Thermophotovoltaic Research Articles

Analysis of a photon-recycling optical cavity in a cylindrical thermophotovoltaic system considering thermal load in cells

Thermophotovoltaics (TPVs) hold immense promise for efficient heat-to-electricity conversion via photons, particularly considering advancements in photon-recycling techniques. We investigated the design of an optical cavity for a TPV system in which the emitters were radially surrounded by cells, with the goal of applying the photon-recycling effect in practical TPV systems. Employing ray-trace simulations, we explored the influence of various parameters on TPV system performance. The optimal optical cavity was designed with the same area of emitter and cell under a constant cell temperature. Incorporating the thermal behavior of the cell into the simulation model yielded significantly different system efficiencies, emphasizing the importance of temperature considerations. Despite some discrepancies, experimental validations and theoretical analyses described the thermal behavior of gallium antimonide cells. Optimization under different conditions indicated a potential for the system efficiency of these cells to exceed 40 %. To further enhance the performance of TPVs, future efforts should focus on increasing the cell’s out-of-band reflectance, improving cooling, and optimizing area ratios for specific cell reflectance and heat dissipation situations. This study provides essential insights into the comprehensive design of a TPV system with an optical cavity by combining photon-recycling and the temperature dependence of the cell.

Directed high-energy infrared laser beams for photovoltaic generation of electric power at remote locations

Transferring energy without transferring mass is a powerful paradigm to address the challenges faced when the access to, or the deployment of, the infrastructure for energy conversion is locally impossible or impractical. Laser beaming holds the promise of effectively implementing this paradigm. With this perspective, this work evaluates the optical-to-electrical power conversion that is created when a collimated laser beam illuminates a silicon photovoltaic solar cell that is located kilometers away from the laser. The laser is a CW high-energy Yb-doped fiber laser emitting at a center wavelength of 1075 nm with ∼1 m2 of effective beam area. For 20 kW illumination of a solar panel having 0.6 m2 of area, optical simulations and thermal simulations indicate an electrical output power of 3000 W at a panel temperature of 550 K. Our investigations show that thermo-radiative cells are rather inefficient. In contrast, an optimized approach to harvest laser energy is achieved by using a hybrid module consisting of a photovoltaic cell and a thermoelectric generator. Finally, practical considerations related to infrared power beaming are discussed and its potential applications are outlined.

Broadband mirrors for thermophotovoltaics.

We present an innovative solution to improve the efficiency of thermophotovoltaic (TPV) devices by tackling the problem of sub-bandgap photon losses. We propose an optimized design for thin-film mirrors using inverse electromagnetic design principles, thereby enhancing the average reflectivity and photon re-use. Our method surpasses the traditional Bragg mirror by employing a gradient-descent based optimization over Bragg mirror geometrical parameters, leveraging the transfer matrix method for derivative calculations. The optimized structure, based on continuously chirped distributed Bragg reflectors proposed herein demonstrates a remarkable increase in reflectivity beyond 98%, over an almost three-octaves bandwidth (0.1eV-0.74eV). We show that the incident power loss in InGaAs TPV cells at an emitter temperature of 1200°C is significantly reduced. While our work shows considerable promise, further exploration is needed to ascertain the practicability and robustness of these designs under various operational conditions. This study thus provides a major step forward in TPV technology, highlighting a new route towards more effective energy conversion systems.

Experimental performance comparison of 0.72 eV-GaSb and 0.59 eV-InGaAs thermophotovoltaic cells under different radiation temperatures

The field of Thermophotovoltaic (TPV) has recently exhibited highly promising characteristics. The performance of thermophotovoltaic (TPV) cells with 0.72 eV-GaSb and 0.59 eV-InGaAs varies at different radiation temperatures, as demonstrated in this experimental study. The research evaluates and compares the TPV cell performance under radiation temperatures ranging from 800 to 1200 °C, and analyses the impact of temperature on critical parameters. The maximum efficiencies achieved for GaSb and InGaAs cells were 23.10% and 28.91%, respectively, under tungsten radiation spectrum at a temperature of 1200 °C. This comprehensive analysis provides valuable insights into optimizing TPV systems for efficient thermal-to-electrical energy conversion using GaSb and InGaAs cells.

First-principles study on the effect of point defects on the magnetic new mechanism and optical properties of the GaN:Be/Mg/Ca system

The new magnetic mechanism and optical properties of Ga vacancies and Hi interstitial in the GaN: Be/Mg/Ca system have not been fully understood, and the use of first principles can solve this problem. The effect of point defects on the magnetic mechanism and optical properties of the GaN: Be/Mg/Ca system was investigated using the first nature principle of the hybridized generalized HSE06 method. Results show that all doped systems have N2− ions in addition to N3− ions, and N2− ions have the dual property of itinerant electrons in the off-domain (donor) and of local electrons (acceptor). The magnetism of magnetic doped systems is generated by the hybrid coupling of Ga4s and N2− 2p states. In comparison with the Ga34MN36 (M = Be/Ca) system, the magnetic moments of Ga34MHiN36 (M = Be/Ca) system are reduced after doping with Hi interstitial. The magnetic properties of the Ga34MgN36 system can be regulated by the presence or absence of Hi interstitial, which is advantageous as a magnetic switch. The absorption spectral distribution of the Ga34MgHiN36 system extends to the mid-infrared optical region. This material has some reference value as infrared thermophotovoltaic cells, infrared photodetectors, or infrared semiconductor lasers.

Exploring combustion characteristics of a novel micro combustor with a fan swirler for thermophotovoltaic system

The combustion and thermal characteristics of non-premixed methane (CH4)–air flames in a swirl micro combustor are numerically investigated for potential use in a thermophotovoltaic (TPV) system. Swirling flows associated with methane–air combustion are simulated using a Reynolds stress model and detailed chemistry of GRI-Mech 3.0. The optimal condition for improved emission and combustion performance is established by taking into account six blade angles and three fuel-lean equivalence ratios, to consider among other geometric features of the fan swirler and operating conditions.The reactive swirling flows in a micro combustor with a fan swirler are modified by altering the blade angle (θb). For θb = 5°, 10°, and 15°, the swirl micro combustor with a two-vortex structure exhibits a significant enhancement of the fuel–air mixing and thermal performance compared to no-swirl combustor. As θb increases, inner vortices close to the fuel stream and outer vortices near the combustor wall behind the fan blades are developed according to the interaction of swirling flows and recirculating flows due to the blockage effect of the fan blades. For θb = 15°, the strong interaction between two vortex regions maximizes combustion efficiency. The combustion efficiency is highest at θb = 15° and lowest at θb = 75°. For three fuel-lean conditions of θb = 15°, the flame zone is enlarged towards the combustion chamber wall, resulting in a high and uniform wall temperature. The resulting radiant emitter efficiencies for a TPV system are increased by 28.3 %–35.1 % compared to that of the no-swirling micro combustor. The results indicate that the fan swirler has recirculation flows for the best emitter and combustion efficiency unlike vane-type swirlers.

Thermophotovoltaics (TPVs), solar and wind assisted hydrogen production and utilisation in iron and steel industry for low carbon productions

To reduce greenhouse gas (GHG) emissions in high-grade steel production plants, this study developed a solar and wind assisted H2-fuelled blast furnace – basic oxygen furnace (BF-BOF) route coupled with the electrolysis of H2O and thermoelectric units. The developed model consists of heat recovery units, water gas shift (WGS), low-temperature electrolysis of H2O, thermophotovoltaic converter, CO2 capture by absorption and oxy-hydrogen firing ovens and furnaces. The recovered thermal energy generated steam and distilled H2O feedstocks for WGS and PEMEC (proton exchange membrane electrolyser cell) units. WGS converted CO to CO2 and increased the H2 production rate before separation from other by-products in the (PSA) pressure swing adsorption column. H2O electrolysis generated more H2 fuel for the coke oven, Fe-CaO oven-sinter, BF and BOF. The result of the proposed system reveals that by utilising H2 as fuel and O2 as oxidant instead of burning natural gas (NG) for thermal decomposition of feedstocks, 1111.4 kg/h of CO2 emission for every 626 kg/h of produced steel can be prevented. The application of CO2 capture by absorption process eliminated CO2 emission footprint from the process. Whereas 61.1 kW was recovered by installing TPV units on ovens and furnaces' walls for the conversion of waste heat into electricity. By incorporating either solar or wind renewable energy systems with a power output of 20 MW, 1290.4 kg/h of H2 fuel and 38.5 kg/h of CH4 were stored for later use and 6754.8 kg/h of CO2 emission was avoided. The steel purchase price of the proposed system is anticipated to be cheaper than the conventional BF-BOF route operating with a CCS unit as ≥10% energy efficiency was recorded. The recycling of more scrap steel is also viable in this developed system because of the high energy density of the utilised H2 fuel for the thermal decomposition of ovens and furnaces’ feeds.

Enhancement of Radiative Power Using a Divergent Splitter Plate Design in H2–Air Non-Premixed Micro-Combustor

Abstract Micro-combustion based power generation devices can be considered as future alternatives to batteries in miniature electronic devices. Micro-combustors operating in non-premixed mode are free from flashback but face the challenge of properly mixing fuel and air within a small volume. In this work, the effect of a divergent fuel–air splitter design on the mixing performance and combustion characteristics of H2–air fueled diffusion micro-combustor is studied. The laminar reacting flow is simulated using the finite volume method and a detailed hydrogen kinetic mechanism. Three divergent splitter designs are compared with the commonly used rectangular splitter to study the effect on radiation power, an essential parameter for thermophotovoltaic power generation. The best-performing divergent and base rectangular splitter designs are investigated in detail. The study shows that the micro-combustor with divergent splitter design reduces mixing distance (Lmix) by 5–23% depending on inlet velocity and channel height. With the divergent splitter, the peak value of the heat release rate also increases slightly, implying enhanced combustion. The divergent splitter increases the high-temperature surface area of the outer wall as compared to the rectangular splitter. This leads to the micro-combustor with divergent splitter producing significantly higher radiation power (>10%) than the rectangular splitter for larger channel heights and higher inlet velocities.

Thermophotovoltaic emitter design with a hyper-heuristic custom optimizer enabled by deep learning surrogates

Micro/nanostructures hold promise for use as emitters to boost the efficiency of thermophotovoltaic (TPV) systems. For example, periodic gratings may alter the spectrum of irradiation on the photovoltaic cell to better match the spectral response of the cell. Photons with energies slightly higher than the bandgap of the semiconductor are the most desired as they generate electron-hole pairs with minimal thermalization losses. This prompts the use of gratings as selective emitters, and the choice of grating geometry has been an active research topic. Even for a one-dimensional (1D) grating, millions of possible geometries exist, and each grating requires computationally intensive full-wave simulation, e.g., the rigorous coupled-wave analysis (RCWA), to calculate the spectral, directional emissivity. Since optimization algorithm performance is problem-dependent, in this work, a hyper-heuristic search enabled by a fully connected neural network surrogate of the native RCWA is employed to select an algorithm for a specific grating design problem. A comparison with existing untuned algorithms for an ideal emitter problem demonstrates that the hyper-heuristically generated algorithm yields superior performance. This algorithm is then employed for the optimization of the emitter for a full TPV system comprising a heated 1D tungsten binary grating paired with a 300 K InGaSb cell. The system is optimized for maximum power or efficiency at 2000 K and 1500 K, respectively, and the grating properties for the optimized cases are analyzed.

On the Performance of Solar Thermophotovoltaics (STPVs) and Wavelength-Selective Thermophotovoltaics (TPVs): Case Study of a High-Rise Residential Building in a Hot and Semi-Arid Climate

Utilizing integrated solar systems and renewable energy sources has the potential to not only decrease the CO2 emissions of buildings but also provide access to more affordable energy alternatives compared to fossil fuels, especially considering the recent rise in prices. Nevertheless, many designers and project decision makers are hesitant to embrace solar technologies due to the uncertainty surrounding the cost–benefit balance. This paper presents a case study of the design process, highlighting the energy-saving and cost–benefit aspects of a solar façade featuring solar thermophotovoltaics (STPVs) and wavelength-selective thermophotovoltaics (TPVs) in a high-rise residential building situated in the semi-arid climate of Tehran, Iran. The simulation methodology consists of EnergyPlus Engines in Rhino, along with the Ladybug and Honeybee plugins throughout the solar façade design process. The solar façade incorporating STPV yielded energy savings of 25 kWh per square meter, marking a 34% reduction compared to a standard façade. In contrast, the TPV demonstrated energy savings of 35 kWh per square meter, indicating a 48% decrease in energy consumption compared to a regular façade. This research indicates that, particularly in semi-arid climatic conditions, TPV systems exhibit a superior performance when integrated into the façades of high-rise structures; yet, due to the low electricity prices in the region, neither STPVs nor TPVs are financially viable. The study contributes to raising awareness, fostering technological innovations, influencing policy discussions, and promoting the adoption of sustainable practices in the field of energy and architecture.

Sub‐0.6 eV Inverted Metamorphic GaInAs Cells Grown on Inp and GaAs Substrates for Thermophotovoltaics and Laser Power Conversion

AbstractInverted metamorphic Ga0.3In0.7As photovoltaic converters with sub‐0.60 eV bandgaps grown on InP and GaAs are presented. Threading dislocation densities are 1.3 ± 0.6 × 106 and 8.9 ± 1.7 × 106 cm−2 on InP and GaAs, respectively. The devices generate open‐circuit voltages of 0.386 and 0.383 V, respectively, under irradiance producing a short‐circuit current density of ≈10 A cm−2, yielding bandgap‐voltage offsets of 0.20 and 0.21 V. Power and broadband reflectance measurements are used to estimate thermophotovoltaic (TPV) efficiency. The InP‐based cell is estimated to yield 1.09 W cm−2 at 1100 °C versus 0.92 W cm−2 for the GaAs‐based cell, with efficiencies of 16.8 versus 9.2%. The efficiencies of both devices are limited by sub‐bandgap absorption, with power weighted sub‐bandgap reflectances of 81% and 58%, respectively, the majority of which is assumed to occur in the graded buffers. The 1100 °C TPV efficiencies are estimated to increase to 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0–2.3 µm atmospheric window. Peak laser power converter efficiencies of 36.8% and 32.5% are estimated under 2.0 µm irradiances of 1.86 and 2.81 W cm−2, respectively.

Spectral regulation in thermophotovoltaic devices

Thermophotovoltaic (TPV) device converts thermal radiation into electricity output through photovoltaic effect. High-efficiency TPV devices have extensive applications in grid-scale thermal storage, full-spectrum solar utilization, distributed thermal-electricity cogeneration, and waste heat recovery. The key to high-efficiency TPV devices lies in spectral regulation to achieve band-matching between thermal radiation of the emitters and electron transition of the photovoltaic cells. The latest advances in nanophotonics, materials science, and artificial intelligence have made milestone progress in spectral regulation and recording power conversion efficiency of up to 40% of TPV devices. Here we systematically review spectral regulation in TPV devices at the emitter end as well as the photovoltaic cell end. At the emitter end, spectral regulation is realized through thermal metamaterials and rare-earth intrinsic emitters to selectively enhance the in-band radiation and suppress the sub-bandgap radiation. At the photovoltaic cell end, spectral regulation mainly focuses on recycling the sub-bandgap thermal radiation through optical filters and back surface reflectors located at the front and back of the photovoltaic cells, respectively. We emphasize the light-matter interaction mechanisms and material systems of different spectral regulation strategies. We also discuss the spectral regulation strategies in near-field TPV devices. Finally, we look forward to potential development paths and prospects of spectral regulation to achieve scalable deployment of future TPV devices.

Envisioning the future: optically transparent metasurface-based solar thermophotovoltaic with high conversion efficiency

Transparent solar thermo-photovoltaic (TPV) technology combines visible transparency and solar energy conversion. They are developed for their potential applications in buildings and vehicles windows, where conventional opaque solar cells may not be feasible. TPV’s offer a promising solution to harness solar energy without compromising aesthetics or functionality of transparent surfaces. Broadband absorption at UV and IR frequencies and simultaneous transmission at visible frequencies can be achieved by fabricating metamaterials that employ semi-conducting oxides. In this study, an optically transparent metasurface (OTM) based STPV composed of indium tin oxide (ITO) is introduced as the transparent metal and ZnS as a substrate layer. Our design offers a cost-effective and scalable solution for large-scale fabrication. The designed OTM structure exhibits exceptional absorption capabilities, achieving an absorption rate of up to 99% in the UV region. Additionally, it achieves over 90% absorptivity in the far infrared range and maintains a high average transmittance at visible frequencies. Furthermore, the absorption remains consistently high, exceeding 90%, even when the incident angle is less than 70° for both TE and TM polarization waves. This innovative design holds promise for various applications requiring high-performance transparent metasurface absorbers/emitters. The proposed transparent metasurface based STPV holds great potential for efficient utilization in combined solar/thermal conversion systems.

Pathways for low carbon hydrogen production from integrated hydrocarbon reforming and water electrolysis for oil and gas exporting countries

Integrated hydrocarbon reforming processes with H2O electrolysis for H2 production have been developed to reduce greenhouse gas (GHG) emissions in oil and gas exporting countries (OGEC). The developed system includes steam methane reforming (SMR), gas-heated autothermal reforming (GH-ATR), autothermal reforming (ATR) and partial oxidation (POX). Each of these systems operates with H2O electrolysis to increase the H2 production rate and feed O2 to the reformers. The combined systems also incorporate thermophotovoltaic (TPV) units, solar cells, a wind turbine system and a Rankine cycle to convert waste heat to electricity and generate more electrical energy. Parabolic dish concentrators (PDCs) produced thermal energy for endothermic reactions. The introduction of H2 fuel cells replaced the burning of natural gas in power plants to generate electricity. The result of this study reveals that using renewable energy sources and energy recovery units to produce distilled H2O and operate other downstream units, the H2 selling price of the proposed system is expected to be <$2.9/kg. Efficiency improvement by ≥10% for each hydrocarbon reforming method operating with other units was also achieved in contrast to the current hydrocarbon reforming processes. The provided integrated method will provide a high-value chain in the H2 generation industry and promote carbon neutrality in oil and gas rich countries.

Screening and optimization of metal-insulator-metal selective emitter in thermophotovoltaic system

In thermophotovoltaic (TPV) systems, it is crucial that the selective emitters can tailor emission spectrum to match the bandgap of photovoltaic (PV) cells and largely enhance the system efficiency. In this work, a metamaterial emitter based on the metal-insulator-metal (MIM) structure is proposed to obtain the high energy conversion efficiencies. The geometric parameters of MIM emitter have been investigated to obtain an excellent radiation spectrum of emitter composed of W and HfO2. The excellent emission performance of MIM emitter is attributed to the excitation of surface plasmon polariton (SPP) and cavity resonance, and the structure of MIM emitter is insensitive for different polarization modes. The 21 material combinations of MIM emitters have been screened to obtain the optimal emitter matching GaSb and InGaAsSb cells. This work identifies the crucial role of structure and materials into the emitter of a TPV system. In the evaluation of MIM emitter and TPV System, when the operating temperature of emitter increases from 1400[Formula: see text]K to 2000[Formula: see text]K, the system efficiency of optimal W/HfO2/W MIM emitter increases from 20.26% to 30.41%, while the output electric power increases from 3.59[Formula: see text]kW/m2 to 42.48[Formula: see text]kW/m2. The phenomenon indicates that the MIM emitter with the optimal material combinations and geometric parameters can significantly improve the matching degree with GaSb and InGaAsSb cells. Our results will be helpful to expand the optimization scope of metamaterial emitters in TPV systems.

Highly reflective and passivated ohmic contacts in p-Ge by laser processing of aSiCx:H(i)/Al2O3/aSiC films for thermophotovoltaic applications

Crystalline germanium (c-Ge) has historically been regarded as a cost-effective alternative to III-V semiconductors for thermophotovoltaic (TPV) device fabrication. However, Ge-based devices have not yet reported high efficiencies, partially due to the lack of an efficient back-surface reflector that turns back to the heat source out-band (sub-bandgap) thermal radiation. The difficulty of implementing back surface reflectors in Ge TPV cells is related to the simultaneous requirement of good back surface passivation, low electrical resistivity, and high out-band optical reflectivity. In this study, we demonstrate a highly reflective ohmic contact to p-type c-Ge (doping concentration of 2 × 1015 cm−3) made of an aSiCx(1 nm)/Al2O3 (50 nm)/aSiC (45 nm) stack that is laser processed using Nd:YVO4 laser emitting at 355 nm to create punctual p+ contacts (locally diffused Al regions). This stack is finally caped with a thick (1000 nm) Al layer that behaves as a metallic mirror and back electrode. As the laser processed area increases from 0.1 to 3 %, which is the typical range in the final devices, the surface recombination velocity increase from 10.5 to 60.0 cm/s, while the effective contact resistance reduces from 0.462 to 0.036 Ω cm2. Moreover, a sub-bandgap reflectance of 90–98 % is achieved. Simulations assuming ideal device configuration indicate that implementing these back contacts could potentially enable TPV cell conversion efficiencies comparable to the reported high-efficiency c-Ge TPV cells operating at similar illumination temperature.

Exploring the potential of GeTe for the application in Thermophotovoltaic (TPV) cell

Germanium telluride (GeTe) having a direct bandgap of 0.6 eV has mainly been used in phase change memory and thermoelectric power generation. In this article, we study the electronic structure of the GeTe by first-principles calculations. The theoretical direct bandgap of GeTe was found to be 0.69 eV which is very close to the experimental value. Then, we demonstrated a single-junction GeTe thermophotovoltaic (TPV) cell based on device transport model with np structure at the black body and cell temperature of 1775 and 300 K, respectively. The device was optimized for the higher performance of the TPV cell. The GeTe TPV cell exhibited an efficiency of 7.9% with JSC = 16.16 A cm−2, VOC = 0.360 V and FF = 75.51%, respectively. These results indicate that GeTe could be a promising material for the fabrication of efficient TPV cell.

Optimized design and application performance analysis of heat recovery hybrid system for radioisotope thermophotovoltaic based on thermoelectric heat dissipation

Radioisotope thermophotovoltaic (RTPV) generators face a fundamental challenge, which is the significant output loss and energy wastage caused by the substantial thermal deposition resulting from the arrival of infrared radiation from the heat source at the energy conversion unit. This contradiction has gradually become a key limiting factor for its further development in recent years. In this work, a novel hybrid system based on the thermophotovoltaic-thermoelectric (TPV-TE) effect is proposed, in which the Bi2Te3-based I/W-type thermoelectric leg plays both heat dissipation and heat recovery for power supply. The heat recovery thermoelectric legs were prepared using a cold pressing mould and sintering method, and their Seebeck effect was used to convert the unwanted heat recover from InGaAs thermophotovoltaic cells into electrical power. The design preparation scheme of the heat recovery hybrid module and its potential application performance at 700–1000 K heat source temperature were investigated through experimental tests and simulations. The W-type TPV-TE heat recovery hybrid system exhibits good gain of 12.9% at 1000 K, and the output power can be increased by additional 21.6% in series mode. Simulations for environmental applications show better heat recovery in deep space than on Earth, with an output gain of 63.7% and an energy conversion efficiency of 11.23%. The heat recovery design enhances the competitiveness of the RTPV and provides new ideas for optimizing other heat recovery applications for thermal energy conversion systems.

Performance evaluation of a thermoradiative device coupled to a solid oxide fuel cell

To improve the efficiency of the energy conversion, a novel coupling model comprising a solid oxide fuel cell (SOFC), a thermoradiative cell (TRC) with sub-bandgap and non-radiative losses, and a regenerator is proposed. The waste heat from the SOFC is utilized by the TRC to produce extra power. Considering the main irreversible losses inside the coupling model, mathematical expressions for power and efficiency are derived, along with revealing the performance characteristics. Numerical calculation results illustrate that the maximum power output density (MPOD) and corresponding efficiency of the coupled model are 0.94 Wm−2 and 41%, respectively, which are improved by 68% and 14%, respectively, compared to a single SOFC. Furthermore, the influences of parameters such as non-radiative losses, temperature, and bandgap are investigated. The optimal operating regions for power output density, efficiency, and current are determined. Finally, compared with the MPODs of the SOFC-based coupled models, the SOFC-TRC can more effectively utilize the waste heat released from the SOFC. This study may provide some new ideas for the waste heat utilization of SOFC.

Performance evaluation of a GaSb thermophotovoltaic converter

In recent years, Gallium Antimonide (GaSb), which has smallest bandgap among III-V semiconductors family, became the subject of extensive investigations in the field of thermophotovoltaic (TPV) converters, because of the recent improvements in optoelectronic technology. This paper describes an analytical process used to evaluate the performance of a GaSb TPV converter under different levels of illumination, taking account of the photons with energy below the cells bandgap by considering the cell’s reflectance to this fraction of incident radiation. The results show that a radiator temperature near 2200 K is most advantageous and a reflectance of 0.98 is necessary for below-bandgap irradiations to obtain conversion efficiency greater than 28%, at 300 K cell temperature. This efficiency will decrease as the cell temperature increase. The obtained results are found to be in good agreement with the available data.