A proposed thermophotovoltaic solar energy conversion system

A solar-electric system is proposed and discussed. This system uses concentrated mirrors focusing on a thermophotovoltaic (TPV) converter. Within the TPV converter the concentrated sunlight heats a refractory radiator. A silicon photovoltaic cell faces the radiator, receives incandescent radiation from it, and converts this radiation into electricity.     

Practical thermophotovoltaic generators

For use in gas-fired thermophotovoltaic systems, a selective emitter made from Yb₂O₃ foam ceramic has been developed. This foam ceramic is mechanically stable, and FTIR spectroscopy showed that 10% of the radiation power emitted by the foam can be converted by Si photocells. The thermal and thermal-shock stability of Yb₂O₃ foam ceramic was analyzed. The foam passed 200 heating/cooling cycles without major damage. Tubes were manufactured from this material and tested in a thermophotovoltaic demonstration system. An electrical power of 86 W was achieved at a thermal power of 16 kW. Using a simulation model, the potential efficiency of a thermophotovoltaic system based on our technology applied for the conversion of concentrated solar radiation was estimated.     

旁瓣对消系统的对消比上限分析

构造了一个理想的旁瓣对消系统和一个理想的信号环境,分析了旁瓣对消系统的对消比上限,推导出了对消比上限的精确表达式及近似表达式,得知对消比的上限取决于辅助通道的干噪比和辅助通道的数量.最后,给出了与理论分析相一致的仿真结果.     

Infrared materials for thermophotovoltaic applications

Thermophotovoltaic generation of electricity is attracting renewed attention due to recent advances in low bandgap (0.5–0.7 eV) III-V semiconductors. The use of these devices in a number of applications has been reviewed in a number of publications.^1–4 Two potential low-bandgap diode materials are In_xGa_1−xAs_ySb_1−y and In_xGa_1−xAs. The performance of these devices are comparable (quantum efficiency, open circuit voltage, fill factor) despite the latter’s long-term development for optoelectronics. For an 1100°C blackbody, nominally 0.55 eV devices at 25°C exhibit average photon-weighted internal quantum efficiencies of 70–80%, open circuit voltage factors of 60–65%, and fill factors of 65–70%. Equally important as the energy conversion device is the spectral control filter that effectively transmits above bandgap radiation into the diode and reflects the below bandgap radiation back to the radiator. Recent developments in spectral control technology, including InGaAs plasma filters and nonabsorbing interference filters are presented. Current tandem filters exhibit spectral utilization factors of ∼65% for an 1100°C blackbody.     

激光辐照下光电电池的极限能量转化效率研究

采用激光输能技术解决临近空间飞行器的能源供给问题的研究正在广泛开展。为研究激光辐照下的光电电池的极限性能,基于细致平衡理论,分析了激光辐照条件下光电材料禁带宽度与最佳激光波长的关系,得到了光电电池各类输出性能参数的计算方法。通过计算,得到了激光波长、激光功率密度和电池温度对能量转化效率等参数的影响规律。结果表明对于特定材料的光电电池存在最佳的激光波长使其获得极限能量转化效率;激光功率密度的增加可以提高光电电池的能量转化效率,但达到一定数值后提高不再明显;对于电池温度的影响,不同的输出性能参数具有不同的温度系数。     

Global optimization of solar thermophotovoltaic systems

In this paper, we present a theoretical model based on the detailed balance theory of solar thermophotovoltaic systems comprising multijunction photovoltaic cells, a sunlight concentrator and spectrally selective surfaces. The full system has been defined by means of 2n + 8 variables (being n the number of sub‐cells of the multijunction cell). These variables are as follows: the sunlight concentration factor, the absorber cut‐off energy, the emitter‐to‐absorber area ratio, the emitter cut‐off energy, the band‐gap energy(ies) and voltage(s) of the sub‐cells, the reflectivity of the cells' back‐side reflector, the emitter‐to‐cell and cell‐to‐cell view factors and the emitter‐to‐cell area ratio. We have used this model for carrying out a multi‐variable system optimization by means of a multidimensional direct‐search algorithm. This analysis allows to find the set of system variables whose combined effects results in the maximum overall system efficiency. From this analysis, we have seen that multijunction cells are excellent candidates to enhance the system efficiency and the electrical power density. Particularly, multijunction cells report great benefits for systems with a notable presence of optical losses, which are unavoidable in practical systems. Also, we have seen that the use of spectrally selective absorbers, rather than black‐body absorbers, allows to achieve higher system efficiencies for both lower concentration and lower emitter‐to‐absorber area ratio. Finally, we have seen that sun‐to‐electricity conversion efficiencies above 30% and electrical power densities above 50 W/cm² are achievable for this kind of systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Upper frequency limit for Gunn oscillators imposed by carrier energy-relaxation time

The small-signal microwave conductivity of semiconductors in the presence of Large steady electric fields is highly frequency-dependent, owing to the finite energy-relaxation time of the heated carriers. Considering the Gunn oscillator, in view of these effects, it can be seen that an upper limit for oscillation frequency exists. Methods useful for calculation of this limit are proposed.     

无线传感装置的太阳能供电系统研究

为了能够实现对安装在无缝钢轨上的无线传感装置无间断供电,详细介绍一种太阳能无间断供电系统的设计方案。供电系统以充放电控制器为核心,采用免维护铅酸蓄电池作为备用电源,实现了对无线传感装置的无间断供电系统设计。在完成系统的同时,还充分考虑了雷击影响,以及无线传感装置电压的远程监控问题,为此,增加了电池防雷模块以及GSM短信开关,保证了该系统的安全可靠性。经现场测试,蓄电池能够在阴雨天气持续作业达7天,该系统具有安全、可靠、防雷击、远程监控等特点。     

Development and experimental evaluation of a complete solar thermophotovoltaic system

We present a practical implementation of a solar thermophotovoltaic (TPV) system. The system presented in this paper comprises a sunlight concentrator system, a cylindrical cup‐shaped absorber/emitter (made of tungsten coated with HfO₂), and an hexagonal‐shaped water‐cooled TPV generator comprising 24 germanium TPV cells, which is surrounding the cylindrical absorber/emitter. This paper focuses on the development of shingled TPV cell arrays, the characterization of the sunlight concentrator system, the estimation of the temperature achieved by the cylindrical emitters operated under concentrated sunlight, and the evaluation of the full system performance under real outdoor irradiance conditions. From the system characterization, we have measured short‐circuit current densities up to 0.95 A/cm², electric power densities of 67 mW/cm², and a global conversion efficiency of about 0.8%. To our knowledge, this is the first overall solar‐to‐electricity efficiency reported for a complete solar thermophotovoltaic system. The very low efficiency is mainly due to the overheating of the cells (up to 120 °C) and to the high optical concentrator losses, which prevent the achievement of the optimum emitter temperature. The loss analysis shows that by improving both aspects, efficiencies above 5% could be achievable in the very short term and efficiencies above 10% could be achieved with further improvements. Copyright © 2012 John Wiley & Sons, Ltd.     

基于热光伏电池GaSb多晶薄膜的可控生长

采用物理气相沉积(PVD)法在ITO透明导电衬底上制备GaSb多晶薄膜.研究了衬底温度及薄膜厚度对GaSb薄膜结构特性、电学特性以及光学特性的影响.在一定条件下生长的GaSb薄膜择优取向由GaSb(111)晶向转变为GaSb(220)晶向, 这是在玻璃衬底上生长GaSb薄膜没有发现的现象.择优取向改变为(220)晶向的GaSb薄膜具有更高的霍尔迁移率.因为这种薄膜材料具有更少的晶粒间界和更少的缺陷.经优化后的GaSb薄膜的光学吸收系数在104 cm-1以上, 适用于热光伏薄膜太阳电池中.     

Gas-fired thermophotovoltaic generator based on metallic emitters and GaSb cells

A prototype compact TPV generator with a propane burner (pressure 2 bar) and a metallic netted emitter has been developed and tested. A photovoltaic generator unit with 24 (1 × 1 cm²) GaSb cells has been fabricated. The fabrication technology of photovoltaic cells has been optimized. It is shown that the data obtained can be used to select the starting bulk material for fabrication of photovoltaic cells with similar output parameters. It has been experimentally demonstrated that, to achieve maximum efficiency, it is necessary, in addition to using photovoltaic cells with similar characteristics, to provide identical conditions of their operation (temperature, illuminance).     

分子束外延生长的GaSb热光伏电池器件特性研究

利用分子束外延的方法在GaSb衬底上生长GaSb热光伏电池单元,制作了两种不同的1 cm×1 cm面积尺寸的热光伏电池单元,它们有着不同的电极形状。通过不断优化分子束外延的生长条件,以期得到高质量的GaSb外延层。AFM图中显示的表面形貌表明器件有着高质量的外延层,其表面形貌的RMS只有1.5 ? （1 ?=0.1 nm）。测量和比较了两种热光伏电池的器件特性,包括开路电压、短路电流密度、光电转换效率、填充因子以及暗电流密度。在一个模拟太阳光照射下,热光伏电池单元有着0.303 V的开路电压和27.1 mA/cm2的短路电流密度。和只有简单电极形状的热光伏电池单元进行对比,有栅形电极形状的热光伏电池单元在短路电流密度和填充因子上具有更优异的表现。在红外光的照射下,有栅形电极形状的热光伏电池达到了一个最优的填充因子56.8%。     

Design of a frequency selective structure with inhomogeneous substrates as a thermophotovoltaic filter

In this paper, the design of a thermophotovoltaic (TPV) filter with high-pass characteristics is presented. The filter is in the form of a frequency selective structure (FSS) with cascaded inhomogeneous dielectric substrates. The goal is to allow for more design flexibility using dielectric periodic structures to deliver a sharper filter response. Therefore, the primary focus is to design a periodic material substrate composition (supporting FSS elements) using a topology optimization technique known as the density method. The design problem is formulated as a general nonlinear optimization problem and sequential linear programming is used to solve the optimization problem with the sensitivity analysis based on the adjoint variable method for complex variables. A key aspect of the proposed design method is the integration of optimization tools with a fast simulator based on the finite element-boundary integral method. The capability of the design method is demonstrated by designing the material distribution for a TPV filter subject to pre-specified bandwidth and compactness criteria.     

Analytical model of solar thermophotovoltaic systems with cylindrical symmetry: Ray tracing approach

An analytical model for a full solar thermophotovoltaic (STPV) system with cylindrical symmetry is presented. In contrast to the already published TPV models, this model considers both TPV optical cavity and absorber (or heating) systems. An analytical ray‐tracing method has been used to analyse the radiation interchange within a TPV optical cavity. It allows us to calculate the portion of the absorbed power in each component within the system. Using this model, a broad analysis of an STPV system has been carried out, finding the optimum configurations and presenting a detailed loss analysis. Copyright © 2009 John Wiley & Sons, Ltd.     

入射光谱及功率密度对GaSb热光伏电池性能的影响

热光伏发电技术具有高效率、高功率密度、适应热源广泛等特征,揭示其核心部件—电池芯片内部光电能量转换物理机制及影响规律,有利于系统性能的进一步提升。针对GaSb热光伏电池内部的光-电转换过程,建立了微观载流子输运模型,采用光-电耦合仿真方法对其性能参数进行了数值模拟。重点探究了入射辐射光谱特性及功率密度对电池性能的影响。结果表明：当入射光谱辐射功率相同时,电池转换效率随光谱变化呈现明显的非单调性,在波长1.42 μm处达到峰值22.50%。同一光谱下,随着辐射功率密度提高,电池最大输出电功率密度等比例增大,转换效率也随之增大但增幅逐渐减小,波长1.42 μm时增幅最大约4.85%。     

Key aspects in the modeling of concentrator III-V solar cells and III-V thermophotovoltaic converters

The development of III-V concentrator solar cells and thermophotovoltaic converters is at a critical point in which both sophisticated technology and an accurate modeling are required. This paper emphasizes the aspects relating to the modeling of multijunction solar cells for the concentration of applications and thermophotovoltaic converters. In the case of solar cells, the key aspects are

• —Necessity of three-dimensional modeling
• —Consideration of real conditions of operation
• —Critical review of material parameters.

For TPV converters, the aforementioned aspects are also to be applied. Preliminarily, the material parameters of the less mature thermophotovoltaic semiconductors must be specified or even measured.     

Enhancing thermal,electrical efficiencies of a miniature combustion‐driven thermophotovoltaic system

Methods to enhance the thermal and electrical efficiencies through novel design of combustion and thermal management of the combustor in a miniature thermophotovoltaic (TPV) system are proposed, discussed, and demonstrated in this paper. The miniature TPV system consists of a swirling combustor surrounded by GaSb PV cell arrays. The swirl combustor design, along with a heat‐regeneration reverse tube and mixing‐enhancing porous‐medium fuel injection, improves the low illumination and incomplete combustion problems associated with typical miniature TPV systems. A reverse tube is used to enforce swirling flame attachment to the inner wall of the emitter by pushing the swirl recirculation zone back into the chamber and simultaneously redirecting the hot product gas for reheating the outer surface of the emitter. The porous medium fuel injector is used as a fuel/air mixing enhancer and as a flame stabilizer to anchor the flame. The miniature TPV system, using different combustor configurations, is tested and discussed. Results indicate that the proposed swirling combustor with a reverse tube and porous medium can improve the intensity and uniformity of the emitter illumination, and can increase the thermal radiant efficiency. Consequently, the overall thermal efficiency and electrical output of the miniature TPV system are greatly enhanced. Copyright © 2009 John Wiley & Sons, Ltd.     

A simple single-step diffusion and emitter etching process for high-efficiency gallium-antimonide thermophotovoltaic devices

A single-step diffusion followed by precise etching of the diffused layer has been developed to obtain a diffusion profile appropriate for high-efficiency GaSb thermophotovoltaic (TPV) cells. The junction depth was controlled through monitoring of light current-voltage (I–V) curves (photovoltaic response) during the post-diffusion emitter-etching process. The measured photoresponses (prior to device fabrication) have been correlated with the quantum efficiencies (QEs) and the open-circuit voltages in the fabricated devices. An optimum junction depth for obtaining the highest QE and open-circuit voltage is presented based on diffusion lengths (or minority carrier lifetimes), carrier mobility, and the typical diffused impurity profile in GaSb.     

A study of the conversion efficiency limit of p+-i-n+silicon solar cells in concentrated sunlight

The conversion efficiency limit of p⁺-i-n⁺silicon solar cells in concentrated sunlight is explored with numerical simulations of an idealized p⁺-i-n⁺cell having field-induced junctions. Conversion efficiencies greater than 30 percent are calculated for this cell operating in sunlight concentrated 1000 times. The relative importance of bulk and surface recombination in limiting the cell conversion efficiency is illustrated for operation in 1 to 1000 suns. For surface recombination velocities below 100 cm/s, it is shown that bulk recombination losses limit the cell performance rather than recombination losses occurring in the p⁺or n⁺regions. The results show that Auger recombination in the bulk region will limit ultimately the cell conversion efficiency.     

Wafer bonding and epitaxial transfer of GaSb-based epitaxy to GaAs for monolithic interconnection of thermophotovoltaic devices

GaInAsSb/AlGaAsSb/InAsSb/GaSb epitaxial layers were bonded to semi-insulating (SI) GaAs handle wafers with SiO_x/Ti/Au as the adhesion layer for monolithic interconnection of thermophotovolatic (TPV) devices. Epitaxial transfer was completed by removal of the GaSb substrate, GaSb buffer, and InAsSb etch-stop layer by selective chemical etching. The SiO_x/Ti/Au provides not only electrical isolation, but also high reflectivity and is used as an internal backsurface reflector (BSR). Characterization of wafer-bonded (WB) epitaxy by high-resolution x-ray diffraction (HRXRD) and time-decay photoluminescence (PL) indicates minimal residual stress and enhancement in optical quality. The 0.54-eV GaInAsSb cells were fabricated and monolithically interconnected in series. A ten-junction device exhibited linear voltage building with an opencircuit voltage of 1.8 V.