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.     

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.     

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).     

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.     

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.     

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.     

Fabrication and characterization of GaSb based thermophotovoltaic cells using Zn diffusion from a doped spin-on glass source

The GaInSb material system is attractive for application in therm ophotovoltaic (TPV) cells since its band gap can be tuned to match the radiation of the emitter. At present, most of the TPV cells are fabricated using epitaxial layers and hence are expensive. To reduce the cost, Zn diffusion using elemental vapors in a semiclosed diffusion system is being pursued by several laboratories. In this paper, we present studies carried out on Zn diffusion into n-type (Te-doped) GaSb substrates in an open tube diffusion furnace. The dopant precursor was a 2000 ? thick, zinc doped spin-on glass. The diffusion was carried out at temperatures ranging from 550 to 600°C, for times from 1 to 10 h. The diffused layers were characterized by Hall measurements using step-and-repeat etching by anodic oxidation, secondary ion mass spectrometry measurements, and TPV device fabrication. For diffusion carried out at 600°C, the junction depth was 0.3 μm, and the hole concentration near the surface was 5 × 10¹⁹/cm³. The external quantum efficiency, measured without any anti-reflection coating of the TPV cells fabricated using mesa-etching had a maximum value of 38%. Masked diffusion was also carried out by opening windows in a Si₃N₄ coated, GaSb wafer. TPV cells fabricated on these structures had similar quantum efficiency, but lower dark current.     

Decimal-binary conversion

A new simple sequential circuit for decimal-binary conversion needs essentially one serial shift register and a special adder.     

Approaching the quantum limit

The knowledge of the wavelike behavior of electrons that emerges as technology drives semiconductor geometries deeper into the nanometer domain is described. How the underlying size quantization effects restrict the scaling down of device dimensions is explained. Studies underway to explore and understand these phenomena are reviewed. Ways in which they can be exploited in new types of device are explored, alone with the fabrication problems attending such applications