Near-field Thermophotovoltaic System Research Articles

Design and optimization of near-field thermophotovoltaic systems using deep learning

Design and optimization of near-field thermophotovoltaic systems using deep learning

Experimental Study of Near-field Radiative Heat Transfer Between Large-Area Polar Dielectric Films

Utilizing polar dielectric films can improve the performance of near-field thermophotovoltaic system. To confirm the enhancement of near-field radiative heat transfer (NFRHT) achieved by large-area polar dielectric films, we built an experimental setup and observe NFRHT between two 250 nm-thick SiO2 films on intrinsic Si substrate with an area 5 × 5 mm2. The two films are supported by SiO2 nano-spacers with a height of 302 nm. Temperature difference between the two films varies from 13 K to 42 K at room temperature. The observed near-field radiative heat flux closely matches the theoretical values, which surpasses substrate-only system by up to 9.87 times and could reach up to 7.45 times that of blackbody radiation due to excitation of the surface photon polaritons modes. These results demonstrate the effectiveness of polar dielectric films in enhancing NFRHT.

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.

Performance investigation of tandem nano-gap thermophotovoltaic system considering the near-field thermal radiation

In this paper, a near-field thermophotovoltaic (TPV) system with a dual TPV cell design is considered and studied numerically. The fluctuational electrodynamics accompanied by the dyadic Green's function are utilized for calculating the radiation heat flux. The generated photocurrent and the conversion efficiency are obtained by solving the photon-coupled charge transport equations. It was discovered that the addition of the second TPV cell to the system, increases the generated photocurrent of the system due to the absorption of the low-energy photons and generation of the electron-hole pairs. By increasing the gap size between the emitter and Si TPV cell from 50 nm to 1 μm, the generated photocurrent decreases because of the reduction of the near-field thermal radiation effects. Finally, the tandem nano-gap TPV system shows higher conversion efficiency compared with the single Si TPV cell for different values of the vacuum gaps, and the gap size of d1 = 200 nm is the optimum gap size due to the improvement of the conversion efficiency of the system.

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.

Performance analysis of near-field tandem thermophotovoltaic systems based on radiative absorption distribution

Fluctuational electrodynamics and minority carrier diffusion equations are applied to evaluate the electric power output of a near-field tandem thermophotovoltaic (TPV) system with a bulk tungsten emitter (at 1500 K) and InGaSb/InAs tandem cells (at 300 K). We give evidence of the superiority of the tandem TPV system over the single-junction TPV system in the near-field regime because a tandem structure can harvest useful photons in a wider spectral range. It is also found that in comparison with the system with a bulk tungsten emitter, the practically achievable indium‑tin‑oxide (ITO) emitter and multilayer emitter could enhance the electricity output by a factor of 1.7 and 2.9 as a result of the supported surface plasmon resonances and hyperbolic modes, respectively. The conversion efficiency of the ITO emitter is 6% higher due to the suppression of the surface recombination loss, but the efficiency of the multilayer emitter is even 15% lower because of the heavy bulk recombination loss. These results highlight the possibilities to enhance the performance of the near-field tandem TPV systems via optimizing the radiative absorption distribution in the tandem cells, which is paramount important for the design and improvement of near-field tandem TPV systems.

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.

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.

Random Rough Surface Effects on the Performance of Near-Field Thermophotovoltaic System

Random Rough Surface Effects on the Performance of Near-Field Thermophotovoltaic System

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.

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.

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.

Spatial profiles of photon chemical potential in near-field thermophotovoltaic cells

Emitted photons stemming from the radiative recombination of electron–hole pairs carry chemical potential in radiative energy converters. This luminescent effect can substantially alter the local net photogeneration in near-field thermophotovoltaic cells. Several assumptions involving the luminescent effect are commonly made in modeling photovoltaic devices; in particular, the photon chemical potential is assumed to be zero or a constant prescribed by the bias voltage. The significance of photon chemical potential depends upon the emitter temperature, the semiconductor properties, and the injection level. Hence, these assumptions are questionable in near-field thermophotovoltaic devices with nanoscale separation distances between the emitter and the cell. In the present work, an iterative solver that combines fluctuational electrodynamics with the drift-diffusion model is developed to tackle the coupled photon and charge transport problem, enabling the determination of the spatial profile of photon chemical potential beyond the detailed balance approach. The difference between the results obtained by allowing the photon chemical potential to vary spatially and by assuming a constant value demonstrates the limitations of the conventional approaches. This study is critically important for performance evaluation of near-field thermophotovoltaic systems with nanoscale vacuum gaps.

Coupled Charge and Radiation Transport Processes in Thermophotovoltaic and Thermoradiative Cells

Energy converters that recover thermal radiation have the potential for high-efficiency solid-state electricity generation. Modeling of these systems to date has made limiting assumptions that do not fully capture the nuances of charge and radiation interactions. To remedy this, the authors propose a fully coupled iterative model to properly account for photon and charge-carrier dynamics in both near-field and far-field thermophotovoltaic and thermoradiative systems. This approach can be used to better understand and design devices and materials that utilize thermal radiation.

Transient performance of a nanowire-based near-field thermophotovoltaic system

• A transient model of the near-field thermophotovoltaics is constructed. • Heating mode beneficial to the system’s thermal stability is proposed. • Start-up power consumption as a new evaluation index is proposed. Great improvement has been made in the optimization on the near-field thermophotovoltaic system at the steady state, whereas system safety evaluation under near-field thermal radiation and transient output performance considering the energy consumption of the cooling device are lacking. Therefore, it is important to extend the investigation on the system to the transient state. Based on the law of energy conservation and theory of near-field radiative heat transfer, a transient energy transfer model of a W nanowire-based near-field TPV system considering the energy consumption of the cooling device was established. The heating process of each component in the system was explored, and the effects of the input power density and cooling medium’s velocity on system performances were analyzed. Moreover, a new evaluation index named the start-up power consumption was proposed to characterize the start-up performance of the system. The results show that based on the common working conditions, the maximum heating rate of the emitter in the transient process is up to 1500 K/s, which makes a serious threat to the experimental safety of the system. However, only the maximal operating temperature of 1752 K can be obtained according to the steady-state model and this temperature is within safe range, indicating the shortcomings of the steady-state model. The maximum heating rate of the emitter can be reduced to 304 K/s by adopting a progressive heating mode with a power supply rate of 20 W/cm 2 /s, showing that the safety operation of the system can be ensured. The highest cell efficiency occurs at the transient moment due to the increasing cell temperature. Increasing the cooling medium’s velocity is expected to maintain the highest cell efficiency until the steady state to improve the net system efficiency. However, the system needs external energy in the start-up stage because the cell is insufficient to produce enough electricity to drive the cooling device during this time, and the start-up power consumption of the system increases sharply with the increasing velocity of the cooling medium, which can be as high as 23405.9 W/m 2 . By comprehensively considering the net system efficiency and start-up power consumption, the alternative velocity was determined as 4 m/s. It can be seen that the transient energy transfer model of the near-field TPV system can be used to better guide the experimental safety design and comprehensively evaluate the system performance compared with the steady-state model.