Thermophotovoltaic Research Articles

Performance Analysis of a Near-Field Thermophotovoltaic Device With a Metallodielectric Selective Emitter and Electrical Contacts for the Photovoltaic Cell

The photon transport and energy conversion of a near-field thermophotovoltaic (TPV) system with a selective emitter composed of alternate tungsten and alumina layers and a photovoltaic cell sandwiched by electrical contacts are theoretically investigated in this paper. Fluctuational electrodynamics along with the dyadic Green's function for a multilayered structure is applied to calculate the spectral heat flux, and the photocurrent generation and electrical power output are solved from the photon-coupled charge transport equations. The tungsten and alumina layer thicknesses are optimized to obtain maximum electrical power output for bare TPV cell. The spectral heat flux is much enhanced when plain tungsten emitter is replaced with the multilayer emitter due to the effective medium intrinsic lossy property and additional surface plasmon polariton coupling in the tungsten thin film, for which the invalidity of effective medium theory to predict photon transport in the near field with multilayer emitters is discussed. Effects of a gold back reflector and indium tin oxide front coating with nanometer thickness, which could practically act as the electrodes to collect the photon-generated charges on the TPV cell, are explored. The conversion efficiency of 23.7% and electrical power output of 0.31 MW/m2 can be achieved at a vacuum gap distance of 100 nm when the emitter and receiver temperature are, respectively, set as 2000 K and 300 K.

Enhanced heat transfer using metal foam liquid supply layers for micro heat spreaders

We propose a nanostructured metal foam liquid supply layer that can efficiently provide operating fluid to evaporator hot spots and can be easily integrated within micro heat spreaders. The liquid supply layer is incorporated onto the micropost evaporator wicks to enhance the capillary performance by combining the high permeability of liquid supply layers and the high capillary pressure of micropost wicks. The coverage ratio (CR) between the liquid supply layer and the evaporator wicks was varied from 15% to 100% to find the proper CR for efficiently increasing the liquid supply performance with minimizing the parasitic thermal resistance. By incorporating the liquid supply layer of CR 33% onto the Cu micropost wicks of ∼0.4 solid fraction, the results show that a high (>6W/cm2K) and stable heat transfer coefficient can be achieve at a high heat flux range (>400W/cm2), which outweighs the performance of previously-reported evaporator wicks. The achieved maximum heat flux was over 150% higher than the same wicks without the liquid supply layer. Our work shows the importance of the efficient liquid supply to hot spots and provides the strategy to increase the heat transfer performance at high heat flux region. The suggested liquid supply layer will help develop micro heat spreaders for the thermal management of high power density microprocessors, IGBTs and thermophotovoltaic cells.

Optimization of spectrally selective Si/SiO2 based filters for thermophotovoltaic devices

Design of a spectrally selective filter based on one-dimensional Si/SiO2 layers is considered for improved performance of thermo-photovoltaic devices. Spectrally selective filters transmit only the convertible radiation from the emitter as non-convertible radiation leads to a reduction in cell efficiency due to heating. The presented Si/SiO2 based filter concept reflects the major part of the undesired range back to the emitter to minimize energy required for the process and it is adaptable to different types of cells and emitters with different temperatures since its cut-off wavelength can be tuned. While this study mainly focuses on InGaSb based thermo-photovoltaic cell, Si, GaSb, and Ga0.78In0.22As0.19Sb0.81 based cells are also examined. Transmittance of the structure is predicted by rigorous coupled wave approach. Genetic algorithm, which is a global optimization method, is used to find the best possible filter structure by considering the overall efficiency as an objective function that is maximized. The simulations show that significant enhancement in the overall system and device efficiency is possible by using such filters with TPV devices. The methodology described in this paper allows for an improved filter design procedure for selected applications.

Suppression of the surface “dead region” for fabrication of GaInAsSb thermophotovoltaic cells

Zn diffusion processes in n-Ga0.78In0.22As0.2Sb0.8 epitaxial films are studied using different diffusion sources, a series of the Zn profiles with single and double humps are obtained. The group V-atoms (As and Sb) are found to have little effect on suppressing the surface “dead region”. The Ga and In atoms have entirely different effect on diffusion although they are both group III-atoms. The “dead region” is suppressed completely under Ga-rich conditions, while the suppression will not occur under In-rich conditions. The GaInAsSb cells are fabricated under pure Zn and Ga-rich conditions. An electrical heating thermophotovoltaic system is fabricated for cell testing. Under the radiation from 1055°C-SiN ceramics emitter, the output power density of the GaInAsSb cell obtained under Ga-rich condition is much larger than that of the cell obtained under pure Zn condition, which demonstrated that the GaInAsSb cells can be fabricated without precise etching under Ga-rich conditions.

Refractory material based frequency selective emitters/absorbers for high efficiency and thermal stable thermophotovoltaics

Refractory materials based frequency selective emission/absorption is extremely encouraging for various applications, especially the combustion based thermophotovoltaics (TPV) and solar-TPV systems. The frequency selective emitters/absorbers also face the challenges of robust operation at elevated temperatures. Here, we report titanium nitride based thermal stable emitters/absorbers with good selectivity in the near infrared wavelength range. The titanium nitride nanocavitity array provides high emissivity/absorptivity at the desired short wavelength range but suppressed emissivity/absorptivity at the long wavelength range, and this design could be used to obtain high power density of the TPV system. The cut-off wavelength could be adjusted by encapsulating the cavities with different dielectrics. Meanwhile, the encapsulating layer protects the nanocavity structure from deformation at high temperatures. The thermal stability at 1073 K and 1273 K for two hours in Argon was demonstrated for the encapsulated nanocavity structure.

Performance characteristics and parametric choices of a solar thermophotovoltaic cell at the maximum efficiency

The overall model of the solar thermophotovoltaic cell (STPVC) composed of an optical lens, an absorber, an emitter, and a photovoltaic (PV) cell with an integrated back-side reflector is updated to include various irreversible losses. The power output and efficiency of the cell are analytically derived. The performance characteristics of the STPVC at the maximum efficiency are revealed. The optimum values of several important parameters, such as the voltage output of the PV cell, the area ratio of the absorber to the emitter, and the band-gap of the semiconductor material, are determined. It is found that under the condition of a relative low concentration ratio, the optimally designed STPVC can obtain a relative large efficiency.

Hybrid energy converter based on swirling combustion chambers: the hydrocarbon feeding analysis

This manuscript reports the latest investigations about a miniaturized hybrid energy power source, compatible with thermal/electrical conversion, by a thermo-photovoltaic cell, and potentially useful for civil and space applications. The converter is a thermally-conductive emitting parallelepiped element and the basic idea is to heat up its emitting surfaces by means of combustion, occurred in swirling chambers, integrated inside the device, and/or by the sun, which may work simultaneously or alternatively to the combustion. The current upgrades consist in examining whether the device might fulfill specific design constraints, adopting hydrocarbons-feeding. Previous papers, published by the author, demonstrate the hydrogen-feeding effectiveness. The project’s constraints are: 1) emitting surface dimensions fixed to 30 × 30 mm, 2) surface peak temperature T > 1000 K and the relative ∆T < 100 K (during the combustion mode), 3) the highest possible delivered power to the ambient, and 4) thermal efficiency greater than 20% when works with solar energy. To this end, a 5 connected swirling chambers configuration (3 mm of diameter), with 500 W of injected chemical power, stoichiometric conditions and detailed chemistry, has been adopted. Reactive numerical simulations show that the stiff methane chemical structure obliges to increase the operating pressure, up to 10 atm, and to add hydrogen, to the methane fuel injection, in order to obtain stable combustion and efficient energy conversion.

Thermoradiative Energy Conversion With Quasi-Fermi Level Variations

An extension of the detailed balance method for the thermoradiative conversion, including the transport properties, is presented in this paper. This formalism is applied to the study of the most important properties (maximum output power, the efficiency at which that power is delivered, and the maximum efficiency) of thermoradiative cells (TRCs) with bandgap values between 0.2 and 0.7 eV. The calculations demonstrate that the consideration of the electronic transport in the calculation of the properties of the TRCs leads to predict better working properties for them.

Experimental demonstration of a novel approach to increase power conversion potential of a hydrocarbon fuelled, portable, thermophotovoltaic system

A novel, compact mesoscale thermophotovoltaic (TPV) power generating system using a combination of porous media combustion and a simple band-pass filtering method has been proposed and experimentally studied. The novelty in this work is the idea of harvesting the heat produced in the microcombustor via the silicon carbide (SiC) porous media which becomes an effective radiator owing to its high emissivity. This is then combined with wafers that have a high optical transmissivity in the part of the spectrum that is useful for power conversion using the TPV system. In this work, we utilised two combustor wafers (silicon and quartz) with suitable thermal and optical properties. Quartz having a high transmissivity of 0.9 compared to silicon (<0.6) proved to be superior in performance when combined with the porous media combustion for TPV power generation. To further illustrate the combination of effects, we also performed experiments in which porous media was not inserted in the combustor and the radiation was effectively from the hot combustor walls. In this case, silicon which has a higher emissivity than the quartz performs better. Overall the TPV power generation was SiC+quartz>SiC+Si>pure Si walls>pure quartz walls.

Figure of merit for front electrodes of solar cells

Conventional figures of merit often characterize the optical and electrical properties of transparent conductive thin films and are widely applied in research and industry. However, an appropriative figure is necessary to evaluate the front electrodes of solar cells. So an appropriate figure of merit is therefore proposed in this paper, specifically designed for the evaluation of the front electrodes of solar cells. The proposed figure is adapted from ΦTC, the most commonly used conventional figure of merit, with two changes. First, external quantum efficiency functions for different types of solar cells are explicitly considered. Second, data from the air mass 1.5 spectrum are used as weight factors to calculate the mean optical transmittance. With these two changes, the proposed figure of merit is shown to be well adapted to the photovoltaic field. To validate it, silicon heterojunction solar cells with different front electrodes were simulated as examples. By comparing the simulation results and the respective values of the proposed figure, it is evident that this figure of merit provides values that are proportional to the conversion efficiencies of solar cells. The proposed figure can therefore be used in the selection and manufacturing of front electrodes of solar cells. In addition, two variations on this figure of merit are proposed, specifically adapted to the characteristics of multiple electron-hole pairs cells and thermophotovoltaic cells.

Thermophotovoltaic energy in space applications: Review and future potential

This article reviews the state of the art and historical development of thermophotovoltaic (TPV) energy conversion along with that of the main competing technologies, i.e. Stirling, Brayton, thermoelectrics, and thermionics, in the field of space power generation. Main advantages of TPV are the high efficiency, the absence of moving parts, and the fact that it directly generates DC power. The main drawbacks are the unproven reliability and the low rejection temperature, which makes necessary the use of relatively large radiators. This limits the usefulness of TPV to small/medium power applications (100We-class) that includes radioisotope (RTPV) and small solar thermal (STPV) generators. In this article, next generation TPV concepts are also revisited in order to explore their potential in future space power applications. Among them, multiband TPV cells are found to be the most promising in the short term because of their higher conversion efficiencies at lower emitter temperatures; thus significantly reducing the amount of rejected heat and the required radiator mass.

Selective emitters for thermophotovoltaic applications

Applying thermophotovoltaic (TPV) technologies to existing energy generators allows us to increase energy output while utilizing present infrastructure by reclaiming the heat lost during the production process. In order to maximize the efficiency of these sources, the conversion efficiency of the TPV system needs to be optimized. Selective emitters are often used to tailor the spectrum of incident light on the diode, blocking any undesirable light that may lead to device heating or recombination. Over the years, many different technologies have been researched to create an ideal selective emitter. Plasmas and rare‐earth emitters provided highly selective spectra early on, but their fixed peaks required tailoring the diode's band gap to the emitter's characteristic wavelength. Recent advances in engineerable materials, such as photonic crystals and metamaterials, allow the opposite to take place; an appropriate selective emitter can be designed to match the TPV diode, allowing the diode structure to be optimized independently from the emitter.

Fabrication of an Omnidirectional 2D Photonic Crystal Emitter for Thermophotovoltaics

In a thermophotovoltaic (TPV) system, a heat source brings an emitter to incandescence and the spectrally confined thermal radiation is converted to electricity by a low-bandgap photovoltaic (PV) cell. Efficiency is dominated by the emitter's ratio of in-band emissivity (convertible by the PV cell) to out-of-band emissivity (inconvertible). Two-dimensional photonic crystals (PhCs) offer high in-band emissivity and low out-of-band emissivity at normal incidence, but have reduced in-band emissivity off-normal. According to Lambert's law, most thermal radiation occurs off-normal. An omnidirectional PhC capable of high in-band emissivity at all angles would increase total in-band power by 55% at 1200°C. In this work, we present the first experimental demonstration an omnidirectional hafnia-filled 2D tantalum PhC emitter suitable for TPV applications such as combustion, radioisotope, and solar TPV. Dielectric filling improved the hemispherical performance without sacrificing stability or ease of fabrication. The numerical simulations, fabrication processes, and optical and thermal characterizations of the PhC are presented in this paper.

An integrated microcombustor and photonic crystal emitter for thermophotovoltaics

Thermophotovoltaic (TPV) energy conversion is appealing for portable millimeter- scale generators because of its simplicity, but it relies on a high temperatures. The performance and reliability of the high-temperature components, a microcombustor and a photonic crystal emitter, has proven challenging because they are subjected to 1000-1200°C and stresses arising from thermal expansion mismatches. In this paper, we adopt the industrial process of diffusion brazing to fabricate an integrated microcombustor and photonic crystal by bonding stacked metal layers. Diffusion brazing is simpler and faster than previous approaches of silicon MEMS and welded metal, and the end result is more robust.

Microgap thermophotovoltaic systems with low emission temperature and high electric output

We theoretically show that a thermophotovoltaic (TPV) system enhanced by a wire metamaterial opens the door to a prospective microgap TPVs which will combine high electric output with relatively low temperatures of the emitter. The suggested system comprises an array of parallel metal nanowires grown on top of a photovoltaic semiconductor and standing free in the vacuum gap between the host dielectric layer and the emitter, so that their ends are sufficiently close to the emitting surface. Due to the resonant near-field coupling between this wire medium and the emitter and due to the optimized layered structure of the whole system, the strongly super-Planckian radiative heat flux of resonant nature is engineered. In the suggested system, heavily doped silicon and indium antimonide are considered as the materials for the emitter and the photovoltaic cell, respectively. Also, the parallel nanowires are made of tungsten. Employing the minority-carrier transport model, it is shown that a power output equal to 26 kW m−2 can be achieved when the temperature of the doped-silicon emitter is only 500 °C.

Entropic and Near-Field Improvements of Thermoradiative Cells.

A p-n junction maintained at above ambient temperature can work as a heat engine, converting some of the supplied heat into electricity and rejecting entropy by interband emission. Such thermoradiative cells have potential to harvest low-grade heat into electricity. By analyzing the entropy content of different spectral components of thermal radiation, we identify an approach to increase the efficiency of thermoradiative cells via spectrally selecting long-wavelength photons for radiative exchange. Furthermore, we predict that the near-field photon extraction by coupling photons generated from interband electronic transition to phonon polariton modes on the surface of a heat sink can increase the conversion efficiency as well as the power generation density, providing more opportunities to efficiently utilize terrestrial emission for clean energy. An ideal InSb thermoradiative cell can achieve a maximum efficiency and power density up to 20.4% and 327 Wm−2, respectively, between a hot source at 500 K and a cold sink at 300 K. However, sub-bandgap and non-radiative losses will significantly degrade the cell performance.

Broadband Polarization-Insensitive Metamaterial Perfect Absorbers Using Topology Optimization

A novel scheme for a perfect hyperbolic metamaterial (HMM) absorber is proposed, and experimental verification is provided. It has been shown previously that tapered HMM stacks can provide adiabatic waveguiding over a wide spectral range and thus are an ideal opaque absorber. Here, nontapered shape-optimized HMM absorbers are proposed, which facilitates the fabrication and promotes the large-area applications such as thermophotovoltaics (TPV). In the synthesis of the optimal patterns, we use 5-harmonic rigorously coupled wave analysis (RCWA) and experimental trials to shorten the trial-and-error time. The best pattern provides an averaged broadband experimental absorption of 88.38% over λ = 1 μm to λ = 2 μm, which is comparable to the state-of-the-art experimental effort using tapered HMM. The nontapered nature can be easier to fabricate from the semiconductor processing viewpoint. The physics behind the pattern-optimized HMM cavity is the broadband light coupling by the air-cavity and the unbounded photonic density of the states (PDOS) associated with the HMM. The topology optimized air cavity effectively couples the incident photons into the metal-dielectric stacking, eliminating the need of sidewall tapers. We believe the proposed topology-optimization methodology benefits the future design of compact metamaterial perfect absorbers (MPA), sensors, antenna, and thermophotovoltaic emitters, and absorbers.

Performance evaluation and parametric optimum design of a molten carbonate fuel cell-thermophotovoltaic cell hybrid system

A new model of the hybrid system composed of a molten carbonate fuel cell (MCFC) and a thermophotovoltaic cell (TPVC) is proposed to recovery the waste heat produced by the MCFC. Expressions for the power output and the efficiency of the hybrid system are analytically derived. The performance characteristics of the hybrid system are evaluated. It is found that when the current density of the MCFC, voltage output of the TPVC, electrode area ratio of the MCFC to the TPVC, and energy gap of the material in the photovoltaic cell are optimally chosen, the maximum power output density of the hybrid system is obviously larger than that of the single MCFC. Moreover, the improved percentages of the maximum power output density of the proposed model relative to that of the single MCFC are calculated for differently operating temperatures of the MCFC and are compared with those of some MCFC-based hybrid systems reported in the literature, and consequently, the advantages of the MCFC-TPVC hybrid system are revealed.

Toward high performance radioisotope thermophotovoltaic systems using spectral control

This work describes RTPV-PhC-1, an initial prototype for a radioisotope thermophotovoltaic (RTPV) system using a two-dimensional photonic crystal emitter and low bandgap thermophotovoltaic (TPV) cell to realize spectral control. We validated a system simulation using the measurements of RTPV-PhC-1 and its comparison setup RTPV-FlatTa-1 with the same configuration except a polished tantalum emitter. The emitter of RTPV-PhC-1 powered by an electric heater providing energy equivalent to one plutonia fuel pellet reached 950°C with 52W of thermal input power and produced 208mW output power from 1cm2 TPV cell. We compared the system performance using a photonic crystal emitter to a polished flat tantalum emitter and found that spectral control with the photonic crystal was four times more efficient. Based on the simulation, with more cell areas, better TPV cells, and improved insulation design, the system powered by a fuel pellet equivalent heat source is expected to reach an efficiency of 7.8%.

Gradient-Doped Colloidal Quantum Dot Solids Enable Thermophotovoltaic Harvesting of Waste Heat

Electromagnetic radiation emitted from hot objects represents a sizable source of energy, one that in most applications is not harvested efficiently. Even for a blackbody at 800 °C, the radiation intensity peaks near 2.7 μm wavelength, and this requires a semiconductor absorber having a band gap in the short-wavelength infrared and beyond to enable thermophotovoltaic (TPV) heat recovery. Here we report the first solution-processed TPV device to harvest efficiently 800 °C heat. The active layer consists of colloidal quantum dots (CQDs), infrared-absorbing nanoparticles synthesized using a scalable solution-based method, having 0.75 eV band gap. We construct rectifying junction devices based on controllably p- and n-doped CQD solids that benefit from a gradient in electron affinity that extends over the devices’ thickness. The gradient-doped architecture relies on engineered charge carrier drift and overcomes the existing limitations of small band gap CQD solids. The devices provide 2.7% efficiency in the c...