Solar thermal energy accumulator and converter

Abstract

The present invention relates to a solar thermal energy accumulator and converter, comprising at least one hollow vessel (11), filled with a phase-change material (12), forming hollow cylindrical cavities (13) around a hollow rectangular cavity (15), a solar absorber (14) arranged in a fixed position and inserted into a cylindrical cavity (13), and a thermophotovoltaic generator (16) which is inserted into the rectangular cavity (15). The vessel (11), the thermophotovoltaic generator (16) and the fixed solar absorber (14) are aligned along a horizontal axis of the accumulator and converter. If the thermophotovoltaic generator (16) moves vertically away from the vessel (11) and the fixed solar absorber (14), the solar absorber (14) transfers thermal energy to the vessel (11); if the vessel (11) moves vertically such as to be aligned with the position reached by the thermophotovoltaic generator (16), the vessel (11) transfers energy to the thermophotovoltaic generator (16); and if the thermophotovoltaic generator (16) moves away from the position reached by the vessel (11), the exchange of thermal energy between the solar absorber (14), the thermophotovoltaic generator (16) and the vessel (11) is prevented, with thermal energy being stored in the vessel (11).

Description

[0001] DESCRIPTION

[0002] Solar thermal energy storage and converter

[0003] Technical field

[0004] The invention belongs to the sector of the accumulation and transformation of solar thermal energy into electricity by means of thermophotovoltaic devices.

[0005] Background

[0006] During the last few years, different types of solar energy storage systems have been described that use latent heat in phase change materials and thermophotovoltaic converters to produce electricity from the stored heat.

[0007] In these systems, solar energy is concentrated onto a high-melting-point material, which, upon changing from a solid to a liquid phase, stores this energy as latent heat of phase change. The photovoltaic converter is used to produce electricity from the incandescent thermal radiation emitted by a material in close contact with the phase-change material.

[0008] The converters are of the type that do not have physical contact with the emitter, allowing the latter to reach very high temperatures. This enables the use of phase-change materials with very high melting points, such as silicon or ferrosilicon, which are characterized by a very high latent heat of phase change, thus allowing for very high energy densities. Therefore, unlike other systems that use turbines to convert heat into electricity, the use of thermophotovoltaic converters allows for the manufacture of very compact and quiet systems, as they lack moving parts and require less maintenance and offer a higher level of safety, since they do not require heat transfer fluids or all the subsystems associated with their use, such as pressurized fluids, valves, and piping.

[0009] In these known systems, the phase-change material is contained in a vessel characterized by one wall, the absorber, being designed to absorb solar radiation, and another wall, the emitter, being designed to emit thermal radiation towards the photovoltaic converter. The remaining walls of the vessel are covered with thermal insulation.

[0010] One aspect of the storage and converter design involves two processes: energy charging and discharging. During charging, sunlight strikes the absorbent surface of the tank, generating heat that is transferred to the phase-change material. This material changes from a solid to a liquid state, storing solar energy as latent heat of phase change. During discharging, the latent heat contained in the liquid phase of this material is transferred to the emitter, which radiates the latent heat as photons to the photovoltaic converter, which directly produces electricity.

[0011] During this discharge process, the phase-change material tends to solidify around the surface of the vessel closest to the emitter, creating a solid crust that impedes heat flow from the liquid phase to the emitter. Therefore, to maximize heat transfer through this layer, thermal fins, cylindrical geometries, or, more generally, the use of phase-change materials with very high solid-phase thermal conductivities are employed.

[0012] It is known that the photovoltaic generator does not produce electrical power during a large part of the system's charging process, until the emitter has reached a sufficiently high temperature. This process can last several hours and, therefore, poses a problem when managing the system's electrical production. This issue arises because the charging and discharging processes occur sequentially on separate surfaces of the vessel: one comprises the absorber (charging process), and the other comprises the emitter (discharging process). These two surfaces are separated by the phase-change material, which acts as a reservoir between the solar energy absorbed on the first surface and the thermal radiation illuminating the photovoltaic converter on the second surface.Therefore, the solar energy absorbed on the absorbing surface is not transferred to the emitting surface immediately, but one must wait until the phase change material has reached a sufficiently high temperature throughout virtually its entire volume.

[0013] If the same surface of the vessel acts as both absorber and emitter, the system will also produce electricity during the charging process because the thermal emitter is heated directly by sunlight, reaching very high temperatures without the phase-change material needing to reach high temperatures throughout its entire volume. In this way, the system charges and power is generated simultaneously through the photovoltaic converter, which is illuminated by the emitter.

[0014] One drawback of these systems is regulating when the system discharges. Whenever the emitter reaches a high temperature, heat is transferred to the photovoltaic converter, which, in turn, produces electricity. If the photovoltaic generator moves in and out of the cavity formed by the walls of the vessel that makes up the emitter, it's possible to manage when the photovoltaic generator is illuminated by the emitter and, therefore, produces electricity. However, this design does not allow for simultaneous charging and discharging of the system. If the photovoltaic generator is inserted into the cavity formed by the emitter, it blocks sunlight from reaching the emitter and thus prevents the system from charging.

[0015] Summary

[0016] According to aspects of certain embodiments, a solar thermal energy accumulator and converter comprises at least one hollow vessel, rectangular in shape with concave corners, filled with a phase-change material; at least four of the contiguous vessels form four hollow cylindrical cavities around a hollow rectangular cavity; a solar absorber, arranged in a fixed position within the accumulator and converter, is inserted into a cylindrical cavity, and a thermophotovoltaic generator is inserted into the rectangular cavity.If the vessel, the photovoltaic generator, and the fixed solar absorber are aligned along a horizontal axis of the accumulator and converter, the fixed solar absorber simultaneously transfers thermal energy to the vessel and the photovoltaic generator. If the photovoltaic generator moves vertically, along an axis orthogonal to the horizontal axis of the accumulator and converter, away from the vessel and the fixed solar absorber, the solar absorber transfers thermal energy to the vessel. If the vessel moves vertically, aligning itself with the position reached by the photovoltaic generator, the vessel transfers energy to the photovoltaic generator. And if the photovoltaic generator moves away from the position reached by the vessel, the exchange of thermal energy between the solar absorber, the photovoltaic generator, and the vessel is prevented, and thermal energy is stored in the vessel.

[0017] The energy accumulator and converter stores solar energy in the form of latent heat of phase change in the high melting point phase change material and directly converts the latent heat into electricity by means of at least one thermophotovoltaic converter.

[0018] In one embodiment of the accumulator and converter (according to a specific operating mode of the accumulator and converter), the charging and discharging of thermal energy occur simultaneously. The process of generating electrical power is managed by providing relative vertical movement between the vessels and the photovoltaic generators, where the solar absorbers, in turn, exhibit independent movement.

[0019] In the first operating mode of the accumulator and converter, the vessel, the thermophotovoltaic generator and the fixed solar absorber are aligned in the highest position of the accumulator and converter, in which the vessel and the thermophotovoltaic generator exchange heat with the solar absorber.

[0020] During this mode of operation, the processes of solar charging or absorption and thermophotovoltaic discharge or generation occur simultaneously.

[0021] Sunlight reaches the solar absorber through a top opening. The solar absorber transfers heat to both the vessel walls and the photovoltaic generator. A thermal emitter absorbs the radiation emitted by the solar absorber and the vessel walls, and some of this energy is re-emitted to a photovoltaic converter, which produces electricity. Furthermore, the proximity between the vessel walls designed to receive the photovoltaic generator and the vessel walls designed to receive the solar absorber allows for highly efficient heat transfer between them, without the need for heat to first flow through the phase-change material. This facilitates heat flow from the solar absorber to the photovoltaic generator and enables the simultaneous charging and discharging of the storage tank and converter.During this operating mode, some of the heat inevitably escapes from the solar absorber through the inlet opening.

[0022] In a second operating mode, the photovoltaic generator moves vertically away from the vessel and the fixed solar absorber. The solar absorber transfers thermal energy to the vessel, which remains aligned with the solar absorber and is therefore positioned higher than the photovoltaic generator, which is located further away from the fixed solar absorber. During this operating mode, only solar absorption charging occurs. Sunlight reaches the solar absorber through the top inlet opening.

[0023] In another embodiment, portions of the thermal insulation are placed on the upper surface of the vessel and on the upper surface of the photovoltaic generator opposite the lower surface of the photovoltaic generator where the photovoltaic converter is located. The portions of the thermal insulation move together with the vessel and the photovoltaic generator.

[0024] The solar absorber transfers heat to the vessel walls, which, in turn, transfer it to the phase-change material. As in the first operating mode, some heat inevitably escapes from the solar absorber through the inlet opening.

[0025] The thermal insulation portion is made of a mirror-like material with low thermal conductivity. This way, when the photovoltaic generator is in the lower position, the thermal insulation portion on top of the generator seals the cavity and prevents heat loss through the walls of the vessel (or crucible).

[0026] The thermal emitter protects the photovoltaic generator from potential contaminants from the vessels and the phase-change material that would otherwise deposit on the photovoltaic cells, darkening them and thus impairing their proper functioning. The upper and lower walls of the vessels and the thermal emitter are covered with sections of thermal insulation material. In a third operating mode, the vessel moves vertically, aligning itself with the photovoltaic generator at its intermediate position. The vessel transfers energy to the photovoltaic generator, allowing it to capture the radiation emitted by the vessel walls. During this operating mode, only the discharge process occurs. The phase-change material transfers the stored heat to the flat walls of the vessel, which, in turn, transfer it to the photovoltaic generator to produce electricity.

[0027] The advantage of placing the vessel in a lower position during this mode of operation is that it eliminates the sight factor between the vessel walls and the inlet opening, and therefore eliminates losses from re-emission of thermal radiation through this opening.

[0028] In a fourth operating mode, the photovoltaic generator moves away from the intermediate position reached by the vessel, preventing the exchange of thermal energy between the solar absorber, the photovoltaic generator, and the vessel. During this mode, thermal energy is stored with minimal losses. As in the previous mode, the line of sight between the vessel walls and the inlet opening is eliminated, thus eliminating losses from the re-emission of thermal radiation through this opening.

[0029] The battery and converter produces electricity during the charging process. This is due to two factors. First, there is an operating mode in which the line of sight between the solar absorber and the photovoltaic generator's vessel (or crucible) is nonexistent. Second, the walls of the vessel (or crucible) dedicated to receiving the solar absorber are contiguous with the walls of the vessel dedicated to receiving the photovoltaic generator, which facilitates heat flow between them. During simultaneous charging and discharging operation, both the photovoltaic generator and the vessel are heated by the sunlight captured by the solar absorber, reaching very high temperatures without the phase-change material needing to reach high temperatures throughout its entire volume.In this way, the system is charged through the walls of the vessel and power is generated through the thermophotovoltaic converter illuminated by the emitter.

[0030] The accumulator and converter manages the times when electricity is produced thanks to the ability to vary the relative positions between the thermophotovoltaic generator and the vessel, reaching a situation between one in which the sight factor between both is zero, and another situation in which the sight factor between both is different from zero.

[0031] The advantage of this accumulator and converter is the reduction of re-emission losses through the opening of solar collection during periods when the system is not being charged, i.e., during discharge and storage periods.

[0032] This advantage is due to the ability of the vessel to be moved to lower positions, as well as the arrangement of the thermal insulation layers, which move along with the vessel, completely eliminating the visibility factor between the vessel and the solar absorber when the vessel is placed in positions distant from the fixed position of the absorber.

[0033] The advantages described above are due to the geometric arrangement of the vessel, the solar absorber, and the thermophotovoltaic generator, as well as the vertical displacement capability of the vessel and the thermophotovoltaic generator.

[0034] First, the system will be able to manage electrical power generation because the solar absorber, the storage vessel, and the photovoltaic generator are positioned along different parallel vertical longitudinal axes of the storage tank and converter. This allows them to move independently and relatively from each other, producing the four operating modes described above.

[0035] Secondly, the system will have few re-emission losses through the solar collection opening during the discharge process because the vessel moves vertically and away from the solar absorber when there is no sunlight, or when the phase change material is completely melted, thus eliminating the vessel's view factor to the solar absorber and preventing heat from flowing in that direction towards the outlet opening, reducing thermal radiation losses through the outlet or solar collection opening during a storage period.

[0036] Brief description of the figures The above and other advantages and features will be more fully understood from the following detailed description of at least one embodiment, with reference to the accompanying figures, in which: Fig. 1 shows in a plan view an embodiment of a solar thermal energy storage and converter. Fig. 2 shows in an elevation view a cross-section along three horizontal axes A, B, C of the embodiment of the solar thermal energy storage and converter illustrated in Fig. 1. Fig. 3 shows in a perspective view a section of the solar thermal energy storage and converter. Fig. 4 shows in an elevation view the cross-section along the horizontal axis C of the embodiment of the solar thermal energy storage and converter illustrated in Fig. 1, and the four operating modes of the solar thermal energy storage and converter.

[0037] Detailed description

[0038] With regard to Figures 1 to 4, where an embodiment of a solar thermal energy accumulator and converter is shown comprising at least one hollow vessel 11 filled with a phase change material 12, at least four of the contiguous vessels 11 forming at least one hollow cylindrical cavity 13 around a hollow rectangular cavity 15 formed by four flat walls 11 1 of four vessels 11 and a solar absorber 14.

[0039] The solar absorber 14 has a cylindrical shape, and the photovoltaic generator 16 has a parallelepiped shape. The solar absorber 14 is fixed in position and is inserted into a cylindrical cavity 13 formed by concave walls. The photovoltaic generator 16 is inserted into the rectangular cavity 15 formed by the flat walls of the vessels 11.

[0040] The photovoltaic generator 16 and the fixed solar absorber 14 are aligned along a horizontal axis of the accumulator and converter. The fixed solar absorber 14 simultaneously transfers thermal energy to vessel 11 and to the photovoltaic generator 16. If the photovoltaic generator 16 is moved vertically away from vessel 11 and the fixed solar absorber 14, this absorber 14 transfers thermal energy to vessel 11.

[0041] If vessel 11 moves vertically, aligning itself with the position previously reached by the thermophotovoltaic generator 16, vessel 11 transfers energy to the thermophotovoltaic generator 16. If the thermophotovoltaic generator 16 moves away from the position previously reached by vessel 11, the exchange of thermal energy between the solar absorber 14, the thermophotovoltaic generator 16, and vessel 11 is avoided, with thermal energy being stored in vessel 11.

[0042] Vessel 1 1 comprises at least eight side walls 1 11 , 112 four flat emitting walls 11 1 , which are arranged to transfer heat to the thermophotovoltaic generator 16 and the other four concave walls 1 12, which have a circular segment shape, are arranged to be heated by solar radiation.

[0043] The flat emitting walls 1 11 are arranged to transfer heat to the thermophotovoltaic generator 16 and the concave walls 1 12 are arranged to be heated by solar radiation from the solar absorber 14.

[0044] The vessels 11 are arranged so that the sight factor is nonexistent between at least one cylindrical cavity 13 and at least one rectangular cavity 15. The upper and lower walls of vessel 11 are coated with a portion of insulating material 181 and the top of the thermophotovoltaic generator 16 is coated with a portion of insulating material 182 (which, as already mentioned, may be a mirror material that returns or reflects thermal radiation and thus functions as an insulator).

[0045] Hollow vessel 11 is made of a refractory material of the ceramic type with a high melting point (>2000 e C) and high thermal conductivity (>20 W / mK) like graphite, which operates in an inert atmosphere of the argon type.

[0046] In another embodiment, the vessel 1 1 is manufactured using silicon carbide or graphite coated with refractory oxides such as aluminum oxide or hafnium oxide, which is operated in an air atmosphere.

[0047] The hollow vessel 11 is filled with a phase change material 12 comprising a high melting point metal >1000 e C, but with a lower melting point than that of vessel 11, high thermal conductivity >20 W / mK of the solid phase at temperatures close to the melting point and high latent heat of phase change >400 Wh / m3, such as silicon, boron, nickel, iron, or an alloy of these elements.

[0048] Solar absorber 4 comprises a refractory material manufactured from a high melting point refractory ceramic >2000 e C, high thermal conductivity >20 W / mK and high emissivity >0.8 like graphite or silicon carbide.

[0049] In another embodiment, the solar absorber 14 is manufactured using a high melting point metal >2000 e C and high thermal conductivity >20 W / mK such as tantalum, tungsten, molybdenum or molybdenum disilicide (MoSi2). The purpose of the solar absorber 14 is twofold: to absorb solar radiation and to transfer the absorbed solar energy to the phase change material and to a thermal emitter 161 which emits it towards the thermophotovoltaic converter 162. The thermal emitter 161 comprises a refractory material.

[0050] In another embodiment, the walls of the vessel 11 are adapted to emit radiation directly towards the thermophotovoltaic converter 162. In another embodiment, the walls of the vessel 11 are adapted to emit radiation directly towards the thermophotovoltaic converter 162 and, furthermore, the thermophotovoltaic converter 162 has an arrangement with a geometry similar to that of the thermal emitter 161.

[0051] In another embodiment, the walls of vessel 11 are adapted to directly absorb solar radiation.

[0052] In another embodiment, the thermal emitter 161 is manufactured from a high melting point refractory ceramic >2000 e C, high thermal conductivity >20 W / mK and high emissivity >0.8 like graphite or silicon carbide.

[0053] In another embodiment, the thermal emitter 161 is manufactured using a high melting point metal >2000 eC and high thermal conductivity >20 W / mK such as tantalum, tungsten, molybdenum or molybdenum disilicide MoSi2. The face of the thermal emitter 161 seen by the thermophotovoltaic converter 162 incorporates a selective emitter so that the radiation received by the thermophotovoltaic converter 162 is spectrally selective and consists mainly of photons with energy higher than the energy corresponding to the bandwidth of the semiconductor used to manufacture the thermophotovoltaic converter 162, which comprises at least one cell.

[0054] The thermophotovoltaic converter 162 comprises at least one thermophotovoltaic cell arranged on a substrate whose purpose is to conduct the heat generated in the cell to a heat sink 163.

[0055] The cell of the thermophotovoltaic converter 162 is manufactured using at least one semiconductor material such as silicon, GaAs, germanium, GaSb, InGaAs, InGaAsSb, etc., with the optimal bandwidth adapted to the light emission spectrum of the emitting flat wall 111 of the vessel 11 (or crucible), dependent on its temperature, and forming at least one p / n junction, cathode / anode, to make the selective contacts of electrons and holes generated internally in the semiconductor material. The converter substrate is manufactured using a direct-bonded copper (DBC) substrate, placed on top of a metal support made of copper or aluminum, which conducts the heat generated in the cell to the heat sink 163, comprising a metal piece through which a coolant circulates. The DBC substrate provides electrical insulation of the cell from the metal support and, at the same time, good conduction of the heat generated in the cell to the heat sink 163.If there is more than one cell, they can be interconnected in series to form a cell matrix.

[0056] An inlet aperture 17 is arranged to collect the incoming solar radiation towards the absorber 14 and confine it in an optical cavity formed by the walls of the solar absorber 14, where it is absorbed.

[0057] In one embodiment, the inlet opening 17 has reflective walls 172 made of a high reflectivity metal >90%, such as aluminum, and is arranged to reflect the incident solar radiation towards the lower inlet opening 173 of the solar absorber 14.

[0058] In one configuration, the inlet opening 17 has a collection window 171 made of a material transparent to solar radiation and with a high melting point >1500 eC, like quartz. This window 171 comprises an interference filter! located on both sides that blocks the passage of infrared radiation emitted by the cavity to the outside, and in turn allows the passage of solar radiation into the interior of the solar absorber 14.

[0059] In another embodiment, the inlet opening 17 has a Fresnel lens that concentrates solar radiation into the lower inlet opening 173 of the solar absorber 14.

[0060] The insulating cover 18 made of thermal material may comprise a refractory material, for example, a mirror. In one embodiment, the cover 18 is manufactured entirely from high-melting-point ceramic compounds (>2000°C). e C and low thermal conductivity <1 W / m- K such as alumina fibers or refractory bricks of alumina or mullite.

[0061] In another embodiment, the insulating cover 18 is manufactured by a combination of a ceramic cover arranged in close proximity to the vessel and a multi-layer composition, in which thin layers of low emissivity and high melting point metal, such as molybdenum or nickel, are interleaved with thin ceramic layers of low thermal conductivity and high melting point, such as quartz or zirconium fiber.

[0062] List of numerical references

[0063] 11 vessel

[0064] 111 flat wall transmitter

[0065] 112 concave wall

[0066] 12 material phase change

[0067] 13 cylindrical cavity

[0068] 14 solar absorber

[0069] 15 rectangular cavity

[0070] 16 thermophotovoltaic generator

[0071] 161 thermal emitter

[0072] 162 thermophotovoltaic converter

[0073] 163 heat sink

[0074] 17 top entrance opening

[0075] 171 catchment area

[0076] 172 reflective wall

[0077] 173 lower entrance opening

[0078] 18 insulating cover

[0079] 181, 182 portions of insulating materials

[0080] 19 sunlight

Claims

CLAIMS 1. A solar thermal energy storage and converter comprises at least one hollow vessel (11) filled with a phase-change material (12), at least four of the contiguous vessels (11) forming four hollow cylindrical cavities (13) around a hollow rectangular cavity (15), a solar absorber (14) arranged in a fixed position being inserted into a cylindrical cavity (13), and a thermophotovoltaic generator (16) being inserted into the rectangular cavity (15), characterized in that if the vessel (11), the thermophotovoltaic generator (16), and the fixed solar absorber (14) are aligned about a horizontal axis of the storage and converter, the fixed solar absorber (14) simultaneously transfers thermal energy to the vessel (11) and to the thermophotovoltaic generator (16), if the thermophotovoltaic generator (16) is moved vertically away from the vessel (11) and the solar absorber (14) fixed, the solar absorber (14) transfers thermal energy to the vessel (11),If the vessel (11) moves vertically, aligning itself with the position reached by the thermophotovoltaic generator (16), the vessel (11) transfers energy to the thermophotovoltaic generator (16). If the thermophotovoltaic generator (16) moves away from the position reached by the vessel (11), the exchange of thermal energy between the solar absorber (14), the thermophotovoltaic generator (16), and the vessel (11) is prevented, and thermal energy is stored in the vessel (11).

2. Accumulator and converter according to claim 1, wherein the vessel (11) comprises at least eight side walls (111, 112), four flat emitting walls (111) arranged to transfer heat to the thermophotovoltaic generator (16) and the other four concave walls (112), which have a circular segment shape (112), arranged to be heated by solar radiation.

3. Accumulator and converter according to claim 2, wherein the flat emitting walls (1 11 ) are arranged to transfer heat to a thermophotovoltaic generator (16) and the concave walls (1 12) are arranged to be heated by solar radiation from the solar absorber (14).

4. Accumulator and converter according to claim 3, wherein the vessel (1 1 ) is arranged so that there is a non-zero sight factor between at least one of a cylindrical cavity (13) and at least one of a rectangular cavity (15).

5. Accumulator and converter according to claim 4, wherein the solar absorber (14) is cylindrical and the thermophotovoltaic generator (16) is a parallelepiped.

6. Accumulator and converter according to claim 2, wherein the upper and lower walls of the vessel (11) are coated with a portion of insulating material (181).

7. Accumulator and converter according to claim 2, wherein the upper part of the thermophotovoltaic generator (16) is coated with a portion of insulating material (182) and / or a reflective or mirror material.

8. Accumulator and converter according to claim 2, wherein the emitting flat walls (11 1 ) are adapted to absorb solar radiation.

9. Accumulator and converter according to claim 2, wherein the flat walls of the vessel (1 1 ) are adapted to emit radiation directly to the thermophotovoltaic converter 162.

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