Thermophotovoltaic Efficiency Research Articles

Sub‐0.6 eV Inverted Metamorphic GaInAs Cells Grown on Inp and GaAs Substrates for Thermophotovoltaics and Laser Power Conversion

AbstractInverted metamorphic Ga0.3In0.7As photovoltaic converters with sub‐0.60 eV bandgaps grown on InP and GaAs are presented. Threading dislocation densities are 1.3 ± 0.6 × 106 and 8.9 ± 1.7 × 106 cm−2 on InP and GaAs, respectively. The devices generate open‐circuit voltages of 0.386 and 0.383 V, respectively, under irradiance producing a short‐circuit current density of ≈10 A cm−2, yielding bandgap‐voltage offsets of 0.20 and 0.21 V. Power and broadband reflectance measurements are used to estimate thermophotovoltaic (TPV) efficiency. The InP‐based cell is estimated to yield 1.09 W cm−2 at 1100 °C versus 0.92 W cm−2 for the GaAs‐based cell, with efficiencies of 16.8 versus 9.2%. The efficiencies of both devices are limited by sub‐bandgap absorption, with power weighted sub‐bandgap reflectances of 81% and 58%, respectively, the majority of which is assumed to occur in the graded buffers. The 1100 °C TPV efficiencies are estimated to increase to 24.0% and 20.7% in structures with the graded buffer removed, if previously demonstrated reflectance is achieved. These devices also have application to laser power conversion in the 2.0–2.3 µm atmospheric window. Peak laser power converter efficiencies of 36.8% and 32.5% are estimated under 2.0 µm irradiances of 1.86 and 2.81 W cm−2, respectively.

Design of efficient thermophotovoltaic system based on meta-material narrow-band emitter for space power supply

Photovoltaic technology has been widely used in spacecraft power supply, but its efficiency is difficult to be greatly improved by Shockley-Queisser limitation. The thermophotovoltaic technology can convert solar radiation energy or high temperature combustion energy into radiation energy with reshaped spectrum for direct photovoltaic power generation. In this study, a meta-material structure composed of metal tantalum, Ta, and dielectric SiO2 is innovatively proposed for shaping narrowband radiation. The results show that the optimized spectral emittance peak of narrowband emitters reaches 0.9998. Narrowband emitter has advantages at high temperatures above 1000 K. The thermophotovoltaic efficiency of InGaAsSb cell and tandem Si/InGaAsSb cells can reach more than 41.67% and 46.26%, respectively. It is significantly higher than published thermophotovoltaic system with broadband emitter. This study demonstrates the notable advantages and potential of narrowband emitter for spectrum reshaping, which provides an important reference for future spacecraft power supply as well as space solar power generation.

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.

Thermophotovoltaic conversion efficiency measurement at high view factors

A standardized method for measuring thermophotovoltaic (TPV) efficiency has not been yet established, which makes the reported results difficult to compare. Besides, most of the TPV efficiencies reported to date have been obtained using small view factors, i.e., large cell-to-emitter distances, so the impact of the series resistance is usually underestimated, and the optical cavity effects, i.e., the multiple reflections taking place between the emitter and the cell, are not accounted for experimentally. In this work, we present an experimental setup able to measure the TPV efficiency under high view factors (up to 0.98), by using small emitter-to-cell distances (< 1 mm). This allows a more accurate direct measurement of the TPV efficiency at higher power densities than previous works. As a result, a TPV efficiency of 26.4+-0.1 % and a power density of 4.3+-0.8 W/cm2 have been obtained for an InGaAs TPV cell with a back surface reflector irradiated by a graphite thermal emitter at 1592 C.

Power dense thermophotovoltaic cells

Class-leading 2-junction (2J) thermophotovoltaic (TPV) cells have been developed with thermophotovoltaic efficiency exceeding 40%. However, these devices have sub-optimal power density because the subcell bandgaps are not matched to the emitter spectrum. Although efficiency is important, power density is also an important metric to gauge TPV cell performance; the greater the power density, the less total area of TPV cells that are needed to satisfy a given power generation target. To quantify the relevance of power density, spectrum-matched 1.04/0.78/0.62/0.48/0.36 eV 5-junction (5J) TPV cells have the potential to be 3.5 times more power dense than state-of-the-art, yet spectrum-mismatched, 1.4/1.2 eV 2J TPV cells when irradiated by a 2400 °C emitter. The proposed 5J TPV cells also have the potential to exceed 40% standard power conversion efficiency.

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.

Effect of oxidizer dilution on thermophotovoltaic performance of premixed ammonia/hydrogen/air combustion in radial micro-combustor

Present research deals with the combustion characteristics, thermal performance, and emissions analyses of a radial micro-combustor for thermo-photovoltaic application. Simulations are carried out for premixed ammonia/hydrogen/air combustion with different inlet velocities and air dilution levels with N2. Influences of input parameters on the thermal performance of micro-combustor, the total energy conversion efficiency of the micro thermo-photovoltaic system, OH distribution, recirculation zone characteristics, flame blowout limits and NOx-N2O emissions are examined. The results imply that elevating inlet velocity in the studied range enhances the uniformity factor of temperature distribution in the fluid and solid domains. The maximum value of the total energy conversion efficiency was obtained to be 8.2 % corresponding to the 6.6 W of output electric power. On the other hand, the present results showed that a higher dilution level of air (up to 84 % N2 on volume basis) results in a significant increase in temperature distribution uniformity and consequently a decrease in mean temperature and total thermo-photovoltaic efficiency. Moreover, such increment in dilution level results in higher N2O emission at lower velocities and consequently a significant decrease in NOx emission level up to 58 %. Finally, reaction pathway analysis showed that the main NO production pathway remains unchanged in NH3 → NH2 → HNO → NO route with increasing inlet velocity and dilution level.

Performance analysis of biomass driven thermophotovoltaic system from energy and exergy perspectives

• Propose a thermo-physical model of biomass-TPV and conduct a parametric analysis. • Present biomass-TPV/T concept for photo-thermal energy cascade conversion. • Effect of fuel and combustion design is analyzed from energy and exergy perspectives. • Increasing the TPV area and the regenerator design improves efficiency by more than 10%. • Provide reference for the design of TPV power device driven by fuel combustion. The new thermophotovoltaic (TPV) technology driven by biomass fuel is an important way of solar indirect use, it can directly convert thermal energy into electrical energy through thermal radiation; and it is more suitable for the cascade utilization of energy when compared to direct solar-TPV. Therefore, to identify the principles of energy conversion within the biomass-TPV, this study conducted a thermodynamic analysis of a biomass-TPV power device from energy and exergy perspectives. A thermo-physical model of TPV verified by experimental data was established. The concept of biomass-thermophotovoltaic/thermal (TPV/T) combined conversion was summarized to achieve cascade utilization. Particular consideration is given to the effects of biomass fuels, combustion chamber designs and selective filters on the thermodynamic performance of biomass-TPV device and biomass-TPV/T conversion. The results show that the largest exergy loss of biomass combustion accounts for 40%. The biomass fuel moisture increases the exergy losses in combustion and flue gas and thus reduces TPV efficiency by more than 3 percent points. Increasing the TPV area and the regenerator design are beneficial for reducing the exergy loss of flue gas and improving TPV efficiency by more than 10 percent points. Narrow-band filters decrease the exergy losses of TPV emitter and cell, while increasing the loss in flue gas. The narrow-band filter is more suitable for TPV/T combined conversion. This study provides a reference for the design and optimization of biomass-driven TPV conversion system.

Thermophotovoltaic efficiency of 40%

Thermophotovoltaics (TPVs) convert predominantly infrared wavelength light to electricity via the photovoltaic effect, and can enable approaches to energy storage1,2 and conversion3–9 that use higher temperature heat sources than the turbines that are ubiquitous in electricity production today. Since the first demonstration of 29% efficient TPVs (Fig. 1a) using an integrated back surface reflector and a tungsten emitter at 2,000 °C (ref. 10), TPV fabrication and performance have improved11,12. However, despite predictions that TPV efficiencies can exceed 50% (refs. 11,13,14), the demonstrated efficiencies are still only as high as 32%, albeit at much lower temperatures below 1,300 °C (refs. 13–15). Here we report the fabrication and measurement of TPV cells with efficiencies of more than 40% and experimentally demonstrate the efficiency of high-bandgap tandem TPV cells. The TPV cells are two-junction devices comprising III–V materials with bandgaps between 1.0 and 1.4 eV that are optimized for emitter temperatures of 1,900–2,400 °C. The cells exploit the concept of band-edge spectral filtering to obtain high efficiency, using highly reflective back surface reflectors to reject unusable sub-bandgap radiation back to the emitter. A 1.4/1.2 eV device reached a maximum efficiency of (41.1 ± 1)% operating at a power density of 2.39 W cm–2 and an emitter temperature of 2,400 °C. A 1.2/1.0 eV device reached a maximum efficiency of (39.3 ± 1)% operating at a power density of 1.8 W cm–2 and an emitter temperature of 2,127 °C. These cells can be integrated into a TPV system for thermal energy grid storage to enable dispatchable renewable energy. This creates a pathway for thermal energy grid storage to reach sufficiently high efficiency and sufficiently low cost to enable decarbonization of the electricity grid.

Sustaining efficiency at elevated power densities in InGaAs airbridge thermophotovoltaic cells

Recent work has demonstrated record-high thermophotovoltaic efficiency using thin-film InGaAs cells, but the power density of devices remains low. Elevated power densities are relevant to many thermophotovoltaic (TPV) applications, ranging from mobile generators to stationary energy storage of renewable electricity, and require effective management of heat and charge carriers. Here we investigate the use of single-junction InGaAs airbridge cells (ABCs) under such conditions. Experimental characterization of an InGaAs ABC with varying emitter and cell temperature is used to develop a predictive device model where carrier lifetimes and series resistances are the sole fitting parameters. The utility of this model is demonstrated through its use in identifying near-term opportunities for improving performance at elevated power densities, and for designing a thermal management strategy that maximizes overall power output. After accounting for the power necessary to cool the cells, this model shows that an InGaAs ABC with high material quality can reach a peak efficiency of ∼41% at 0.5 W/cm2, corresponding to an emitter temperature of 1070 °C, and sustain efficiencies above 36% up to 1.5 W/cm2.

Semiconductor-based selective emitter with a sharp cutoff for thermophotovoltaic energy conversion.

A semiconductor emitter can possibly achieve a sharp cutoff wavelength due to its intrinsic bandgap absorption and almost zero sub-bandgap emission without doping. A germanium-wafer-based selective emitter with front-side antireflection and backside metal coating is studied here for thermophotovoltaic (TPV) energy conversion. Optical simulation predicts the spectral emittance above 0.9 in the wavelengths from 1 to 1.85 µm and below 0.2 in the sub-bandgap range with a sharp cutoff around the bandgap, indicating superior spectral selectivity behavior. This is confirmed by excellent agreement with indirectly measured spectral emittance of the fabricated Ge-based selective emitter sample. Furthermore, the TPV efficiency by pairing the Ge-based selective emitter with a GaSb cell is theoretically analyzed at different temperatures. This Letter facilitates the development of the semiconductor-based selective emitters for enhancing TPV performance.

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.

Optoelectronic analysis of spectrally selective nanophotonic metafilm cell for thermophotovoltaic energy conversion

This work theoretically studies a spectrally selective nanophotonic cell based on an asymmetric Fabry-Perot resonance cavity structure with sub-100-nm GaSb layer for improving the thermophotovoltaic (TPV) energy conversion performance. The simulated spectral property of the ultrathin metafilm cell structure exhibits a high absorption peak above the bandgap due to the interference effect with electromagnetic field enhanced inside the GaSb layer between top and bottom silver electrodes, while the sub-bandgap absorption is as low as a few percent because of high reflectivity of the metal. An absorption enhancement nearly 20 times at particular frequency above bandgap is achieved within the sub-100-nm GaSb layer with the nanophotonic cell structure compared to the free-standing one. Besides, a thin layer of MoOx is incorporated into the metafilm cell structure as a hole transport layer to consider the charge collection in practice. With rigorous optoelectronic analysis by considering both radiative and nonradiative recombinations (Shockley-Read-Hall and Auger), the nanophotonic cell is predicted to achieve a TPV efficiency of 22.8% and output power of 0.62 W/cm2 with a black emitter at 1500 K due to spectrally enhanced in-band absorption and low sub-bandgap absorption. With an ideal selective emitter the efficiency can be further improved to 28% by eliminating sub-bandgap photons. While selective emitters still endure the challenges in perfect spectral emittance and high-temperature stability in practice, the proposed wavelength-selective nanophotonic metafilm cell could be a viable route to achieve high-efficiency and low-cost TPV energy conversion.

Accurate calibration of thermophotovoltaic efficiency

The new record efficiency in Thermophotovoltaics relies upon a highly reflective rear mirror. The excellent rear mirror boosts voltage by enhancing the luminescence extraction, and separately also reflects low energy photons, which would otherwise be useless in thermophotovoltaics. The reflected low energy photons reheat the thermal emitter, and regenerate above-bandgap energy photons. The efficiency calibration for such regenerative thermophotovoltaics depends on several factors, yet predominantly on the accurate measurement of the rear mirror reflectivity. Here, we report on the technique for accurate measurement of mirror reflectivity, and of record thermophotovoltaic efficiency 29.1 ± 0.6%, at 1207 °C.

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%.

Theoretical design of nanoparticle-based spectrally emitter for thermophotovoltaic applications

Wavelength-selective emitter is a critical component in thermophotovoltaic (TPV) systems. How to evaluate and further enhance TPV efficiency for different PV cells by designing selective emitter is still needed for realistic applications. Hence, the impact of the nanoparticle-based spectrally selective emitter on the TPV efficiency was investigated in this work. The nanoparticle material, radius, volume fraction and thickness of emitter was optimized based on the spectral cut-off efficiency ηs and effective emittance efficiency ηe for GaSb and InGaAsSb PV cells respectively. Results showed that W emitter achieved better TPV performance than Ti and Ni emitters. The maximized ηsηe achieved 0.415 and 0.535 for GaSb and InGaAsSb PV cells at 1200 °C, indicating that this nanoparticle-based structure emitter was more suitable for the InGaAsSb PV cell with the low bandgap. These results give valuable insights into design opportunity for the selective emitter in various TPV systems.

Effects of photon recycling, trapping, and reuse on thermophotovoltaic conversion efficiency and output power

Photonic management is a key issue for the optimization of thermophotovoltaic (TPV) energy conversion systems. It is realized by selective emitters, front surface filters on TPV cells, and back surface reflectors (BSRs). Photonic management modifies photon energy transfer from the emitter to the TPV cell due to photon reuse and energy conversion processes in the TPV cell due to photon recycling and trapping in the cell. Our work has developed a comprehensive thermodynamic theory of photonic management in the TPV cell and in whole TPV systems to elucidate key optimization parameters. Our approach is based on the exact Lambert function solution of the generalized Shockley–Queisser model and the corresponding fundamental formulas of endoreversible thermodynamics for maximal electric power, emitted optical power, and dissipation losses. The model includes interrelated processes of photon recycling, photon trapping, nonradiative recombination, and parasitic absorption of the BSR. Optimization of a TPV system with photon reuse should take into account that the cell thickness that provides maximal output power does not correspond to the thickness, which gives the maximal conversion efficiency. The theory predicts the important limits for TPV efficiency and output power determined by the Auger recombination in low-bandgap semiconductor materials, various parasitic losses in the cell and conductive layers, and photon escape from the TPV system. For example, we consider the TPV system based on 0.6 eV InGaAs cells with a BSR and a front surface photon scattering layer, which provides Lambertian light trapping.

(Invited) Macroscopically Aligned Carbon Nanotubes for Efficient Thermophotovoltaic Energy Conversion

Single-walled carbon nanotubes (SWCNTs) exhibit many unique properties arising from their quantum confinement. However, the lack of a controllable large-scale alignment procedure had limited the exploration of their unique properties. Our recent work has overcome this problem by demonstrating a vacuum-filtration based technique to align SWCNTs on wafer-scale. These aligned SWCNT films exhibit many interesting optical properties such as extreme optical anisotropy or hyperbolic dispersion. Besides, aligned SWCNTs are flexible, refractory (stable up to 1900 K), inexpensive, and based on earth-abundant carbon making them an excellent refractory infrared nanophotonic material platform. Such a refractory infrared material platform is exactly what is necessary to enable efficient thermophotovoltaic energy conversion.Thermophotovoltaics is a solid-state heat-to-electricity conversion technique. It relies upon photovoltaic conversion of thermal light emanating from a hot surface to electricity. The broadband nature of thermal light makes the thermophotovoltaic conversion inefficient. Optical resonators, filters, spectrally selective thermal emitters are a few methods proposed to increase the efficiency of thermophotovoltaics. Selective thermal emitters are very promising due to their ease of design and integration. Selective thermal emitters are hot surfaces that emit thermal radiation in a narrow spectral band above the bandgap of the photovoltaic cell. The suppression of thermal emission below the bandgap and enhancement above the bandgap are the key requirements for efficient thermophotovoltaic conversion. Generally, optical properties of materials at high temperatures limit the performance of such selective emitters and the current efficiency record is about 8%. But, the hyperbolic optical property of aligned SWCNTs together with their thermal stability make aligned SWCNTs ideal for efficient thermophotovoltaics.Here in this work, we study the infrared optical properties of aligned SWCNTs at high temperatures (1000 K) and demonstrate spectrally selective thermal emission from plain and nano-patterned thin films. Using deep sub-wavelength sized optical cavities, we show that aligned SWCNTs exhibit hyperbolic dispersion in almost of all of infrared and enhance thermal photon density by at least 100 times. Such a large enhancement in thermal photon density leads to a large suppression of unwanted sub-bandgap thermal photons and a large enhancement of above-bandgap photons. Our analysis shows that aligned SWCNTs enable 30% or higher thermophotovoltaic efficiency when operating at 1300 K with no concentration. With near-field concentration, aligned SWCNTs can allow conversion efficiencies over 50%.

Efficiency Enhancement of a Thermophotovoltaic System Integrated With a Back Surface Reflector

This work aims to investigate the various factors which may affect a thermophotovoltaic (TPV) system's performance, with a special focus on the importance of incorporating a back surface reflector (BSR), which enables below-bandgap photons' recycling. The possible extent to which common PV materials can be used in TPV applications is investigated by comparing them on a Planck distribution curve. The effects of varying BSR reflectivity, TPV cell's external quantum efficiency, and emitter temperature are investigated on the TPV module's efficiency using open-circuit voltage, empirical relations for fill factor, maximum voltage, and photogenerated current. It is shown that TPV applications require materials with smaller (e.g. 0.6 eV 26%) above-bandgap without a spectral control and a small difference between peak and bandgap wavelength. It is shown that the inclusion of a BSR (reflectivity = 1) results in an increase of 15% in TPV efficiency. The results show that by the collective changes of an added BSR, high emitter temperature (>2000 K), and improved external quantum efficiency (EQE ≈ 1), the present TPV systems can attain efficiency values more than 30% which makes them a favorable prospective choice for Concentrated Solar Power.

Ultraefficient thermophotovoltaic power conversion by band-edge spectral filtering

Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking solar efficiency. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high thermophotovoltaic efficiency. This mirror reflects low-energy infrared photons back into the heat source, recovering their energy. Therefore, the rear mirror serves a dual function; boosting the voltage and reusing infrared thermal photons. This allows the possibility of a practical >50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we report a thermophotovoltaic efficiency of 29.1 ± 0.4% at an emitter temperature of 1,207 °C.