Thermophotovoltaic Research Articles

Four-layer metallodielectric emitter for spectrally selective near-field radiative transfer in nano-gap thermophotovoltaics

A four-layer metallodielectric planar thermal emitter is designed and optimized to tailor the spectral near-field radiative flux in a nano-gap thermophotovoltaic (TPV) system. The structure contains an ultra-thin refractory metallic layer (tungsten) sandwiched by two dielectric (hafnia) layers sitting on a metallic substrate. The theoretical calculation showed that the lossy metallic thin-layer acted as a perfect absorber that stimulated strong thermal emission, with its frequency and amplitude further modulated by the Fabry–Perot resonances in the dielectric layer. The spectral peak of near-field radiative flux can be manipulated by layer thickness to suit different cell bandgaps and emitter temperatures. The combination of optimized emitter and cell layer thickness results in selectively high near-field radiative flux above the bandgap, reaching a conversion efficiency of 50.2% (at radiative limit) with emitter temperature at 1600 K and 200 nm vacuum gap, surpassing or comparing with the efficiency and output power of state-of-the-art plasmonic and hyperbolic emitter designs. By tailoring the radiative heat flux mainly in the propagating and frustrated components, the design also circumvents possible electrical losses due to enhanced surface recombination when the radiative transfer is dominated by surface modes. This study offers a straightforward strategy to manipulate the near-field radiative transfer with great flexibility by a few-layer planar emitter and emphasizes the different consideration for designing a nano-gap TPV emitter from far-field PV/TPV emitter designs.

Dynamics of methane/air premixed flame in a mesoscale diverging combustor with/without a cylindrical flame holder

A mesoscale diverging combustor with a cylindrical flame holder is developed to improve the flame stability and obtain the steadily symmetrical flame, which can be used as heat source for the mesoscale TPV (Thermophotovoltaic) system. The present work investigates the flammability limits based on equivalence ratio and the flame dynamics in the combustors with/without a cylindrical flame holder. For the combustor without the holder, four typical flame propagation modes, the stable slant, oscillating, spinning x-shape and flame blow-off, are observed, and all these flame patterns are asymmetrical. On the other side, the introduction of the cylindrical flame holder can significantly suppress the unstable flame and improve the flame stability probably due to enhanced flame-wall coupling and/or flow recirculation effects, as expected. The stable flame in the combustor with the holder can remain symmetrical under a quite wide range of equivalence ratio and the spinning x-shape mode does not occur here. Although the oscillating flames are also observed in relative narrow range of equivalence ratio in the combustor with the holder, their behaviors still keep symmetrical and their oscillating frequencies are probably dependent on the enhanced flame-wall coupling effect instead of the flame speed. In addition, the flame blow-off dynamics in the combustors with/without a cylindrical flame holder are also demonstrated and discussed in detail. In summary, the cylindrical flame holder can significantly improve the flame stability and makes the flame always remain symmetrical, which is beneficial for extensive application of the combustor with the diverging structure.

Numerical modeling of a cogeneration system based on a direct carbon solid oxide fuel cell and a thermophotovoltaic cell

A new combined system model is proposed to numerically assess the feasibility and effectiveness of using a thermophotovoltaic cell (TPVC) for waste heat recovery from a direct carbon solid oxide fuel cell (DC-SOFC). The system model mainly consists of an irreversible TPVC model and a 2D tubular DC-SOFC model considering the ionic/electronic charge transport, mass transport, momentum transport, and chemical/electrochemical reactions. Mathematical expressions of the power density and efficiency for the proposed system are derived and the performance characteristics of the combined system are revealed. The impacts of operation conditions and designing parameters including the operating temperature of the DC-SOFC, distance between the carbon layer and anode, and band-gap energy of the TPVC on the combined system performance are examined. It is found that the TPVC can efficiently harvest the waste heat from the DC-SOFC. The maximum power density of the combined system is approximately 56.2% larger than that of the stand-alone DC-SOFC. Moreover, a higher operating temperature of the DC-SOFC and a smaller distance between the carbon layer and the anode are beneficial to the performance enhancement of the combined system. The power density of the combined system can be further improved through designing the TPVC with an optimum band gap.

Numerical investigation of key parameters of the porous media combustion based Micro-Thermophotovoltaic system

Power generation with porous media driven Micro-Thermophotovoltaic (MTPV) was investigated and effects of changes to key parameters of the system investigated. The micro combustor had the dimensions of length–15 mm, width–10 mm, height–1 mm and wall thickness of 0.5 mm. The distance between the outside wall of combustor and the TPV cell was fixed at 1 mm. Variation in distance from 1 to 6 mm between the outside wall of the combustor and the thermophotovoltaic cell (TPV cell) caused a reduction of 13.75% and 1.4% in the radiation heat transfer efficiency and the TPV cell conversion efficiency respectively. An increase in the mixture flow rate from 300 mL/min to 1800 mL/min caused an increase in the radiation heat transfer efficiency, TPV cell conversion efficiency and the total system efficiency. As the flow rate increased, the system’s power output also increased. At 600 mL/min, the output power was 560 mW but rose to 3.2 W at the flow rate of 1800 mL/min. The cooling load of the system showed a linear growth as the flow rate increased. At 1800 mL/min the cooling load of the system was 12.4 W which is three times the cooling load at 900 mL/min.

High Electron Mobility and Insights into Temperature-Dependent Scattering Mechanisms in InAsSb Nanowires.

InAsSb nanowires are promising elements for thermoelectric devices, infrared photodetectors, high-speed transistors, as well as thermophotovoltaic cells. By changing the Sb alloy fraction the mid-infrared bandgap energy and thermal conductivity may be tuned for specific device applications. Using both terahertz and Raman noncontact probes, we show that Sb alloying increases the electron mobility in the nanowires by over a factor of 3 from InAs to InAs0.65Sb0.35. We also extract the temperature-dependent electron mobility via both terahertz and Raman spectroscopy, and we report the highest electron mobilities for InAs0.65Sb0.35 nanowires to date, exceeding 16,000 cm2 V-1 s-1 at 10 K.

Endoreversible model of thermal to radiative energy converters

In this work, we present a general endoreversible model to calculate the efficiency and the output power of energy emissive harvesters. Our model improves previous models since it includes the entropy losses associated to the energy transport happening in the energy emissive harvesters. Our model is developed for both harvesters exchanging energy with the environment through the full spectrum and when the exchange is mediated by a bandgap. For cells exchanging energy through a bandgap, we use the concept of electronic temperature previously introduced by Strandberg to model the operation of the cells at different polarization voltages and to make a model for thermoradiative cells. We calculate the output power and the efficiency of cold-carrier emissive energy harvesters working at temperatures between 300 and 500 K in environments with temperatures between 50 and 300 K. In the last part of the paper, we calculate the output power and the efficiency of thermoradiative cells working at different polarization voltages. We compare our results to those obtained in the works of Byrnes and Strandberg.

Performance Assessment and Optimization of a Thermophotovoltaic Converter–Thermoelectric Generator Combined System

To evaluate the feasibility of the performance enhancement of a thermophotovoltaic (TPV) converter by using a thermoelectric generator (TEG), a new model of a combined system is established, where the TEG is attached on the backside of the TPV converter to harvest the heat produced in the TPV converter. The effects of the voltage output of the TPV converter, band gap energy of the TPV converter, dimensionless current of the TEG, and emitter temperature on the performance of the combined system are examined numerically. It is found that the performance of the TPV converter can be enhanced by using the TEG. The percentage increment of the maximum power output density is larger than that of the maximum efficiency. There are optimally working regions of the converter voltage, dimensionless current, and band gap energy. The elevated emitter temperature results in the increase of the power output density of the combined system. However, there is an optimal emitter temperature that yields the maximum efficiency of the combined system. Moreover, the TEG is not suitable to harvest the heat produced in the TPV converter when the emitter temperature is sufficiently high.

High-efficiency thermophotovoltaic energy conversion enabled by a metamaterial selective emitter

Thermophotovoltaics (TPV) is the process by which photons radiated from a thermal emitter are converted into electrical power via a photovoltaic cell. Selective thermal emitters that can survive at temperatures at or above ∼1000°C have the potential to greatly improve the efficiency of TPV energy conversion by restricting the emission of photons with energies below the photovoltaic (PV) cell bandgap energy. In this work, we demonstrated TPV energy conversion using a high-temperature selective emitter, dielectric filter, and 0.6 eV In0.68Ga0.32As photovoltaic cell. We fabricated a passivated platinum and alumina frequency-selective surface by conventional stepper lithography. To our knowledge, this is the first demonstration of TPV energy conversion using a metamaterial emitter. The emitter was heated to >1000°C, and converted electrical power was measured. After accounting for geometry, we demonstrated a thermal-to-electrical power conversion efficiency of 24.1±0.9% at 1055°C. We separately modeled our system consisting of a selective emitter, dielectric filter, and PV cell and found agreement with our measured efficiency and power to within 1%. Our results indicate that high-efficiency TPV generators are possible and are candidates for remote power generation, combined heat and power, and heat-scavenging applications.

Optimization of a near-field thermophotovoltaic system operating at low temperature and large vacuum gap

The present work successfully achieves a strong enhancement in performance of a near-field thermophotovoltaic (TPV) system operating at low temperature and large-vacuum-gap width by introducing a hyperbolic-metamaterial (HMM) emitter, multilayered graphene, and an Au-backside reflector. Design variables for the HMM emitter and the multilayered-graphene-covered TPV cell are optimized for maximizing the power output of the near-field TPV system with the genetic algorithm. The near-field TPV system with the optimized configuration results in 24.2 times of enhancement in power output compared with that of the system with a bulk emitter and a bare TPV cell. Through the analysis of the radiative heat transfer together with surface-plasmon-polariton (SPP) dispersion curves, it is found that coupling of SPPs generated from both the HMM emitter and the multilayered-graphene-covered TPV cell plays a key role in a substantial increase in the heat transfer even at a 200-nm vacuum gap. Further, the backside reflector at the bottom of the TPV cell significantly increases not only the conversion efficiency, but also the power output by generating additional polariton modes which can be readily coupled with the existing SPPs of the HMM emitter and the multilayered-graphene-covered TPV cell.

Performance evaluation of thermophotovoltaic GaSb cell technology in high temperature waste heat

In this study, waste heat was evaluated and examined by means of thermophotovoltaic systems with the application of energy production potential GaSb cells. The aim of our study is to examine GaSb cell technology at high temperature waste heat. The evaluation of the waste heat to be used in the system is designed to be used in the electricity, industry and iron and steel industry. Our work is research. Graphic analysis is done with Matlab program. The high temperature waste heat graphs applied on the GaSb cell are in the results section. Our study aims to provide a source for future studies.

Examination of thermophotovoltaic GaSb cell technology in low and medium temperatures waste heat

In this study, waste heat was evaluated and examined by means of thermophotovoltaic systems with the application of energy production potential GaSb cells. The aim of our study is to examine GaSb cell technology at low and medium temperature waste heat. The evaluation of the waste heat to be used in the system is designed to be used in the electricity, industry and iron and steel industry. Our work is research. Graphic analysis is done with Matlab program. The low and medium temperature waste heat graphs applied on the GaSb cell are in the results section. Our study aims to provide a source for future studies.

Near-field thermophotovoltaic energy conversion using an intermediate transparent substrate.

We propose a scheme for near-field thermophotovoltaic (TPV) energy conversion, where thermal emission from an emitter is extracted by an intermediate transparent substrate attached to the top of a photovoltaic (PV) cell. The addition of an intermediate transparent substrate suppresses the unwanted heat transfer from the emitter to the PV cell due to the surface modes on the PV cell while maintaining the enhancement in the interband absorption. We confirm that our scheme is applicable for near-field TPV systems using a silicon (Si) or tungsten (W) emitter. As a specific example, we designed a near-field TPV system composed of a one-dimensional Si photonic crystal thermal emitter, an InGaAs PV cell, and an intermediate Si substrate, and displayed that our scheme could realize both high power density (>5 × 104 W/m2) and high power conversion efficiency (>40%) at a 50-nm gap between the emitter and the intermediate substrate.

Thermodynamic analysis of thermophotovoltaic systems used in waste heat recovery systems: an application

Thermodynamic analysis of the thermophotovoltaic (TPV) system was carried out in our research and the results are presented. First, the TPV system was analyzed in three different regions. In the analysis, each part of the system is taken separately, while the whole system is handled separately. Within the thermodynamic analysis of each region, energy and exergy analysis were carried out and the system was analyzed from part to part. As a result of this method, a general energy and exergy efficiency of the whole TPV system is determined. Our results are supported by formulas. The In0.2Ga0.8As0.18Sb0.82 cell has a higher efficiency compared to the GaSb cell at the same source temperature. This is because the reverse saturation current and the energy band gap are low and the short-circuit current is high. If TPV systems are applied for waste heat energy potential in the Turkish iron and steel industry, the energy efficiency of GaSb cell systems is 66 192 MJ per year with energy efficiency of 2.04%, the energy efficiency of In0.2Ga0.8As0.18Sb0.82 cell systems is 7.31% with annual energy efficiency of 189 971 MJ can be recovered. It is aimed that the work done will be an alternative to the existing electricity generation and will form a resource for future works.

Maximum power output and parametric choice criteria of a thermophotovoltaic cell driven by automobile exhaust

Abstract An unreported model of the cylindrical thermophotovoltaic cell (TPVC) composed of an emitter and a photovoltaic (PV) cell is proposed and used to recycle the waste heat released by the automobile exhaust pipe to generate electricity. To theoretically analyze the performance of this system, expressions of the power output and the conversion efficiency of the thermophotovoltaic cell driven by the automobile exhaust are derived analytically. The optimal functions of the temperature distributions of the automobile exhaust pipe and TPVC are obtained by the variational method and the modified Lagrangian formulation. The maximum power output is calculated. The performance characteristics of the whole system at the maximum power output are represented. The optimal regions of the voltage output of the PV cell, the energy gap of the material in the PV cell, and the inlet heat flow of the gas pipe are determined. The effects of the size of the gas pipe on the performance of the TPVC are discussed. The theoretical efficiencies of the TPVC and thermoelectric generator driven by the automobile exhaust are compared. The advantages of the TPVC are revealed. The results obtained show that the optimally designed TPVC can significantly harvest the waste heat of the automobile exhaust.

A coherent description of thermal radiative devices and its application on the near-field negative electroluminescent cooling

Using the transimissivity between two thermal reservoirs and the generalized Planck distributions, we describe the devices that use radiative energy transfer between thermal reservoirs in a unified formalism. Four types of devices are distinguished. For power generators that use the temperature difference between reservoirs, photovoltaic (PV) and thermoradiative (TR) devices respectively use the low-temperature photovoltaic cell and high-temperature thermoradiative cell to generate electricity. For active cooling, the electroluminescent (EL) cooling devices apply a forward bias voltage on the object we want to cool, whereas the negative EL cooling devices apply a reverse bias voltage to the heat sink. The relationship among these four devices is explicated. The performance of the negative EL cooling is analyzed, both in the Shockley-Queisser (blackbody spectrum and radiative recombination) framework and the near-field enhancement. The “impedance match” condition derived for PV systems is applied to the negative EL devices. One advantageous feature of the negative EL cooling is that it does not apply the voltage to the target object which we want to cool, and the near-field enhancement can apply to various target materials that support the surface resonant modes.

Surface structure for manipulating the near-field spectral radiative transfer of thermophotovoltaics

To improve the efficiency and output power of the nano-gap thermophotovoltaic (TPV) power generation system, surface rectangular grating structures are added to the top surface of the group Ⅲ-V semiconductor cell to control the spectrum of near-field radiative transfer. Doped zinc oxide that supports surface waves at near-infrared wavelengths is selected as the TPV emitter. When paired with GaSb grating structures, the surface plasmon polariton excited by the emitter and the light trapping effect by the grating tunnels will be coupled, which results in a significantly and selectively enhanced near-field radiative heat flux within a narrow spectral region above the cell bandgap, thereby fulfilling the design purpose. This physical mechanism is explained by a direct finite-difference time-domain (FDTD) simulation based on the Langevin approach. The material volume meshgrids filled with random dipole sources can act as the thermal emission source and the radiative heat flux is calculated by solving the Maxwell equations numerically. The spectral results show that adding rectangular grating structures to GaSb not only increases radiative transfer in the expected wavelength region over the unstructured case, resulting in a heat flux surpassing that of a far-field blackbody source at the same temperature, but also suppresses the unwanted long-wavelength heat flux that causes radiative loss and cell heating. With a vacuum gap of 200 nm between the emitter and the cell, using a bulk GaSb cell with rectangular gratings can double the spectral flux of the blackbody emitter case, and using an ultrathin GaSb cell with surface structures and back reflectors further increases this ratio to 2.84 due to the total internal reflection controlled by the cell thickness. The amplitude and wavelength of the spectral peak are controlled by the grating size parameters. Low filling ratio gratings with lower-aspect-ratio grating channels generally have sharper enhancement peaks but lower total radiative heat flux, while high filling ratio structures with higher-aspect-ratio channels have better heat flux improvement but might also result in lower conversion efficiency due to the broader spectrum. The rigorous approach reveals the detailed physical mechanism that is otherwise unseen with effective medium approaches for inhomogeneous structures or the Derjaguin proximity approximation. Overall the results of this study enable an enhancement of near-field radiative heat flux limited within a narrow wavelength range shorter than the cell bandgap, offering practical benefit to the application of TPV power generation with higher feasible power and conversion efficiency.

InAs thermophotovoltaic cells with high quantum efficiency for waste heat recovery applications below 1000 °C

InAs thermophotovoltaic (TPV) cells with external quantum efficiency at the peak wavelengths reaching 71% at low temperature and 55% at room temperature are reported, which are the highest values to date for InAs. The TPV exhibited 10% power conversion efficiency at 100K cell temperature. The dark and light current-voltage characteristics were measured at different cell temperatures (100–340K) in response to heat sources in the range 500–800°C. The resulting dependences of the output voltage and current as well as the spectral response of the InAs TPV have been extensively characterized for waste heat recovery applications. The performance of these cells is strongly determined by the dark current which increases rapidly with increasing cell temperature originating from bandgap narrowing, which resulted in a reduction of open circuit voltage and output power.

Performance comparison between photovoltaic and thermoradiative devices

Photovoltaic (PV) and thermoradiative (TR) devices are power generators that use the radiative energy transfer between a hot and a cold reservoir. For PV devices, the semiconductor at the cold side (PV cell) generates electric power; for TR devices, the semiconductor at the hot side (TR cell) generates electric power. In this work, we compare the performance of the photovoltaic and thermoradiative devices, with and without the non-radiative processes. Without non-radiative processes, PV devices generally produce larger output powers than TR devices. However, when non-radiative processes become important, the TR can outperform the PV devices. This conclusion applies to both far-field and near-field based devices. A key difference in efficiency between PV and TR devices is pointed out.

Plasmonic light trapping for enhanced light absorption in film-coupled ultrathin metamaterial thermophotovoltaic cells

Ultrathin cells have gained increasing attention due to their potential for reduced weight, reduced cost and increased flexibility. However, the light absorption in ultrathin cells is usually very weak compared to the corresponding bulk cells. To achieve enhanced photon absorption in ultrathin thermophotovoltaic (TPV) cells, this work proposed a film-coupled metamaterial structure made of nanometer-thick gallium antimonide (GaSb) layer sandwiched by a top one-dimensional (1D) metallic grating and a bottom metal film. The spectral normal absorptance of the proposed structure was calculated using the rigorous coupled-wave algorithm (RCWA) and the absorption enhancement was elucidated to be attributed to the excitations of magnetic polariton (MP), surface plasmon polariton (SPP), and Fabry-Perot (FP) resonance. The mechanisms of MP, SPP, and FP were further confirmed by an inductor-capacitor circuit model, dispersion relation, and phase shift, respectively. Effects of grating period, width, spacer thickness, as well as incidence angle were discussed. Moreover, short-circuit current density, open-circuit voltage, output electric power, and conversion efficiency were evaluated for the ultrathin GaSb TPV cell with a film-coupled metamaterial structure. This work will facilitate the development of nextgeneration low-cost ultrathin infrared TPV cells.

Design of broadband absorber using 2-D materials for thermo-photovoltaic cell application

Present study is done to analyze a nano absorber for thermo-photovoltaic cell application. Optical absorbance of two-dimensional materials is exploited to achieve high absorbance. It is found that few alternating layers of graphene/transition metal dichalcogenide provide high absorbance of electromagnetic wave in visible as well as near infrared region. Four transition metal dichalcogenides are considered and found that most of these provide perfect absorbance for almost full considered wavelength range i.e. 200–1000 nm. Demonstrated results confirm the extended operating region and improved absorbance of the proposed absorber in comparison to the existing absorbers made of different materials. Further, absorber performance is improved by using thin layers of gold and chromium. Simple geometry of the proposed absorber also ensures easy fabrication.