Thermophotovoltaic Research Articles

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.

Exploiting the waste heat in graphene-based thermionic energy converter by means of thermophotovoltaic cell

A coupled system comprising of a graphene-based thermionic energy converter (GTEC) and a thermophotovoltaic cell (TPVC) is proposed to recover the waste heat from the anode of GTEC for additional power generation. Based on energy balance principle, the operating temperature of the anode is numerically calculated. Neglecting the non-ideal factors, the local maximum power density 4.46 W cm −2 and efficiency 0.573 of the system are determined for given related parameters. An overall maximum efficiency 0.584 is obtained by optimizing the cathode’s temperature, while local maximum power density is increased with increase of the cathode’s temperature. The comparisons between the present work and the previous works are made. The present work demonstrates that the proposed system can achieve higher performances than those of the single GTEC and TPVC operating at the same temperature differences. The effects of the thermal loss at the anode and the non-radiative recombination losses of the PV cell on the system’s performances are discussed. The local maximum power density 2.06 W cm −2 and efficiency 0.251 are obtained. An overall maximum efficiency 0.307 can be achieved by optimizing the cathode’s work function. The present coupled system is very promising for renewable energy utilization. • A graphene thermionic power device-thermophotovoltaic cell hybrid system is proposed. • The performances of the system are improved by comparing to the previous works. • The influences of the non-ideal factors on the system’s performances are revealed. • The power density 2.06 W cm −2 and efficiency 0.251 of non-ideal system are obtained. • The proposed coupled system is helpful for the utilization of renewable energy.

Numerical investigation of an ultra-broadband, wide-angle, and polarization-independent metasurface light absorber.

In this paper, we propose and numerically investigate an ultra-broadband, wide-angle, and polarization-independent metasurface absorber based on periodic hexagon-latticed titanium (Ti) nanoring arrays over a continuous Ti film. The proposed absorber can achieve more than 90% absorptivity under normal incidence, ranging from 350 to 1453 nm, and the average absorption is up to 95.6%. Additionally, the absorptivity still remained beyond 70% when the incident angles varied from 0° to 60°. The simulations of electric field distributions indicate that the broadband absorption performance can be ascribed to the superposition of the localized surface plasmon resonance (LSPR) originated from the nanopillars and nanoholes, respectively. The proposed approach is simple and inexpensive, and the metal material is optional. Therefore, we believe that the proposed absorber will be a candidate for many potential applications, such as thermophotovoltaic cells, thermal emitters, and optoelectronic devices.

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.

Near-perfect photon utilization in an air-bridge thermophotovoltaic cell.

Thermophotovoltaic cells are similar to solar cells, but instead of converting solar radiation to electricity, they are designed to utilize locally radiated heat. Development of high-efficiency thermophotovoltaic cells has the potential to enable widespread applications in grid-scale thermal energy storage1,2, direct solar energy conversion3-8, distributed co-generation9-11 and waste heat scavenging12. To reach high efficiencies, thermophotovoltaic cells must utilize the broad spectrum of a radiative thermal source. However, most thermal radiation is in a low-energy wavelength range that cannot be used to excite electronic transitions and generate electricity. One promising way to overcome this challenge is to have low-energy photons reflected and re-absorbed by the thermal emitter, where their energy can have another chance at contributing towards photogeneration in the cell. However, current methods for photon recuperation are limited by insufficient bandwidth or parasitic absorption, resulting in large efficiency losses relative to theoretical limits. Here we demonstrate near-perfect reflection of low-energy photons by embedding a layer of air (an air bridge) within a thin-film In0.53Ga0.47As cell. This result represents a fourfold reduction in parasitic absorption relative to existing thermophotovoltaic cells. The resulting gain in absolute efficiency exceeds 6 per cent, leading to a very high power conversion efficiency of more than 30 per cent, as measured with an approximately 1,455-kelvin silicon carbide emitter. As the out-of-band reflectance approaches unity, thethermophotovoltaic efficiency becomes nearly insensitive to increasing cell bandgap or decreasing emitter temperature. Accessing this regime may unlock a range of possible materials and heat sources that were previously inaccessible to thermophotovoltaic energy conversion.

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.

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%.

One-dimensional comparison of Tio2/SiO2 and Si/SiO2 photonic crystals filters for thermophotovoltaic applications in visible and infrared

In this paper, two photonic crystal structures composed of Si/SiO2 and TiO2/SiO2are optimized and compared for potential use as thermophotovoltaic (TPV) optical filters. The performance of the proposed one-dimensional structures comprises eight periods each. The infrared radiation included is in the wavelength range of 300 nm to 900 nm for the visible and from 800 nm to 2000 nm for the infrared. The limiting temperature of the system is 2000 K. The transfer matrix method is used to analyze the behavior of the frequency for a structure comprising a layer of defects. This method permitted to observe the behavior of the peak of light transmission when the period and angle of incidence change. We have subsequently found that the band gaps of the Si/SiO2structure were wider than those of the TiO2/SiO2structure.

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.

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.

Near-Field Electroluminescent Refrigeration System Consisting of Two Graphene Schottky Diodes

Abstract It has been shown that the performance of a thermal radiative device, such as a thermophotovoltaic (TPV) and an electroluminescent (EL) refrigerator, can be significantly enhanced when the vacuum gap between a reservoir and a semiconductor diode becomes nanoscale. Recently, several studies have reported the integration of a TPV and a light emitting diode (LED) in one near-field thermal radiative device to improve the operation efficiency. However, surface polaritons were hardly exploited in previous research because bare semiconductor diodes were used. In this paper, we propose a TPV-LED integrated near-field EL refrigeration system consisting of two graphene-semiconductor Schottky diodes. A substantial refrigeration rate (101.9 kW/m2) is achieved owing to the coupling of surface plasmon-phonon polaritons excited by a symmetric configuration of graphene-polar materials. Moreover, the cooling coefficient of performance (COP) of the system can be enhanced up to 2.65 times by recycling the electrical power generated in the TPV cell. The cooling performance is further investigated in relation to design parameters, namely the doping concentration of Si and insulator thickness.

Integrated near-field thermo-photovoltaics for heat recycling

Energy transferred via thermal radiation between two surfaces separated by nanometer distances can be much larger than the blackbody limit. However, realizing a scalable platform that utilizes this near-field energy exchange mechanism to generate electricity remains a challenge. Here, we present a fully integrated, reconfigurable and scalable platform operating in the near-field regime that performs controlled heat extraction and energy recycling. Our platform relies on an integrated nano-electromechanical system that enables precise positioning of a thermal emitter within nanometer distances from a room-temperature germanium photodetector to form a thermo-photovoltaic cell. We demonstrate over an order of magnitude enhancement of power generation (Pgen ~ 1.25 μWcm−2) in our thermo-photovoltaic cell by actively tuning the gap between a hot-emitter (TE ~ 880 K) and the cold photodetector (TD ~ 300 K) from ~ 500 nm down to ~ 100 nm. Our nano-electromechanical system consumes negligible tuning power (Pgen/PNEMS ~ 104) and relies on scalable silicon-based process technologies.

Parametric Optimum Design and Performance Improvement of a Thermophotonic Cell

An irreversible model of the thermophotonic cell (TPC) with a heated forward-based light-emitting diode (LED) as an emitter and a photovoltaic cell (PVC) is established. The performances of the TPC are optimized and compared with those of the thermophotovoltaic cell (TPVC) based on finite-time thermodynamics. The results obtained show that there exist the optimally operating ranges of the voltages and band-gap energies of the LED and PVC. The thermal flow emitted from the heated LED dramatically exceeds that from the pure thermal emitter. The maximum efficiency of the TPC is less than that of the TPVC, whereas the maximum power density of the TPC surpasses that of the TPVC under an arbitrary emitter temperature. The TPC has the advantages of the relatively high optimal band-gap energies and relatively low-operating temperatures so that it is easier to be made than the TPVC.

Bi2Te3 / Si Thermophotovoltaic Cells Converting Low-Temperature Radiation into Electricity

The thermophotovoltaic cells which can convert the infrared radiation from room-temperature surroundings 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. During the formation of the heterojunctions at room temperature, both electrons and holes diffuse in 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 room-temperature heat source. This work proves that it is possible to convert the infrared radiation from dark and room-temperature surroundings into electricity through narrow bandgap thermophotovoltaic cells.

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.

Study on crystal growth of In<sub><italic>x</italic></sub>Ga<sub>1−</sub><sub><italic>x</italic></sub>Sb under microgravity

In x Ga1− x Sb is a ternary alloy with tunable properties. The wavelength of In x Ga1− x Sb can be varied in the range of 1.7– 6.8 μm, which is in the infrared (IR) region and makes that In x Ga1− x Sb can be used as a substrate for IR detector and thermophotovoltaic (TPV). The phase diagram reveals that there is a large temperature gap between liquidus and solidus lines, which leads to constitutional supercooling and the formation of crystal defects during the solidification process of InGaSb crystal. Moreover, convection caused by gravity will increase the inhomogeneity of transport in the liquid region near the crystal growth interface, which makes it quite difficult to grow In x Ga1− x Sb crystal. The convection will be restrained in microgravity environment, and thus, it is very beneficial for crystal growth. This article introduces the effect of microgravity on the growth of In x Ga1− x Sb crystal and the results of the space growth experiment of In x Ga1− x Sb ternary crystal with a high concentration of In in SJ10 recoverable scientific experimental satellite.

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.

Performance analysis of narrow-bandgap interband cascade thermophotovoltaic cells

We present the detailed characterization and performance analysis of two sets of four interband cascade thermophotovoltaic devices with InAs/GaSb type-II superlattice (SL) absorbers. The bandgaps of the SL absorbers in the four devices were between 0.22 and 0.25 eV at 300 K. With different numbers of cascade stages and individual absorber thicknesses, we are able to show how possible material quality variations and current mismatch between stages can affect the device performance. Based on the calculated effective quantum efficiency in each stage and experimental results, it is shown that the current mismatch between stages due to the deviation of absorption coefficient from the designed value can be substantial with more stages. On the other hand, the collection efficiency can be much improved with more stages and thin individual absorbers. Also, the carrier lifetime is extracted through measured dark current density to evaluate the material quality. The effects of material quality, current mismatch and collection efficiency on device performance are quantified and a comparison shows that material quality plays the most important role.