Thermophotovoltaic Research Articles

Radioisotope Thermophotovoltaic Generator Design Methods and Performance Estimates for Space Missions

This work provides the design methodology of a radioisotope thermophotovoltaic system (RTPV) using spectral control for space missions. The focus is on the feasibility of a practical system by using two-dimensional micropatterned photonic crystal emitters, selecting the proper thermophotovoltaic cell and insulation material to exclude material incompatibilities, to optimize the system efficiency by impedance matching and to design a radiator with minimum mass. In the last section, a design example is presented based on the tested indium gallium arsenide antimonide (InGaAsSb) cells. It is shown computationally that, in using the experimentally tested InGaAsSb cells, the RTPV generator is expected to reach an efficiency of 8.6% and a specific power of with advanced radiators. Using the more efficient InGaAs cells, the system can expect to triple the figure of merits of the radioisotope thermoelectric generator, promising to reach and , respectively. With a high performance device, the results of this work can lead to a functional prototype for further research focusing on manufacturability and reliability.

Thermoradiative-Thermionic Hybrid Energy Electron Devices

In this article, the conceptual model of a hybrid energy converter consisting of a thermoradiative cell (TRC) and a thermionic cell (TIC) is established, where the TRC is made of p-type and n-type semiconductor materials of group III–V, and two metal-based electrodes, while the TIC is composed of an anode and a cathode (one electrode of the TRC). According to the semiconductor physics and the Planck’s radiation theory, the expressions for the overall power output density and energy conversion efficiency of the hybrid device are given. The output voltages of the two subcells are optimized to obtain the optimal power output density and efficiency. Furthermore, the optimum thicknesses of the TRC are determined to obtain the maximum power output density and efficiency of the hybrid energy converter. Making tradeoff between the power output density and efficiency, the parametric optimum regions are determined. This article provides a route to efficiently convert the medium-grade thermal energy into electricity.

Design of an All-Semiconductor Selective Metamaterial Emitter in the Mid-IR Regime with Larger Feature Sizes for Thermophotovoltaic Energy Conversion Applications

A thermophotovoltaic (TPV) system converts heat that is absorbed via conduction, convection, and/or radiation to electricity. The efficiency of TPV energy conversion can be improved with a narrowband selective emitter that emits photons at just above the bandgap energy towards the TPV photodiode. We numerically report a selective metamaterial (MM) emitter design with a single layer of cylindrical structures of p-type silicon (boron-doped). Our design (substrate-free) features a peak absorbance of 94.8% at the wavelength of 3.47 μm with the smallest lateral dimension of 0.8 μm. The absorption is found to be due to the resonance of electric and magnetic fields in the structure. The larger dimensions of our selective MM emitter design make it significantly easier to pattern than many of previously reported selective MM emitters operating at similar wavelengths to that of our work. We believe that our work demonstrates a path forward for future research on larger-area all-semiconductor selective MM emitters with a variety of peak absorbance wavelengths for TPV applications.

Evaluation of performance of near-field thermophotovoltaic systems based on entropy analysis

Near-field radiative heat transfer can significantly improve the output power of thermophotovoltaic (TPV) systems. Therefore, it is crucial to explore how to increase the energy conversion efficiencies of near-field TPV systems. In this study, based on the fluctuation–dissipation theorem with the effective medium theory, we evaluate the performance of a near-field TPV system from the formulation of thermodynamics. It is found that a near-field TPV system consisting of InAs or GaSb cell can achieve higher heat flux or efficiency limit, respectively. Moreover, the TPV system with a hyperbolic metamaterial (HMM) emitter composed of nanowire or nanohole arrays can achieve higher heat flux and efficiency limit compared to that of the bulk reference. This is attributable to the HMMs being able to support hyperbolic modes, and the radiation with the HMM emitter exhibits a favorable entropy content for the energy conversion efficiency. This work provides an approach to determine the efficiency limit and establish a target for efficiency of the near-field TPV system without considering how the system works. The results of this study will facilitate the design and application of the HMM emitter and material of the TPV cell to improve the efficiency of near-field TPV systems.

Harvesting waste heat produced in solid oxide fuel cell using near-field thermophotovoltaic cell

A coupled system consisting of a regenerator, a solid oxide fuel cell (SOFC), and a near-field thermophotovoltaic cell (NFTC) is proposed to recovery the waste heat from the SOFC. Based on the theories of electrochemical and fluctuation electrodynamics, analytical formulas for the power output and the energy efficiency of the coupled system are derived. The dependence of the matching area ratio between the subsystems on the key parameters is discussed. As the SOFC works at 1073 K, the two voltages of the subsystems are optimized. The maximum power output density of 1.01 Wcm−2 and an efficiency of 0.402 are achieved for the proposed system. By comparing to the performance of the single SOFC, the optimally working regions of the coupled system are determined. The effects of the SOFC's temperature and the NFTC's vacuum gap on the maximum power output density and the optimum operating conditions of the system are studied. One can find that the maximum power density in the near-field is 1.3 times than that in the far-field and better than the most of the other SOFC-based systems. The present work can provide a new route to efficiently utilize the waste heat of SOFC to achieve higher energy efficiency.

High temperature ultra-small modular reactor: Pre-conceptual design

This paper describes the pre-conceptual design for a high temperature (>1300 °C), ultra-small (1–10 MWe) modular reactor with a high efficiency (>50%) thermophotovoltaic (TPV) power-block. The integration of the TPV is attractive for increased efficiency over the heat cycles of traditional nuclear power plants (NPP) and increased inherent safety from the elimination of a working fluid. Allowing for heat removal through radiative and passive convective cooling, the design must operate at low power densities. A preliminary sampling of the design space was performed using coupled thermal and neutronic analysis on a simplified model. This study shows the viability of the design and reveals the favorability of using uranium nitride fuel that allow the core to operate safely at approximately 1–2 W/cm3 within the desired temperature limits. Although preliminary, the depletion and economic analyses reveal an operational time on the order of ten thousand days and approximately 43% savings in NPP capital costs.

Broadening Near-Field Emission for Performance Enhancement in Thermophotovoltaics.

The conventional notion for achieving high efficiency in thermophotovoltaics (TPVs) is to use a monochromatic emission at a photon energy corresponding to the band gap of the cell. Here, we prove theoretically that such a notion is only accurate under idealized conditions and further show that, when nonradiative recombination is taken into account, efficiency improvement can be achieved by broadening the emission spectrum, due to an enhancement in the open-circuit voltage. Broadening the emission spectrum also improves the electrical power density, by increasing the short-circuit current. Hence, broadening the emission spectrum can simultaneously improve the efficiency and power density of practical TPV systems. To illustrate these findings, we focus on surface polariton-mediated near-field TPVs. We propose a versatile design strategy for broadening the emission spectrum via stacking of multiple plasmonic thin film layers. As an example, we consider a realistic ITO/InAs TPV and predict a conversion efficiency of 50% simultaneously with a power density of nearly 80 W/cm2 at a 1300 K emitter temperature. The performance of our proposed system far exceeds previous works in similar systems using a single plasmonic layer emitter.

High-efficiency solar thermophotovoltaic system using a nanostructure-based selective emitter

In this work, we present the design, fabrication, optimization, and experimental results of a high-efficiency planar solar thermophotovoltaic (STPV) system utilizing a micro-textured absorber and a nanostructure multilayer metal-dielectric coated selective emitter fabricated on a tungsten (W) substrate. Light absorptance of more than 90% was achieved at visible and near-infrared wavelengths using the microtextured absorbing surface. The nanostructure selective emitter consists of two thin-film optical coatings of silicon nitride (Si3N4) and a layer of W in between to increase the surface emissivity in spectral regimes matching the quantum efficiency of the thermophotovoltaic (TPV) cells. Gallium antimonide (GaSb)-based TPV cells are used in our STPV design. The experiment was conducted at different operating temperatures using a high-power continuous wave laser diode stack as a simulated source of concentrated incident radiation. Our experimental setup measured a maximum electrical output power density of 1.71 W/cm2 at 1676 K STPV temperature, and the overall power conversion efficiency of 8.4% after normalizing the output power density to the emitter area. This is the highest STPV system efficiency reported so far for any experimental STPV device. The incident optical laser power on the absorber side was 131 W. This is equivalent to a solar concentration factor of ~2100, which is within the practical limit and readily achievable with Fresnel lens setup.

A novel thermophotovoltaic optical cavity for improved irradiance uniformity and system performance

It has been demonstrated that a single thermophotovoltaic (TPV) cell can produce a high-output power density. In practical applications, many such cells are arrayed to obtain a high level of output. However, the irradiance on the surface of each cell of an array may be different. This nonuniformity causes mismatch loss in the array and reduces the reliability of the cells. To address these problems, a novel TPV optical cavity with reflectors was proposed to improve the irradiance uniformity of the cell array. A model TPV system using the proposed cavity was built and evaluated for comparison with traditional systems. TracePro and MATLAB software were used to examine the irradiance uniformity of cell arrays and the electrical performance of TPV systems that incorporate these arrays. Besides system efficiency ηsystem and maximum output power Pmax, several critical parameters degree of uniformity N, network efficiency ηnet, and average cell electric power density Wave, were used to evaluate the performance of the TPV systems. The results indicate that the proposed cavity improved the irradiance uniformity of the TPV cell array, thereby reducing mismatch loss and enhancing system performance, and it did so with a reduced number of cells.

Electronic states and interband tunneling conditions in type-II quantum well heterostructures

Type-II quantum well (QW) heterostructures based on the InAs/GaSb/AlSb material system have important applications in interband tunneling and midinfrared optoelectronic devices. From the perspective of interband tunneling conditions in widely used type-II QWs such as N-QW, W-QW, and M-QW structures, this work studies how energy levels for electronic and light-hole states are varied with various combinations of semiconductor material layers. Using a two-band model, analytical expressions are derived for the transition well widths that correspond to the critical layer thicknesses for bound- or quasibound states to appear in the interband tunneling region in type-II heterostructures, which provide meaningful connections to semiconductor material parameters. Calculations of transition well widths are carried out for several InAs/GaSb/AlSb-based type-II QW structures. The results and understanding gained from the study may be helpful in designing and developing midinfrared optoelectronic devices such as interband cascade lasers, photodetectors, and thermophotovoltaic cells.

Efficiency Enhancement of a Thermophotovoltaic System Integrated With a Back Surface Reflector

This work aims to investigate the various factors which may affect a thermophotovoltaic (TPV) system's performance, with a special focus on the importance of incorporating a back surface reflector (BSR), which enables below-bandgap photons' recycling. The possible extent to which common PV materials can be used in TPV applications is investigated by comparing them on a Planck distribution curve. The effects of varying BSR reflectivity, TPV cell's external quantum efficiency, and emitter temperature are investigated on the TPV module's efficiency using open-circuit voltage, empirical relations for fill factor, maximum voltage, and photogenerated current. It is shown that TPV applications require materials with smaller (e.g. 0.6 eV 26%) above-bandgap without a spectral control and a small difference between peak and bandgap wavelength. It is shown that the inclusion of a BSR (reflectivity = 1) results in an increase of 15% in TPV efficiency. The results show that by the collective changes of an added BSR, high emitter temperature (>2000 K), and improved external quantum efficiency (EQE ≈ 1), the present TPV systems can attain efficiency values more than 30% which makes them a favorable prospective choice for Concentrated Solar Power.

Recent Development of Thermophotovoltaic System for Waste Heat Harvesting Application and Potential Implementation in Thermal Power Plant

Rapid depletion of fossil fuels due to the growing demand for energy has resulted in a worldwide concern to improve energy conversion efficiency. Yet, the energy conversion of conventional fossil fuel power generation plants remains relatively low (less than 40%) and a huge amount of energy is wasted in the form of heat, leading to global warming issues. Recycling and recuperating even a small portion of energy losses could provide a huge impact on energy saving and minimize the reliance on fossil fuels. Thermophotovoltaic system appears to be a potential candidate to capture, recover, and convert waste heat energy into useful electricity. This paper presents an overview of the recent development of thermophotovoltaic technology for waste heat recovery applications. Each component in the thermophotovoltaic system including thermophotovoltaic generator/heat source, thermal emitter, spectral filter and thermophotovoltaic cells is vital and can be engineered to achieve a better heat-to-electricity conversion efficiency. Recently, researchers have shown great interest in near-field thermophotovoltaic systems where higher power intensity can be captured by the thermophotovoltaic cell, thus improving the overall system performance. Furthermore, the potential locations for energy scavenging in thermal power plants is investigated based on the on-site temperature measurement. In Malaysia, it is estimated that around 3,831 GWh of waste heat energy could be saved in operational thermal power plants. This review will contribute to the knowledge for future development thermophotovoltaic systems in waste heat recovery applications while summarizing the potential locations for energy scavenging in thermal power plants.

High-performance near-field thermophotovoltaic device with CaF<sub>2</sub>/W multilayer hyperbolic metamaterial emitter

Thermophotovoltaic (TPV) device is a thermoelectric conversion method with great application prospects. In the far-field regime, the thermoelectric power is usually small due to the Planck blackbody radiation limit, but can be substantially enhanced in the near-field regime where evanescent waves will participate in the heat transfer by tunneling. In this aspect, the surface optical density of state is a key factor that will determine the transfer conversion of thermal photons. Plasmonic or phononic resonance materials have been discussed in the literature to acquire large heat flux. Besides, metamaterial is another way to pursue the design freedoms for the same purpose. In this work, we propose a [CaF<sub>2</sub>/W]<i><sub>n</sub></i> multilayer based infrared hyperbolic metamaterial (HMM) with high surface density of states as an emitter of a high-performance TPV cell made of an InSb p-n junction (energy bandgap = 0.17 eV). The effective medium theory (EMT) is utilized to describe the electromagnetic behavior of the HMM. The near-field heat flux is calculated based on electrodynamic wave theory and Green's function method, and the photocurrent of thermophotovoltaic device is derived using diffusion equation for semiconductor. For comparison, we design three different radiators, i.e. tungsten film (W), [GaF<sub>2</sub>/W]<i><sub>n</sub></i> multilayer hyperbolic metamaterial (HMM), and tungsten-grounded HMM (WHMM). Compared with the pure tungsten radiator, the artificial structure exhibits the hyperbolic dispersion characteristic in a wide frequency range, which gives rise to a higher local density of states, in particular in the hyperbolic-to-elliptic spectral transition region. As a result, the radiation power and the energy conversion efficiency are greatly enhanced, which are more easily realized by a matched emission band achieved by the structural design. We find that the thermophotovoltaic device with WHMM radiator has a similar power and conversion efficiency to that with the HMM radiator. The influence of the substrate can be ignored when the hyperbolic metamaterial is thicker than 140 nm, very beneficial to the actual fabrication of the device. By our system, with multilayer hyperbolic metamaterial (HMM) radiator, a high electric power >1 W/m<sup>2</sup> and a conversion efficiency about 11% can be obtained at a bias temperature of 200 K and a 100 nm vacuum gap. Compared with nanowire arrays or natural hyperbolic material, the multilayer structure proposed in this paper has obvious advantages in bandwidth and manufacturing and may find important applications in near-field thermophotovoltaic device and other relevant areas.

Biosyngas-fueled platinum reactor applied in micro combined heat and power system with a thermophotovoltaic array and stirling engine

Currently, alternative fuels, derived from biomass and urban maximize the overall power output, have been increasingly attracting more attention in the industrial and residential sectors. Micro-combined heat and power (CHP) systems with high energy efficiency have been developed, particularly for the use in remote and rural areas. This study used a micro-CHP system, with a combustion-driven thermophotovoltaic (TPV) cell array and a Stirling engine-driven power system, within which methane and bio-syngas fuels were deployed strategically. This micro-CHP system harvests energy generated through thermal radiation from the reactor surface and harvests thermal energy from hot flue gas. Eventually, the micro-CHP prototype was proven that all energies are converted to electricity, and hot water can be simultaneously supplied. High incandescent surface and high-temperature flue gas of platinum reactor provide the thermal sources for TPV cell array and Stirling engine. The overall efficiency of the micro-CHP system was 35.0% for 50%H2+50%CO, and the generated power included 2.7, 3.5, and 272.1 W from the Stirling engine-driven power system, GaSb TPV cell array, and hot water supply system, respectively. The systematic performance of the micro-CHP system, the combustion features, radiation efficiency, fuel conversion rate, total electricity output, and corresponding overall efficiencies were examined thoroughly.

Investigation on premixed H2/C3H8/air combustion in porous medium combustor for the micro thermophotovoltaic application

The study of premixed H2/C3H8/air combustion in the combustor inserted with porous medium (PM), which involved ten kinds of PM and varied with porosity, wire diameter and pores per inch (PPI), is experimentally and numerically conducted. Moreover, in order to boost the electrical power output and efficiency of the micro thermophotovoltaic (TPV) system, the effects of flow rate and propane blended ratio of the fuel on combustion and the thermal performance are investigated. The results indicate that the flame stability and heat transfer of premixed H2/C3H8/air combustion can be significantly enhanced in PM. The 10% propane blended fuel effectively changes the flame location and temperature, as well as the thermal performance of the combustor and the outer wall temperature distribution, while the thermal properties of the combustor are less decreased with 15% propane added. It is also found that the combustor inserted with PM, which has an areal density of 0.0524–0.0551 g/cm2 (PM #30-0.2, #60-0.14 and #80-0.12) and a porosity of 0.9, achieves the highest mean wall temperature and provides a high and uniform wall temperature at different inlet flow rates. The combustor inserted with PM #30-0.2 achieves the electrical power output 2.02 W and efficiency 1.6% for the micro TPV system under the condition of fuel flow rate 400 mL/min, propane blended ratio 12% and equivalence ratio 0.8.

Omnidirectional broadband absorber for visible light based on a modulated plasmonic multistack grating

We present an ultra-thin film absorber made of a metallic/dielectric (Ni/SiO2) metamaterial with a half cylinder cross section. The proposed structure exhibits high absorption for both polarization modes. The total average absorption is over 96% and 92% for transverse electric (TE) and transverse magnetic (TM) polarization, respectively, for normal incidence in all the visible spectrum. This property is well maintained over a wide range of angles of incidence from 0° to 80°. The effects of the structure periodicity are also investigated for both modes and the results show small changes for variations of the order of 200 nm. We also analyzed the structure for a different number of metal-dielectric pairs considering different layers thicknesses. The results show that the absorber can maintain an excellent absorption even with possible errors during the fabrication process. The proposed structure can be used for a wide range of applications, such as thermal emitters, thermophotovoltaics (TPV), optical energy harvesting, thermal detectors among others where a broadband absorption is required.

Schottky junction-based thermophotovoltaic-thermionic devices

A conceptual model of a Schottky junction (SJ)-based thermophotovoltaic-thermionic device (SJTTD) that can simultaneously convert thermally emitted electrons and photons into electricity is proposed, where the thermionic cell (TIC) is composed of a tungsten-based cathode and anode, and the thermophotovoltaic cell (TPVC) is made of a thermal emitter and an SJ. The thermal emitter simultaneously acts as the cathode of the TIC and the anode/p-type silicon interface forms the SJ. The impacts of space charge effect on the TIC and radiative recombination on the TPVC are taken into account. According to the theory of statistical physics, analytical expressions for the total power output density and energy conversion efficiency of the SJTTD are derived. The maximum power output density and efficiency are calculated numerically by optimizing the key parameters of the two subsystems. Further, the parametric optimum criteria are provided. The optimally operating states of the ideal and non-ideal SJTTDs are compared. The results show that selecting cathode materials with high emissivity can significantly enhance the performance. The proposed model may be helpful for developing high-efficiency hybrid electron devices.

Thermophotovoltaic systems for achieving high-solar-fraction hybrid solar-biomass power generation

Medium operating temperature hybrid solar-biomass TPV power plant design requires complex integration of multiple high temperature processes with low band-gap TPV cells. A 0.72 eV band-gap GaSb TPV cell has been used in thermophotovoltaic (TPV) systems operating at temperatures above 1400 °C. Low band-gap TPV cells, such as InGaAs (Eg = 0.55 eV) and InAs (Eg = 0.36 eV) could enable a TPV system to operate optimally at temperatures ≈1000 °C. To examine this, two hybrid solar-biomass TPV system configurations are studied using TRNSYS simulation that incorporates a new algorithm for TPV. It is found that in a high solar fraction CSP power plant, a TPV system could recover surplus thermal energy gained from solar energy at mid-days that would otherwise be unused.

Thermophotovoltaic applications in waste heat recovery systems: example of GaSb cell

Abstract In this study, it is aimed at evaluating real data in high temperature GaSb cell thermophotovoltaic (TPV) systems. The TPV systems are considered as an alternative energy source in terms of efficient use of waste heat, cost and efficiency. The TPV system can be defined as a system that converts waste heat energy emitted from heat sources into electrical energy at high temperature. In this context, efficiency and parameters of TPV GaSb cells have been determined in laboratory conditions. The conversion of the high temperature applied to the cell to electrical energy has been investigated by selecting the GaSb photovoltaic cell as the cell type. According to the analysis have been done so far, TPV high-temperature real graphics have been obtained using GaSb cell. The temperature parameters used are, namely, cell temperature and source temperature. With these graphs, energy efficiency, fill factor, effect of open-circuit voltage and short-circuit current values have been determined. While the efficiency value of the GaSb TPV cell systems was calculated, the radiation source temperature values have been taken in increments of 300 K between 1300 and 3100 K. In this analysis, the optimum energy conversion efficiency values of GaSb solar cell structure have been detected to be 21.57%. Opinions about the feasibility, efficiency and development of thermophotovoltaic energy conversion systems are stated, and suggestions are presented.

Bi2Te3/Sb2Te3 thermophotovoltaic cells for low temperature infrared radiation

The thermophotovoltaic cells which convert the low temperature (below 500 K) infrared radiation into electricity are of significance due to their potential applications in many fields. In this work, narrow bandgap Bi2Te3/Sb2Te3 thin film thermophotovoltaic cells were fabricated, and the formation mechanism of Bi2Te3/Sb2Te3 p-n heterojunctions was investigated. At room temperature, the heterojunctions are formed through the diffusion of electrons and holes along the same direction from n-type Bi2Te3 thin films to p-type Sb2Te3 thin films rather than conventional bi-directional diffusion. Because the strong intrinsic excitation generates a large number of intrinsic carriers which weaken the built-in electric field of the heterojunctions, their I-V curves become similar to straight lines. It is also demonstrated that Bi2Te3/Sb2Te3 thermophotovoltaic cells can output electrical power under the infrared radiation from a low temperature heat source. Although the open-circuit voltage and the short-circuit current of the cells are low, this work provides a possible way to utilize the low temperature radiation.