 1. A thermal energy storage device comprising: a powder bed having an electrical resistivity in the range of -500 Ωm to 50,000 Ωm; at least two electrodes embedded in the powder bed and arranged to heat the powder bed by providing an electric current between the at least two electrodes; - at least one heat transfer tube arranged to contain a heat transfer fluid, the at least one heat transfer tube having an inlet and an outlet connectable to a thermal energy consumer, said heat transfer tube and said powder bed being thermally coupled via an electrically insulating material.

2. 10. The thermal energy storage device of claim 1, wherein the powder bed comprises a semiconductor material.

3. 3. The thermal energy storage device of claim 2, wherein the semiconductor material comprises a silicon carbide matrix that is undoped or doped with nitrogen, phosphorous, beryllium, boron, aluminum, or gallium.

4. 3. The thermal energy storage device of claim 1, wherein the electrodes are in direct contact with the powder bed.

5. 3. The thermal energy storage device of claim 1 or 2, wherein the electrodes comprise graphite or sintered silicon carbide.

6. 3. The thermal energy storage device of claim 1, wherein the electrically insulating material is an electrically insulating layer of bulk material such as silicon carbide, sand, quartz, or iron ore, and the heat transfer tube is embedded in the bulk material.

7. a plurality of thermally coupled modules, each module comprising: a heating layer comprising said powder bed and said at least two electrodes; - a heat transfer tube and a heat dissipation layer comprising the electrically insulating material.

8. 3. The thermal energy storage device of claim 1 or 2, further comprising a buffer layer thermally coupled to the powder bed and separated from the heat transfer tube by at least the powder bed.

9. 10. The thermal energy storage device of claim 8, wherein the buffer layer comprises a phase change material.

10. a plurality of thermally coupled modules, each module comprising: a first heating layer comprising said powder bed and said at least two electrodes; a heat dissipation layer comprising said heat transfer tube and said electrical insulating material; a second heating layer comprising said powder bed and said at least two electrodes; - the buffer layer.

11. 3. The thermal energy storage device of claim 1 or 2, further comprising an insulating underlayer supporting at least the powder bed, the heat transfer tube, and the electrically insulating material.

12. 12. The thermal energy storage device of claim 11, further comprising a cooling tube embedded in the insulating lower layer, the cooling tube connectable to the heat transfer tube.

13. 3. The thermal energy storage device of claim 1, wherein the at least two electrodes are directly connected to a high voltage supply having a voltage greater than 1,000 volts.

14. 1. A method of heating a heat transfer fluid, comprising: - providing a thermal energy storage device according to claim 1 or 2, comprising a powder bed, at least two electrodes, and at least one heat transfer tube thermally coupled to said powder bed via an electrically insulating material, - passing an electric current between said at least two electrodes, thereby generating heat in said powder bed and heating said electrically insulating material; passing said heat transfer fluid through said at least one heat transfer tube, thereby heating said heat transfer fluid with heat from said electrically insulating material.

15. The method of claim 14 , wherein the current varies over time.

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