Thermophotovoltaic Research Articles

Design and evaluation of a quasi-monochromatic and high-energy flow thermophotovoltaic test system

The energy conversion efficiency of thermophotovoltaic (TPV) cells is quite different for different spectral energy and related to the spectral energy intensity. To obtain the energy conversion relationship of the thermophotovoltaic cell for various spectral energy with high-density flux, a test system was designed, which consists of a blackbody radiation source, spectrum control device, temperature control device, thermophotovoltaic cells, and cooling device. To evaluate the performance of the test system, the Monte Carlo ray tracing method and the finite volume method is applied to analyze the spectral energy transfer and temperature of the primary devices. The quasi-monochromatic and high-energy flow that reaches the cell surface can be obtained. The energy conversion performance of thermophotovoltaic cells under a single high-energy spectrum can be thoroughly investigated through this designed test system. The results show that the energy transfer efficiency of this newly designed test system is greater than 14 %. When the radiation source temperature is 1000 K, 1500 K, 2000 K, and 2500 K, the design efficiency of the testing system is 14.11 %, 14.38 %, 14.58 %, and 14.87 %, respectively. When the temperature of the blackbody radiation source is 2000 K and 2500 K, the energy conversion efficiency of the GaSb cell is 20.37 % and 22.85 %, respectively. This work provides theoretical guidance for developing a quasi-monochromatic and high-energy flow thermophotovoltaic test system.

Theoretical analysis of composite nanostructure with three-dimensional photonic lattice for ultra-broadband and polarization-independent absorption

In this study, we present a composite nanopillar array with an average absorption efficiency near 100% in an ultra-broad wavelength range from 300 nm to 2000 nm. In particular, the average absorption in the band range from 400 nm to 1000 nm is close to 99% due to the strong coupling between the composite nanopillars and a complementary absorption generated by the two kinds of nanopillars in one period. The real and imaginary parts of the absorber’s impedance present a fluctuation near 1 and 0 in the entire wavelength range, which indicates an ultra-weak reflection of the absorber. The device also shows insensitivity to the polarization state and angle of incidence. The research offers a novel strategy for achieving high-efficiency light absorption in an ultra-broad wavelength range and boosts the potential value of composite nanostructures for thermophotovoltaic cells, electromagnetic stealth, and stray light suppression.

Computational study of III–V direct-gap semiconductors for thermoradiative cell applications

We investigate the performance of thermoradiative (TR) cells using the III–V group of semiconductors, which include GaAs, GaSb, InAs, and InP, with the aim of determining their efficiency and finding the best TR cell materials among the III–V group. The TR cells generate electricity from thermal radiation, and their efficiency is influenced by several factors such as the bandgap, temperature difference, and absorption spectrum. To create a realistic model, we incorporate sub-bandgap and heat losses in our calculations and utilize density-functional theory to determine the energy gap and optical properties of each material. Our findings suggest that the absorptivity of the material, especially when the sub-bandgap and heat losses are considered, can decrease the efficiency of TR cells. However, careful treatment of the absorptivity indicates that not all materials have the same trend of decrease in the TR cell efficiency when taking the loss mechanisms into account. We observe that GaSb exhibits the highest power density, while InP demonstrates the lowest one. Moreover, GaAs and InP exhibit relatively high efficiency without the sub-bandgap and heat losses, whereas InAs display lower efficiency without considering the losses, yet exhibit higher resistance to sub-bandgap and heat losses compared to the other materials, thus effectively becoming the best TR cell material in the III–V group of semiconductors.

An Opto-Electro-Thermal Model for Black-Silicon Assisted Photovoltaic Cells in Thermophotovoltaic Applications

Black silicon (b-Si)-assisted photovoltaic cells have textured b-Si surfaces, which have excellent light-trapping properties. There has been a limited amount of work performed on the theoretical modelling of b-Si photovoltaic cells, and hence, in this work, a coupled optical-electrical-thermal model has been proposed for the simulation of b-Si photovoltaic cells. In particular, the thermal aspects in b-Si photovoltaic cells have not been discussed in the literature. In the proposed model, the finite-difference time-domain (FDTD) method was used to study the optical response of the b-Si photovoltaic cell. Semiconductor equations were used for the electrical modelling of the cell. For the thermal model, the Energy Balance Transport Model was used. The developed model was used to simulate b-Si photovoltaic cells under thermophotovoltaic sources. The impacts of heat generation on the electrical performance of thermophotovoltaic cells are discussed. Simulation results from this study showed that black silicon layer improved efficiency and power output in thermophotovoltaic cells compared to thermophotovoltaic cells with no surface texture. In addition, heat generation due to Joule heating and electron thermalization in b-Si-assisted thermophotovoltaic cells reduced the open-circuit voltage and electrical performance.

Modelling and performance analysis of a novel thermophotovoltaic system with enhanced radiative heat transfer for combined heat and power generation

Solid-state energy conversion technologies such as thermophotovoltaics (TPV) have attracted much interest recently in the development of high-performance combined heat and power generation (CHP) systems because of the merits such as vibration-free operation, long working life, and versatile fuel source. However, current work is limited in power density, system efficiency, and power to heat generation ratio that are constrained by the device structure and working principle of existing techniques. In this work, we propose to apply zero-gap TPV (zTPV) to create a new type of CHP system that can potentially address the limitations in existing systems. zTPV, different from conventional TPV with a vacuum or gas-filled gap between the thermal emitter and photovoltaic cell, possesses an infrared-transparent intermediate solid layer. Our theoretical analysis, combining electromagnetic, thermal, and power generation modelling in zTPV, has confirmed that an one-order-of-magnitude higher thermal radiation dictates the energy conversion in zTPV. The significantly higher thermal radiation is due to the energy transfer of low-loss high-wavevector electromagnetic modes. In conventional TPVs, these modes are not allowed to transport in the gap, and thus not able to contribute to energy conversion. In contrast, the inserted infrared transparent layer allows the transport of these high-density modes, leading to much higher power density (over one order of magnitude enhancement), higher power conversion efficiency (nearly 25% improvement), and high power to heat ratio that is relevant to the system efficiency improvement in CHP applications. The novel device configuration of zTPV allows us to propose a hybrid CHP design that integrates zTPV with thermoelectric (TE) arrays into an ultra-compact CHP device. The effect of different system parameters is modelled and analyzed to demonstrate the high performance of the hybrid CHP.

Materials and device designs for thermophotovoltaic power conversion

AbstractTwo IV–VI semiconductor alloys, Pb0.81Sn0.19Se and Pb0.8Sr0.2Se, are proposed for use in designing multiple quantum well (MQW) materials and devices for thermophotovoltaic (TPV) power conversion. These materials can be epitaxially grown on silicon substrates, so they offer the potential for a low cost TPV device manufacturing technology. MQW materials examples are provided for fabricating triple junction TPV devices for power conversion with a 1400°C radiator. Optimal n‐type and p‐type layer thicknesses for each junction were determined using internal quantum efficiency expressions derived assuming all photogenerated charge collection is by diffusion. Depending on MQW material quality and assumed optical absorption levels, predicted power generation for current matched triple junction devices ranged from 1.7 to 4.6 W/cm2 and power conversion efficiencies ranged from 15.9% to 35.7%. These device performance parameters were used in a levelized cost of energy (LCOE) calculation to predict the cost of energy provided by TPV devices with a molten silicon thermal energy storage system. Results show that even for TPV devices fabricated from low quality materials, such energy storage systems will have a low enough LCOE to be competitive with other forms of energy storage and power generation.

Performance analysis of a novel solar radiation cascade conversion system for combined heat and power generation based on spectrum splitting and reshaping

In this paper, a novel cascading solar photovoltaic system with concentrating spectrum splitting and reshaping for combined heat and power generation is proposed for the first time to break through the limitations of photovoltaic efficiency. Two spectral splitters divide the solar spectrum into three parts, and each part of the spectrum is used by photovoltaics, thermophotovoltaics, and heat exchange fluids according to the photon grade. In addition, the heat exchange fluid also recovers the waste heat of concentrating photovoltaic and thermophotovoltaic cells. Therefore, the system achieves solar energy cascade utilization and has high electrical and thermal efficiency simultaneously. The effect of the fluid flow rate, solar irradiance, and cutoff wavelength of the spectral splitter on the hybrid system is discussed. The results show that under the conditions of 1000 sunlight and outlet fluid temperature of 60 °C, the solar energy conversion rate and exergy efficiency of the hybrid system are 90.17% and 39.84%, which are 63.97 percentage points and 11.62 percentage points higher than those of the single photovoltaic system, respectively.

GEOMETRIC OPTIMIZATION OF A NANOSTRUCTURED W-SiO2-W SELECTIVE EMITTER WITH TEMPERATURE DEPENDENT EMISSIVITY FOR THERMOPHOTOVOLTAIC APPLICATIONS

Metal-Insulator-Metal (MIM) nanostructures provide tunable multiple absorption/emission peaks desirable for spectroscopy, light sensing and thermophotovoltaic (TPV) applications. The efficiency of TPV systems can be improved by employing MIM emitters with resonators that allow high emission above PV cell bandgap and low emission elsewhere. Although there have been attempts to design MIM emitters for TPV systems, a comprehensive study that investigates and optimizes different resonator shapes is lacking. In this study, broadband TPV emitters with W-SiO2-W nanostructures are optimized for pairing with GaSb PV cells. A numerical approach is followed utilizing finite-difference time-domain method and particle swarm optimization scheme, MIM emitters with four resonator shapes: disk, square, pyramid, and cone are dimensionally optimized to attain an emissivity spectrum that overlaps with high quantum efficiency region of the GaSb cell. The optimized emitters are compared for efficiency, power output, material consumption, as well as their optical response to temperature and angular effects. At an emitter temperature of 1600 K, electrical power outputs of 2.335-2.418 W·cm-2 and spectral efficiencies of 56.2-57.6% are obtained. It is found that flat resonators tend to achieve similar performance to that of pointy resonators with shorter heights. Among the considered shapes, disk emitter demonstrates the highest efficiency with minimum material consumption. Compared to a plain W emitter at the same temperature, the disk MIM emitter exhibits significantly higher spectral efficiency and electrical power output (34% and 215% respectively). The results demonstrate the successful use of nano-elements in TPV systems, and the potential for fabricating and realizing such structures.

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.

Multi-field coupling characteristics of the thermophotovoltaic cell under high-intensity laser beam radiation

Improving energy conversion efficiency is a crucial challenge in the development of thermophotovoltaic (TPV) technology, particularly under high-intensity radiation, where enhancing energy reuse can have significant environmental benefits. The current study aims to address this challenge by constructing an optical-thermal-electric coupled conversion model of a single thermophotovoltaic (TPV) cell under high-intensity laser beam radiation. The primary connection parameter in the coupling model is a spectral response (SR). Therefore, the SR experimental platform has been established to obtain accurate parameter values. A fitting formula between SR, temperature, and wavelength was established and validated by testing. Based on the suitable fitting formula, the finite element method (FEM) has been used to study the energy distribution of incident optical and thermal heat transfer, as well as the electric characteristics of the TPV cell. The performance of a thermophotovoltaic (TPV) cell can be characterized by its I-V curve, which is influenced by various factors such as temperature, incident radiation, and spot size. In particular, it was found that when the incident radiation energy is the same as the spot size increases from 4 to 7 cm, the change in current is not proportional to the spot diameter. The results showed that the maximum power output could be achieved when the spot diameter was 6 cm. However, the temperature is the decisive factor affecting the conversion performance of the cell. The energy conversion efficiency can rise by 40.54% if the convective heat transfer coefficient is doubled. Therefore, it is important to maintain the TPV cell at a low temperature to achieve optimal electric conversion efficiency.

An Efficient Photon Utilization Radioisotope Thermophotovoltaic Based on Curled Reflectors

For radioisotope thermophotovoltaic (RTPV) to produce higher output power, it is often required that the irradiance incident on the thermophotovoltaic (TPV) cell array be more uniform. However, the irradiance received by each cell of the TPV array is relatively different under the conventional cell array surround structure of RTPV, resulting in a serious overall electrical output mismatch loss and a lower output power utilization rate. Herein, an RTPV with curled reflectors optimized by finite‐element method is proposed. It is shown in the simulation results that the irradiation uniformity of the cell array can reach 87.5%, the total output power reaches 88.12 W, which is 39.8% higher than that prior to optimization, and the mismatch loss is reduced by 69.2%. In addition, the impact of the reflectivity of reflectors is also targeted from the perspective of practical applications, and it can still produce higher output power than that of the conventional structure even at reflectivity as low as 70%. Ultimately, the RTPV prototype with reflectors is developed, and the electrical performance is tested to verify the effect of irradiance improvement. Herein, reflectors with positive gain for RTPV are proposed, which provides a new idea for the development of high‐efficiency power supplies.

Limits to the Energy-Conversion Efficiency of Air-Bridge Thermophotovoltaics

As the energy economy becomes increasingly decarbonized, low-cost energy storage grows ever more important. Thermal batteries in combination with thermophotovoltaic (TPV) cells are one major source of storage. The lowest-loss TPV cells utilize an air bridge (AB) with a gold back reflector. In this work, the authors determine a 55.5% thermodynamic efficiency limit for AB-TPVs at an emitter temperature of 1400 K, based on detailed balance. Including losses from nonradiative recombination, finite resistance, and free-carrier absorption, the practical efficiency limit is 48.6%. This work provides a road map for evaluating and limiting losses, leading to even higher AB-TPV efficiencies.

Study of one-dimensional of W/SiO2 grating selective thermal emitter for thermophotovoltaic applications

In this paper, a one-dimensional multilayer is optimized for potential applications as thermophotovoltaic (TPV) selective emitter. The effect of the diffraction orders and plane of incidence on the spectral emittance of the proposed TPV emitter is calculated numerically by using the rigorous coupled-wave analysis (RCWA). The emittance spectrum of the proposed TPV selective emitter shows three close to unity emission peaks which are explained by the surface plasmon polariton (SPP), gap plasmon polariton (GPP) and magnetic polariton (MP) excitation. The strong emittance at short wavelengths occurs due to three peaks, one close to unity at 1.2μm wavelength, and two equal to unity at 0.8 and 1.59μm wavelengths. The proposed structure can be used as a selective emitter for TPV applications.

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.

Bayesian-optimized infrared grating for tailoring thermal emission to boost thermophotovoltaic performance

Thermophotovoltaic (TPV) devices, which can break the Shockley–Queisser limit (33.7%) and enhance the thermal energy utilization efficiency, have garnered increasing attention in recent decades. Structuring the emitter surface has been demonstrated to be powerful for tailoring thermal emission to enhance the power density and system efficiency of a TPV system. However, the design and optimization of the broad parameters of the surface nanostructures manually remain to be thorny issues. In this paper, the Bayesian algorithm under the framework of material informatics was coupled with a rigorous coupled wave analysis to optimize the geometry of the infrared grating nanostructure to achieve wavelength-selective emission to boost the TPV performance. It is demonstrated that only less than 0.173% of the total candidate structures were calculated to find out the optimal structure with high spectral emittance in the range of 0.3–1.708 μm, and the power density and system efficiency of the TPV system were enhanced to 4.20 W/cm2 and 35.37%, respectively. The present machine-learning-based optimization of a multi-parameter nanostructure can improve the performance of the TPV system significantly and can be extended to other physical fields in a feasible manner.

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.

How thermophotovoltaics can help decarbonize the grid

Thermophotovoltaics (TPV) are a technology that can turn light into electricity, just like solar panels. However, instead of taking their energy from the sun, TPVs make use of the light emitted by a hot object in close proximity. We demonstrated a new record-breaking efficiency of more than 40% for TPVs, enabling the viability of this technology for ‘thermal battery’ energy storage.

Bifacial Thermophotovoltaic Energy Conversion

Thermophotovoltaic (TPV) energy conversion efficiency has recently surpassed 30%. The key behind such high efficiency is the inclusion of a highly efficient mirror in the rear of the TPV cell that turns back to the thermal emitter the outband energy photons. Efficiencies over 50% could be theoretically attainable by approaching a mirror reflectance of 100%. However, the very few percent of outband absorption significantly deteriorate the conversion efficiency, especially at low emitter temperatures. Thus, current research focuses on developing advance mirror designs able to reach an extreme high outband reflectance over 95%. In this article I propose a bifacial TPV cell that enables very efficient photon recycling without using mirrors and that is less sensitive to outband optical losses. The key to this design is that the cell is introduced in a thermal emitter enclosure where it is irradiated from both sides. Then, outband photons transmit through the cell and are reabsorbed in the emitter. Therefore, the optical losses linked to the mirror/cell interface are eliminated, potentially enabling higher photon recycling efficiencies. This article presents a detailed balance simulation of an edge-cooled bifacial TPV cell to demonstrate that bifacial configuration enables higher conversion efficiencies and twice much as power density than monofacial designs, the latter being an advantage for moderate temperature and low-cost TPV power generation. Therefore, bifacial TPV cells are appealing for developing practical high-efficient and low-cost TPV devices for power generation in an extended range of heat source temperatures.

Module-Level Polaritonic Thermophotovoltaic Emitters via Hierarchical Sequential Learning.

Thermophotovoltaic (TPV) generators provide continuous and high-efficiency power output by utilizing local thermal emitters to convert energy from various sources to thermal radiation matching the bandgaps of photovoltaic cells. Lack of effective guidelines for thermal emission control at high temperatures, poor thermal stability, and limited fabrication scalability are the three key challenges for the practical deployment of TPV devices. Here we develop a hierarchical sequential-learning optimization framework and experimentally realize a 6″ module-scale polaritonic thermal emitter with bandwidth-controlled thermal emission as well as excellent thermal stability at 1473 K. The 300 nm bandwidth thermal emission is realized by a complex photon polariton based on the superposition of Tamm plasmon polariton and surface plasmon polariton. We experimentally achieve a spectral efficiency of 65.6% (wavelength range of 0.4-8 μm) with statistical deviation less than 4% over the 6″ emitter, demonstrating industrial-level reliability for module-scale TPV applications.

Simulation of edge effects in thermophotovoltaic efficiency by Monte Carlo calculation

Thermophotovoltaic (TPV) is one of the efficient ways that convert heat to electricity. The high-temperature emitter emits infrared photons omnidirectionally, and the view factor would be very larger compared with photovoltaic. This means that the effect of non-normal incidence will be very significant. In this work, we constructed a four-layer model consisting of the reflector, filter, anti-reflection-coating, and TPV cell. Based on the Monte Carlo algorithm, randomly generated photons that obey the black-body radiation distribution law on the surface of the emitter were tracked, and finally, the reached positions on the TPV cell were recorded. Calculations show that the density of photons received at the edge of the TPV is significantly less and depends on the interlayer spacing. We also analyzed the perimeter effect in the two-dimensional situation and gave possible optimization strategies.