Thermophotovoltaic Generation Research Articles

Design of thermophotovoltaics for tolerance of parasitic absorption.

With advances in thermophotovoltaic (TPV) cells enabling recycling of sub-bandgap photons, a key barrier to reaching high prototype efficiencies has become radiative losses to parasitic high-emissivity regions, such as heavily doped contact regions, defects in coatings, and inactive areas. Here, we examine the impact of such radiative losses on the performance of various candidate cell materials, including GaAs, Si, InGaAsP, InGaAs, GaSb, and InGaAsSb. The ability of a TPV design to resist this performance loss is termed "radiation-sink tolerance" (RST). We show that RST is directly proportional to the spectral overlap between the absorptance profile of the cell and the emission profile of the emitter, which can be improved by adding a lower-bandgap absorber, increasing the emitter temperature, and utilizing a selective emitter. Our RST expressions can be used to estimate the efficiency of a prototypical TPV generator based on a component-level measurement.

A Review of Advances in Thermophotovoltaics for Power Generation and Waste Heat Harvesting

ABSTRACTThe vast majority of power generation in the United States today is produced through the same processes as it was in the late-1800s: heat is applied to water to generate steam, which turns a turbine, which turns a generator, generating electrical power. Researchers today are developing solid-state power generation processes that are more befitting the 21st-century. Thermophotovoltaic (TPV) cells directly convert radiated thermal energy into electrical power, through a process similar to how traditional photovoltaics work. These TPV generators, however, include additional system components that solar cells do not incorporate. These components, selective-emitters and filters, shape the way the radiated heat is transferred into the TPV cell for conversion and are critical for its efficiency. Here, we present a review of work performed to improve the components in these systems. These improvements will help enable TPV generators to be used with nearly any thermal source for both primary power generation and waste heat harvesting.

Porosity optimization of rare-earth oxide emitters for thermophotovoltaics

Porous rare-earth oxides are promising emitters for thermophotovoltaic generation because rare-earth ions exhibit narrow-band thermal emissions in a photovoltaic-convertible wavelength range and the pores scatter light of unwanted wavelengths. However, the effects of pores on the emitter performance have not been studied well due to the complexity. In this letter, we reveal the effects by analyzing the thermal emission spectra of garnet (Yb3Ga5O12) emitters by using a Kubelka-Munk-(KM) like two-flux model. The model incorporates the effects of pores as dependences on porosity of the absorption and scattering coefficients of the emitters. The absorption and scattering coefficients are determined uniquely by fitting the model to the experimental results with the help of the KM transform. We demonstrate that the emitter performance can be maximized by optimizing the porosity.

Super-Planckian Thermophotovoltaics Without Vacuum Gaps

Thermophotovoltaic (TPV) systems could be very attractive for energy efficiency, as they allow the harvesting of radiative waste heat as electricity. The authors describe an efficient TPV generator whose spectrum of radiative thermal power is frequency-selective and exceeds the Planckian limit. Their device does not require a vacuum gap between the hot and cold parts of the system, and can be handily realized with existing nanofabrication methods. Thus this work shows the way to efficient TPV generators that are quite practical for application.

Evaluation of elliptical optical cavity for a combustion thermophotovoltaic system

A reflective cavity with elliptical cross-section is proposed to couple a burner to three arrays of thermophotovoltaic (TPV) cells. Burner and photoconverter, with triangular section, are located near the two foci of the ellipse. The elliptical cavity is designed to be inserted into a thermo-electric micro-cogenerator for residential use, which exploits the TPV cells to convert visible and infrared radiation generated by the burner into electricity. Taking into account the key construction constraints, several cavity configurations, which differ for components or optical properties, are analysed by ray tracing simulations. The maximum radiation collection efficiency is 62% for the elliptical cavity and is reduced to 48% by introducing quartz protections around the burner and the receiver. A germanium-based photoconverter is developed and manufactured by metalorganic chemical vapour deposition techniques. Each photoconverter can deliver 180mW/cm2 (at 230mV voltage and 55°C) when illuminated by a blackbody spectrum at 1473K. The manufacture of the TPV receiver is described and the thermal design aspects are considered. The use of a dichroic filter for a TPV generator configuration based on an elliptical chamber is also evaluated.

Preparation and performance study of Er2O3 film selective thermal emitter

Solar thermophotovoltaic (STPV) generator is a popular energy converter due to providing low noise, low thermal mechanical stress and portability. It has the ability to exceed the efficiency of pure solar photovoltaic system. An idealized STPV generator is a reversible heat-engine, offering a theoretical efficiency of over 80%, but the actual conversion efficiency of STPV generator is still low due to the mistuned spectral property between the thermal selective emitter and the TPV cell. One key issue in developing the STPV generator with high performance is the spectral matching between the thermal radiation spectrum of radiator and the spectral response of photovoltaic cell in visible and near-infrared region, which usually lies between the visible and the near-infrared region. High-temperature spectral emissivity of rare earth oxide is of special interest, because the radiation has a narrow band of wavelengths in the near infrared and infrared region from 900 to 3000 nm. In this work, the thermal-selective film Er2O3 emitter is fabricated by post-oxidation of Er film deposited on Si substrate through using electron-beam gun evaporation. Based on the X-ray diffraction results, the Er2O3 film is of cubic phase structure and well-crystallized when the oxidation temperature is 700℃, and the Si substrate has no obvious influence on the crystal structure of Er2O3 film. According to the X-ray photoelectron spectroscopy results of the Er2O3 film after thermal oxidation at 700℃, the atomic ratio of Er/O is stoichiometric. In order to obtain the selective emission characteristic of the Er2O3 film, a measurement system is designed. The system consists of two major portions, i.e., one is a near infrared spectrometer purchased from Ocean Optics, the other is a high-temperature emission characteristic tester which can provide oxyhydrogen flame to heat the sample by using an electronic impulse ignition to torch the hydrogen-oxygen mixture. The oxyhydrogen flame passes through the nozzle and sprays vertically on the surface of the thermal-selective emitter sample. The facula of the oxyhydrogen flame convergence is very small (facula diameter:~0.8 cm), and the highest temperature achieved is about 2500℃. The measurement condition of selective emission performance of the Er2O3 film emitter coincides with the application characteristic of STPV generator. The emission performance result of the film emitter at 700℃ shows a typical gray-body emission characteristic. The measurements carried out at 900 and 1100℃ show that the Er2O3 film has a distinct emittance spectrum at 1550 nm corresponding to Er3+, and the intensity of the selective emission peak strengthens with the measuring temperature or film thickness increasing. The thermal-selective film Er2O3 emitter is found to have emission spectrum suitable for efficient matching with the infrared response of GaSb photovoltaic cell.

Measurements of Hydrogen-Enriched Combustion of JP-8 in Open Flame

Hydrogen enrichment is presented as a control parameter to improve JP-8 combustion. Research in fuel reforming gives an opportunity for hydrogen production at the point of use. Hydrogen-enriched combustion of JP-8 seeks to take advantage of the energy density of JP-8 and the combustibility of hydrogen. At low power output (<2 kWe), technologies such as Stirling engines, thermoelectric, and thermophotovoltaic generators have the potential to compete with diesel engines, but require reliable JP-8 combustion. Experiments were conducted with atomized JP-8 in a 5 kWth open flame, based on a 500 W power source. JP-8 is sprayed through an air-atomizing nozzle. Hydrogen was added to either the atomizing air or to a concentric tube supplying the main combustion air. In these experiments, hydrogen represented up to 26% of the fuel energy contribution (EC). During hydrogen enrichment, JP-8 flow rate was reduced to maintain constant fuel energy input. Temperature is measured vertically and laterally through the flame. Temperature profiles show that combustion shifts toward the nozzle as hydrogen is added. Hydrogen in the secondary air maintains diffusion flame behavior, but earlier in the flame. Hydrogen in the nozzle air creates a premixed pilot flame structure in the center of the flame. This premixed hydrogen and air flame provides initial energy to speed droplet heating and vaporization, producing higher peak temperatures than the other cases studied. Gaseous emissions are measured above the visible flame. Hydrogen enrichment by both methods reduced unburned hydrocarbon emissions by up to 70%. The advantages provided by hydrogen enrichment represent opportunities for reduced size, improved operational reliability and control, and reduced pollutant emissions.

Ultra-compact combustion-driven high-efficiency thermophotovoltaic generators

The confluence of high-temperature (~1600–3200K) flameless combustion chambers and vertical-junction (VJ) photovoltaics creates the possibility of ultra-compact high-efficiency thermophotovoltaic (TPV) generators. The actively-cooled cells can be sited at the chamber's aperture without contaminating or damaging them. The flux density, however, is then well above values at which conventional TPV cells can operate efficiently due to the series resistance losses at photo-generated current densities of the order of tens to hundreds of A/cm2. Generally, this problem is resolved by distancing the cells from the radiator, but at the price of dramatically increasing the requisite cell area and system size. VJ cells offer an intrinsically low-current low-series-resistance configuration that obviates the problem. But carrier dynamics then mandate that only indirect-bandgap semiconductors can provide practical solutions. Silicon and Germanium are the candidates analyzed here, with simulated performance rivaling or exceeding current TPV generators. We also evaluate the degree to which efficiency can be increased by introducing a cell back-surface mirror that recirculates sub-bandgap photons back to the combustion region. The unobvious energetic advantage of recirculating photons by (1) increasing combustion chamber temperature at fixed fuel consumption rate, vs. (2) reducing fuel consumption rate at fixed combustion chamber temperature is also evaluated.

Performance assessment of thermophotovoltaic application in steel industry

The potential for using Thermophotovoltaic (TPV) generators as an alternative for recovering energy losses in steel production industry is assessed. A mathematical model for the assessment of the performance of TPV application in the iron and steel industry has been developed. In order to support the mathematical model, a sample TPV apparatus in laboratory scale based on an IR emitter has been designed and assembled. The key modeling parameters of TPV generator include: the open circuit voltage, the short circuit current density and fill factor of the TPV cell. These parameters have been considered in the model as functions of several variables such as: the emitter (hot steel slab) temperature, the cell temperature, the distance between the cells and emitter, the spectral response and the cell energy gap. The External Quantum Efficiency (EQE) as an important indicator of the cell׳s spectral response is included in the model. Moreover, the variation of the emitter temperature has been considered. Several tests have been carried out for different values of the cell-emitter gap. It has been found that when the GaSb cells are used for energy recovery, a minimum temperature of 873°C is required. The upper limit of the emitter temperature is usually determined in steel production process associated with hot rolling process which has a temperature around 1250°C. Finally, the total efficiency of the system was obtained to 4.12%, when GaSb cell with temperature of 27°C and slab emitters with temperature of 1257°C are used. The results of the simulation of the model in a casting process at the Mobarakeh Steel Complex have shown a potential of energy recovery of 26.987MJ per year.

Monte Carlo simulation of epitaxial growth of GaInAsSb films

Material engineering finds an important support on simulation methods. The study of semiconductors growth techniques through simulation allows the determination of the influence of some growth parameters on the film properties. Experimentally, the variations of these parameters are difficult due to the high experimental demands and expenses. In this work we present the numerical simulation of the epitaxial growth of GaInAsSb by three methods. Devices based on this semiconductor material are thermophotovoltaic generators. The solid-on-solid approximation was used, considering the unit cell formed by the four constituent elements, in the establish proportions according to the choose stoichiometry. Through the Kinetic Monte Carlo method we obtained a high coincidence between the simulated film morphology and the obtained in the experimentally grown films.

Implementation of a TPV integrated boiler for micro-CHP in residential buildings

There is a growing interest in direct thermal-to-electric energy conversion using solid state devices such as thermophotovoltaic (TPV) generators. TPV devices convert thermal radiation from heat sources into electricity without involving any moving parts. TPV opens up possibility for efficient and stand-alone power generation in boilers and furnaces. In this paper, a TPV integrated boiler was designed, built and investigated for micro combined heat and power (micro-CHP) application in residential buildings. A full size gas fired residential boiler was used as a precursor for integration with TPV devices. Experiments were conducted with the prototype TPV boiler so as to evaluate various issues related to this new technology. The electric output of TPV modules installed in the boiler was characterized under different operating conditions. The TPV cell modules generated 246.4W at an emitter temperature of 1265°C, which would be enough to power the electrical components of the whole heating system. Moreover, such a TPV integrated boiler could be employed to form a micro-CHP system in residential homes, providing an effective means for primary energy savings, on-site power and energy security.

Overview and Status of Thermophotovoltaic Systems

In the last decade thermophotovoltaic (TPV) generator has gained an increasing attention as cogeneration system for the distributed generation sector. Nevertheless, these systems are not fully developed and studied: several aspects need to be further investigated and completely understood.The aim of this study is to give a complete overview and the status of the art of thermophotovoltaic generation considering both the research developments and the experiences field. More in details, in this study, the characteristics of a TPV generator are analyzed with a particular attention to the physical relationships which govern the behavior of its main components. Moreover, the current technologies regarding the combustor, the emitter, the optical filter and the photovoltaic cells are investigated by taking into account both the role of each component and also their integration in the whole system. Finally, a critical review of the realized prototypes is presented and discussed.

Thermo – Photo – Voltaic Generator Development

The growing demand of energy coupled with an increasing attention to the environmental impact have forced, in the last decades, toward the study and the development of new strategies in order to reduce primary energy consumptions.The cogeneration (CHP) and the on-site generation (also known as distributed generation) could be the key strategy to achieve this goal; CHP systems allow to reduce the fuel consumption and pollutant emissions (in particular the greenhouse gases) compared to separate generation; moreover on-site-generation contributes to the reduction of the energy which is lost in electricity transmission, and increases the security in the energy supply.In this scenario the Thermo–Photo–Voltaic generation (TPV) is obtaining an increasing attention; TPV is a system to convert into electrical energy the radiation emitted from an artificial heat source (i.e. the combustion of fuel) by the use of photovoltaic cells. A domestic gas furnace based on this technology can provide the entire thermal need of an apartment and can also contributes to satisfy the electrical demand.In this paper, the main research activities on thermophotovoltaic generation developed in the last years at Institute of Materials for Electronics and Magnetism (IMEM) of Italian National Research Council (CNR), of will be presented and discussed.

Thermophotovoltaic energy conversion: Analytical aspects, prototypes and experiences

In the last decade Thermo-Photo-Voltaic (TPV) generators have gained an increasing attention as cogeneration systems for the distributed generation sector. Nevertheless, these systems are not fully developed and studied: several aspects need to be further investigated and completely understood.The aim of this study is to highlight and outline the current state of the art of the TPV generation; more in details, in this study, the characteristics of a TPV generator will be analyzed with a particular attention to the physical relationships which govern the behavior of its main components. Further, the current technology regarding the combustor, the emitter, the optical filter and the photovoltaic cells will be analyzed and investigated taking into account both the role of each component and also their integration in the whole system. Finally, a comprehensive review off all the prototypes realized up to now and reported in literature will be presented and discussed.

Simulation of dual firing of hydrogen-rich reformate and JP-8 surrogate in a swirling combustor

Flame control, particularly at very lean conditions, is a critical requirement for external combustion power sources such as thermoelectric and thermophotovoltaic generators. The availability of in-situ produced hydrogen from JP-8 fuel reforming presents a potential supply of hydrogen at the application site, such as the battlefield, without adding a second fuel to the logistics system. This research investigates the use of small amounts of hydrogen co-fired with JP-8 for improved energy efficiency, operational flexibility and environmental protection. Numerical simulations were conducted in a 6 kW (thermal) swirling flow combustor under different conditions. The simulations use hydrogen and a single hydrocarbon surrogate for JP-8 with a 4-step reaction mechanism. It also examined the effect of using hydrogen-rich reformate containing hydrogen, carbon monoxide and other gases dual-fired with JP-8. Results showed that at lean conditions, dual firing of hydrogen or reformate with JP-8 provided improved fuel conversion, better flame stability and higher fuel burnout. Dual firing hydrogen-rich reformate and JP-8 led to increased burner capacity, reduced system size and weight requirements. However, dual firing with hydrogen provided little benefit to JP-8 combustion under stoichiometric or fuel rich conditions because they were limited by mixing rather than chemical kinetics. These results indicate that dual firing of hydrogen with JP-8 is a promising method for improving lean flame stability and burner control. This has the potential to enable small scale power applications with specific temperature requirements such as thermoelectric and thermophotovoltaic generators.

The critical role of emitter size in thermo-photovoltaic generators

The influence of emitter size on the fuel to electrical conversion efficiency in thermo-photovoltaic generators is investigated. Based on simple and reasonable physical assumptions such as a black body emitter and thermal equilibrium between emitter and exhaust gases we evaluate the electrical efficiency as a function of the emitter area for thermo-photovoltaic generators equipped with Ge, GaSb and Si photovoltaic cells.It is found that for a given thermal energy introduced there are optimal values of the emitter surface which maximize the generator efficiency. The corresponding values of energy per unit area of the emitter are higher than in the realized prototypes. This suggests that it is possible to obtain more efficient thermo-photoconverters with reduced emitter and photovoltaic cell area.

High-efficiency GaSb photocells

High-current solar cells based on gallium antimonide and intended for use in solar modules and systems with solar-spectrum splitting at large solar light concentration ratios, in thermophotovoltaic generators with a high-temperature emitter, and in laser energy converters have been designed and fabricated by the diffusion of zinc from the gas phase. The influence exerted by the thickness of the p+ diffusion layer on the basic characteristics of the solar cell has been studied. The optimal doping profile and the p-n-junction depth providing a high photovoltaic conversion efficiency at photocurrent densities of up to 100 A cm−2 have been determined.

Ceramic IR Emitter with Spectral Match to GaSb PV Cells for TPV

ABSTRACTA high temperature ceramic selective emitter for thermophotovoltaic (TPV) electric generators is described with a spectral match to GaSb IR cells. While solar cells generate electricity quietly and are lightweight, traditional solar cells are used with sunlight and only generate electricity during the day. Workers at JX Crystals invented the GaSb IR cell as a booster cell to demonstrate a solar cell conversion efficiency of 35%. JX Crystals now makes these IR cells. In TPV, these cells can potentially be used with flame heated ceramic emitters to generate electricity quietly day and night. One of the most important requirements for TPV is a good spectral match between the ceramic IR emitted and the IR PV cells. The first problem is to find, demonstrate, and integrate a doped ceramic IR emitter with a spectral match to these GaSb cells. Recently, nickel oxide and cobalt oxide doped MgO-based ceramics have been shown experimentally and theoretically to have spectral selectivity but no attempts have been made to integrate these ceramic IR emitters into a fully operational TPV generator. Herein, we review the history of TPV and note that a key to future progress will be the integration of an appropriate ceramic emitter with cells and a burner to demonstrate an operational TPV generator. Integrating TPV into a residential boiler is discussed as a potential future large volume commercial market.

Thermophotovoltaic Generation of Electricity through Evanescent Wave Effect Using GaSb Cells

Thermophotovoltaic Generation of Electricity through Evanescent Wave Effect Using GaSb Cells

Development and experimental evaluation of a complete solar thermophotovoltaic system

ABSTRACTWe 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.