Solar thermophotovoltaic device

Abstract

The invention relates to a thermophotovoltaic device, comprising: - an absorber layer configured to absorb photonic radiation, particularly daylight, within a focal spot as a part of a surface of the absorber layer and convert it into heat energy; - an emitter layer positioned adjacent to the absorber layer or integrally formed with it, configured to emit thermal radiation due to the heat energy via a surface of the emitter layer; - at least one photovoltaic cell positioned to receive the emitted thermal radiation from the emitter layer and convert it into electrical energy using the photoelectric effect; - a heat spreading layer embedded within the absorber layer and / or emitter layer configured to distribute heat energy lateral over the emitter surface.

Description

[0001] Solar thermophotovoltaic device

[0002] Technical field

[0003] The present invention relates to thermophotovoltaic devices, particularly measures to increase efficiency of such thermophotovoltaic device.

[0004] Technical background

[0005] Mainstream technology for energy harvesting is represented by the standard silicon photovoltaic cells. Traditional photovoltaic systems suffer from a mismatch between the sun's broad spectrum and the PV band gap. The Shockley-Queisser limit of silicon photovoltaic cells indicates that only solar photons with energies more significant than the photovoltaic cell's bandgap may form electron-hole pairs. The Shockley-Queisser limit is the theoretical upper limit of conventional solar cells. The solar spectrum's broadband nature generally extends from 200 to 2500 nm, causing low conversion efficiency in siolicon photovoltaic cells. In other words, photovoltaic cells, in fact, absorb and convert photons with higher energy than the band gap into electron holes, but photons with lower energy do not generate electricity. With no concentration of sunlight, these effects result in a 33% upper limit for single junction solar cell efficiency and a 41% upper limit for maximum solar concentration. So the limitation of photon conversion in silicon photovoltaic cells is affected by energy losses due to low-bandgap photons and hot carrier thermalization.

[0006] A new approach to solar energy harvesting is solar thermophotovoltaics (STPV). Thermophotovoltaic systems are efficient energy transfer technology that overcomes the Shockley-Queisser limit by absorbing the whole solar spectrum emitting suitable energy photons to excite charge carriers for power generation. Solar thermophotovoltaic systems may have higher efficiency and output power than traditional photovoltaic systems. Thermophotovoltaic systems are solid-state heat engines that convert heat to electricity by thermal radiation emission and conversion. These systems can use a spectrally selective emitter material heated to over 1000° K and a photovoltaic cell to capture the radiant emission as e.g. described in Gamel, M. M. A. et al., “A review on thermophotovoltaic cell and its applications in energy conversion: Issues and recommendations”, 2021, Materials (Vol. 14, Issue 17), https: / / doi.org / 10.3390 / ma14174944 and Wang, Y. et al., “Solar thermophotovoltaics: Progress, challenges, and opportunities”, 2019, APL Materials (Vol. 7, Issue 8). https: / / doi.Org / 10.1063 / 1.5114829.

[0007] Solar thermophotovoltaics is an effective solar energy transfer technology that has the potential to overcome the Shockley-Queisser limit by absorbing sunlight across the entire solar spectrum and emitting narrowband radiation to photovoltaic cells. In terms of Carnot efficiency, a solar thermophotovoltaic device can theoretically achieve a maximum of an efficiency of 85%. The ideal temperature for operating the photovoltaic cell was found to be 2544° K, with the ultimate efficiency of 85%.

[0008] The technology of solar thermophotovoltaic systems consists of two components. The solar concentrator and absorber / emitter are the first components, and the photovoltaic system to produce power is the second. The interaction between the sun and the absorber is the first one, with the solar-to-thermal efficiency determining the upper limit. Solar thermophotovoltaic devices uses the solar concentrator to concentrate sunlight on the solar absorber. The solar absorber basically absorbs all of the incoming concentrated light. It converts it to thermal radiation, whereas the selective thermal emitter, physically coupled to the absorber, emits thermal radiation toward a photovoltaic cell.

[0009] The second part is the energy transfer between the emitter and the photovoltaic cell, with the Carnot efficiency determining the upper limit. Because no direct sunlight reaches the photovoltaic cell in a solar thermophotovoltaic device, the efficiency of multiple intermediary energy conversion steps is critical to the performance of solar thermophotovoltaic device. The absorber converts optically concentrated sunlight into heat which is conducted to the emitter. The emitter thermally radiates towards the photovoltaic cell, where radiation finally excites charge carriers and generates electricity. The emitter should be optimized for a photovoltaic cell such that emitted photons have energy slightly higher than the bandgap because photons with energies less than the bandgap cannot produce electricity, and photons with very high energies would cause high-energy electrons to fall to the bandgap's edge, resulting in excess energy loss such as heat. So, the emittance of a spectrally selective emitter should be high for energy above the photovoltaic bandgap and low for energy below the bandgap. The emitter temperature should ideally be high enough that Planck's blackbody peak approaches the bandgap, as determined by Wien's displacement law, to excite enough thermal modes for considerable emission above the bandgap, as known from Lenert, A. et al., “A nanophotonic solar thermophotovoltaic device. Nature Nanotechnology”, 2014, 9(2). https: / / doi.org / 10.1038 / nnano.2013.286.

[0010] The emitter's high-temperature operation presents two significant challenges for successful solar thermophotovoltaic power conversion: efficiently collecting sunlight and maintaining spectral selectivity at high temperatures. However, effective sunlight collection in the absorber and spectrum control in the emitter is challenging at high working temperatures. The operating temperature is the most critical factor in deciding which kind of photovoltaic cell is to be used in a solar thermophotovoltaic device. The optimum efficiency of a solar thermophotovoltaic system occurs when the emitting surface's blackbody peak is near the photovoltaic cell's energy gap. As a result, the photovoltaic cell may absorb a large portion of the emitted heat energy, boosting the system's efficiency. Wien's displacement law determines the wavelength of a blackbody's peak emission at a specific temperature.

[0011] Because most of the emitted thermal radiation in practical thermophotovolatic systems (T < 2000 K) is in the near-to mid-infrared (IR) spectrum, it is commonly accepted that developing low bandgap thermophotovoltaic devices is the way to increase thermophotovoltaic efficiencies. Gallium antimonide (GaSb), indium gallium arsenide (InGaAs), and indium gallium arsenide antimonide (InGaAsSb) cells have all been used in experiments. Although GaAs, InGaAs, and InGaAsSb cells are excellent for thermophotovoltaic applications, their fabrication procedures are expensive and complex. On the other hand, for silicon photovoltaic cells in solar thermophotovoltaic devices, a high-temperature emitter is required. Silicon cells need a temperature of roughly 2500 K to function correctly, making them unsuitable for most solar thermophotovoltaic devices. Therefore, attempts to use silicon cells for solar thermophotovoltaic energy conversion systems have proven almost non-existent due to the larger bandgap energy of silicon, which requires higher operating temperatures for effective energy conversion. However, as commercial silicon photovoltaic cells are numerous, inexpensive, and commercially available in large sizes, they are attractive to be used for such systems.

[0012] It is therefore an object of the present invention to provide a solar thermophotovoltaic device capable of being used with large bandgap photovoltaic cells and with reduced manufacturing costs.

[0013] Summary of the invention

[0014] This object has been achieved by the solar thermophotovoltaic device according to claim 1 and the method for manufacturing a solar thermophotovoltaic device according to the further independent claim.

[0015] Further embodiments are indicated in the depending subclaims.

[0016] According to a first aspect a thermophotovoltaic device is provided, comprising: an absorber layer configured to absorb photonic radiation, particularly daylight, within a focal spot as a part of a surface of the absorber layer and convert it into heat energy; an emitter layer positioned adjacent to the absorber layer or integrally formed with it, configured to emit thermal radiation due to the heat energy via a surface of the emitter layer; at least one photovoltaic cell positioned to receive the emitted thermal radiation from the emitter layer and convert it into electrical energy using the photoelectric effect; a heat spreading layer embedded within the absorber layer and / or emitter layer configured to distribute heat energy lateral over the emitter surface. The above thermophotovoltaic device may comprise an optical concentrator, an absorber, an emitter, and a photovoltaic cell. The optical concentrator can be formed with optical components such as at least one of one or more optical lenses such as Fresnel lenses, a lens array, metasurface, a mirror, and one or more parabolic reflectors in trough, tower, and dish systems to concentrate / focus sunlight onto the focal spot of the absorber.

[0017] The absorber layer and emitter layer may form a single or combined element having a surface directed to the concentrator and an opposite surface directed to a photovoltaic cell for radiating thermal radiation onto the photovoltaic cell.

[0018] In operation, focussed sunlight is radiated onto the absorber surface. The focused sunlight heats the absorber. As the emitter is thermally closely coupled with the absorber, the absorber temperature is transferred to the emitter which emits thermal radiation onto a photovoltaic cell.

[0019] Depending on the material of the semiconductor of the photovoltaic cell, the temperature of the emitter, the size of the concentrator and the materials of absorber and emitter have to be carefully selected. Conventionally, it has been tried to use low bandgap materials, semiconductor materials, such as lll-V semiconductors, such as GaSb, InGaAs and InGaAsSb. Due to the significantly higher energy gap of a silicon semiconductor, higher absorber temperatures are required for effective energy conversion. Solar thermophotovoltaic devices based on silicon or other high bandgap materials (bandgap of more than 0,9 eV) have not been applied yet.

[0020] However, for high bandgap materials thermal radiation in a higher temperature range have to be produced. For silicon photovoltaic cells preferred temperatures are in a range of 2500°K which impose challenges to the absorber / emitter unit.

[0021] The solar concentrator usually focuses sunlight on a focal spot on the absorber. One issue which has been addressed by the above thermophotovoltaic device, is to the spread of the high temperature uniformly across the absorber and emitter surfaces. As the efficiency of the thermophotovoltaic device and therefore its power output are highly depending on the temperature of the components and the uniformity of the temperature, the above solar thermophotovoltaic device include measures to provide an increased heat spreading over the area of the absorber and emitter.

[0022] In conventional thermophotovoltaic devices, the concentrated sunlight typically heats a central focal spot on the surface of the absorber radiating energy towards the emitter and the photovoltaic cell. However, the heat energy also radiates laterally (in parallel to the surfaces) outward from this focal spot and the temperature drops significantly due to thermal diffusion. This results in a high temperature core surrounded by areas of progressively lower temperature to a sub-optimal thermal radiation and decreased overall efficiency.

[0023] Due to rapid thermal dissipation / high temperature gradient over the distance from the heat source of the focal spot, achieving and maintaining these high temperatures on the emitter over a lager area is difficult. Non-uniform temperature distribution reduces the efficiency and results in a lower power output as lower temperature areas do not significantly contribute to electric power generation in the photovoltaic cell.

[0024] According to the above solar thermophotovoltaic device, the absorber and emitter are configured with a heat spreading layer to provide an improved spreading of the heat energy from the focal spot of concentrated sunlight to laterally neighboring areas of the absorber / emitter.

[0025] It may be provided that the heat spreading layer includes an arrangement of microchannels configured to circulate fluid to maintain a uniform temperature distribution across the surfaces of the absorber and emitter layers.

[0026] Furthermore, the microchannels may be designed in a bifurcated, fractal-like, or branching pattern to enhance fluid distribution and thermal performance.

[0027] So, microchannels may be arranged in I embedded into the absorber / emitter layers of the thermophotovoltaic device. The spreading of the temperature of the radiated spot to neighboring areas of the absorber / emitter may be made by microchannels fabricated using techniques of silicon planar technology. A fluid circulating system may be integrated into the absorber layer and / or the emitter layer or fabricated separately and connected to the microchannels. The fluid circulating system may be configured to actively or passively provide a fluid circulation or fluid flow thereby transporting heat energy. The arrangement of microchannels itself can form the fluid circulating system as the fluid may be moved by convection. Particularly, the fluid circulating system may be implemented as a thermosyphon or as a high-temperature micro-pump, or by using electrohydrodynamics, electrofluid dynamics, or capillary effects.

[0028] By means of integrating microchannels into the heat spreading layer, and having a thermal fluid flown through these microchannels, heat may be absorbed from the central focal spot of concentrated light and evenly redistributed across the surface / lateral extension of the absorber / emitter. This allows to have a temperature distribution which is more uniform over the absorber / emitter layers. This allows maintaining a higher temperature over a larger area of the emitter / absorber increasing the efficiency of the thermal radiation irradiated from the surface of the emitter layer to the photovoltaic cell thereby improving the overall conversion efficiency of the photovoltaic cell.

[0029] A thermal fluid being capable to withstand a high temperature of up to 3000 K may be actively or passively circulated through these microchannels ensuring efficient heat transfer away from the irradiated focal spot.

[0030] Due to the heat radiation emitted by the emitter, the absorber / emitter arrangement has a temperature difference between the absorber and emitter, which can be significant. An arrangement of microchannels filled by an appropriate thermal fluid may be actively driven by convection caused by significant density and pressure differences due to the high temperature gradient. The high temperature creates a buoyancy force that propels the fluid with the less dense heated fluid rising and the denser cooler fluid descending.

[0031] For the active structure, the height of the microchannel may be more than 80% of the thickness or almost equal to the thickness of the entire structure, or it should be designed in a way that covers the entire structure efficiently. Basically, a significant component may be a micro-sized fluid circulating system regulating fluid flow through the microchannels. This system could be implemented by applying a thermosyphon, electrohydrodynamics, electrofluiddynamics, capillary action, or even a high-temperature micropump. Using a thermosyphon may be preferred as it allows to passively circulate the fluid inside the integrated absorber / emitter layer structure.

[0032] The performance key factor of such a circulating system effecting the fluid flow through the microchannels is the material selected for the thermal fluid. The fluid must remain stable and effective at these elevated temperatures. Possible materials which may withstand high temperatures may include molten metals, molten salts comprising sodium, potassium, and mercury.

[0033] According to another embodiment the heat spreading layer may include microchannels filled with a solid material, particularly including copper, having a higher thermal conductivity than the absorber layer and / or the emitter layer.

[0034] Additionally, or alternatively, the spreading of the temperature within the absorber / emitter layers may be made with a passive structure comprising microchannels filled with a solid, high thermal conductivity material such as graphite, graphene, graphene powder, or metals, such as copper, with enhanced response to conventional emitter / absorber units.

[0035] According to an alternative embodiment, a passive structure features a microchannel height that differs from the active structure. This design requires filling the microchannel with material processing high thermal conductivity which may be including graphite, graphene, copper, or other metals.

[0036] It may be provided that between the emitter layer and the at least one photovoltaic cell a vacuum is emptied to provide a vacuum.

[0037] Brief description of the drawings Embodiments are described in more detail in conjunction with the accompanying drawings in which:

[0038] Figure. 1 a schematic perspective view on a thermophotovoltaic device according to the present invention,

[0039] Figure 2 a cross-sectional view on an absorber / emitter structure to be applied in the thermophotovoltaic device of Figure 1 with microchannels to laterally convey heat energy for spreading the temperature;

[0040] Figures 3a to 3c top down cross-sectional views on an examplary absorber / emitter units to be applied in the thermophotovoltaic device of Figure 1 with microchannels to laterally convey heat energy over the full area of the surface the absorber layer for spreading the temperature.

[0041] Description of embodiments

[0042] Figure 1 shows a perspective view on a thermophotovoltaic device 1 which converts daylight radiation 2 into thermal radiation 3 to be transformed into electrical energy by means of a silicon photovoltaic cell 4.

[0043] The thermophotovoltaic device 1 comprises an optical concentrator 5 such as a Fresnel lens to concentrate sunlight onto a focal spot F. Other optical components may be used including a mirror, such as a parabolic reflector in trough, tower, and dish arrangement or any optical lens, or array of lenses and metasurfaces.

[0044] The optical concentrator 5 direct the concentrated sunlight onto a absorber / emitter unit 6 which comprises an absorber layer 61 and an emitter layer 62 which are closely attached or being formed in an integrated manner.

[0045] As shown in the cross-sectional view of the absorber / emitter unit 6 of Figure 2 and the plain view of a cross-section of the absorber / emitter unit 6 , the absorber layer 61 and the emitter layer 62 of the absorber / emitter unit 6 sandwich a heat spreading layer 63 of microchannels 64 or embed a layer of microchannels 64 extending over the lateral dimensions of the absorber / emitter unit 6.

[0046] Examples of the lateral spreading of the microchannels 64 are shown in Figures 3a-3c. Figure 3a shows a single layer of microchannels each extending in parallel to each other and parallel to the surface of the emitter layer 61. Figure 3b shows two layers of microchannels 61 wherein in each layer the microchannels extend in to each other and parallel to the surface of the emitter layer 61 , wherein the microchannels of the two layers extend perpendicular to each other. Figure 3c shows microchannels radially extending in parallel to the surface of the emitter layer 61.

[0047] The microchannels 64 may have a height of between 80 -95% of the total thickness of the absorber / emitter unit 6 particularly when a thermal fluid is used for circulating. The width of such microchannels may be 10-50 pm to allow convection of the thermal fluid.

[0048] The absorber layer 61 is preferably made of a metallic material with a high melting temperature, such as tungsten, ceramic or the like, to withstand the heat of the focal spot when sunlight is radiated thereon. The emitter layer 62 is preferably selected to have a high thermal conductivity such as silicon carbide, graphite, or the like.

[0049] To improve lateral spreading or propagation of heat energy from the focal spot onto which the sunlight is focused towards neighboring areas of the absorber / emitter unit 6, the heat spreading layer 63 may include an arrangement of microchannels 64 embedded in the absorber / emitter unit 6 in a sandwich manner.

[0050] The microchannels 64 may include a thermal fluid which is actively or passively circulated through the microchannels 64 to manage heat distribution from the focal spot onto the neighboring areas of the emitter layer 62.

[0051] Figure 3 schematically shows a possible arrangement of microchannels 64 to convey heated liquid away from the focal spot onto the neighboring areas of the absorber / emitter structure. The transportation of the liquid can be made by means of a thermosyphon, electrohydrodynamics., electrofluiddynamics, capillary action or a high-temperature micropump system. The thermosyphon could be promising as it could passively circulate the liquid inside the arrangement of microchannels 64 at high temperatures.

[0052] As the thermal fluid, it is essential that the material of the thermal liquid is stable at high temperatures of up to 2,600°K. The materials may include metals such as sodium and mercury or salts thereof and shall maintain a liquid state up to a high temperature range of several hundred degrees °K.

[0053] The microchannels 64 may also include a solid material with a high thermal conductivity having a higher thermal conductivity than the thermal conductivity of the material of the absorber layer 61 . Such a solid material may include metals such as copper to manage the distribution of heat from the focal spot onto the neighboring areas of the emitter layer 62.

[0054] Apart from comprising microchannels 64 the heat spreading 63 may be formed as a plain or smooth layer of the solid high thermal conductivity material.

[0055] The absorber / emitter unit 6 may be heated by the focused sunlight up to high temperatures of about 2,400 K. Due to the low thickness of the absorber / emitter unit 6 of between 200 pm and 800 pm, the thermal energy introduced into the absorber / emitter unit 6 by the focused sunlight is directly transferred to the emitter layer 62 below the focal spot F. Also, by means of the heat spreading layer 63 the heat energy is spread in lateral directions to areas outside of the focal spot F. The temperature is thus more uniformly spread / evenly distributed over the entire surface of the absorber / emitter unit 6.

[0056] Heat energy is irradiated by the emitter layer 62 as heat radiation from the outer surface of the emitter layer 62 onto the photovoltaic cell 4 which may be provided as a standard silicon photovoltaic cell or other high bandgap photovoltaic cell with a sensitivity of light in a bandwidth including the blackbody heat radiation emitted by the absorber / emitter unit 6.

[0057] The absorber / emitter unit 6 and the photovoltaic cell 4 may be stacked with a distance of between 50pm to 500 pm which is enclosed to accommodate a vaccuum so as to avoid convection losses and to ensure material stability. Particularly, the absorber / emitter unit 6 and the photovoltaic cell 4 may be hold in a frame which seals the edges of the absorber / emitter unit 6 and the photovoltaic cell 4.

[0058] The formation of the microchannels can be made with standard photolithographic technology by masking, etching, deep-etching, dry-etching to form the microchannels and close the microchannels to form the absorber layer 61. Before closing the microchannels 64 can be filled with the liquid thermal fluid or the solid material to produce the heat spreading layer 63.

[0059] The surface of the absorber layer 61 facing the optical concentrator 6 be structured with inverted pyramids or the like to reduce reflectivity to increase the absorption rate for any photon wavelength received.

Claims

Claims1. A thermophotovoltaic device, comprising: an absorber layer configured to absorb photonic radiation, particularly daylight, within a focal spot as a part of a surface of the absorber layer and convert it into heat energy; an emitter layer positioned adjacent to the absorber layer or integrally formed with it, configured to emit thermal radiation due to the heat energy via a surface of the emitter layer; at least one photovoltaic cell positioned to receive the emitted thermal radiation from the emitter layer and convert it into electrical energy using the photoelectric effect; a heat spreading layer embedded within the absorber layer and / or emitter layer configured to distribute heat energy lateral over the emitter surface.

2. The thermophotovoltaic device cell of claim 1, wherein the heat spreading layer include a high thermal conductivity material.

3. The thermophotovoltaic device cell of claim 1 or 2, wherein the heat spreading layer includes microchannels configured to circulate fluid to maintain a uniform temperature distribution across the surfaces of the absorber and emitter layers.

4. The thermophotovoltaic device of claim 3, wherein the microchannels are designed in a bifurcated, fractal-like, or branching pattern to enhance fluid distribution and thermal performance.

5. The thermophotovoltaic device of any of the claims 3 to 4, wherein a fluid circulating system is integrated into the absorber layer and / or the emitter layer or fabricated separately and connected to the microchannels.

6. The thermophotovoltaic device of claim 5, wherein the fluid circulating system is implemented as a thermosyphon or as a high-temperature micro-pump, or by using electrohydrodynamics, electrofluid dynamics, or capillary effects.

7. The thermophotovoltaic device of any of the claims 3 to 6, wherein the fluid is selected from one of a metal or a molten salt.

8. The thermophotovoltaic device cell of claim 1, wherein the heat spreading layer includes microchannels filled with a solid material, particularly including copper, having a higher thermal conductivity than the absorber layer and / or the emitter layer.

9. The thermophotovoltaic device of any of the claims 1 to 8, wherein the absorber layer, emitter layer, and microchannels at least one of graphite, silicon carbide (SiC), tungsten, and diamond-like carbon.

10. The thermophotovoltaic device of any of the claims 1 to 9, wherein between the emitter layer and the at least one photovoltaic cell a vacuum is emptied to provide a vacuum.

11. The thermophotovoltaic device of any of the claims 1 to 10, wherein an optical concentrator is provided to focus daylight onto the focal spot, wherein the optical concentrator includes at least one of one or more optical lenses, a lens array, a metasurface, a mirror, and one or more parabolic reflectors in trough, tower, and dish systems.

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