Thermophotovoltaic System Research Articles

Inverted metamorphic AlGaInAs/GaInAs tandem thermophotovoltaic cell designed for thermal energy grid storage application

We demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice-mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for high-temperature thermophotovoltaic (TPV) applications. This device differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer that otherwise filters light from the junctions below and absorbs sub-bandgap light via free-carrier absorption. Sub-bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells to enable band edge spectrum filtering. Three components required development to enable this device: (1) a lattice-mismatched 1.2 eV AlGaInAs junction, (2) a metamorphic contact layer grown after the graded buffer, and (3) a transparent tunnel junction that sits in front of the 1.0 eV GaInAs junction. Growth conditions that minimize oxygen defect incorporation maximize AlGaInAs cell quality, enabling a 0.41 V bandgap open circuit voltage offset at 22 mA/cm2 under AM1.5D. A mismatched GaInAs:Se layer is developed as a low resistance contact. Lastly, we develop a GaAsSb:C/GaInP:Se tunnel junction suitable for high-power densities with more transparency than the GaAsSb:C/GaInAs:Se structure used in past IMM cells. We characterize the tandem device under a high-intensity spectrum that approximates the emission from a 2150 °C blackbody radiator and deduce a projected ideal TPV efficiency of 39.9% at ∼30% of the blackbody irradiance and 36% ideal TPV efficiency under the full 118 W/cm2 irradiance. Improvements to the back-surface reflectivity and series resistance are expected to increase the ideal TPV efficiency well above 40%.

Theoretical design of nanoparticle-based spectrally emitter for thermophotovoltaic applications

Wavelength-selective emitter is a critical component in thermophotovoltaic (TPV) systems. How to evaluate and further enhance TPV efficiency for different PV cells by designing selective emitter is still needed for realistic applications. Hence, the impact of the nanoparticle-based spectrally selective emitter on the TPV efficiency was investigated in this work. The nanoparticle material, radius, volume fraction and thickness of emitter was optimized based on the spectral cut-off efficiency ηs and effective emittance efficiency ηe for GaSb and InGaAsSb PV cells respectively. Results showed that W emitter achieved better TPV performance than Ti and Ni emitters. The maximized ηsηe achieved 0.415 and 0.535 for GaSb and InGaAsSb PV cells at 1200 °C, indicating that this nanoparticle-based structure emitter was more suitable for the InGaAsSb PV cell with the low bandgap. These results give valuable insights into design opportunity for the selective emitter in various TPV systems.

Performance Analysis of GaSb Cell and Thermophotovoltaic System Under Near-Field Thermal Radiation

An energy transfer model of a thermophotovoltaic (TPV) system, considering the variation of GaSb cell parameters under the near-field thermal radiation, was constructed. The effects of emitter–cell distances with different cooling systems on cell parameters and TPV system performances were analyzed. The results show that if the cell temperature is set at 300 K, the required convective heat transfer coefficient of the cooling system should be as high as 8.1 × 105 W/(m2·K), which is a severe requirement for the existing cooling system. As the cell temperature increases, the photocurrent density decreases, because the internal quantum efficiency decreases due to the decreasing carrier life. The photocurrent density is significantly affected by the emitter–cell distance, because the distance has a strong impact on the thermal radiation. Similarly, the open-circuit voltage decreases with the increasing cell temperature and emitter–cell distance. To ensure a safe operation, the extreme emitter–cell distance is 12 nm for the best cooling system. When the emitter–cell distance decreases from 100 nm to 12 nm, the cell efficiency, which is around 0.25 at the far-field condition, decreases under the near-field thermal radiation, decreasing the TPV system efficiency which is obviously lower than that compared with the cell temperature of 300 K. Thus, the system performances with the cell temperature set at 300 K are overestimated. The results reveal that the variation of cell temperature needs to be considered to actually evaluate the performance of the near-field TPV system, and highly efficient cooling systems are urgently needed for the near-field TPV system.

Ga0.84In0.16As0.14Sb0.86/InAs0.91Sb0.09 Dual-Junction Device for Thermophotovoltaic Energy Conversion

We have theoretically shown thermophotovoltaic (TPV) energy conversion with Ga0.84In0.16As0.14Sb0.86/InAs0.91Sb0.09 tandem cell, for which both subcells have the same lattice matched to GaSb substrate and can be experimentally fabricated in a high quality. Under the illumination of blackbody-like thermal spectra, the doping profiles of this device should be controlled as ${N}_{d{(}{a}{)}} = {12}$ (6) $\times 10^{{17}}$ cm−3 for top subcell while 1018(17) cm−3 for bottom one, and energy conversion efficiency superior to 13% is expectable for radiator temperature ranging from 1700 to 2000 K, showing the enhancement in both efficiency and power density output when comparing with those for Ga0.84In0.16As0.14Sb0.86 single-junction cell. We have thus demonstrated a new type of experimentally available dual-junction cell to boost the thermal radiation conversion of TPV system.

AgGaTe2 – The thermoelectric and solar cell material: Structure, electronic, optical, elastic and vibrational features

Silver gallium telluride AgGaTe2 is a promising and one of few multifunctional energy convertor crystal for processing solar energy or waste heat into electricity (by the thermoelectric, photovoltaic and recently intensively studied thermophotovoltaic systems). Its electronic structure, optical, elastic and vibrational properties were consistently and systematically studied in the framework of the first principles calculations. It should be emphasized that the results of our investigations were verified by comparison with the collected available experimental and vestigial theoretical data. The main spectral features of the electronic band structure and origin of the electronic levels affecting the physical and chemical properties are analyzed in detail. Spectral dependencies of both real ε1 and imaginary ε2 components of dielectric function ε(ω) have been calculated showing a good agreement with spectral ellipsometry measurements. The anisotropy of optical and elastic properties, which is important for possible applications, has been highlighted by means of the 2D and 3D graphs. Phonon dispersion curves and corresponding density of states are studied in the frame of linear response approach based on the density functional perturbation theory. Unified approach focused on the analysis of a wide-range physical properties of anisotropic crystals not only results in good agreement with the experimental data, but also allows to explain the observed optical effects at the microscopic level. Therefore, the presented results should be very useful for future possible improvements in the desired properties of the base material both by changing the chemical composition and/or introducing stresses by epitaxial growth of thin films.

Transparent planar indium tin oxide for a thermo-photovoltaic selective emitter

Designing an efficient emitter design is an important step for achieving a highly efficient TPV conversion process. Wavelength-selective emissivity, spectra match between the emitter and TPV cells, and high thermal stability are three main characteristics that must be considered before implementing the emitter. In this work, an indium tin oxide (ITO)/sapphire emitter structure is investigated for TPV application over the temperature range from 200°C to 1000°C. A 1-µm-thick ITO layer is deposited on a 650-µm-thick sapphire substrate. In addition, 50-nm-thick SiO2 is deposited on top of the ITO to enhance the performance of emitter at high temperatures. High-temperature emissivity and absorptivity measurement of the emitter samples are obtained using FTIR and a Hitachi U-4100 spectrophotometer, respectively. The resultant SiO2/ITO/sapphire/stainless-steel planar emitter structure has selective emission with high emissivity of ∼0.8 in the 1–1.6 µm wavelength regime at 1000°C. This emission range lies at the bandgap edge of silicon TPV cells and thus can be used to harness the true potential for making a low-cost thermophotovoltaic system.

DESIGN AND OPTIMIZATION OF ONE-DIMENSIONAL TiO2/SiO2 PHOTONIC CRYSTAL FOR THERMOPHOTOVOLTAIC FILTER

Thermophotovoltaic (TPV) energy conversion systems convert solar energy into electricity via thermally radiated photons at tailored wavelengths to increase energy conversion efficiency. In this study, we report the design and analysis of a TPV system with one-dimensional photonic crystals (1DPCs) filter using thermal electrical model that includes the thermal coupling between the emitter/filter (PhCs)/PV cell. The performance of the proposed structure, one-dimensional photonic crystals (TiO2/SiO2) 6 with P periods of Titanium Dioxide (TiO2)and Silicon Dioxide (SiO2) of thermophotovoltaic system, has been studied by Mbakop et al [1] and Banerjee et al[2]. The temperature distribution and electrical power output of TPV cell are calculated at a heat varying from 1000 K to 1600 K using parametric solver at a regular interval of 25 K. Furthermore, voltaic efficiency distribution in a concentrating thermal photovoltaic system is presented.

Effects of photon recycling, trapping, and reuse on thermophotovoltaic conversion efficiency and output power

Photonic management is a key issue for the optimization of thermophotovoltaic (TPV) energy conversion systems. It is realized by selective emitters, front surface filters on TPV cells, and back surface reflectors (BSRs). Photonic management modifies photon energy transfer from the emitter to the TPV cell due to photon reuse and energy conversion processes in the TPV cell due to photon recycling and trapping in the cell. Our work has developed a comprehensive thermodynamic theory of photonic management in the TPV cell and in whole TPV systems to elucidate key optimization parameters. Our approach is based on the exact Lambert function solution of the generalized Shockley–Queisser model and the corresponding fundamental formulas of endoreversible thermodynamics for maximal electric power, emitted optical power, and dissipation losses. The model includes interrelated processes of photon recycling, photon trapping, nonradiative recombination, and parasitic absorption of the BSR. Optimization of a TPV system with photon reuse should take into account that the cell thickness that provides maximal output power does not correspond to the thickness, which gives the maximal conversion efficiency. The theory predicts the important limits for TPV efficiency and output power determined by the Auger recombination in low-bandgap semiconductor materials, various parasitic losses in the cell and conductive layers, and photon escape from the TPV system. For example, we consider the TPV system based on 0.6 eV InGaAs cells with a BSR and a front surface photon scattering layer, which provides Lambertian light trapping.

Three-junction tandem photovoltaic cell for a wide temperature range based on a multilayer circular truncated cone metamaterial emitter

To improve the conversion efficiency of thermophotovoltaic devices, we designed a thermophotovoltaic system based on an InAs/InGaAsSb/GaSb three-junction tandem cell. The tandem cell can recover photons in the wavelength range of 200–3650 nm and therefore enhance the output power of the system. To further improve system performance, we designed a multilayer circular truncated cone metamaterial emitter matching the tandem cell. Existing TPV systems based on multi-junction tandem PV cells can achieve conversion efficiencies of 33.3%–41%, while the thermophotovoltaic system coupled with the multilayer circular truncated cone metamaterial can recover more photons of 1.44 mol/(m2·s) and achieve a higher conversion efficiency of 52.8% at 1773 K. The thermophotovoltaic system designed here demonstrates an extremely high energy conversion efficiency and has good application prospects.

Optimal design of molten carbonate fuel cell combined cycle power plant and thermophotovoltaic system

A novel hybrid system consisting of a molten carbonate fuel cell (MCFC), a gas turbine (GT), a steam cycle and a thermophotovoltaic (TPV) system to generate power is introduced and investigated. For this purpose, a GT is coupled to a MCFC in order to utilize unused fuel, raise the gas components temperature, supply the required carbon dioxide, and further power generation. Moreover, in order to boost the power generation and ameliorate the overall efficiency, a TPV system together with a steam cycle has been added to the system for heat recovery from the combustion chamber radiation and cathode output stream, respectively. The simulation of the hybrid system was accomplished through ASPEN HYSYS and MATLAB software. The effect of different influential parameters on each sub-system as well as the proposed hybrid system was examined comprehensively through sensitivity analysis. The results obtained revealed that increasing MCFC temperature decreased the anode, cathode, and ohmic overpotentials, while augmented the output voltage of the MCFC led to enhanced MCFC performance. Furthermore, the output voltage, power density, and efficiency of the MCFC enhanced with increasing the operating pressure for both of the fuel consumption rates. The output voltage and power density of the MCFC decreased as the fuel consumption rate incremented due to increased polarization losses, while the electrical efficiency of the MCFC enhanced. Optimizing the output temperature of the first and second heat exchangers increased the power of the steam turbine by about 16 kW. The highest power density and performance of the TPV system was obtained at cell and emitter distance of 1 cm. Incorporating the GT to the MCFC enhanced the system’s efficiency to 54.83%. The efficiency of the proposed hybrid system was found to be 67.3%.

Design and validation of a high-efficiency planar solar thermophotovoltaic system using a spectrally selective emitter.

This paper presents a detailed-balance analysis required for the achievement of a high-efficiency spectral selective STPV system utilizing thermodynamic and optical modeling approaches. Key parameters affecting the design and optimization of spectrally selective surfaces that are essential for high-efficiency STPV applications are investigated. A complete GaSb-based planar STPV system utilizing a micro-textured absorber and a nanostructure multilayer metal-dielectric coated selective emitter was fabricated and evaluated. The micro-textured absorber features more than 90% absorbance at visible and near-infrared wavelengths. The selective emitter, consisting of two nanolayer coatings of silicon nitride (Si3N4) and a layer of W in between, exhibits high spectral emissivity at wavelengths matching the spectral response of the GaSb cells. The performance of the STPV system was evaluated using a high-power laser diode as a simulated source of concentrated incident radiation. When operated at 1670 K, an output power density of 1.75 W/cm2 and a system efficiency of 8.6% were recorded. This system efficiency is higher than those of previously reported experimental STPV systems. Optical and thermal losses that occurred at multiple stages of the energy transport process were modeled and quantified. Essential guidelines to mitigate these losses and further enhance the system performance are also provided.

Consideration of temperature-dependent emissivity of selective emitters in thermophotovoltaic systems

Spectral emissivity control is paramount for designing a high-efficiency selective emitter surface required for thermophotovoltaic (TPV) applications. Owing to the temperature dependency of materials optical constants, the spectral properties of a selective emitter surface changes with the emitter temperature. This paper presents the fabrication of a multilayer metal-dielectric ( S i 3 N 4 / W / S i 3 N 4 ) coated tungsten selective emitter aimed for GaSb-based TPV systems and studies the dependence of its surface spectral emissivity, ε ( λ ) , upon a temperature ranging from 300 K to 1500 K. Both the simulation and experimental methods were used to characterize ε ( λ ) as a function of temperature. For wavelengths less than 1.4 µm, ε ( λ ) was found to have a minimal dependence on temperature. Beyond 1.4 µm, ε ( λ ) increases with the temperature. At 1.55 µm, the simulation and experimental data estimated a ∼ 4 % greater emissivity at 1500 K than at room temperature. At 1500 K, the increased ε ( λ ) at longer wavelengths lowered the spectral conversion efficiency of the selective emitter from 58% to 47%. The output power density, sub-bandgap loss, and TPV conversion efficiency ( η T P V ) for a GaSb cell illuminated by the selective thermal emitter at 1500 K were estimated. η T P V drops from 13.7% to 11% due to the increased sub-bandgap emission at 1500 K. Essential approaches for mitigating the sub-bandgap losses to further improve η T P V are also discussed.

Performance analysis of solar thermophotovoltaic system with selective absorber/emitter

Selective spectra of absorber and emitter have great effects on the solar thermophotovoltaic (STPV) performance. In this work, a systematic thermal analysis of the STPV system was conducted to optimize the selective absorber and emitter. For the ideal absorber and emitter, the maximum STPV efficiency can be achieved at the cutoff wavelength of 1.2 μm for the ideal absorber and emitted spectrum width of 1.0 μm for the ideal emitter. Increasing concentrated factor of solar radiation from 1 to 100 can enhance the total STPV efficiency from 10.4% to 20.3%, indicating that the large incident solar intensity can enhance the STPV efficiency. In addition, increasing ratios of length to gap (eg: l/d to be ~ 30) and length to width (eg: n to be ~ 6) can further increase the STPV efficiency by reducing the radiation loss. Compared with an original meta-surface serving as both absorber and emitter, the designed selective absorber and emitter can achieve a STPV efficiency of 9.5%, which ~ 20% higher than that of the original meta-surface (7.9%) while it is still lower than that of ideal absorber/emitter. These results indicate that the selective spectrum of the absorber and emitter is of significant to improve the STPV performance, giving valuable insights into design opportunity for the selective absorber and emitter in the STPV system.

Evaluation of Applicability of Thermophotovoltaic System in Combi Boiler

In this study, thermophotovoltaic (TPV) system components are examined and introduced, and theoretically, it is given information about the applicability of the system in combi boiler. The latest developments in the components of the TPV system, heat source, emitter, filter and TPV cell, are explained. In addition, the study state how the TPV system should be integrated into a combi boiler. It is estimated that TPV system integrated into the combi boiler can produce 36.96 kWh per month in the case that it has 2% electrical efficiency. The study is stated that the higher TPV system electric power output can be achieved if the emitter, the filter and the TPV cell are matched in harmony with each other in the combi boiler, the distances of these components and the emitter-filter dimensions are arranged. Thus, a standard family in Turkey can be supplied a certain amount of the monthly energy demand. In addition, it is explained in the study the issues that should be taken into consideration when this system is integrated into the combi boiler.

Effects of bluff-body on the thermal performance of micro thermophotovoltaic system based on porous media combustion

To improve thermophotovoltaic system efficiency, a porous media combustor with a bluff-body as the flame holder was proposed. A two-dimensional numerical simulation of premixed hydrogen/air combustion inside a planar porous media channel with the local thermal non-equilibrium model was conducted. Effects of the bluff-body flame holder on mean wall temperature and system efficiency were examined. By inserting the bluff-body, the blowout limits of the planar porous media combustor for ϕ = 0.6, 0.8, 1.0, 1.2 are increased by 33%, 19%, 12%, 20%, respectively. Numerical results show that the flame stabilization is dominated by flow and heat recirculation. Further analysis reveals the mechanism of heat transfer on flame stabilization due to three heat recirculation paths. Considering the view factor and the external quantum efficiency, the system efficiency with the bluff-body is increased by 14.72% as compared to the one without the bluff-body at ϕ = 0.8, vin = 4 m/s.

Machine learning-optimized Tamm emitter for high-performance thermophotovoltaic system with detailed balance analysis

Abstract Light-matter interaction upon nanophotonic structures in the infrared wavelength has drew increasing attentions due to the extensive potential applications. Among them, thermophotovoltaic (TPV) systems can exhibit higher efficiency over the Shockley-Queisser limit due to the nanophotonic structure-enabled tunable narrowband thermal emission rather than the broadband incident spectrum. However, two long-standing issues remain formidable as bottlenecks for achieving better performances of TPV system. One is the competing role of the power density and the system efficiency of TPV system, and the other is the magnanimity possibilities of structures, configurations, dimensions, and materials of thermal emitters that disables the manual optimization of TPV system. Here, we attempt to achieve high-performance TPV system by employing the machine learning algorithm under the framework of material informatics. The power density and system efficiency are well modelled through the detailed balance analysis with full considering the photocurrent generation in the PV cells. Through optimization, the non-trial aperiodic Tamm emitters are obtained and the metal-side one is preferable in terms of the TPV performance. The present work is demonstrated to be feasible and efficient in optimizing the TPV performance, and opens a new door for the optimization problems in other fields.

Parametric analysis on the combustion and thermal performance of a swirl micro-combustor for micro thermophotovoltaic system

A swirl micro-combustor which has strong capability to stabilize the flame has been developed and is promising to be used in micro thermophotovoltaic power generators in this work. Heat recirculation plays a requisite role in flame stability and thermal performance of micro combustors. Thus, the effects of three parameters (center inlet radius w1, swirler width w2, and step height w3) that determine the size and location of the recirculation zones in a premixed hydrogen/air flame are investigated numerically. The results show that inner recirculation zone and corner recirculation zone generated by a swirler are critical for stabilizing the flame. The parameter w1 has an insignificant influence on the thermal performance. Enlarging the inner recirculation zone by decreasing w2 provokes enhanced heat transfer, and increases the temperature level and temperature non-uniformity of combustor walls. The size of corner recirculation zone is increased by increasing w3, which raises the outer wall temperature uniformity. Based on the parametric analysis, the newly designed micro combustor possesses a high wall temperature level. Additionally, the energy output increment is between 9.1% and 14.2% at hydrogen mass flow rate of 0.0006 g/s when stoichiometric hydrogen/air ratio ranges from 0.6 to 1.4.

Thermal stability of tungsten based metamaterial emitter under medium vacuum and inert gas conditions

Commercial deployment of thermophotovoltaics (TPV) is lacking behind the implementation of solar PV technology due to limited thermal stability of the selective emitter structures. Most of the TPV emitters demonstrated so far are designed to operate under high vacuum conditions (~10−6 mbar vacuum pressure), whereas under medium vacuum conditions (~10−2 mbar vacuum pressure), which are feasible in technical implementations of TPV, these emitters suffer from oxidation due to significant O2 partial pressure. In this work, the thermal stability of 1D refractory W-HfO2 based multilayered metamaterial emitter structure is investigated under different vacuum conditions. The impact of the O2 partial pressure on thermal stability of the emitters is experimentally quantified. We show that, under medium vacuum conditions, i.e. ~10−2 mbar vacuum pressure, the emitter shows unprecedented thermal stability up to 1300 °C when the residual O2 in the annealing chamber is minimized by encapsulating the annealing chamber with Ar atmosphere. This study presents a significant step in the experimental implementation of high temperature stable emitters under medium vacuum conditions, and their potential in construction of economically viable TPV systems. The high TPV efficiency, ~50% spectral efficiency for GaSb PV cell at 1300 °C, and high temperature stability make this platform well suited for technical application in next-generation TPV systems.

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.

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.