Thermophotovoltaic Power Generation Research Articles

Enhancement of Radiative Power Using a Divergent Splitter Plate Design in H2–Air Non-Premixed Micro-Combustor

Abstract Micro-combustion based power generation devices can be considered as future alternatives to batteries in miniature electronic devices. Micro-combustors operating in non-premixed mode are free from flashback but face the challenge of properly mixing fuel and air within a small volume. In this work, the effect of a divergent fuel–air splitter design on the mixing performance and combustion characteristics of H2–air fueled diffusion micro-combustor is studied. The laminar reacting flow is simulated using the finite volume method and a detailed hydrogen kinetic mechanism. Three divergent splitter designs are compared with the commonly used rectangular splitter to study the effect on radiation power, an essential parameter for thermophotovoltaic power generation. The best-performing divergent and base rectangular splitter designs are investigated in detail. The study shows that the micro-combustor with divergent splitter design reduces mixing distance (Lmix) by 5–23% depending on inlet velocity and channel height. With the divergent splitter, the peak value of the heat release rate also increases slightly, implying enhanced combustion. The divergent splitter increases the high-temperature surface area of the outer wall as compared to the rectangular splitter. This leads to the micro-combustor with divergent splitter producing significantly higher radiation power (>10%) than the rectangular splitter for larger channel heights and higher inlet velocities.

All-top-contact 0.59 eV InGaAs thermophotovoltaic cells and modules

InGaAs cells are ideal converters for low temperature thermophotovoltaic power generation systems. Commonly, the positive and negative electrodes of InGaAs cells are deposited on the top and bottom of the cells, respectively. In the engineering of level modules, solder strips are used to connect the top and bottom of adjacent cells, the distance between the cells is considerable in order to prevent short circuits from the side wall of the cells, and the area ratio of cells to modules is relatively low. In this paper, we design and fabricate a novel all-top-contact InGaAs cell, with the positive electrode deposited on top of the p-InGaAs epitaxial layer, and the negative electrode deposited on top of the n-InPAs-grade epitaxial layer by precise etching of the p–n junction layers. The novel all-top-contact cells show good efficiencies compared to traditional cells with top-and-bottom electrodes under the same testing conditions. Novel flat and compact cell modules were fabricated using all-top-contact InGaAs cells, the distance between the adjacent cells was controlled within tens of microns, and the area ratio of cells to modules was improved efficiently.

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.

Semitransparent thermophotovoltaics for efficient utilization of moderate temperature thermal radiation

Recent advances in thermophotovoltaic (TPV) power generation have produced notable gains in efficiency, particularly at very high emitter temperatures. However, there remains substantial room for improving TPV conversion of waste, solar, and nuclear heat streams at temperatures below 1,100°C. Here, we demonstrate the concept of transmissive spectral control that enables efficient recuperation of below-bandgap photons by allowing them to transmit through the cell to be absorbed by a secondary emitter. We fabricate a semitransparent TPV cell consisting of a thin InGaAs-InP heterojunction membrane supported by an infrared-transparent heat-conducting substrate. The device absorbs less than 1% of below-bandgap radiation, resulting in a TPV efficiency of 32.5% at an emitter temperature of 1,036°C. To our knowledge, this represents an 8% absolute improvement (~33% relative) in efficiency relative to the best TPV devices at such low temperatures. By enabling near-zero photon loss, the semitransparent architecture facilitates high TPV efficiencies over a wide range of applications.

Air-Bridge Si Thermophotovoltaic Cell with High Photon Utilization

Thermophotovoltaic (TPV) cells convert photons emitted from hot surfaces into electrical power. Unlike solar cells, TPV cells can be placed in close proximity to the heat source, allowing below-bandgap (i.e., out-of-band, OOB) photons to be reflected and reabsorbed by the emitter. As the reflectance of OOB photons approaches unity, the spectral efficiency of the TPV becomes increasingly insensitive to the bandgap of the cell and the source temperature. Here, we employ air-bridge structures with a lateral junction Si TPV cell as a means of increasing OOB reflectivity, which allows for efficient operation at low thermal source temperatures that were long inaccessible to efficient Si TPV power generation. The devices exhibit an OOB reflectance of 98.0 ± 0.1% and an air-bridge scalability to at least 6 cm × 6 cm. Compared to devices with a Au back surface reflector (BSR), devices featuring an air-bridge BSR exhibit a 25% relative increase in power conversion efficiency at a thermal source temperature of 1988 K. Such performance improvements in TPV cells made with scalable and relatively low-cost Si can potentially expedite the widespread use of TPV systems in both energy storage and generation systems.

Prospects and requirements for thermophotonic waste heat energy harvesting

Thermophotovoltaic (TPV) power generators offer great possibilities for thermal energy conversion when thermal sources with temperatures nearing or exceeding 1000 K are available. While the power density of conventional TPV systems is generally determined by Planck’s law in the far field, their fundamental performance is known to be dramatically affected by near field effects between the thermal emitter and the photovoltaic cell. Another potentially disruptive enhancement to the performance may be reached by transforming the thermal emitter to exploit electroluminescence. Taking advantage of an electroluminescent emitter as the source of radiation fundamentally alters the thermodynamics of the system. This allows boosting the achievable power densities by orders of magnitude, and also provides access to electroluminescent coolers, thermophotonic (TPX) heat pumps, and TPX power generation devices that can outperform both TPV and thermoelectric heat engines, especially at the low-grade waste heat (LGWH) temperature range (300–500 K) containing in total the majority of recoverable energy. In reality, functional TPX devices are yet to be demonstrated experimentally, due to several material and design bottlenecks. Here, we discuss the thermodynamics, ideal characteristics and advantages of TPX heat engines, and quantify how non-idealities such as non-radiative recombination, optical and resistive losses affect their performance. Our results suggest that, at LGWH temperatures, TPX heat engines start to outperform the best TPV systems when reaching quantum efficiencies of the order of 90%; beyond this threshold, TPX systems become increasingly efficient and powerful. • Showing the promise of thermophotonic (TPX) heat engines (HEs) for heat recovery. • Capturing TPX-HEs’ thermodynamics by developing and using a holistic model. • Quantifying the impact of non-radiative recombination, optical and resistive losses. • TPX-HEs outperform idealized thermophotovoltaic systems at low temperatures (<600 K). • Functional TPX-HEs are within reach with existing semiconductor technologies.

Eco-Friendly and Renewable Power Generation from Heat Using Thermophotovoltaic Technology

Renewable energy sources are the need of the hour, considering climate change’s impact on our planet. Many sources are currently being researched, none of them are commercially stable enough to compete with the existing non-renewable energy sources, with the main reason being performance for the price. We aim to develop a practical system based on the current research progress in infrared-based thermophotovoltaic power generation and storage. Similar systems already exist for harnessing solar energy, but we plan to overcome the inability of solar energy to produce electricity in the absence of sunlight by developing an infrared-based system. Another challenge is the unavailability of enough thermal radiation in the vicinity to support the system from different sources. The main objectives of this work are to identify the advantages and disadvantages of such a system, its feasibility, and its efficiency for working in the real world through theoretical predictions, simulation, and experimentation.

Investigation of thermal performance and energy conversion in a novel planar micro-combustor with four-corner entrances for thermo-photovoltaic power generators

A novel planar micro-combustor with four-corner entrances is proposed by considering high and uniform outer wall temperature distribution used in micro thermo-photovoltaic generator. For this, a three-dimensional numerical model with detailed chemical mechanism, transport, and heat transfer and a thermo-photovoltaic energy conversion model are developed to examine its performance. Results show that the proposed micro-combustor has excellent wall temperature uniformity because a uniform distribution of high-temperature zones of four individual flames is formed in the combustion chamber. Thermal performance and energy efficiency of the proposed micro-combustor are better than conventional micro-combustor at a low flow rate ( m ) and large equivalence ratio ( φ ). Additionally, the implemented plate-insertion strategy effectively overcome the deterioration of combustor performance under higher m or lower φ . Proposed micro-combustor with plates insertion has the largest wall temperature and energy efficiency, no matter what φ and m are adopted. Plates extend the fluid ravel path and produce some recirculation zones, which promotes heat transfer, preferential diffusion, and flame anchoring, while its pressure loss is larger. Of all the tested cases, the system efficiency reaches a maximum of 6.39%, corresponding to 12.1 W electricity output. This work provides effective combustor design and research insights to develop high-efficiency micro-power system. • A novel micro-combustor with four entrances is proposed for micro-TPV system. • Excellent performance of wall temperature uniformity is obtained. • Inserting plates boosts thermal performance and energy conversion efficiency. • A maximum system efficiency of 6.39% is achieved with 12.1 W electric output.

Optimal design and economic analysis of a hybrid process of municipal solid waste plasma gasification, thermophotovoltaic power generation and hydrogen/liquid fuel production

In this paper, an integrated process of hydrogen, methanol and electricity production from municipal solid waste biomass feed is investigated and analyzed. Also, the computational model for recovering the energy from the internal radiation of the combustion chamber in a thermophotovoltaic system is developed. The main parameters in the thermophotovoltaic modeling system are open circuit voltage, short circuit current density and cell density coefficient. These parameters are considered as a function of emitter temperature, cell temperature, the interval between cell and emitter, spectral response and the cell energy gap. After designing the integrated process, efficiency of the overall proposed hybrid system is calculated about 68.17% which is increased approximately 3.5% by heat recovery from the combustion chamber of thermophotovoltaic system. The results showed that increasing the temperature of the WGS reactor has a negative impact on the overall productivity of the system. Also increasing the rate of inlet plasma air decreases H2/CO ratio of the synthesized gas, which accordingly decreases the methanol production rate. Based on the economic analysis results in a period of 2 years and 9 months the total investment cost is returned to the investor.

Evaluation of thermo photovoltaic performance through aluminum-fueled combustor with partially porous medium and different geometric cross-sections

Thermo photovoltaic (TPV) power generation systems have attracted a great deal of attention in the field of portable electronic devices. The high and uniform temperature of TPV combustor plays a decisive role in the improvement of system efficiency. In this study, aluminum is utilized as energy source of TVP. To model the premixed combustion of aluminum-air, an asymptotic model of flame structure including four zones of solid, liquid, and gaseous aluminum and post-flame and three asymptotic fronts of melting, evaporation, and flame is proposed. Then, the conservation equations and boundary conditions are derived and the combustion characteristics of aluminum are obtained. Combustion is evaluated for simple and porous media with trapezoidal, rectangular, triangular, and circular cross-sections. Finally, the influence of parameters on the TVP performance is assessed. The results indicated that in the medium with a porosity factor of 0.8, the total efficiency is higher than the simple medium for all cross-sections. In the medium with a porosity factor of 0.6, however, the triangular and circular cross-sections exhibit lower efficiency compared to the simple medium. In the case with a porosity factor of 0.4, only the efficiency of the channel with a trapezoidal cross-section is higher than the simple medium.

Ultrathin High Quality‐Factor Planar Absorbers/Emitters Based on Uniaxial/Biaxial Anisotropic van der Waals Polar Crystals

AbstractTailoring the absorption/emission through nanostructures from broadband to narrowband has attracted increasing attention due to the merits of high efficiency and compactness. However, most of the reported narrowband emitters require either complex fabrication process or thick coating of the suitable materials (~mm). In this paper, by exciting Berreman mode through a combination of Au mirrors and ultra‐thin layers of low‐loss uniaxial/biaxial anisotropic 2D van der Waals polar crystal hBN/α‐MoO3, a narrowband absorption/emission peak is realized in the mid‐infrared region. The measured quality‐factor of Berreman mode for hBN/Au and α‐MoO3/Au structure can reach up to 134 and 164, respectively. The deep subwavelength thickness (~nm) and bilayer architecture simplify the fabrication process. Furthermore, taking advantage of the in‐plane birefringence originating from α‐MoO3, the polarization of the absorbed/emitted radiation can be controlled through this planar configuration. Such ultrathin narrowband absorbers/emitters provide new opportunities for the advancements in thermal sources, infrared sensors, and thermophotovoltaic power generation systems.

Near-field thermophotovotaic devices with surrounding non-contact reflectors for efficient photon recycling.

Near-field thermophotovoltaic (TPV) power generation has been attracting increasing attention as a promising approach for efficient conversion of heat into electricity with high output power density. Here, we numerically investigate near-field TPV devices with surrounding reflectors for efficient recycling of low-energy photons, which do not contribute to the power generation. We reveal that the conversion efficiency of a near-field TPV system can be drastically increased by introducing a pair of reflectors above and below the system, especially when the two mirrors are not in contact with the emitter and absorber. In addition, we investigate the influence of non-perfect photon recycling on the TPV efficiency and reveal that near-field TPV systems are more robust against the decrease of the reflectivity of the reflectors than the far-field TPV systems.

Thermal performance improvement of premixed hydrogen/air fueled cylindrical micro-combustor using a preheater-conductor plate

A micro-combustor is the key component of the micro thermo photovoltaic power generation system. The energy conversion efficiency of the system strongly depends on the outer wall temperature and the exergy efficiency of the micro-combustor. A novel preheater-conductor plate was designed to be inserted into a cylindrical micro-combustor to improve the outer wall temperature and the exergy efficiency by preheating premixed hydrogen/air mixture via the preheater plate and conducting out the heat in the combustion chamber via the conductor plate. The impact of the width of preheater plate, the gradually increased and decreased thickness with divergent angle and convergent angle of conductor plate, and the inserted number of conductor plate on the thermal performance of the micro-combustor were investigated. It is found that the gradually varying thickness of the conductor plate has a slight influence on micro-combustor performance compared to other factors. Inserting a preheater-conductor plate with the 0.5 mm-wide preheater plate and the three 0.5 mm-thick conductor plates improve the mean outer wall temperature and the exergy efficiency of the micro-combustor by 173 K and 22.9% respectively. It is indicated that the proposed preheater-conductor plate is capable of considerably improving the thermal performance of premixed hydrogen/air fueled cylindrical micro-combustor.

Parametric analysis on the combustion and thermal performance of a swirl micro-combustor for micro thermophotovoltaic system

A swirl micro-combustor which has strong capability to stabilize the flame has been developed and is promising to be used in micro thermophotovoltaic power generators in this work. Heat recirculation plays a requisite role in flame stability and thermal performance of micro combustors. Thus, the effects of three parameters (center inlet radius w1, swirler width w2, and step height w3) that determine the size and location of the recirculation zones in a premixed hydrogen/air flame are investigated numerically. The results show that inner recirculation zone and corner recirculation zone generated by a swirler are critical for stabilizing the flame. The parameter w1 has an insignificant influence on the thermal performance. Enlarging the inner recirculation zone by decreasing w2 provokes enhanced heat transfer, and increases the temperature level and temperature non-uniformity of combustor walls. The size of corner recirculation zone is increased by increasing w3, which raises the outer wall temperature uniformity. Based on the parametric analysis, the newly designed micro combustor possesses a high wall temperature level. Additionally, the energy output increment is between 9.1% and 14.2% at hydrogen mass flow rate of 0.0006 g/s when stoichiometric hydrogen/air ratio ranges from 0.6 to 1.4.

Economic appraisal of hybrid solar–biomass thermophotovoltaic power generation

The techno-economic parameters that influence the commercial deployment of hybrid thermophotovoltaic (TPV) solar power generation are determined using annual system simulations. It has been found that a TPV cell price of €5/cm2 or less together with a TPV operating temperature under 800°C is required for a hybrid solar–biomass TPV power plant to be economically competitive with the state-of-the-art hybrid solar–biomass Rankine cycle power plants.

Design and optimization of a combined solar thermophotovoltaic power generation and solid oxide electrolyser for hydrogen production

This study proposes a novel integrated solar-TPV device with a solid oxide electrolyzer cell to utilize the solar energy for hydrogen production. It explores the possibility of employing a high temperature solar-thermal-photovoltaic power generation technology as a power source for a steam electrolyzer as a high-efficiency and applicable hydrogen production method. Mathematical and electrochemical modeling of the subsystems is conducted and performance of the system in different operating conditions such as current density, temperature, and steam mole fraction of SOEC is analyzed. An STPV device of multiwalled carbon nanotube (MW-CNT) absorber and 1D Si/SiO2 PhC emitter and InGaAsSb PV cell is employed to maximize the solar energy utilization. A detailed system level model in this part is conducted and the solar to electrical efficiency of the scaled-up STPV device reached to 17%. The results show that this STPV device can provide the power demand in SOEC system. A planar cathode-supported high-temperature electrolyzer cell was designed to perform in an exothermic mode and the result was validated by the experimental data precisely. The sensitivity analysis showed that 7458 kg/h hydrogen can be produced in the proposed system with 54% electrical for SOEC efficiency. The major implementation challenges are presented to provide a comprehensive insight into performance, potential development, limitations and challenges of the integrated system. The proposed combined system shows the overall efficiency can reach to 34%. This high efficiency makes this novel hybrid system a competitive option in solar-based hydrogen production technologies.

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

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

Micro combustion in a porous media for thermophotovoltaic power generation

This work delved into porous media combustion (PMC) TPV with H2/O2 as fuel with much focus on experiment and numerical assessment of the TPV generator. The effects of some major parameters on PMC namely flow velocity, equivalence ratio and conductivity of the solid matrix were also numerically investigated.The results indicated a reduction in combustion efficiency upon the increment in inlet velocity. It was as a result of reduction in the residence time. The average wall temperature decreased with increase in the solid matrix thermal conductivity. Increment in cell temperature decreased the forbidden band whiles the cut-off wavelength increased. Temperature variation of the PV cell also caused a 35% decline in output power of the system. For any 10 K increase in cell temperature, the cell efficiency and power output reduced by 7% and 0.14 W respectively. A projected electrical output power and power density of the complete system were 2.7W and 0.72W cm-2 respectively when the cell temperature is kept at 300 K and the spacing between the radiant wall and the PVC is 1 mm. The experiment produced 1.703W electrical power which was in consonance with what was predicted with the model.

Simultaneous fabrication of a microcavity absorber–emitter on a Ni–W alloy film

A process for the simultaneous fabrication of microcavity structures on both sides of a film was proposed and demonstrated to develop a free-standing-type integrated absorber–emitter for use in solar thermophotovoltaic power generation systems. The absorber–emitter-integrated film comprised a heat-resistant Ni–W alloy deposited by electroplating. A two-step silicon mould was fabricated using deep reactive-ion etching and electron beam lithography. Cavity arrays with different unit sizes were successfully fabricated on both sides of the film; these arrays are suitable for use as a solar spectrum absorber and an infrared-selective emitter. Their emissivity spectra were characterised through UV–vis–NIR and Fourier transform infrared spectroscopy.

A dielectric-encapsulated 2D photonic crystal based solar thermophotovoltaic power generator

Photovoltaic is a common method in converting solar energy into electricity. However, due to the existence of the bandgap of PV cells, only half of the solar energy could be used for power generation. Solar-thermphotovoltaic (solar-TPV) system offers a solution to use extended solar wavelength range up to infrared. The key component of a solar-TPV system is a selective absorber and emitter. The conventional 2D metallic photonic crystal (PhC) based selective absorber and emitter is limited to the fabrication quality in deep etching as well as the material reservation such as Tungsten and Tantalum. Here we propose a dielectric-encapsulated 2D PhC selective absorber and emitter which is made of Titanium nitride (TiN) for solar-TPV system. The encapsulating dielectrics with different refractive index easily tune the cut-off wavelength of the selective absorber and emitter which is free of modifying the size of the cavity. Besides, the TiN material could be obtained by chemical vapor deposition by Titanium which has large world reservation. In this study, the optimal absorber cut-off wavelength is calculated at various solar concentrating factors. A selective absorber and emitter is designed and fabricated with the SiO2 and Al2O3 encapsulated PhC structure, the fabricated selective absorber and emitter show good frequency selectivity and thermal stability.