Near-field thermophotovoltaic system containing non-polar photonic crystal film

Abstract

The invention provides a near-field thermophotovoltaic system containing a non-polar photonic crystal film in order to adjust the radiation transmission spectrum distribution of the near-field thermophotovoltaic system, enable the radiation transmission spectrum distribution to be well matched with a semiconductor band gap and reduce the potential influence of phonon polaritons on the surface of a thermophotovoltaic cell at the same time. Relates to the technical field of thermophotovoltaics. According to the invention, the surface of the thermophotovoltaic cell is covered with a surface structure of a non-polar two-dimensional photonic crystal, and a nano grid grating is selected as a pattern of a silicon intermediate layer. Determining two key parameters, namely the cycle length and the filling rate which influence the uniformity of the nanostructure, and adjusting a thermal radiation transmission spectrum and realizing good matching between a radiation spectrum and a semiconductor cell band gap by adjusting the parameters. Undoped silicon is used as a photonic crystal medium material, and a non-polar intermediate layer is added between a polar medium and a vacuum gap, so that excitation of a surface mode is inhibited, and additional radiation absorption loss is reduced. The method is suitable for improving the output power and the conversion efficiency of the near-field thermophotovoltaic system.

Description

Technical Field

[0001] The invention relates to the technical field of thermophotovoltaic technology, and in particular to a near-field thermophotovoltaic system containing a non-polar photonic crystal film. Background Art

[0002] In modern industry, a large part of the energy consumption is released into the environment in the form of waste heat. The recycling of this part of waste heat will help improve the efficiency of energy use and reduce the consumption of other fossil energy sources to a certain extent. There are many ways to recover low-temperature waste heat, including heat-engine cycles based on working fluids, and solid-state power generation technologies such as thermoelectricity and thermophotovoltaics. The latter has the advantage of miniaturization because there are no moving parts and no noise is generated during operation. Thermoelectric technology uses the Seebeck effect to directly convert thermal energy into electrical energy. However, commonly used thermoelectric materials are often unstable at high operating temperatures. Thermophotovoltaic system is a solid-state energy conversion device that collects thermal radiation energy through the photovoltaic effect and converts it into electrical energy. It has a higher upper limit of operating temperature than thermoelectricity. Thermophotovoltaic system usually includes a high-temperature radiator and a low-temperature thermophotovoltaic cell, which are separated by a vacuum gap.

[0003] In the far field, heat can only be transferred between the radiator and the thermophotovoltaic cell through propagation waves, and the radiation heat transfer is limited by the blackbody limit radiation limit. If the distance is reduced to less than the characteristic thermal wavelength (about 10μm at room temperature), heat can be transferred not only through propagation wave radiation, but also through evanescent wave radiation, that is, a near-field thermophotovoltaic system is formed. In this case, the radiation heat transfer can even exceed the blackbody radiation limit by several orders of magnitude. The enhanced energy input leads to the realization of high power density of near-field thermophotovoltaics, which is expected to become an important way in the field of medium and low temperature waste heat recovery power generation.

[0004] The conversion efficiency of traditional far-field thermophotovoltaics has exceeded 44% so far. The output power of near-field thermophotovoltaics is very high, but the conversion efficiency is relatively low, at most about 14%. The low conversion efficiency will bring great pressure to the cooling of the system's thermophotovoltaic cells. For near-field thermophotovoltaics, the extreme nanogap makes it very difficult to maintain the temperature difference between the emitter and the absorber, which makes the radiator temperature extremely limited. Under this temperature configuration, only a small part of the radiation spectrum is distributed above the band gap. Only this part of the radiation transmission can be absorbed by the thermophotovoltaic cell and converted into electrical energy. The rest of the energy is converted into waste heat, which creates a problem of mismatch between the radiation spectrum and the band gap.

[0005] Another reason for low efficiency is the excitation of surface modes of polar semiconductor cells in near-field radiation transfer mode. III-V semiconductors are often used as photovoltaic cell materials in thermophotovoltaic systems because of their low bandgap characteristics. III-V semiconductors are polar media that support surface modes such as surface phonon polaritons, which will enhance heat transfer near the optical phonon frequency. However, the photon energy corresponding to this frequency is usually lower than the semiconductor bandgap energy. Therefore, this part of the radiation cannot be used by thermophotovoltaic cells. Summary of the invention

[0006] The present invention proposes a near-field thermophotovoltaic system containing a non-polar photonic crystal film in order to adjust the radiation transmission spectrum distribution of the near-field thermophotovoltaic system to form a better match with the semiconductor band gap and reduce the potential impact on the surface phonon polaritons of the thermophotovoltaic cell.

[0007] To achieve the above object, the present invention provides the following technical solutions:

[0008] The present invention provides a near-field thermophotovoltaic system containing a non-polar photonic crystal film, wherein the near-field thermophotovoltaic system comprises a semi-infinite radiator and a semi-infinite thermophotovoltaic cell;

[0009] The nonpolar medium is patterned over the surface of the semi-infinite thermophotovoltaic cell;

[0010] A vacuum gap distance d is set between the semi-infinite radiator and the non-polar medium.

[0011] Furthermore, the above non-polar dielectric material must meet the following requirements:

[0012] The real part of the dielectric constant is positive at all frequencies;

[0013] The medium is transparent at all frequencies and maintains near-field radiation enhancement.

[0014] Furthermore, the above-mentioned non-polar medium is realized by non-doped silicon.

[0015] Furthermore, the above-mentioned non-doped silicon pattern can be designed as a nanowire array, a two-dimensional square grating, a one-dimensional grating structure or other lateral periodic structures.

[0016] Furthermore, when the non-doped silicon is a nano-grid grating, the height of the non-doped silicon is h=300 nm.

[0017] Furthermore, the uniformity of the nano-square grating structure is determined according to the period length P of the square grating and the width a of the square grating.

[0018] Furthermore, the non-doped silicon filling rate f = (a / P) 2 ; and 0<f≤1.

[0019] Furthermore, when f=1, the nanogrid grating is a uniform silicon thin film structure.

[0020] Furthermore, the vacuum gap distance d is in the range of 200 nm to 10 μm, and the best effect is achieved when d is 200 nm.

[0021] Furthermore, the radiator material may be tungsten, silicon, graphite, GZO, ITO or other materials with high temperature stability;

[0022] The above-mentioned thermophotovoltaic cell material can be a III-V semiconductor, such as gallium antimonide GaSb, gallium indium arsenide In 0.53 Ga 0.47 As, indium arsenide InAs, or a material with a narrow band gap.

[0023] Furthermore, the above narrow band gap ranges from 0.172 to 0.726 eV.

[0024] The beneficial effects of the present invention are as follows:

[0025] The present invention proposes a near-field thermophotovoltaic system containing a non-polar photonic crystal film, by covering the surface of the thermophotovoltaic cell with a non-polar two-dimensional photonic crystal surface structure and selecting a nano-grid grating as the pattern of the silicon intermediate layer. The two key parameters that affect the uniformity of the nanostructure, the period length and the filling rate, are determined. By adjusting the grating parameters, the thermal radiation transmission spectrum is adjusted to achieve a good match between the radiation spectrum and the band gap of the semiconductor cell. At the same time, undoped silicon is used as the photonic crystal dielectric material, and a non-polar intermediate layer is added between the polar medium and the vacuum gap, which suppresses the excitation of the surface mode and reduces the additional radiation absorption loss.

[0026] Furthermore, compared with the prior art, the present invention introduces non-polar undoped silicon as an intermediate layer material, thereby suppressing harmful low-frequency surface mode radiation transmission and improving system performance parameters. At the same time, the introduced undoped silicon is nano-patterned to adjust the thermal radiation spectrum and overcome the spectrum mismatch problem of near-field thermal photovoltaic radiators under temperature constraints.

[0027] The present invention is suitable for improving the output power and conversion efficiency of a near-field thermal photovoltaic system. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 It is a structural schematic diagram of a near-field thermophotovoltaic system containing a non-polar photonic crystal film according to the present invention;

[0030] Figure 2 It is the spectral heat flux distribution of the near-field thermophotovoltaic system described in the present invention;

[0031] Figure 3 It is the calculation of spectral heat flow ratio under different period lengths P described in the present invention;

[0032] Figure 4 is the output power P of the present invention pv and conversion efficiency η. DETAILED DESCRIPTION

[0033] In the following description, specific details such as specific system structures and technologies are provided for the purpose of illustration rather than limitation, so as to provide a thorough understanding of the embodiments of the present application. However, it should be clear to those skilled in the art that the present application may also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to prevent unnecessary details from obstructing the description of the present application.

[0034] The specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several changes and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

[0035] Implementation method 1, see Figure 1 This embodiment is described. In order to adjust the radiation transmission spectral distribution of the near-field thermophotovoltaic system to form a better match with the semiconductor band gap, while reducing the potential impact on the surface phonon polaritons of the thermophotovoltaic cell, this embodiment proposes a near-field thermophotovoltaic system containing a non-polar photonic crystal film.

[0036] The specific structure of the near-field thermophotovoltaic system is as follows: Figure 1 As shown, it consists of a semi-infinite radiator and a semi-infinite thermophotovoltaic cell.

[0037] Among them, the radiator material can be tungsten, silicon, graphite, GZO, ITO or other materials with certain high-temperature stability. In this embodiment, tungsten is selected as the radiator material. At the same time, since the actual thickness of the radiator and the thermophotovoltaic cell is limited, but the thickness is large enough, it is equivalent to a semi-infinite structure in theoretical analysis to simplify the analysis process, that is: semi-infinite tungsten radiator.

[0038] Thermophotovoltaic cell materials are III-V semiconductors, such as gallium antimonide GaSb, gallium indium arsenide In 0.53 Ga 0.47 As, indium arsenide InAs or materials with narrow band gaps, the band gap range is 0.172-0.726eV, which is a polarized medium and can excite surface modes at specific optical frequencies. This embodiment takes gallium antimonide GaSb as an example, whose band gap is 0.726eV, and the corresponding angular frequency is 1.101×10 15 rad / s, the optical phonon frequency is 4.34×10 13 rad / s, that is: semi-infinite GaSb thermal photovoltaic cell.

[0039] In order to suppress harmful low-frequency surface mode radiation transmission, this embodiment covers the surface of the GaSb thermophotovoltaic cell with a non-polar medium. There is a vacuum gap distance d between the semi-infinite tungsten radiator and the non-polar medium.

[0040] Specifically, the selection of non-polar dielectric materials needs to meet the following conditions:

[0041] (1) The real part of the dielectric constant should be positive at all frequencies to prevent the excitation of surface modes.

[0042] (2) The medium is as transparent as possible at all frequencies to minimize absorption losses within the medium.

[0043] (3) Maintain the original near-field radiation enhancement effect.

[0044] In this embodiment, non-doped silicon is selected as the non-polar medium, such as Figure 1 As shown, this embodiment uses non-doped silicon as an example to demonstrate the effect of the non-polar intermediate layer. In terms of structural selection, nanowire (hole) arrays, two-dimensional square grid gratings, one-dimensional grating structures, and other transverse periodic structures can all adjust the radiation transmission spectrum, and can be selected based on the adjustment effect and processing difficulty.

[0045] Furthermore, although the non-polarized dielectric layer is covered on the surface of the thermophotovoltaic cell, the ideal conditions of the intermediate medium are met, the near-field enhancement of thermal radiation transfer is maintained, the excitation of the surface mode is prevented, and the absorption loss inside the intermediate medium is minimized. However, it is also necessary to enhance the flexibility of radiation spectrum regulation to achieve an ideal spectral heat flux distribution. Therefore, this embodiment takes a nano-grid grating as an example, and its height h = 300nm. By adjusting the period length P of the grid grating and the width a of the grid grating, the uniformity of the nano-grid grating structure is adjusted.

[0046] Furthermore, this embodiment also introduces another supplementary parameter: filling rate f (i.e., the volume ratio of silicon in the middle layer), the specific formula is: f = (a / P) 2The filling rate f ranges from 0 (not included) to 1 (inclusive), and in particular, when f=1, it is a uniform silicon thin film structure.

[0047] The vacuum gap distance d between the tungsten radiator and the silicon upper surface is within 10μm, that is, the near field range, so it is designed to be 200nm~10μm; in particular, when d is 200nm, the effect is better. Among the above parameters, the filling rate f, the period length P and the width a of the square grating are only applicable to the case of covering the silicon grating structure, the height h of the undoped silicon layer is applicable to the case of covering the silicon grating structure and the thin film, and the spacing d is for all cases.

[0048] Implementation Method 2: See Figures 2 to 4 This embodiment is described, and this embodiment verifies and illustrates the role of the silicon square grating layer in adjusting the radiation spectrum and suppressing the transmission of harmful surface modes;

[0049] This embodiment calculates the spectral heat flux distribution and compares the results of the original flat plate structure without silicon covering and the case where it is covered with a silicon film.

[0050] The thickness of the silicon film and the silicon square grating is set to h = 300nm, and the vacuum spacing of the three structures is kept at d = 200nm. Figure 2 As shown. Figure 2 It can be seen that the frequency coverage range of the left half of the figure is 4.2~4.5×10 13 rad / s. Without silicon covering, the spectral heat flux is about 4.34×10 13 There is a peak at the frequency of rad / s, which is caused by the surface mode of GaSb. The spectral heat flow caused by the surface mode is suppressed by covering the silicon film and silicon square grating layer. Figure 2 The frequency coverage range in the right half of the figure is 0.045~2.5×10 15 rad / s, the dotted line indicates the position of the bandgap corresponding to the frequency, and only part of the radiation transmission on the right side of the dotted line can be utilized. Compared with the case without silicon coverage, the radiation flux covered with silicon film is slightly reduced below the bandgap, and there is almost no change above the bandgap, indicating that it can maintain near-field enhancement, but the effect of adjusting the radiation spectrum is limited. In contrast, the coverage of silicon square grating layer significantly suppresses the absorption of radiation energy below the bandgap. At the same time, the selective enhancement above the bandgap is maintained, which is conducive to improving the conversion efficiency of TPV (thermal photovoltaic system).

[0051] Figure 3The spectral heat flow ratio under different period lengths is shown, that is, the ratio of the case with silicon grating coverage to the case without silicon coverage, under the conditions of grating height h = 300nm and filling rate f = 0.7. The vertical dotted line indicates the position of the frequency corresponding to the band gap, and only part of the radiation transmission on the right side of the dotted line can be utilized. The horizontal dotted line represents the case where the ratio is 1. Above 1 means enhanced radiation transmission, and below 1 means suppressed radiation transmission. Calculations show that covering the surface of the thermophotovoltaic cell with a silicon grating structure can enhance radiative heat transmission in the available high-frequency region while suppressing unavailable low-frequency photons, thereby playing a role in regulating the spectrum. However, as the period length increases, this effect tends to weaken. The period length P is selected as 200nm, 400nm, and 600nm. Under the three period lengths, P = 200nm has a better effect in regulating the spectrum.

[0052] Furthermore, the power density and conversion efficiency of the near-field thermophotovoltaic system were calculated under the condition of an emitter temperature of 1400K, which is the temperature that can be achieved in the current experimental research of near-field thermophotovoltaic systems. Figure 4 As shown, the data and its corresponding coordinate axes are represented by the same color. The filling rate f is used as an independent variable, and the selected values ​​are 0.5, 0.7, 0.8, 0.9, and 1. The other parameters remain unchanged, including the period length P = 200nm and the grating height h = 300nm. The calculation results of the original flat plate structure without silicon coverage are represented by the dotted line. The covered grating structure significantly improves the output power and conversion efficiency of the near-field thermophotovoltaic system. Specifically, in the case of the original flat plate structure without silicon coverage, the output power is 0.954×10 4 W / m 2 , the conversion efficiency is 14.8%. When the surface of the thermal photovoltaic cell is covered with silicon light, the output power reaches 1.094×10 4 W / m 2 , the conversion efficiency is 21.1%. Compared with the design of heat exchange structure between plates without grating intermediate layer, the output power is increased by 14.7%, and the conversion efficiency is increased by more than 42%. When f = 0.5, the output power is 1.091×10 4 W / m 2 , the conversion efficiency η = 23.9%. Compared with the flat plate heat exchange structure design without a grating intermediate layer, the output power is increased by 14.4% and the conversion efficiency is increased by more than 61%.

[0053] In summary, the near-field thermophotovoltaic system containing a non-polar photonic crystal film proposed in the present invention covers the surface structure of a non-polar two-dimensional photonic crystal on the surface of gallium antimonide, and selects a nano-grid grating as the pattern of the silicon intermediate layer. The harmful low-frequency surface mode radiation transmission is suppressed, and the system performance parameters are improved. At the same time, the introduced undoped silicon is nano-patterned to adjust the thermal radiation spectrum and overcome the spectrum mismatch problem of the near-field thermophotovoltaic radiator under temperature limitation. The conversion efficiency of the near-field thermophotovoltaic system is increased by 61%.

[0054] It should be understood that when used in this specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or groups thereof.

[0055] The above description is only the implementation mode of the present invention and is not limited to the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the scope of the claims of the present invention.

Claims

1. A near-field thermophotovoltaic system containing a non-polar photonic crystal film, characterized in that: Includes semi-infinite radiators and semi-infinite thermophotovoltaic cells; The nonpolar medium is patterned over the surface of the semi-infinite thermophotovoltaic cell; A vacuum gap distance d is set between the semi-infinite radiator and the non-polar medium.

2. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 1, characterized in that: Non-polar dielectric materials must meet the following requirements: The real part of the dielectric constant is positive at all frequencies; The medium is transparent at all frequencies and maintains near-field radiation enhancement.

3. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 2, characterized in that: The non-polar medium is realized by non-doped silicon; the non-doped silicon pattern can be designed as a nanowire array, a two-dimensional square grating, a one-dimensional grating structure or other lateral periodic structures.

4. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 3, characterized in that: When the non-doped silicon is a nano-grid grating, the height of the non-doped silicon is h=300 nm.

5. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 4, characterized in that: The uniformity of the nano-square grating structure can be determined according to the period length P of the square grating and the width a of the square grating.

6. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 5, characterized in that: Undoped silicon filling rate f = (a / P) 2 ; and 0<f≤1.

7. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 6, characterized in that: When f=1, the nanosquare grating is a uniform silicon thin film structure.

8. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 1, characterized in that: The vacuum gap distance d ranges from 200 nm to 10 μm.

9. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 1, characterized in that: The radiator material can be tungsten, silicon, graphite, GZO, ITO or other materials with high temperature stability; Thermophotovoltaic cell materials can be III-V semiconductors, such as gallium antimonide GaSb, gallium indium arsenide In 0.53 Ga 0.47 As, indium arsenide InAs, or a material with a narrow band gap.

10. A near-field thermophotovoltaic system containing a non-polar photonic crystal film according to claim 9, characterized in that: The narrow band gap ranges from 0.172-0.726 eV.

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