Near-field Thermophotovoltaic Research Articles

Spectral regulation in thermophotovoltaic devices

Thermophotovoltaic (TPV) device converts thermal radiation into electricity output through photovoltaic effect. High-efficiency TPV devices have extensive applications in grid-scale thermal storage, full-spectrum solar utilization, distributed thermal-electricity cogeneration, and waste heat recovery. The key to high-efficiency TPV devices lies in spectral regulation to achieve band-matching between thermal radiation of the emitters and electron transition of the photovoltaic cells. The latest advances in nanophotonics, materials science, and artificial intelligence have made milestone progress in spectral regulation and recording power conversion efficiency of up to 40% of TPV devices. Here we systematically review spectral regulation in TPV devices at the emitter end as well as the photovoltaic cell end. At the emitter end, spectral regulation is realized through thermal metamaterials and rare-earth intrinsic emitters to selectively enhance the in-band radiation and suppress the sub-bandgap radiation. At the photovoltaic cell end, spectral regulation mainly focuses on recycling the sub-bandgap thermal radiation through optical filters and back surface reflectors located at the front and back of the photovoltaic cells, respectively. We emphasize the light-matter interaction mechanisms and material systems of different spectral regulation strategies. We also discuss the spectral regulation strategies in near-field TPV devices. Finally, we look forward to potential development paths and prospects of spectral regulation to achieve scalable deployment of future TPV devices.

Near-field thermophotovoltaic energy conversion analysis based on enhanced radiative absorption distribution

Near-field radiation has been widely shown to greatly boost the electrical power of thermophotovoltaic (TPV) cells. However, there is a lack of theoretical analysis exploring the important influences of near-field effects on radiative absorption distributions as well as TPV energy conversion performances. This work investigates the electrical performances of near-field TPV cells made of InGaSb coupled with different practical emitters such as plain tungsten (W), indium tin oxide (ITO) film, and alternate W and alumina multilayer in detail. A comprehensive analysis is conducted to systematically compare the impacts of evanescent wave tunneling, surface plasmon resonance, and hyperbolic modes on spatial distributions of radiative absorption and the profiles of local carrier concentrations. The detailed and accurate analysis reveals the crucial role of near-field radiation emitted by various emitters in charge collection efficiency, thermal photon flux penetration depth, and photocurrent generation. Thus, the results certify that the electric power could be enhanced by utilizing ITO and multilayer emitters instead of a plain W emitter. The efficiency for an ITO emitter increases with decreasing vacuum gap owing to the suppressed bulk recombination but decreases when the vacuum gap falls below 18 nm due to increased surface recombination. While the efficiency for a multilayer emitter is comparatively lower due to the larger sub-bandgap photons and inefficient n-region. Furthermore, we verify the strategies for performance improvement via decreasing the surface recombination and optimizing the p-region thickness. The underlying mechanism is interpreted based on the spatial distribution and the collection efficiency of minority carriers.

A novel oxy-enrich near-field thermophotovoltaic system for sustainable fuel: Design guidelines and thermodynamic parametric analysis

Developing new technologies is a key to achieve renewable energy utilization. The near-field thermo-photovoltaics (NFTPV), an emerging power generation device, is investigated from the energy perspective in this paper. A novel oxy-enrich NFTPV system is proposed, and a thermo-physical model is established for sustainable fuel gas based on energy balance. The effects of parameters on system performance are numerically explored, including the furnace size, oxygen ratio, voltage and vacuum gap, etc. Moreover, special attention is paid to the effects of oxygen ratio on system performance under 2 atm of O2/N2 and O2/CO2. Besides, exergy analysis is conducted. The results show that it is more appropriate to control the power density of furnace at 50 kW/m2 in system design. The efficiency of NFTPV system is about twice greater than that of far-field TPV system. The power density of NFTPV system increases by more than 2.5 times when the oxygen ratio increases; the TPV volume can increase by 2–3 times and the efficiency is also improved. The results indicate that oxy-enrich combustion with higher oxygen ratio matches a larger NFTPV system, which reduces the manufacturing difficulty and is significant in engineering. This study provides new ideas and references for NFTPV practical application.

Medium-Bridge Near-Field Thermophotovoltaic System

ABSTRACT The energy conversion performance of thermophotovoltaic (TPV) systems can be improved when the vacuum gap between the emitter and the TPV cell is at the near-field owing to the photon tunneling of evanescent waves. Among them, the back-gapped-reflector TPV systems have gained interest as a method of improving their conversion efficiency by optimizing the spectral absorption of TPV cells. In this work, we introduce an alternative concept for the back-gapped-reflector TPV systems, namely the medium-bridge near-field TPV system, by building a medium bridge between the metal reflector and the TPV cell using SU8 nanofilm. The SU8 medium-bridge achieves a noticeable improvement in output performance by increasing the spectral absorption of the InAs cell and reducing parasitic absorption losses of the Au substrate. The results indicate that, as a consequence of the improved effect of the medium-bridge, the output power density and efficiency of this system exceed those of the conventional TPV system (which lacks a medium-bridge) by 26.4% and 36.5%, respectively. Moreover, we systematically analyze the modulation of medium-bridge thicknesses and cell thickness on output performance and clarify how both affect energy losses of this near-field TPV system. Our work offers a strategy to improve the energy conversion performance of the near-field TPV system, opening new opportunities for developing near-field energy conversion.

Topological Transitions Enhance the Performance of Twisted Thermophotovoltaic Systems

Twisted bilayer two-dimensional electronic systems give rise to many exotic phenomena and unveil a new frontier for the study of quantum materials. In photonics, twisted two-dimensional systems coupled via near-field interactions offer a platform to study localization and lasing. Here, we theoretically propose that twisting can be an unprecedented tool to tune the performance of near-field thermophotovoltaic systems. Remarkably, through twisting-induced photonic topological transitions, we achieve significant tuning of the thermophotovoltaic energy efficiency and power. The underlying mechanism is related to the change of the photonic isofrequency contours from elliptical to hyperbolic geometries in a setup where the hexagonal-boron-nitride metasurface serves as the heat source and the indium-antimonide $p$-$n$ junction serves as the cell. We find that a notably high energy efficiency, nearly 53 of the Carnot efficiency, can be achieved in our thermophotovoltaic system, while the output power can reach up to $1.1\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.2em}{0ex}}\mathrm{W}/{\mathrm{m}}^{2}$ without requiring a large temperature difference between the source and the cell. Our results indicate the promising future of twisted near-field thermophotovoltaics and paves the way towards tunable, high-performance thermophotovoltaics and infrared detection.

Near-Field Thermophotovoltaic Energy Conversion: Progress and Opportunities

Thermophotovoltaic (TPV) energy conversion is a promising power-generation technology for converting heat to electricity. Recent studies have explored TPV devices featuring nanoscale gaps, which take advantage of near-field effects that enable much larger radiative fluxes and power density. The authors review the physics of near-field thermal radiation, and assess theoretical and experimental advances in predicting and validating near-field enhancements of power output and efficiency in TPV devices. Their discussion of the near-field photonic heat engines presented here will help to guide future engineering solutions in developing practical near-field energy-conversion devices.

Enhancement of a Graphene-Based Near-Field Thermophotovoltaic System by Optimization Algorithms and Dynamic Regulations

Thermophotovoltaics (TPVs), a heat recovery technique, is faced with low efficiency and power density. It has been proven that graphene helps add new functionalities to optical components and improve their performance for heat transfer. In this work, I study Near-field radiative heat transfer in TPVs based on a composite nanostructure composed of Indium Tin Oxide (ITO) sheet and a narrow bandgap photovoltaic cell made from Indium Arsenide (InAs). I introduce a new way to calculate nonradiative recombination (NR) and compare the performance with and without the NR being considered. By comparing graphene modulated on the emitter (G-E), on the receiver (G-R), and on both the emitter and the receiver (G-ER), I find the G-ER case can achieve the highest current density. However, constrained by the bandgap energy of the cell, this case is far lower than the G-E case when it comes to efficiency. After applying variant particle swarm optimization (VPSO) and dynamic optimization, the model is optimized up to 43.63% efficiency and 11 W/cm2 electric power at a 10 nm vacuum gap with a temperature difference of 600 K. Compared with before optimization, the improvement is 8.97% and 7.2 W/cm2, respectively. By analyzing the emission spectrum and the transmission coefficient, I find that after optimization the system can achieve higher emissivity above the bandgap frequency, thus achieving more efficient conversion of light to electricity. In addition, I analyze the influence of temperature difference by varying it from 300 K to 900 K, indicating the optimized model at a 900 K temperature difference can achieve 49.04% efficiency and 52 W/cm2 electric power. By comparing the results with related works, this work can achieve higher conversion efficiency and electric power after the optimization of relevant parameters. My work provides a method to manipulate the near-field TPV system with the use of a graphene-based emitter and promises to provide references in TPV systems that use low bandgap energy cells.

Random rough surface effects on the performance of near-field thermophotovoltaic system

Surfaces roughness exists inevitably during manufacturing process, while how it affects the performance of the near-field thermophotovoltaic (NF-TPV) system is still unclear. In this work, we investigate the effect of rough surfaces on the performance of NF-TPV systems theoretically. Two typical NF-TPV systems with the same plasmonic emitter (Ga-doped ZnO, GZO) and different TPV cells (GaSb and InAs) are considered. The effective multilayer approach and analytical approximation model are applied to analyze near-field radiative heat transfer and the performance of the NF-TPV system. Near-field radiative heat transfer between the ideal GZO and TPV cell with smooth surfaces is through the TR-SPPs modes (interaction between the evanescent waves generated by total reflection and the surface plasmon polaritons), while the surface roughness excites the red-shift and the coupling of the TR-SPPs modes. The influence of surface roughness on the performance of the NF-TPV system depends on the cooperation between the resonance frequency of the emitter and the bandgap of TPV cell. Generally, roughness increases the power output for a given average emitter-cell distance, while the efficiency can be improved or reduced. This work will help the understanding and evaluation of the performance of NF-TPV systems with rough surfaces.

A multi-junction-based near-field solar thermophotovoltaic system with a graphite intermediate structure

A solar thermophotovoltaic (STPV) system can transform incident concentrated solar energy into electrical energy with an efficiency that could be higher than the Shockley–Queisser limit. Near-field thermophotovoltaic (NF-TPV) devices can generate larger electrical power output than traditional far-field TPV devices with the aid of photon tunneling. Moreover, multi-junction PV cells can boost the performance of TPV devices by effectively distributing the absorbed photon energy inside the PV cell. In this work, we design a multi-junction-based near-field STPV system with a practical and high-temperature stable graphite intermediate structure. To optimize the system configuration, we employ a genetic algorithm and a surrogate model based on an artificial neural network, which enables us to suggest a better design approach for the multi-junction-based NF-STPV system between the power output density and power conversion efficiency maximization scenarios. When the concentration factor of the incident solar energy is 5000 and the absorber-to-emitter area ratio is 3, we can achieve a system efficiency of 23%. By introducing a material whose emissivity is as high as a blackbody on the solar absorber, the system efficiency can be further enhanced up to 35%.

Parametric optimization of a novel solar concentrating photovoltaic-near field thermophotovoltaic hybrid system based on cascade utilization of full-spectrum solar energy

Based on cascade utilization of full-spectrum solar energy, a novel solar concentrating photovoltaic and near-field thermophotovoltaic hybrid system (CPV-NFTPVS) is proposed to achieve efficient solar power generation. The full-spectrum solar energy is split into two parts by spectrum splitter: The high-frequency part is provided to concentrating photovoltaic module while the rest to near-field thermophotovoltaic module. The effect of operating parameters such as concentration ratio, split wavelength, applied voltage, and vacuum gap on the energy conversion performance of the hybrid system is investigated. Starting with a concentration ratio of only 210, the efficiency of the hybrid system exceeds the maximum efficiency of near-field solar thermophotovoltaic system (NF-STPVS) with a concentration ratio of 2000. Considering the technical difficulties of the ultrahigh concentration ratio and ultrahigh cooling rate, a concentration ratio of 1000 is a practical choice for the hybrid system and its efficiency reaches 56.20%, which is about 1.12 times higher than the maximum efficiency of NF-STPVS. The law of the optimal value of the operating parameters is further explored and it's found that the optimal gap distance is always 10 nm and the optimal split wavelength needs to be adjusted according to the concentration ratio. The significant value of CPV-NFTPVS based on cascade utilization in solar power generation is indicated and practical application of the near-field thermophotovoltaic is guided.

Modeling and experiments of near-field thermophotovoltaic conversion: A review

This article provides an in-depth review of charge transport models utilized to analyze the performance of near-field thermophotovoltaic (NF-TPV) devices, and a comprehensive examination of NF-TPV experiments performed to date. Researchers have used several models to theoretically analyze NF-TPV devices; however, each model can result in different performances of NF-TPV devices, particularly due to characteristics of near-field thermal radiation. For clarification, we sort up-to-date analysis models into four categories; the analytical approximation model, detailed balance analysis, minority carrier separation (MCS) model, and Poisson-drift–diffusion (PDD) model. Key assumptions of each model are enumerated, and the calculated results by each model are compared. For the review of experimental works, we first sort NF-TPV experiments according to the manner of gap control into positioner-, spacer-, and MEMS-based approaches. Then, we introduce the primary design configuration of each approach and the compelling experimental results achieved by state-of-the-art studies. In addition, advanced concepts of hybrid devices and multi-junction PV cells, proposed to further improve NF-TPV performance, are briefly introduced. • A review of charge transport models used to analyze the NF-TPV device is provided. • Models are classified into analytical approximation, detailed balance, MCS, and PDD. • A comprehensive review of all NF-TPV experiments performed to date is conducted. • NF-TPV experiments are sorted into positioner-, spacer-, and MEMS-based approaches.

Thermophotovoltaic Energy Conversion in Far-to-Near-Field Transition Regime

Recent experimental studies on near-field thermophotovoltaic (TPV) energy conversion have mainly focused on enhancing performance via photon tunneling of evanescent waves. In the sub-micron gap, however, there exist peculiar phenomena caused by the interference of propagating waves, which is seldom observed due to the dramatic increase of the radiation by evanescent waves in full spectrum range. Here, we experimentally demonstrate the oscillatory nature of near-field TPV energy conversion in the far-to-near-field transition regime (250-2600 nm), where evanescent and propagating modes are comparable due to the selective spectral response by the PV cell. Noticeably, it was possible to produce the same amount of photocurrent at different vacuum gaps of 870 and 322 nm, which is 10\% larger than the far-field value. Considering the great challenges in maintaining nanoscale vacuum gap in practical devices, this study suggests an alternative approach to the design of a TPV system that will outperform conventional far-field counterparts.

Improved performance of a near-field thermophotovoltaic device by a back gapped reflector

Various spectral control techniques can be applied to improve the performance of a thermophotovoltaic (TPV) system. For example, a back surface reflector (BSR) can improve the performance of TPV systems. A conventional metal BSR structure enhances the photogeneration rate by increasing the absorption probability of photons via back surface reflection, affording a second chance for absorption. However, surface passivation and external luminescence effects introduced by BSR structures have been previously ignored, which potentially decreases the performance of TPV systems. Recently, a back gapped reflector (BGR) structure was proposed to greatly improve the performance of far-field TPV systems by reducing reflection loss at the semiconductor-metal interface. In the present work, the performance improvement on a thin-film, near-field InAs TPV system with a BGR is investigated, comparing its performance to that with a conventional metal BSR. Surface passivation conditions are also investigated to further improve the performance of TPV systems with back reflectors. The output power and efficiency are calculated using an iterative model combining fluctuational electrodynamics and the full drift-diffusion model. For the well-passivated condition, when the BSR is replaced by the BGR, the calculated conversion efficiency was improved from 16.4% to 21% and the output power was increased by 10% for the near-field regime. Finally, the reflection loss and external luminescence loss are analyzed to explain the performance improvement.

Comprehensive analysis of an optimized near-field tandem thermophotovoltaic converter

It is well known that performance of a thermophotovoltaic (TPV) device can be enhanced if the vacuum gap between the thermal emitter and the TPV cell becomes nanoscale due to the photon tunneling of evanescent waves. Having multiple bandgaps, multi-junction TPV cells have received attention as an alternative way to improve its performance by selectively absorbing the spectral radiation in each subcell. In this work, we comprehensively analyze the optimized near-field tandem TPV system consisting of the thin-ITO-covered tungsten emitter (at 1500 K) and GaInAsSb/InAs monolithic interconnected tandem TPV cell (at 300 K). We develop a simulation model by coupling the near-field radiation solved by fluctuational electrodynamics and the diffusion-recombination-based charge transport equations. The optimal configuration of the near-field tandem TPV system obtained by the genetic algorithm achieves the electrical power output of 8.41 W/cm$^2$ and the conversion efficiency of 35.6\% at the vacuum gap of 100 nm. We show that two resonance modes (i.e., surface plasmon polaritons supported by the ITO-vacuum interface and the confined waveguide mode in the tandem TPV cell) greatly contribute to the enhanced performance of the optimized system. We also show that the near-field tandem TPV system is superior to the single-cell-based near-field TPV system in both power output and conversion efficiency through loss analysis. Interestingly, the optimization performed with the objective function of the conversion efficiency leads to the current matching condition for the tandem TPV system regardless of the vacuum gap distances.

Thermal Degradation of Tungsten Nanowire-Based Hyperbolic Metamaterial Emitters for Near-Field Thermophotovoltaic Applications

Thermal Degradation of Tungsten Nanowire-Based Hyperbolic Metamaterial Emitters for Near-Field Thermophotovoltaic Applications

THERMAL ENERGY, PROCESS, AND TRANSPORT INTENSIFICATION - A BRIEF REVIEW OF LITERATURE IN 2021 AND PROSPECTS

This review provides a brief literature survey recorded by the Web of Science (WoS) in 2021 on the studies of heat transfer enhancement (HTE), thermal energy, process and transport intensification technologies. A topic search containing “heat transfer” or “heat transport” or “thermal transport” in the WoS resulted in over 31,360 papers published in 2021 and more than 30% were relevant to HTE, thermal energy, process and transport intensification, which indicates the importance and rapid development of these technologies and their applications. The chosen studies focus on HTE, thermal energy, process and transport intensification, which are grouped into heat conduction enhancement, convection HTE, boiling and condensation HTE, radiation HTE, thermal energy and process intensification and cross disciplinary research involving two or more techniques. Heat conduction enhancement research covers the research topics on thermal conductivity augmentation using various methods for thermal interface materials, phase change materials (PCMs), composite materials with nano additives, nano coatings, nanofluids, finned metal foam and fins in heat exchangers. Convection HTE research includes passive, active and compound techniques, and their engineering applications. Boiling and condensation HTE research is critical in the development of advanced and cutting-edge thermal energy and process intensification technologies. Radiation HTE research focuses on near-field radiation and thermophotovoltaic. Thermal energy and process intensification research involves thermal and power systems, energy conversion, energy storage, thermal processes, renewable energy and thermal management

Thermodynamics of Light Management in Near-Field Thermophotovoltaics

Thermophotovoltaic (TPV) systems hold promise for recycling heat into electricity, but a key challenge is to lower the operating temperature to the range of industrial waste heat, at high efficiency. The authors offer a thermodynamic analysis of the open-circuit voltage of TPV systems operating in the far and near fields, and compare to solar PVs. An analytic model shows that although nonradiative losses increase in the near field, luminescence efficiency improves significantly in this range. This work suggests that an ultrathin near-field TPV system can exhibit ultrahigh open circuit voltage and high efficiency, for emitter temperatures much lower than its far-field counterpart.

Integrated Near-Field Thermophotovoltaic Device Overcoming Blackbody Limit

Near-field thermal radiation transfer overcoming the blackbody limit has attracted significant attention in recent years owing to its potential for drastically increasing the output power and conve...

Near-field thermophotovoltaics for efficient heat to electricity conversion at high power density

Thermophotovoltaic approaches that take advantage of near-field evanescent modes are being actively explored due to their potential for high-power density and high-efficiency energy conversion. However, progress towards functional near-field thermophotovoltaic devices has been limited by challenges in creating thermally robust planar emitters and photovoltaic cells designed for near-field thermal radiation. Here, we demonstrate record power densities of ~5 kW/m2 at an efficiency of 6.8%, where the efficiency of the system is defined as the ratio of the electrical power output of the PV cell to the radiative heat transfer from the emitter to the PV cell. This was accomplished by developing novel emitter devices that can sustain temperatures as high as 1270 K and positioning them into the near-field (<100 nm) of custom-fabricated InGaAs-based thin film photovoltaic cells. In addition to demonstrating efficient heat-to-electricity conversion at high power density, we report the performance of thermophotovoltaic devices across a range of emitter temperatures (~800 K–1270 K) and gap sizes (70 nm–7 µm). The methods and insights achieved in this work represent a critical step towards understanding the fundamental principles of harvesting thermal energy in the near-field.

Parametric characteristics and optimization of a near-field thermophotovoltaic system considering cooling consumption

Near-field thermophotovoltaic (NF-TPV) system based on the photon tunneling effect can effectively increase the output power, which has shown great application potential in solar energy utilization. In order to evaluate the performance of NF-TPV system more reasonably, this study established an optimization model of NF-TPV combined with a water-cooling system to better reflect the physical process of system running. Taking into account the energy consumption of cell water cooling, the performance of NF-TPV system with tungsten emitter and InAs cell was analyzed and optimized through numerical simulation. The influence of operating parameters such as emitter and cell temperatures, applied voltage and vacuum gap on the system performance was investigated. It is found that the operating parameters of NF-TPV combined with water cooling system have their optimal ranges, which can ensure the high efficiency and output power of the system simultaneously. When NF-TPV is applied in waste heat recovery, the cooling energy consumption could be ignored. In solar energy conversion, the emitter temperature should be kept as high as possible, and the cooling energy consumption must be considered when the emitter temperature is above 1000 K. If fuel combustion is used as NF-TPV heat source, the emitter temperature should not exceed 1500 K. This study provides guidance for the practical application of NF-TPV systems.