Patent Application
20150155413
I. Field of the Disclosure
The present disclosure relates to an apparatus and method for generating electric power, and, in particular, generating electric power from electromagnetic radiation energy using a thermoelectric generator.
2. Description of the Related Art
Many solid state electrical devices, such as photovoltaic cells or photoelectric cells or solar cells, are already used for generating electrical energy from incident solar radiation in the visible or near visible spectrum. Although photovoltaic cells are popular solution for converting solar energy to electrical energy, they are also expensive (on a cost per watt generated basis). The expensive nature of photovoltaic cells ma be traced back to complex fabrication processes, high cost of production, space constraints, efficiency, material costs, etc. Furthermore, common photovoltaic cells absorb a narrow band of optical electromagnetic radiation instead of the entire solar electromagnetic spectrum that reaches the surface of the Earth. Advanced photovoltaic cells, such as triple junction solar cells, have even more complicated fabrication processes and resulting higher costs. What is needed is an apparatus designed to capture a broader range of frequencies without adding more complexity to the fabrication process of the light to electrical energy converting apparatus.
In aspects, the present disclosure is related to at apparatus and method for generating electric power, and, in particular, generating electric power from electromagnetic radiation using a thermoelectric generator. In some aspects, the present disclosure is related to generating electric power using a selected wavelength absorber with the thermoelectric generator.
One embodiment according to the present disclosure includes an apparatus for generating electric power from electromagnetic radiation, the apparatus comprising: a thermoelectric gene r, the thermoelectric generator having a hot side and a cold side; and an electromagnetic radiation absorber in thermal communication with the hot side of the thermoelectric generator and configured to convert electromagnetic energy into beat energy,
The electromagnetic radiation absorber may have high absorbance and low emittance over an operating temperature range of the thermoelectric generator. The electromagnetic radiation absorber may be configured to absorb electromagnetic radiation in the visible spectrum. The electromagnetic radiation absorber may be configured to have low emittance of electromagnetic radiation in the infra-red spectrum. The electromagnetic radiation absorber may comprise: a plurality of absorber layers; and a plurality of dielectric layers, wherein the absorber layers and the dielectric layers alternate in placement. The absorber layers may comprise a titanium dioxide layer and a magnesium oxide layer. The dielectric layers may comprise molybdenum. The absorber and dielectric layers may be formed into a pyramidal shape, and the pyramidal shape may be dimensioned based on a selected range of electromagnetic radiation that is to be absorbed.
The apparatus may also include a housing, wherein the thermoelectric generator and the electromagnetic radiation absorber are disposed in the housing, and wherein the housing is transparent to a selected range of electromagnetic radiation on a side of the housing that is between an electromagnetic radiation source and the electromagnetic radiation absorber. The housing may be configured maintain to a vacuum or be filled with aerogel.
The thermoelectric generator comprises at least one thermocouple. The at least one thermocouple may include at least one n-type thermoelement in thermal communication with the electromagnetic radiation absorber; a first substrate layer in thermal communication with the at least one n-type thermoelement; at least one p-type thermoelement in thermal communication with the electromagnetic radiation absorber; a second substrate layer in thermal communication with the at least one p-type thermoelement, and a foil layer in thermal communication with the first substrate layer and the second substrate layer. An optional first radiation shield may be disposed between the electromagnetic radiation absorber and the thermoelement. Each thermoelement may have an optional metal substrate layer disposed between the thermoelement and the electromagnetic radiation absorber. The thermocouple may also include an n-side second radiation shield disposed between the at least one n-type thermoelement and the first substrate layer; and a p-side second radiation shield disposed between the at least one p-type thermoelement and the second substrate layer. The foil layer may be an anodized metal. The foil layer may have a thermal expansion coefficient substantially similar to the thermal expansion coefficient of the housing. The foil layer may be configured to give structural support to the thermocouple,
Each of the thermoelements may include a constricted contact; a diffusion barrier disposed on the constricted contact, a lower electrical contact disposed on the first diffusion barrier; a plurality of thin-film thermoelectric layers (n-type or p-type depending on the thermoelement) in thermal communication with the first metal substrate; and an upper electrical contact disposed between the plurality of n-type thin-film thermoelectric layers and the first metal substrate. The electrical contacts may be high power factor electrodes. The n-type layers may include one or more of: Bi2Te2SSe0.2. PbTe, AgP18SbTe20, PbTe/SrTe—Na, Ba0.08Yb0.09Co4Sb12, Mg2Si0.4Sn0.6, TiNiSn, SrTiO3, P-doped Si0.8Ge0.2, and La3Te4. The p-type layers may include one or more of: Bi0.5Sb1.5Te3, Zn4Sb3, CeFe3.5Co0.5Sb12, Yb14MnSb11, MnSi1.73, NaCo2O4, B-doped Si, and B-doped Si0.8Ge0.2.
Another embodiment according to the present disclosure includes a method of convening electromagnetic radiation to heat energy, the method comprising the steps of: receiving the electromagnetic radiation with an apparatus, the apparatus comprising: a thermoelectric generator, the thermoelectric generator having a hot side and a cold side; and an electromagnetic radiation absorber in thermal communication with the hot side and configured to convert electromagnetic energy into heat energy. The method may also include one or more steps of: it concentrating the electromagnetic radiation on the electromagnetic radiation absorber and ii) redirecting the electromagnetic radiation from an electromagnetic source on to the electromagnetic radiation absorber.
Another embodiment according to the present disclosure includes a method of manufacturing an electromagnetic radiation driven thermoelectric generator, the method comprising the steps of: forming an electromagnetic radiation absorber; and disposing the electromagnetic radiation absorber in thermal communication with a hot side of a thermoelectric generator. The forming step may include: depositing a silicon dioxide layer on a silicon substrate; removing a part of the silicon dioxide layer to expose the silicon substrate; forming trenches in the silicon substrate; removing a remainder of the silicon dioxide layer from the silicon substrate; depositing a barrier layer on the silicon substrate; depositing alternating layers of electromagnetic absorber material and dielectric material on the barrier layer; depositing a nickel layer on the alternating layers; thinning the silicon substrate; and removing the barrier layer from the alternating layers.
Examples of the more important features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood and in order that the contributions they represent to the art may be appreciated. There are, of course, additional features of the disclosure that will be described hereinafter and which will form the subject of the claims appended hereto.
For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein:
Generally, the present disclosure relates to an apparatus and method for generating electric power, and, in particular, generating electric power from electromagnetic radiation using a thermoelectric generator with a selective wavelength absorber. In some aspects, the present disclosure is related to generating, electric power using incident solar radiation. The, present disclosure is susceptible to embodiments of different forms. They are shown in the drawings, and herein will be described m detail, specific embodiments of the present disclosure with the understanding, that the present disclosure is to be considered an exemplification of the principles of the present disclosure and is not intended to limit the present disclosure to that illustrated and described herein.
The thermoelectric generators (TEG) may be a cost effective solution (e.g., low cost incurred per watt of power generation) for converting incident solar radiation energy to electric energy because of simpler device structures when compared to photovoltaic cells. Some TEGs may be fabricated by thin film wafer-based manufacturing techniques to reduce cost. TEGs may be combined with efficient electromagnetic radiation absorbers that collect energy from the solar spectrum, including sections of the spectrum that are typically not captured by photovoltaic solar cells.
Generally, the conversion of incident solar radiation into electrical energy is a two step process. First, the solar radiation or light is convened to heat. Second, the heat energy is converted to electrical energy by the TEG.
To enhance the performance of the capture of incident solar radiation on the hot side of the TEG, the incident solar radiation may be focused or captured through the use of radiation absorbers, optical concentrators, and thermal concentrators. The efficient capture and conversion of solar radiation to heat on the hot side of the TEG may increase the efficiency of the overall system.
Generally, the power output of the TEG is related to the design of the TEG and the temperature differential between the hot and cold sides of the. TEG. The heat to electrical energy efficiency of a TEG is generally calculated by:
where ηh denotes the efficiency of the solar TEG, Th is temperature of a hot side of the solar TEG, Tc is the temperature of a cold side of the solar TEG, Tm is the average temperature across the solar TEG, and ZTm is the figure of merit of the thermoelectric materials in the thermoelements. From equation (1), it may be observed that the efficiency of the solar thermoelectric generators depends On temperature differential and the figure of merit in some embodiments, the TEG may be configured to operate effectively with a temperature differential of 300 degrees Celsius or more. Additionally., a solar thermoelectric generator may also use thermoelectric devices with high figure of merit, i.e. ZTm>1.
The selective wavelength absorber 130 may be configured to absorb electromagnetic radiation hum the Sun or other electromagnetic sources that have color temperatures in the visible light spectrum (typically the Sun has a color temperature around 6000K). The selective wavelength absorber 130 may also be configured to have a low emittance of light in the infrared spectrum, such as in the color temperature range of 500-800K. Thus, the incoming photons are converted to heat that is transmitted to the thermocouple 115, ii) reflected to another part of the selective wavelength absorber 130 or into free space, or iii) reemitted as less energetic, photons (infrared).
The metal substrates 120 (when present) may be configured to conduct heat from the selective wavelength absorber 130 to the thermoelements 110. The metal substrates 120 are both electrically and thermally conductive, however, the metal substrates 120 are not limited so solely metal materials. In some embodiments, the metal substrates 120 may include, but are not limited to, composite structures with metal layers such as copper or tungsten bonded over ceramics. In some embodiments, the metal substrates 120 may be configured as thermal contractors to focus the heat energy on the thermoelements 110.
A primary radiation shield 140 may be disposed between the selective wavelength absorber 130 and the metal substrates 120. The primary radiation shield 140 may be configured to reduce radiation loss from the backside of the selective wavelength absorber 130. In some embodiments, inclusion of the one or both of the primary radiation shield 140 and the metal substrates 120 may be optional, so long as the thermoelements 110 remain in thermal communication with the selective wavelength absorber 130.
The thermoelements 110 and the metal substrates 120 may be surrounded by a vacuum 150. The housing 105 may be configured to maintain the vacuum 150. Part of the evacuated housing 105 may be an optional lens 170 which may be disposed between incoming electromagnetic radiation 180 and the selective wavelength absorber 130. The lens 170 may be transparent to the incoming electromagnetic radiation 180 on the selective wavelength absorber 130. In some embodiments, the lens 170 may also be configured to concentrate the incoming electromagnetic radiation 180 on the selective wavelength absorber 130. In some embodiments, the lens 170 may include one or more of i) a parabolic trough, ii) mirrors, and iii) a Fresnel lens. In some embodiments, the concentration of the incoming electromagnetic radiation 180 may be achieved using compound parabolic-concentrators (not shown).
The thermoelements 110 may be disposed on a set of secondary radiation shields 160. The secondary radiation shields 160 may include a secondary radiation shield 163 associated with the n-type thermoelements 113 and a secondary radiation shield 167 associated with the p-type thermoelements 117. In some embodiments, inclusion of one or both of the radiation shields 140, 160 may be optional In some embodiments, the radiation shields 140, 160 may be used. Then the operating temperature of the selective wavelength absorber 130 is about 200 degrees Celsius and higher. The radiation shields 140160 may be configured to prevent radiative heat transfer losses. In some embodiments, the radiation shields 140, 160 may be made of gold and/or platinum. The use of gold and/or platinum as the radiation shields 140,160 is exemplary and illustrative only,as other thermally conductive, low emissivity materials may be used as would be understood by a person of ordinary skill in the art with the benefit of the present disclosure. In some embodiments, the primary radiation shield. 140 may be suitably conductive so as to render inclusion of the metal substrates 120 optional. The radiation shields 140, 160 may have low emissivity. In some embodiments, both sides of the radiation shields 140, 160 may be polished to further lower its emissivity.
The substrate layers 190 may includes a substrate layer 193 in thermal communication with one or more n-type thermoelements 113 and a substrate layer 197 in thermal communication with one or more p-type thermoelements 117. The substrate layers 190 may be electrically and thermally conductive. In some embodiments, the substrate layers 190 may be of the same material as metal substrate layers 120. Each of the secondary radiation shields 163, 167 may be configured to allow their respective thermoelements 113, 117 to he in thermal contact with their respective substrate layers 193, 197. The secondary radiation shields 163, 167 may be disposed between their respective thermoelements 113, 117 and their respective substrate layer 193, 197. The substrate layers 190 may be disposed on a foil layer 195. The foil layer 195 may be made of a material that is thermally conductive and electrically insulating. The foil layer 195 may be made of or include an anodized aluminum foil. The use of anodized aluminum for the foil layer 195 is exemplary and illustrative only, as other suitable materials, such as anodized nickel and anodized tungsten, may be used as well. In some embodiments, the foil layer 195 may be configure(to have a thermal expansion coefficient that is substantially identical to the e thermal expansion coefficient of the housing 105.
Each thermoelectric layer 100a, 200b, 200c, 200d may be separated from the other by a phonon blocking layer 160a, 260b, 260c. The phonon blocking layers 269a, 260b, 260c (collectively 260) are configured to reduce heat conduction between the thermoelectric layers 200a, 200b, 200c, 200d via phonon transport. The phonon blocking layers 260a, 260b, 260c may include thin layers of metals or oxides disposed between the thermoelectric layers 200. The phonon blocking layers 260 may reduce the heat conduction is phonon transport m the thermoelement layers 200 without increasing the electrical resistance of the thermoelement layers 200. The electronic transport across the phonon blocking layers 260 may occur by tunneling. Since the speed of propagation of an acoustic phonon is much lower in liquids than in solids, low melting point metals (e.g. tin, indium) are suitable phonon blocking layers. In some embodiments, the phonon blocking may be enhanced when the thermoelement 110 is operating at temperatures close to the melting temperature of the phonon blocking layer material. The phonon blocking layers 260 may be made of, but are not limited to, one or more of: 1) titanium,) ii) titanium tungsten, iii) gallium, iv) indium, v) tin, and s aluminum oxide.
The constricted contacts 210 are electrically and thermally conducting structures of geometric dimensions much smaller than the thickness of the metal substrate 120. The constricted contacts 210 are typically cylindrical in shape with diameters of about 50 microns or less. The constricted contacts 210 may be configured to control the electrical and thermal resistance of the thermoelement 110.
The diffusion barriers 220 may be configured to reduce or eliminate the diffusion of metals constituting the constricted contact 210 into the thermoelectric layers 200. Exemplary diffusion barrier materials may include, but are not limited to one or more of i) tantalum, ii) tantalum nitride, iii) titanium, iv) titanium nitride, v) titanium tungsten, and vi) zirconium.
A first electrode 240 may be disposed between the metal substrate 120 and the thermoelectric layers 200. A second electrode 250 may be disposed between the thermoelectric layers 200 and the diffusion barrier 220. The electrodes 240, 250 may be made of as high power factor material. The power factor is expressed as S2σ, where S is the Seebeck coefficient and σ is the electrical conductivity for the material. In some embodiments, the electrodes 240, 250 may have power factors of about or greater than 0.01 W/m-K2. A set of exemplary high power factor materials for use as electrodes 240, 250 is shown in Table 2.
In some embodiments, the thermoelectric layers 200 may include one or more thin-film layers. A part of the thermoelectric layers 290 may be, optionally, formed into a hemisphere 205 around part: of the diffusion barrier 220. This hemisphere 205 may increase heat spreading along the surface of the layers 200. Each of the layers 200a, 200b, 200c, 200d may have similar or different thicknesses and may operate in different temperature ranges. For example, the innermost thermoelectric layer 100d, closest to the illusion barrier 220, may have a temperature range of 200 degrees C. to 50 degrees C., where the hot side is at 200 degrees C. and cold side is at 50 degrees C. The outermost layer 100a, which is closest to the metal substrate 120, may have a temperature range of about 650 degrees C. to about 400 degrees C., with the hot side of the outermost layer being at about 650 degrees C. and the cold side of the outermost layer 100a being at about 400 degrees C. The plurality of thermoelectric layers 200 may comprise two or more layers, and the temperature ranges and thicknesses of the thermoelectric layers 200 may be varied, as would be understood by a person of ordinary skill in the art with the benefit of the present disclosure.
The number of thermoelectric layers and the number of phonon barriers can vary with the desired power generation level per thermoelement 110. The thermoelectric layer thickness may depend on the electron-phonon thermolization length and the nature of material grain growth. Exemplary thermoelectric layer thicknesses may be but are not limited to is range of 0-500 nanometers. In some embodiments, one or more of the thermoelectric layers 200 may have sub-layers. Table 3 shows characteristics of an exemplary set of thermoelectric layers with thicknesses for a three-layer embodiment of a thermoelement.
Some exemplary materials that may be used as the thermoelectric layers 200a, 200b, 200c, 200d include intrinsically disordered tellurides such as LAST (AgPb18SbTe20), and antimonides, such as β-Zn4Sb3 have shown reduced mean free paths for phonons and ZT>18. At higher temperatures (400-700 degrees C.), the filled skutterudites such as Ba0.08Yb0.09Co4Sb12, CeFe3.5Co0.5Sb12) and clathrates (such as Ba8Ga16Ge30) with rattling weakly-bound atoms, polar zintl phases (such as Yb14MnSb11), semiconducting oxides (such as NaCo2O4), and metal oxides (such as SrTiO3) with complex structures and increased optical phonon modes, have varied degree of performance with ZTs>1. Rattling refers to a property of atoms in a material where the atoms are weakly bound within a lattice cage. Rattling atoms may have modes, such as low frequency modes, where they are more efficient at scattering acoustic phonons, resulting in lower thermal conductivity.
In some embodiments, the thermoelectric layers 200a, 200b, 200c, 200d may be deposited using a combination of Physical Vapor Deposition (PVD) sputtering and Atomic Layer Deposition (ALD)/Chemical Vapor Deposition (CVD) techniques. The phonon blocking layers may be deposited using ALL) or CVD techniques.
Aerogels are synthetic porous materials derived from alcogels, where the liquid component of the gel is replaced by air through supercritical drying. Silica aerogels (prepared b hydrolysis and condensation of methanol diluted TMOS) are the most common aerogels that consist of nanostructured Silicon dioxide network with a porosity of up to 99%. In terms of space occupied, the interconnected backbone can be as little as 0.01% of the structure, with the remainder being comprised of air. Due to its extraordinary small pore sizes (varying between 50 and 100 nm) and high porosity, aerogels achieve their structural properties (ultra low density 3 kgm-3, high compression strength up to about 3 bar, but very low tensile stress). Aerogels may also demonstrate thermal (thermal conductivity ˜0.0129 W m-1 K-1 is much lower than that of still air ˜0.024 W m-1K-1) and optical properties (˜95% transparency in the visible region). Because of the ultra-low thermal conductivity and high transmittance of daylight, aerogels are considered as highly suitable thermal insulation materials for windows and solar collectors. Pure silica aerogels, though suitable for low temperature insulating applications, are transparent to radiation wavelengths between 3 to 8 micrometers, where radiative heat transfer may be significant.
In some embodiments, mineral powders, such as titanium dioxide, silicon carbide, and carbon black may be incorporated into the silicon dioxide backbone of the silica aerogel improve resistance to structural deformation and cracking due to high temperatures. The use of silicon dioxide as the backbone material is exemplary and illustrative only, as other backbone materials may be used, such as ZrSiO4. In some embodiments, a small amount (about 20% by weight or less) of carbon powder may be added to the aerogel backbone to increase elasticity without decreasing or only nominally decreasing hardness. The mean extinction coefficient, which characterizes radiative attenuation, of silica aerogel with 20 wt % carbon is about 100 m2/kg. By comparison, pure silica aerogel has a mean extinction coefficient of about 20 m2/kg; silica aerogel with 20 wt % of silicon carbide ha a mean extinction coeffient of about 52.5 m2/kg; and silica aerogel with 40 wt % of ZrSiO4 has a mean extinction coefficient of about 21.4 m2/kg. In some embodiments, multiple aerogel layers of different types may be combined to capitalize on their properties (pure silica aerogel is highly optically transparent, silica aerogel with 20 wt % carbon has a high mean extinction coefficient, silica aerogel with 20 wt % silicon carbide has high thermal stability).
These layers may include absorption layers and dielectric layers. The dielectric layers may act as optical spacers. A first dielectric layer 620 of the absorber 610 ma disposed on a first absorber layer 630. The first absorber layer 630 may be disposed on a second dielectric layer 640, which may be disposed on a second absorber layer 650. These layers 620, 639, 640, 650 of absorbers and dielectrics may alternate for as many layers as is desired. Typical embodiments may include 4-10 layers. Each of the layers 620630, 640, 650 may vary in thickness, usually between 5 and 100 nanometers. The number and thickness of the layers 620, 630, 640, 650 may provide flexibility in maximizing after of the absorber 610 for a desired operation temperature, where α is the absorptance and is the emittance of the absorber 610. The interference of photons between these layers 620, 630, 640, 650 may result in enhanced absorption in the desired spectral range.
The dielectric layers 620, 640 may be made of a suitable material with a high dielectric cons ant, a high refractive index, and good thermal stability against long term oxidation. The first dielectric layer 620 may be made of titanium dioxide, and the second dielectric layer 640 may be made of magnesium oxide. The use of titanium dioxide and magnesium dioxide as the dielectric layers, and their respective order, are exemplary and illustrative only, as other suitable materials, such as i) titanium aluminum nitride, ii) titanium aluminum ox nitride, iii) TiNOX, iv) metal-dielectric composites (i.e. nanometer-sized metal particles embedded in a ceramic host matrix, including, Pt—Al2O3, Ni—Al2O3 can be used as selective absorber coatings), and v) other transition metal oxides, may be used its understood by a person of ordinary skill in the art with the benefit of the present disclosure.
The absorber layers 630, 650 may be comprised of a material selected for thermal stability at about 700 degrees K, good infra-red wavelength reflectance and visible wavelength absorbance. In some embodiments, the absorber layers 630, 650 may be made of molybdenum. In a multilayer metal-dielectric stack as shown in
While the disclosure has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying, out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.
This application claims the benefit of Provisional U.S. Patent Application No. 61/647,435 filed May 15, 2012) which application is hereby incorporated by reference in its entirety,
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US13/41132 | 5/15/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61647435 | May 2012 | US |
