Efficiency Of Thermophotovoltaic Systems Research Articles

A comprehensive study of effective parameters on the thermal performance of porous media micro combustor in thermo photovoltaic systems

Low thermal performance is one of the biggest challenges of using micro combustor in Thermo-photovoltaic (TPV) system. In this study, a novel micro-combustor with porous media was designed and installed to enhance energy and exergy performance. Furthermore, the effects of several effective parameters including different porous media materials, length, porosity coefficient, and inlet mass flow on energy efficiency, exergy, entropy generation, wall temperature and its uniformity were studied. A comprehensive CFD model for using porous media in the micro-combustor of TPV systems was proposed. Results showed utilizing porous media significantly improve the exergy efficiency and energy output of TPV system. Therefore, using a 6 mm long porous media increased the average wall temperature by 111 K compared to the case without porous media. Additionally, the uniformity coefficient of the wall temperature decreased by 80.05 %, from 4.58 % to 0.89 %. This reduction increased the temperature uniformity of the outer wall of the micro-combustor with porous media compared to the case without a media, increasing the system’s lifetime. Moreover, the 6 mm-long porous media enhanced radiation efficiency and exergy efficiency by 37 % and 79.7 %, respectively, compared to the conventional micro combustor. The total energy conversion efficiency from the fuel chemical energy to electric power increased from 8.9 % to 12.32 %.

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.

Tailorable bandgap-dependent selective emitters for thermophotovoltaic systems

Selective emitters are crucial in thermophotovoltaic (TPV) systems as they can selectively tailor the incident light to match with the bandgap of photovoltaic (PV) cell, thus greatly increasing the system efficiency. However, further progress has been hampered by the limitation in designing high performance selective emitters with spectrally emissive selectivity and compatible ability to different PV cells. In this study, we propose a design strategy to realize tailorable bandgap-dependent selective emitters based on metasurfaces by combing the two-resonance response of surface plasmon polaritons (SPPs) and magnetic polaritons (MPs). The emission spectrum can be manipulated and matched to various PV cells with different bandgap by simply changing the size and period of the meta-atoms in metasurface. We design and experimentally demonstrate a selective emitter appealing to GaSb PV cells, which presents near-perfect emissivity (above 0.96) above the bandgap wavelength (from 0.9μm to 1.54μm) with polarization-insensitive and angle-insensitive features. Moreover, the emitter shows a high figure of merit (FOM, of 0.85), and the efficiency of TPV system can be expected to be 27% at 1473K. The proposed strategy and designed emitter may pave a reliable route for improving the performance of TPV systems.

Inverted metamorphic AlGaInAs/GaInAs tandem thermophotovoltaic cell designed for thermal energy grid storage application

We demonstrate an inverted metamorphic multijunction (IMM) photovoltaic cell comprising lattice-mismatched 1.2 eV AlGaInAs and 1.0 eV GaInAs junctions optimized for high-temperature thermophotovoltaic (TPV) applications. This device differs from traditional IMM solar cells because the mismatched junctions are grown at a single lattice constant. This architecture enables removal of the compositionally graded buffer that otherwise filters light from the junctions below and absorbs sub-bandgap light via free-carrier absorption. Sub-bandgap absorption dramatically reduces the efficiency of TPV systems using high reflectivity cells to enable band edge spectrum filtering. Three components required development to enable this device: (1) a lattice-mismatched 1.2 eV AlGaInAs junction, (2) a metamorphic contact layer grown after the graded buffer, and (3) a transparent tunnel junction that sits in front of the 1.0 eV GaInAs junction. Growth conditions that minimize oxygen defect incorporation maximize AlGaInAs cell quality, enabling a 0.41 V bandgap open circuit voltage offset at 22 mA/cm2 under AM1.5D. A mismatched GaInAs:Se layer is developed as a low resistance contact. Lastly, we develop a GaAsSb:C/GaInP:Se tunnel junction suitable for high-power densities with more transparency than the GaAsSb:C/GaInAs:Se structure used in past IMM cells. We characterize the tandem device under a high-intensity spectrum that approximates the emission from a 2150 °C blackbody radiator and deduce a projected ideal TPV efficiency of 39.9% at ∼30% of the blackbody irradiance and 36% ideal TPV efficiency under the full 118 W/cm2 irradiance. Improvements to the back-surface reflectivity and series resistance are expected to increase the ideal TPV efficiency well above 40%.

Performance Analysis of GaSb Cell and Thermophotovoltaic System Under Near-Field Thermal Radiation

An energy transfer model of a thermophotovoltaic (TPV) system, considering the variation of GaSb cell parameters under the near-field thermal radiation, was constructed. The effects of emitter–cell distances with different cooling systems on cell parameters and TPV system performances were analyzed. The results show that if the cell temperature is set at 300 K, the required convective heat transfer coefficient of the cooling system should be as high as 8.1 × 105 W/(m2·K), which is a severe requirement for the existing cooling system. As the cell temperature increases, the photocurrent density decreases, because the internal quantum efficiency decreases due to the decreasing carrier life. The photocurrent density is significantly affected by the emitter–cell distance, because the distance has a strong impact on the thermal radiation. Similarly, the open-circuit voltage decreases with the increasing cell temperature and emitter–cell distance. To ensure a safe operation, the extreme emitter–cell distance is 12 nm for the best cooling system. When the emitter–cell distance decreases from 100 nm to 12 nm, the cell efficiency, which is around 0.25 at the far-field condition, decreases under the near-field thermal radiation, decreasing the TPV system efficiency which is obviously lower than that compared with the cell temperature of 300 K. Thus, the system performances with the cell temperature set at 300 K are overestimated. The results reveal that the variation of cell temperature needs to be considered to actually evaluate the performance of the near-field TPV system, and highly efficient cooling systems are urgently needed for the near-field TPV system.

Machine learning-optimized Tamm emitter for high-performance thermophotovoltaic system with detailed balance analysis

Abstract Light-matter interaction upon nanophotonic structures in the infrared wavelength has drew increasing attentions due to the extensive potential applications. Among them, thermophotovoltaic (TPV) systems can exhibit higher efficiency over the Shockley-Queisser limit due to the nanophotonic structure-enabled tunable narrowband thermal emission rather than the broadband incident spectrum. However, two long-standing issues remain formidable as bottlenecks for achieving better performances of TPV system. One is the competing role of the power density and the system efficiency of TPV system, and the other is the magnanimity possibilities of structures, configurations, dimensions, and materials of thermal emitters that disables the manual optimization of TPV system. Here, we attempt to achieve high-performance TPV system by employing the machine learning algorithm under the framework of material informatics. The power density and system efficiency are well modelled through the detailed balance analysis with full considering the photocurrent generation in the PV cells. Through optimization, the non-trial aperiodic Tamm emitters are obtained and the metal-side one is preferable in terms of the TPV performance. The present work is demonstrated to be feasible and efficient in optimizing the TPV performance, and opens a new door for the optimization problems in other fields.

Evaluation of performance of near-field thermophotovoltaic systems based on entropy analysis

Near-field radiative heat transfer can significantly improve the output power of thermophotovoltaic (TPV) systems. Therefore, it is crucial to explore how to increase the energy conversion efficiencies of near-field TPV systems. In this study, based on the fluctuation–dissipation theorem with the effective medium theory, we evaluate the performance of a near-field TPV system from the formulation of thermodynamics. It is found that a near-field TPV system consisting of InAs or GaSb cell can achieve higher heat flux or efficiency limit, respectively. Moreover, the TPV system with a hyperbolic metamaterial (HMM) emitter composed of nanowire or nanohole arrays can achieve higher heat flux and efficiency limit compared to that of the bulk reference. This is attributable to the HMMs being able to support hyperbolic modes, and the radiation with the HMM emitter exhibits a favorable entropy content for the energy conversion efficiency. This work provides an approach to determine the efficiency limit and establish a target for efficiency of the near-field TPV system without considering how the system works. The results of this study will facilitate the design and application of the HMM emitter and material of the TPV cell to improve the efficiency of near-field TPV systems.

Highly efficient sub-100-nm thermophotovoltaic cells enhanced by spectrally selective two-dimensional metasurface

Great efforts have been devoted to the investigation of selective emitters and filters for improving the conversion efficiency of thermophotovoltaic (TPV) systems. This work proposes a spectrally selective ultrathin cell with two-dimensional (2-D) metasurface for high-efficiency TPV energy conversion. The cell is composed of Ag substrate, ultrathin gallium antimonide (GaSb) photovoltaic layer, and 2-D metasurface comprising symmetric 2-D metallic grating. The spectral-normal absorptance of the proposed structure is calculated by the finite-element method and the results show significant enhancement in the photon absorption comparing to the free-standing GaSb layer with the same thickness, which is attributed to the excitation of magnetic polariton and Fabry–Perot resonance. The effects of using indium tin oxide and graphene as front electrodes on the absorptance of the proposed ultrathin cell are investigated. The TPV system efficiency is also evaluated using the proposed ultrathin cell. The results show that the TPV efficiency with the proposed ultrathin cell can approach 29.1% with a black emitter at 1500 K and can be further increased to be 35.1% using an ideal selective emitter. This work will facilitate the development of next-generation high-efficiency TPV systems.

Investigation on the effects of wall thickness and porous media on the thermal performance of a non-premixed hydrogen fueled cylindrical micro combustor

An investigation on non-premixed H2/air combustion in a cylindrical micro combustor has been carried out. The effects of porous media and outer wall thickness on the combustion characteristics, flame location, thermal performance and energy conversion efficiency of the thermo photovoltaic (TPV) system were investigated. For the application of micro TPV system, a high and uniform outer wall temperature distribution is indispensable for the sustaining output, and the high energy efficiency is desirable. The results indicate that the setting of porous media or the increase of outer wall thickness can enhance the heat transfer in micro combustor and affects the flame stability, and the micro combustor with porous media and outer wall thickness b = 0.2 mm obtains the lowest flame location. They are also conducive to the improvement of outer wall temperature and energy efficiency, the outer wall temperature of the micro combustor with a thicker outer wall and porous media is relatively uniform and the energy conversion efficiency of micro TPV system is also improved, the micro combustor with b = 0.6 mm and porous media is more suitable for the application of micro TPV system. Additionally, the external thermal environment also can improve the outer wall temperature profile and the working performance of the micro combustor.

Thermophotovoltaic emitters based on a two-dimensional grating/thin-film nanostructure

Thermophotovoltaic (TPV) devices can convert thermal radiation directly into electricity. To improve the efficiency of TPV systems, wavelength-selective emitters are designed to take thermal energy from various heat sources and then emit photons to the TPV cells. A two-dimensional grating/thin-film nanostructure is proposed as an efficient emitter, whose performance is enhanced by the excitations of both surface plasmon polaritons (SPPs) and magnetic polaritons (MPs). Rigorous coupled-wave analysis is used to predict the emittance as well as the electromagnetic field and current density distributions. The normal emittance of the proposed nanostructure is shown to be wavelength-selective and polarization-insensitive. The mechanisms of SPP and MP excitations in the nanostructure are elucidated for different polarizations. The current–density loop further confirms the existence of magnetic resonances. Furthermore, the effect of azimuthal and polar angles on the emittance spectra is also investigated, suggesting that the proposed structure has high emittance not only in the normal direction but also at large oblique angles.

Application of micro/nanoscale thermal radiation to thermophotovoltaic system

Thermophotovoltaic (TPV) system has been regarded as one promising means to alleviate current energy demand because it can directly generate electricity from radiation heat via photons. However, the presently available TPV systems suffer from low conversion efficiency and low throughput. A viable solution to increase their efficiency is to apply micro/nanoscale radiation principles in the design of different components to utilize the characteristics of thermal radiation at small distances and in microstructures. Several critical issues are reviewed, such as photovoltaic effect, quantum efficiency and efficiency of TPV system. Emphasis is given to the development of wavelength-selective emitters and filters and the aspects of micro/nanoscale heat transfer. Recent progress, along with the challenges and opportunities for future development of TPV systems are also outlined.

Concept and combustion characteristics of the high-luminescence flame for thermophotovoltaic systems

The thermophotovoltaic (TPV) power system mainly consists of a heat source, an emitter and a photovoltaic (PV) cell array. One of the major deficiencies leading to the relatively low overall efficiency of the TPV power system is the spectral mismatch between the emitter radiation and the quantum efficiency peak of the PV cells. Therefore, application of high-luminescent-flame in the visible range is a cost-effective method to enhance the radiant efficiency of a small TPV system. The concept of the high-luminescent-flame combustor is proposed with central-porous liquid fuel-film injection of n-heptane and trace amount of iron pentacarbonyl, and an emitter tube. The metal-porous fuel-film injection is an effective method to increase contact surface and thermal conduction for liquid fuel vaporization and for flame stabilization. Chemiluminescence measurement and Abel deconvolution are further performed to identify the flame structure in two different porous materials, bronze and stainless steel, respectively. Flame structure and stabilization mechanism in the chamber can be related to the tribrachial flame. The radiant intensity of the iron pentacarbonyl flame is significantly enhanced by fivefold, and flame color turns to sliver-white. Inserting an emitter tube in the combustor can further effectively integrate the radiation from the flame luminosity and emitter incandescence, and improve the radiant efficiency of the TPV system.

Enhancing thermal, electrical efficiencies of a miniature combustion‐driven thermophotovoltaic system

AbstractMethods to enhance the thermal and electrical efficiencies through novel design of combustion and thermal management of the combustor in a miniature thermophotovoltaic (TPV) system are proposed, discussed, and demonstrated in this paper. The miniature TPV system consists of a swirling combustor surrounded by GaSb PV cell arrays. The swirl combustor design, along with a heat‐regeneration reverse tube and mixing‐enhancing porous‐medium fuel injection, improves the low illumination and incomplete combustion problems associated with typical miniature TPV systems. A reverse tube is used to enforce swirling flame attachment to the inner wall of the emitter by pushing the swirl recirculation zone back into the chamber and simultaneously redirecting the hot product gas for reheating the outer surface of the emitter. The porous medium fuel injector is used as a fuel/air mixing enhancer and as a flame stabilizer to anchor the flame. The miniature TPV system, using different combustor configurations, is tested and discussed. Results indicate that the proposed swirling combustor with a reverse tube and porous medium can improve the intensity and uniformity of the emitter illumination, and can increase the thermal radiant efficiency. Consequently, the overall thermal efficiency and electrical output of the miniature TPV system are greatly enhanced. Copyright © 2009 John Wiley & Sons, Ltd.

Thermophotovoltaic power generation systems using natural gas-fired radiant burners

Thermophotovoltaic (TPV) power generation in gas-fired furnaces is attracting technical attention. Considerable work has been done in the area of low bandgap GaSb cell-based TPV systems as well as silicon solar cell-based TPV systems. Previous investigations have shown that a radiant burner with a high conversion level of fuel to radiation energy must be developed to realize an efficient TPV system. In our work, we investigated different natural gas-fired radiant burners in order to raise the conversion of fuel energy to thermal radiation. These burners were used as radiation sources to establish and test two TPV prototype systems. It was found that for a non-surface combustion radiant burner, the radiation output can be enhanced using a thermal radiator with a porous structure. Also, we developed a cascaded radiant burner that generates two streams of radiation output. One stream illuminates silicon concentrator solar cells while the other drives low bandgap GaSb cells. In this way, useful radiation output and thus TPV system efficiency are significantly increased due to the cascaded utilization of combustion heat and optimized thermal management.

Optical characteristics of one-dimensional Si∕SiO2 photonic crystals for thermophotovoltaic applications

This article presents a detailed exploration of the optical characteristics of various one-dimensional photonic crystal structures designed for use as a means of improving the efficiency and power density of thermophotovoltaic (TPV) devices. The crystals being investigated have a ten-layer quarter-wave periodic structure, and are based on Si∕SiO2 and Si∕SiON material systems. For TPV applications the crystals are designed to act as filters, transmitting photons with wavelengths below 1.78μm to a GaSb photodiode, while reflecting photons of longer wavelengths back to the source of thermal radiation. In the case of the Si∕SiO2 structure, the Si and SiO2 layers were designed to be 170 and 390nm thick, respectively. This structure was fabricated using low-pressure chemical vapor deposition. Reflectance and transmittance measurements of the fabricated Si∕SiO2 photonic crystals were taken from 0.8to3.3μm for both polarizations and for a range of incident angles. Measurement results were found to correlate well with simulation results for the ideal structure. Measurement results were used to predict the TPV system efficiency, power density and spectral efficiency using an ideal thermodynamic model of a TPV system.

Design and optimization of one-dimensional photonic crystals for thermophotovoltaic applications.

We explore the optical characteristics and fundamental limitations of one-dimensional (1D) photonic crystal (PhC) structures as means for improving the efficiency and power density of thermophotovoltaic (TPV) and microthermophotovoltaic (MTPV) devices. We analyze the optical performance of 1D PhCs with respect to photovoltaic diode efficiency and power density. Furthermore, we present an optimized dielectric stack design that exhibits a significantly wider stop band and yields better TPV system efficiency than a simple quarter-wave stack. The analysis is done for both TPV and MTPV devices by use of a unified modeling framework.