Thermophotovoltaic System Research Articles

Thermal degradation of the multilayer Mo/HfO2 emitter induced by the oxygen diffusion at high temperature in vacuum

The metal layer in a multilayer emitter can still be oxidized in vacuum at a high temperature over 1400 K, limiting the application of the emitter in a thermophotovoltaic system. To take a further insight into the above phenomenon, multilayer Mo/HfO2 emitters were fabricated and annealed at a temperature range of 1000∼1500 K under a constant pressure of 3×10−4 Pa. Then, the measurements of the structure morphologies, compositions and spectral emittances on the emitters before and after thermal annealing were conducted to investigate the oxygen diffusion process and its effect on the spectral emittances. Moreover, an improved model based on the transfer matrix method was proposed to predict the spectral emittance of the degraded emitter. It is indicated that the emitter can keep the efficient selective emission up to 1300 K, while it exhibits a thermal degradation at 1500 K. The main reason is that Mo is oxidized at the high temperature, and the generated oxides sublimate to form holes, resulting in significant changes in both the structure and composition of the emitter. With an analysis on these changes, it is inferred that the pinholes produced by the internal stress at the boundary between Mo and HfO2 account for the oxygen diffusion. The predicated spectral emittances after the thermal annealing show a similar trend with the measured ones, providing a further demonstration for the reason of the thermal degradation. This study gives valuable insights into the oxygen diffusion process and thermal degradation of the multilayer emitters, and it can pave the way for the design of the thermophotovoltaic emitter with a high-temperature resistance.

Porous alumina thermophotovoltaic film for output voltage enhancement of silicon solar cell at room temperature

Antimony doped porous alumina film deposited over indium tin oxide (ITO) coated glass substrate can be helpful in the thermophotovoltaic system. The film pore dimensions of the system developed are: pore radius of 200–400 nm, pore height ∼200 nm, inter pore distance ∼400 nm, and pore wall thickness ∼80 nm. These dimensions match to a large extent of specially prepared design values of optimized HfO2-filled photonic crystal for cutoff wavelength (λc) of 1.70 μm. The X-ray diffraction analysis of the synthesized films reveals direct evidence that crystalline alumina coexists with ITO. The trace amount of antimony was detected by X-ray photoelectron spectroscopic analysis. However, other optical analysis techniques, e.g., absorbance and photoluminescence, cannot detect such a trace amount. The developed thermophotovoltaic film was applied to cover the top surface of silicon solar panels. The output voltage of the panel was compared to that of panel without thermophotovoltaic film. The transient voltage response of the covered panel after several measurements confirms an increase in voltage (∼0.15 V) with VOC = 10 V and ISC = 0.4 Amp under solar radiation of ∼600 W/m2. The increase in voltage by applying the thermophotovoltaic film is a novel outcome and has immense potential for industrial application in the future.

An effective design of thermophotovoltaic metamaterial emitter for medium-temperature solar energy storage utilization

Solar energy is one of the most important new energy sources with huge reserves. However, it has the disadvantage of discontinuous and instability, which can be compensated with a molten salt energy storage system. In this study, a novel molten salt energy storage-solar thermophotovoltaic integrated system was proposed for the application in small-scale distributed energy utilization. To adjust the radiation spectrum in the operating temperature of 800–1000 K to better match with InAs cell, a Ta-based stacked-cross pyramid broadband emitter was designed and geometrically optimized. Its effective impedance and magnetic field distribution at different wavelength were analyzed to reveal its radiation mechanism. By introducing the emitter, the spectral efficiency of the system increased from 51% to 88%, and the TPV efficiency increased from 11.4% to 24.24% at 1000 K compared with blackbody radiation and maintains above 20% at the range of 800–1000 K. Compared with narrowband emitters, the broadband emitter has better comprehensive performance when integrated with the molten salt energy storage system, with the TPV efficiency slightly reduced but the output power significantly increased from 4.96 kW·m−2 to 2.26 kW·m−2 at 1000 K.

Optimal design and economic analysis of a hybrid process of municipal solid waste plasma gasification, thermophotovoltaic power generation and hydrogen/liquid fuel production

In this paper, an integrated process of hydrogen, methanol and electricity production from municipal solid waste biomass feed is investigated and analyzed. Also, the computational model for recovering the energy from the internal radiation of the combustion chamber in a thermophotovoltaic system is developed. The main parameters in the thermophotovoltaic modeling system are open circuit voltage, short circuit current density and cell density coefficient. These parameters are considered as a function of emitter temperature, cell temperature, the interval between cell and emitter, spectral response and the cell energy gap. After designing the integrated process, efficiency of the overall proposed hybrid system is calculated about 68.17% which is increased approximately 3.5% by heat recovery from the combustion chamber of thermophotovoltaic system. The results showed that increasing the temperature of the WGS reactor has a negative impact on the overall productivity of the system. Also increasing the rate of inlet plasma air decreases H2/CO ratio of the synthesized gas, which accordingly decreases the methanol production rate. Based on the economic analysis results in a period of 2 years and 9 months the total investment cost is returned to the investor.

Numerical investigation on the self-ignition and combustion characteristics of H2/air in catalytic micro channel

The self-ignition of hydrogen/air is an important process in the micro thermophotovoltaic system. The transient numerical models of gas-phase reaction and catalytic reaction in the various catalytic micro combustors were built and verified. The self-ignition process of gas-phase reaction caused by catalytic reaction in the catalytic micro channel with conventional heat dissipation was studied. The self-ignition process could be divided into four stages, fuel diffusion stage - pure catalytic reaction stage - flame front moving stage - stable combustion stage. The ignition time and temperature limit at different inlet temperatures, inlet velocities and channel heights were analyzed. The results showed that the wall quenching effect, thermal effect and flame propagation effect are dominant at low temperature, medium temperature and high temperature respectively. The catalyst length and the mixture internal energy were the main factor at low inlet velocity and high inlet velocity respectively. The steady-state time was also studied in the various operation conditions. Finally, the catalytic combustion characteristics in the stable combustion stage were analyzed. The influence of inert section length, inlet temperature and inlet velocity on the maximum temperature and fuel conversion ratio were investigated.

Effect of pins and exit-step on thermal performance and energy efficiency of hydrogen-fueled combustion for micro-thermophotovoltaic

Micro thermophotovoltaic system, a chemical energy to electrical output device, has been challenged for application because of burning instability, high heat loss ratio and low energy conversion efficiency. A combustor coupling with pins and exit-step configurations is developed to improve system performance. Effects of pins arrangement and exit-step length on the combustion characteristics and thermal performance are investigated with the eddy-dissipation concept (EDC) model and detailed H2/air combustion mechanism. The results indicate that the premixed flame anchors at combustor upstream near the smaller pins where favorable ignition conditions are established. Besides, the appropriate exit-step length improves the heat transfer of gas-solid and energy efficiency, where the length ratio of exit-step to combustor δ = 0.66 is the best for reducing exhaust loss. Furthermore, the combustor coupling with pins and exit-step achieves a stable flame, high energy efficiency and thermal performance. For the micro thermophotovoltaic system with the combustor and InGaAsSb PV cells, radiation efficiency 35.6 % and electric output 2.26 W can be obtained, that is, the system efficiency is 18.7 % higher than that of the straight-channel combustor at hydrogen flow rate 3.75 g/h and equivalence ratio Φ = 1.0.

Theoretical and Numerical Analysis of Active Switching for Narrow-Band Thermal Emission with Graphene Ribbon Metasurface

Components smaller than the wavelength of electromagnetic waves are called meta-atoms. Thermal emission can be controlled by an artificial structure in which these meta-atoms are arranged on the surface. This artificial structure is called a metasurface, and its optical properties are determined by the materials and shapes of the meta-atoms. However, optical devices may require active control of thermal emission. In the present study, we theoretically and numerically analyze a wavelength-selective emitter using a graphene ribbon metasurface. The graphene ribbon metasurface consists of a graphene ribbon array, potassium bromide thin film, and silver substrate. The geometric parameters of the graphene metasurface are determined based on an equivalent circuit model that agrees well with the results of the electromagnetic field analysis (rigorous coupled-wave analysis). The proposed emitter causes impedance matching depending on the conductivity of the graphene ribbon in a very narrow wavelength range. The conductivity of graphene can be actively controlled by the gate voltage. Therefore, the proposed emitters may realize near-perfect emission with a high quality factor and active controllable switching for various wavelengths. In addition, the quality factor can be changed by adjusting the electron mobility of graphene. The proposed emitter can be used for optical devices such as thermophotovoltaic systems and biosensing.

Performance analysis of indium antimonide thermophotovoltaic system with varied material and geometrical properties

Thermophotovoltaic (TPV) system generates electricity using the selective emission of radiation on a photovoltaic (PV) cell by heating an emitter to a very high temperature. In this work, Indium Antimonide (InSb) is used as it has a low bandgap of 0.23 eV and the performance of the InSb TPV system is compared to an Indium Gallium Arsenide (InGaAs) TPV system. TPV system of different geometries like cylindrical, flat planar, and polygonal shapes are simulated to understand the influence of TPV structure in the efficiency and reliable operating conditions of the TPV system. To enhance the efficiency of the TPV cell, a photonic crystal (PhC) filter is also inserted between the cell and the emitters that consists of a blackbody, Erbium Oxide (Er2O3), and Tungsten to allow only selective radiation from the emitter to the PV cell. The proposed TPV system has a simple structure, it is thermally stable, easy-to-manufacture, and it contains no moving parts. In summary, the performance of an InSb TPV system of different geometries is analysed by matching it with a filter and emitters made of different materials. The performance of the InSb TPV system is then benchmarked against an InGaAs TPV system.

A novel selective thermophotovoltaic emitter based on multipole resonances

Thermophotovoltaic systems can harvest electric energy from heat sources with a potential efficiency exceeding the Shockley–Queisser limit due to the selective emission of an elaborate thermal emitter. In this work, a two-dimensional nanodisks/thin-film metamaterial is proposed as a wavelength-selective emitter, which can coordinate well with the photovoltaic cell in a thermophotovoltaic system. Compared to conventional emitters based on surface plasmon polaritons, the emittance peaks of the proposed emitter are realized by the excitations of both electric dipole and anapole modes in silicon nanodisks, which can be easily tailored due to the non-dispersive optical constants of dielectric materials. Meanwhile, the effect of polarization and polar angle on the emittance spectra is also investigated, suggesting that the proposed emitter has high emittance and efficiency not only in the normal direction but also at large oblique angles. Electromagnetic field and current density distributions reveal that the coupling between multipole resonances and the bottom tungsten layer can induce a magnetic dipolar resonance. Therefore, the wavelengths of both emittance peaks are sensitive to the period paralleled to the incident electric field. Besides, the anapole-induced mode can couple with the lattice resonance, resulting in higher emittance. Moreover, the proposed emitter is successfully fabricated, and the measured spectra agree well with the theoretical results. The fundamental understanding and insights obtained here will facilitate the active design and application of novel multipole-based emitters in enhancing energy conversion.

Ultra-broadband perfect absorber based on nanoarray of titanium nitride truncated pyramids for solar energy harvesting

A metamaterial perfect absorber working in the ultraviolet to near-infrared band is proposed based on a triple-layer structure composed of titanium nitride (TiN) periodic nanoarray, dielectric film, and TiN bottom layer. Each unit cell of the nanoarray is a truncated pyramid structure with its width tapered linearly from the bottom to the top. The physical mechanism of the absorber was explained by analyzing the electric field distributions at different wavelengths, and the absorption performance was simulated as a function of its geometric parameters and the angle and polarization of incident light as well. Optimal geometric parameters are presented. The absorber implemented using the optimal parameters would display an excellent performance with almost perfect absorption (above 99%) over the entire spectral region from 300 to 2500 nm. The ultra-broadband perfect absorption behavior is attributed to the localized surface plasmon resonance effect of the TiN pyramid, the intrinsic loss of the TiN material and the coupling of resonance modes between two adjacent pyramids. Besides, the absorber also exhibits a polarization-independent feature at normal incidence and good angular acceptance capability of incident light. The reported ultra-broadband metamaterial absorber shows great application prospects in the solar energy harvesting industry and solar thermophotovoltaic systems.

Evaluation of thermo photovoltaic performance through aluminum-fueled combustor with partially porous medium and different geometric cross-sections

Thermo photovoltaic (TPV) power generation systems have attracted a great deal of attention in the field of portable electronic devices. The high and uniform temperature of TPV combustor plays a decisive role in the improvement of system efficiency. In this study, aluminum is utilized as energy source of TVP. To model the premixed combustion of aluminum-air, an asymptotic model of flame structure including four zones of solid, liquid, and gaseous aluminum and post-flame and three asymptotic fronts of melting, evaporation, and flame is proposed. Then, the conservation equations and boundary conditions are derived and the combustion characteristics of aluminum are obtained. Combustion is evaluated for simple and porous media with trapezoidal, rectangular, triangular, and circular cross-sections. Finally, the influence of parameters on the TVP performance is assessed. The results indicated that in the medium with a porosity factor of 0.8, the total efficiency is higher than the simple medium for all cross-sections. In the medium with a porosity factor of 0.6, however, the triangular and circular cross-sections exhibit lower efficiency compared to the simple medium. In the case with a porosity factor of 0.4, only the efficiency of the channel with a trapezoidal cross-section is higher than the simple medium.

Nanostructures for Achieving Selective Properties of a Thermophotovoltaic Emitter.

This paper focuses on the research and development of a suitable method for creating a selective emitter for the visible and near-infrared region to be able to work optimally together with silicon photovoltaic cells in a thermophotovoltaic system. The aim was to develop a new method to create very fine structures beyond the conventional standard (nanostructures), which will increase the emissivity of the base material for it to match the needs of a selective emitter for the VIS and NIR region. Available methods were used to create the nanostructures, from which we eliminated all unsuitable methods; for the selected method, we established the optimal procedure and parameters for their creation. The development of the emitter nanostructures included the necessary substrate pretreatments, where great emphasis was placed on material purity and surface roughness. Tungsten was purposely chosen as the main material for the formation of the nanostructures; we verified the effect of the formed structure on the resulting emissivity. This work presents a new method for the formation of nanostructures, which are not commonly formed in such fineness; by this, it opens the way to new possibilities for achieving the desired selectivity of the thermophotovoltaic emitter.

Exploiting zirconium nitride for an efficient heat-resistant absorber and emitter pair for solar thermophotovoltaic systems.

A perfect absorber in the visible-infrared regime maintaining its performance at elevated temperatures and under a harsh environment is needed for energy harvesting using solar-thermophotovoltaic (STPV) systems. A near-perfect metasurface absorber based on lossy refractory metal nitride, zirconium-nitride (ZrN), having a melting-point of 2,980°C, is presented. The numerically proposed design with metal-insulator-metal configuration exhibits an average of > 95% for 400-800 nm and 86% for 280-2200 nm. High absorption is attributed to magnetic resonance leading to free-space impedance matching. The subwavelength structure is polarization- and angle-insensitive and is highly tolerant to fabrication imperfections. An emitter is optimized for bandgap energy ranging from 0.7 eV-1.9 eV.

Ultrathin High Quality‐Factor Planar Absorbers/Emitters Based on Uniaxial/Biaxial Anisotropic van der Waals Polar Crystals

AbstractTailoring the absorption/emission through nanostructures from broadband to narrowband has attracted increasing attention due to the merits of high efficiency and compactness. However, most of the reported narrowband emitters require either complex fabrication process or thick coating of the suitable materials (~mm). In this paper, by exciting Berreman mode through a combination of Au mirrors and ultra‐thin layers of low‐loss uniaxial/biaxial anisotropic 2D van der Waals polar crystal hBN/α‐MoO3, a narrowband absorption/emission peak is realized in the mid‐infrared region. The measured quality‐factor of Berreman mode for hBN/Au and α‐MoO3/Au structure can reach up to 134 and 164, respectively. The deep subwavelength thickness (~nm) and bilayer architecture simplify the fabrication process. Furthermore, taking advantage of the in‐plane birefringence originating from α‐MoO3, the polarization of the absorbed/emitted radiation can be controlled through this planar configuration. Such ultrathin narrowband absorbers/emitters provide new opportunities for the advancements in thermal sources, infrared sensors, and thermophotovoltaic power generation systems.

Radiative Flux of a Spectral Distribution at the Surface of a TPV Thermal Emitter Resulting from the Combustion of Biomass: Case of Palms Nut Shells

Thermo PhotoVoltaic (TPV) system convert into electrical energy the radiation emitted from an artificial heat source (i.e., solar or combustion) by the use of photovoltaic cells. In the last decade, TPV system 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. In this study, we modeled and investigated by performing numerical simulation on the feasibility of the TPV system exploiting waste heat energy during biomass combustion. To achieve this goal, we considered a TPV system in which we evaluated and analyzed the waste heat flux resulting from the combustion of the palm nuts shells as well as the corresponding temperature received at the surface of the TPV Thermal-emitter. Furthermore, we evaluated the heat intensity. Results obtained shows that the average temperature at the TPV absorber-emitter is around 1600k. Different variation of the net heat flux at the TPV absorber-emitter are observed for different position. The maximum heat intensity is around 15*1010 W.m-2.µm-1 for a wave length of 2000 nm. These results indicate that the model presented herein is therefore suitable for the design of the TPV system.

A Review on Thermophotovoltaic Cell and Its Applications in Energy Conversion: Issues and Recommendations.

Generally, waste heat is redundantly released into the surrounding by anthropogenic activities without strategized planning. Consequently, urban heat islands and global warming chronically increases over time. Thermophotovoltaic (TPV) systems can be potentially deployed to harvest waste heat and recuperate energy to tackle this global issue with supplementary generation of electrical energy. This paper presents a critical review on two dominant types of semiconductor materials, namely gallium antimonide (GaSb) and indium gallium arsenide (InGaAs), as the potential candidates for TPV cells. The advantages and drawbacks of non-epitaxy and epitaxy growth methods are well-discussed based on different semiconductor materials. In addition, this paper critically examines and summarizes the electrical cell performance of TPV cells made of GaSb, InGaAs and other narrow bandgap semiconductor materials. The cell conversion efficiency improvement in terms of structural design and architectural optimization are also comprehensively analyzed and discussed. Lastly, the practical applications, current issues and challenges of TPV cells are critically reviewed and concluded with recommendations for future research. The highlighted insights of this review will contribute to the increase in effort towards development of future TPV systems with improved cell conversion efficiency.

High-Temperature Carbonized Ceria Thermophotovoltaic Emitter beyond Tungsten.

Thermophotovoltaics (TPVs) require emitters with a regulated radiation spectrum tailored to the spectral response of integrated photovoltaic cells. Such spectrally engineered emitters developed thus far are structurally too complicated to be scalable, are thermally unstable, or lack reliability in terms of temperature cycling. Herein, we report wafer-scale, thermal-stress-free, and wavelength-selective emitters that operate for high-temperature TPVs equipped with GaSb photovoltaic cells. One inch crystalline ceria wafers were prepared by sequentially pressing and annealing the pellets of ceria nanoparticles. The direct pyrolysis of citric acid mixed with ceria nanoparticles created agglomerated, pyrolytic carbon and ceria microscale dots, thus forming a carbonized film strongly adhering to a wafer surface. Depending on the thickness of the carbonized film that was readily tuned based on the amount of citric acid used in the reaction, the carbonized ceria emitter behaved as a tungsten-like emitter, a graphite-like emitter, or their hybrid in terms of the absorptivity spectrum. A properly synthesized carbonized ceria emitter produced a power density of 0.63 W/cm2 from the TPV system working at 900 °C, providing 13 and 9% enhancements compared to tungsten and graphite emitters, respectively. Furthermore, only the carbonized ceria emitter preserved its pristine absorptivity spectrum after a 48 h heating test at 1000 °C. The scalable and facile fabrication of thermostable emitters with a structured spectrum will prompt the emergence of thermal emission-harnessed energy devices.

Parametric characteristics and optimization of a novel near-field thermophotovoltaic and thermoelectric hybrid system for energy harvest

In this study, a novel near-field thermophotovoltaic (NFTPV) and thermoelectric (TE) hybrid system is proposed to achieve efficient energy cascade conversion, where TE is used to harvest the thermal energy discharged by NFTPV. Based on the physical model, the performance of the hybrid system with tungsten emitter and InAs cell is analyzed and optimized through numerical simulation. The effect of operating parameters such as emitter and cell temperature, applied voltage of TPV cell and vacuum gap on the performance of hybrid system is investigated. The results indicate that the NFTPV-TE hybrid system has significant advantages over single NFTPV system. When the emitter temperature is 1000 K, the efficiency of the NFTPV-TE hybrid system is about 3.5–9.5 percent points higher than that of single NFTPV, and the optimal system power increases by 6–16%. Each parameter of NFTPV-TE hybrid system has its optimal range, which can ensure the high efficiency and output power of the system simultaneously. The NFTPV-TE hybrid system is not suitable for the waste heat recovery under low-temperature conditions. However, it is very suitable for the application of NFTPV-TE in solar energy conversion since the optimized cell temperature is low. NFTPV-TE hybrid system can overcome the limitation of single NFTPV efficiency under high-temperature combustion conditions. The efficiency of the hybrid system can reach 45.8% when the emitter temperature is 2000 K, which is 6 percentage points higher than that of single NF-TPV system. This study indicates the significant value of NFTPV-TE hybrid system in high-temperature engineering and provides guidance for the practical application of NFTPV system.

Design and Theoretical Study of a Metamaterial Absorber-Emitter Pair Matched with a Low-Bandgap PV Cell for an STPV System

The conversion efficiency of conventional semiconductor photovoltaic (PV) devices is restricted to a low level due to the Shockley-Queisser (S-Q) limit. A solar thermophotovoltaic (STPV) system, which consists of a spectrally selective absorber, narrowband emitter, and low-bandgap PV cell, can serve as an alternative to overcome this theoretical limit. However, the stringent requirements of absorber and emitter (like angular and polarization insensitivity and high-temperature resistance) require a careful selection of appropriate material and optimization of structures. To further improve the efficiency of the overall STPV system and ensure high performance, we designed a metamaterial-based absorber-emitter pair matched with InGaAsSb cells for an STPV system. Our numerical simulations demonstrate that the designed absorber provides ultrahigh broadband absorption in the spectral range of 300-2000 nm with a sharp decrease in absorptivity beyond 2000 nm, improving the spectral efficiency to 98%. For the emitter, narrowband emission is ultimately achieved, which prevents both sub-bandgap emission and thermalization loss in the PV cell. Moreover, the pair composed of refractory materials is insensitive to the incident angle and polarization angle. According to detailed balance calculations, the system efficiency can exceed the S-Q limit (41%) since a low temperature (1453 K) and the maximum efficiency (45.94%) outperforms previously reported demonstrations of STPV systems with InGaAsSb cells.

Selective emitter with core–shell nanosphere structure for thermophotovoltaic systems

For thermophotovoltaic (TPV) systems, traditional thermal emitters waste considerable energy due to the mismatch between the thermal radiation power spectrum and the bandgap of the photovoltaic (PV) cells. Here, a core–shell nanosphere (CSN) structured selective emitter was designed and optimized on the basis of numerical calculation. By iteratively optimizing the geometric parameters of the CSN emitter, the average emissivity of the CSN emitter exceeds 0.93 within the bandgap and is suppressed to 0.13 outside the bandgap. The mechanism of selective emission characteristics is elucidated in detail. Additionally, the simulation results demonstrate that the CSN emitter exhibits emissivity insensitivity to polarization and incident angle. For the TPV system with the InGaAs cells, an output power density of 0.594 W/cm2 and a system efficiency of 12.83% are achieved at a CSN emitter temperature of 1338 K. The obtained output power density is relatively high, i.e., 3.2 times that of the previously reported record (0.184 W/cm2). And ultimately, the core–shell structural nanoparticles were synthesized by solution–processed and their performance was tested. This research improves the performance of the selective emitter and paves the way for a more efficient design of TPV systems.