Solar Thermophotovoltaic System Research Articles

A Highly Efficient, Selective, and Thermally Stable Dielectric Multilayer Emitter for Solar Thermophotovoltaics

Herein, the design, optimization, fabrication, and characterization of a highly efficient selective emitter (SE) for solar thermophotovoltaic systems is presented. An SE consisting of three layers (SiNx–SiO2–TiO2) deposited on a tungsten substrate is optimized for use with photovoltaic (PV) cells based on III–V semiconductors, such as GaSb, InGaAs, and InGaAsSb. The fabricated SE shows an emitter efficiency (ηSE) of 50% when coupled with a PV cell having an energy bandgap of 0.63 eV. After thermal treatment carried out at 1000 °C for 8 h in a vacuum environment, ηSE of 46% is recorded, demonstrating the thermal stability of the proposed SE. Its behavior at high temperatures has also been studied using simulations based on the transfer matrix method and on refractive indices experimentally measured at different temperatures (up to 1000 °C). The results show ηSE of 44% in the energy bandgap range of 0.55–0.63 eV, proving that the proposed structure is promising and can operate at high temperatures. In addition, the behavior of a real PV cell is simulated, and calculations show a maximum PV cell efficiency of 15% at 1000 °C and 25% at 1600 °C, exceeding the Shockley–Queisser limit.

A high-temperature solar selective absorber based on one-dimensional multilayer nanostructures

Selective absorbers play a crucial role in harvesting solar energy and realizing effective solar-thermal energy conversion in concentrated solar power systems and thermophotovoltaic systems. This study focuses on obtaining a high-temperature solar selective absorber which can well balance its thermal performance as well as structural complexity. Based on this, a selective absorber with simple one-dimensional multilayer nanostructures is designed to adapt to the high-temperature operation environments. The obtained absorber exhibits a total solar absorptance of 0.9504 to the AM1.5 solar radiation. Taking into account the infrared radiation loss, the solar-thermal efficiency of the absorber is found to be 91.2% under 1000 suns at 1273 K, and the total emittance is 0.1504 at the corresponding temperature. The absorber shows high insensitivity to both incident polarization angles and wide-angle incidence. Analysis of impedance and electromagnetic field distribution indicates that the selective absorption performance of the absorber is attributed to its impedance matching properties, resulting from the coupling effect of surface plasmon polaritons and magnetic polaritons.

Highly efficient molybdenum nanostructures for solar thermophotovoltaic systems: One-step fabrication of absorber and design of selective emitter

Efficient solar energy harvesting materials or structures have become a topic of great interest in response to the ever-increasing energy demand. Refractory transition metals have been found to exhibit attractive properties in broadband absorptance, making them highly effective in solar thermophotovoltaic (STPV) systems that aim to surpass the Shockley-Queisser limit. Typically, broadband absorbers are made from hybrid materials or complex geometrical structures, which require multi-step fabrication technologies. In this study, we successfully developed a highly efficient broadband absorber based on molybdenum (Mo) nanosheets using a one-step physical vapor deposition process. The nanosheets result in the high absorber efficiencies of 96.1 % for blackbody radiation (BBR) at 5778 K and of 96.4 % for air mass (AM) 1.5 spectrum, and are insensitive to polarization and incident angle. After annealing at 1650 K for 24 h, the Mo nanosheets absorber demonstrates outstanding thermal stability without compromising the efficiency of the STPV system. To complete the STPV system, a selective narrowband Mo-based gratings emitter was further designed with a high emissivity of 96 % at a wavelength of 2.17 μm, resulting in a high PV cell efficiency of 41.8 % around 1723 K. Through a series of theoretical calculations, the proposed STPV system can achieve an overall conversion efficiency of 40.2 % by a trade-off between the structure temperature and the sun concentration. Furthermore, an efficiency of 35.5 % can be achieved at a low temperature of 1073 K with a low solar concentration of 500 suns.The results provide tremendous opportunities for widespread applications of high-performance STPV systems.

UV-VIS-NIR absorber to harvest energy for solar thermophotovoltaics

Ideal ultraviolet-visible-infrared (UV-VIS-NIR) absorbers with consistent performance at elevated temperatures and severe climate conditions are crucial to harvest energy for solar-thermophotovoltaic systems (STPVs). As solar energy promises to fulfill the power demands, its efficient utilization through high-performing light-absorbing devices is inevitable. The requirement of high-temperature durability makes conventional plasmonics an infeasible choice, and those highly thermostable refractory metals/their derivatives suitable ones. In this work, a lossy refractory plasmonic material i.e. Zirconium-Nitride-based subwavelength, ultra-broadband, wide-angle, polarization-insensitive, and free-space impedance-matched metasurface absorber in a three-level Pythagorean fractal structure is demonstrated. A comprehensive investigative study is conducted with the successful attainment of more than 90% absorption between ∼ 500–900 nm with a peak of more than 98% at 655 nm. The mean absorption for wideband (200–2500 nm) is 86.01% and it is 91.37% for visible range. The proposed study provides an efficient choice of meta-absorbers for realizing highly efficient STPVs.

Performance of new designed meta-material absorbers/emitters reshaping solar spectrum to double-narrowband emissive spectrum for STPV power generation

Solar thermophotovoltaic system can reshape the solar spectrum and improve the conversion efficiency of photovoltaic. In this study, the characteristics of ideal selective absorber for AM0 and AM1.5 solar spectra are compared, and the principles are summarized to design a metamaterial absorber for different spectra. The absorption efficiency can reach more than 80 % at the high temperature of 1700 K. This study innovatively proposes a STPV system that matching a double-narrowband (DNB) emitter with Si/InGaAsSb tandem cells. The theoretical efficiency of STPV with a DNB emitter is about 4 % higher than that with a wide-band (WB) emitter by analyzing thermodynamic model. Accordingly, a metamaterial DNB emitter is designed. The results show that the DNB metamaterial selective emitter designed in this study can achieve a TPV efficiency of near 50 %. The concentrating ratio and temperature have an inverse effect on the absorption efficiency and TPV efficiency. The optimized STPV system efficiencies for AM0 and AM1.5 are 37.29 % and 37.20 % under typical condition, respectively, which are a great improvement compared to the solar PV cell. The proposed metamaterial absorber and DNB emitter have great application potential in power generation for both space and ground conditions.

A tungsten-based metamaterial emitter for solar thermophotovoltaic systems.

Solar thermophotovoltaic systems are capable of showing efficient photoelectric conversion and are expected to surpass the Shockley-Queisser limit, owing to the spectrum-selective functionality of metamaterial selective emitters. Generally, metamaterial emitters are manufactured from multifarious materials, which also makes their manufacturing process complicated. Here, we propose a tungsten-only emitter composed of two rectangular bars with different widths and heights arranged in a cruciform structure, featuring a rectangular cavity at the top. Results from the simulations reveal that the emissivity of the metamaterial emitter exceeds 90% at the wavelength of 950-1590 nm and drops below 20% for wavelengths exceeding 2025 nm, which can effectively match GaSb photovoltaic cells. The outstanding emission performance is attributed to the coupling effect of surface plasmon resonance, cavity resonance and guided mode resonance, as evidenced by the analysis of electric and magnetic fields. We also explored the radiation spectrum in the 500-2500 K temperature range and found that it performed best at 1400 K. It is concluded that the emission performance is slightly affected by structural parameters and angles. This study presents a meaningful exploration of efficient solar utilization.

Core–shell structure selective emitter doped with rare earth elements for solar thermophotovoltaic system

Aiming at the low utilization of radiation photons in the solar thermophotovoltaic system, a rare earth core–shell (REC) structure selective thermal emitter is designed to achieve selective emission. The 3D symmetrical opal core–shell structure and rare earth elements play an important role in the selective emission characteristic of the REC emitter. The opal core–shell structure of the REC emitter is optimized by finite difference time domain simulation and numerical calculation, and the optimal size is determined. The REC emitter can adjust the characteristic peak of rare earth elements to produce a narrow–band radiation peak with an emissivity approaching 1 at a wavelength just before the cutoff wavelength, and maintain low emissivity in the long and short–wave bands. The conversion efficiency of the solar thermophotovoltaic system based on the REC emitter and InGaAs cell is 17.29 % at 1400 K, and the power density can reach 428.3 mW/cm2. The mechanism of the selective emission characteristic of the REC emitter is revealed, and its emissivity is insensitive to polarization and incidence angle. The REC emitter further improves the selective emission characteristic and makes the radiation spectrum better match the thermophotovoltaic cell. The power density and conversion efficiency of the solar thermophotovoltaic system are increased by improving the utilization of the thermophotovoltaic cell to the radiation photon.

Evaluation of spectral regulation by selective emitter and filter under both ideal and actual conditions for solar thermophotovoltaic systems

Selective emitter and filter are different spectral modules in a solar thermophotovoltaic (STPV) system, whereas the guidelines of their applications for various operation conditions are lacking, especially when the thermalization of the photovoltaic cell is considered. To conduct a comprehensive analysis, an energy transfer model of the STPV system was established based on spectral modules under both ideal and actual conditions, and in particular the cooling consumption was considered. It is observed that when an emitter (E system) or emitter combined with a filter (E + F system) is used, the system efficiency slowly increases with increasing concentration ratio (CR). When a filter (F system) is utilized, an initial increase followed by a subsequent decrease in the system efficiency is shown. The InGaAs cell is suitable for the E + F system, the GaSb cell is preferred for the E system, and the Si cell is better suited for the F system. The cooling consumption occupies up to 1100 W, which can cause the system efficiency to be below 0, indicating that the generated power cannot drive the cooling device. In particular, ideal spectral regulation is not superior to the actual spectral regulation when PV cell thermalization is considered. A foundation for optimization of spectral modules and application of STPV systems can be established by this work.

Functional materials for solar thermophotovoltaic devices in energy conversion applications: a review

Fossil fuels are now used to meet over 80% of the world’s energy demands, but they have the disadvantages of being unsustainable economically and polluting the environment. Solar energy is also one of the most desired alternative forms of renewable energy due to the quantity of direct sunlight among these sources. Due to the difficulties with solar cells, less than 1% of this energy is harvested and transformed into electricity. Notably, solar thermal and photovoltaic systems are the traditional methods for converting solar energy into electricity. It can be challenging to turn the solar energy captured by these systems into power. In contrast to conventional conversion methods, which involve converting solar energy directly into electricity, this article conducts a thorough investigation of solar thermophotovoltaic devices and the high-tech materials used in solar thermophotovoltaic systems as a solution to the conversion challenges.

Performance analysis and selective emitter design for an efficient concentrated solar thermophotovoltaic system based on cavity-structured absorber and high-bandgap cell

In order to break through the limitation of solar photovoltaic (PV) cell efficiency, this study proposes a concentrated solar thermophotovoltaic (STPV) system based on cavity-structured absorber, which can reshape the solar spectrum to match PV cell. An energy-balance model of the system is established for performance analysis, indicating that the theoretical power generation efficiency of system can reach 52.25% when using ideal selective emitter to match Si cell with high bandgap. The efficiency of STPV system using Si cell is 12–13 percentage points higher than that using low-bandgap cells such as GaSb. Furthermore, a metamaterial selective emitter is designed based on Ta and Al2O3 to match the high-bandgap Si cell for the first time, the simulation method is also verified by comparison with previous studies. The concentrated STPV system which based on Si cell achieves an efficiency of 37.18% with the designed selective emitter. The available proportion of spectrum for Si cell increases from 16.52% to 72.69% through reshaping blackbody radiation by designed selective emitter. The results of this study point out that using cavity-structured absorber and metamaterial selective emitter with high-bandgap PV cell is an effective STPV conversion path with the advantages of simple structure and high theoretical efficiency. It provides reference and inspiration for the application of STPV system.

Highly efficient Vanadium Nitride based metasurface absorber/emitter for solar-thermophotovoltaic system

Highly efficient Vanadium Nitride based metasurface absorber/emitter for solar-thermophotovoltaic system

Spheroid-based optical cavities for tunable photon recycling and emitter temperature control in robust solar thermophotovoltaic systems

The theoretical efficiency of solar thermophotovoltaic (STPV) systems is much greater than their efficiencies achieved in practice. Optical cavities can improve the performance of STPV systems by increasing the emitter-to-PV cell view factor and by facilitating photon recycling, whereby photons are reflected back to the emitter. Photon recycling reduces losses and increases the temperature of the emitter, thereby increasing efficiency. Our study presents STPV systems comprising optical cavities in the form of oblate and prolate spheroids. The geometry of the optical cavity can be tuned to control the degree of photon recycling, emitter temperature, emission losses, and the emitter-to-PV cell effective view factor and separation distance without using complex nano- or microstructured materials or optical filters. Numerical analysis shows an optical cavity in the form of a prolate spheroid, prolate spheroid with a middle annular aperture specular reflector, and integrated oblate- and prolate-spheroid can be used to achieve efficiencies of 17.7%, 18.9%, and 22%, respectively, under solar irradiation at a concentration factor of 1500X. These robust spheroid-based optical cavities can be used to design improved STPV systems with increased durability and higher performance.

An ultra-broadband and wide-angle absorber based on a TiN metamaterial for solar harvesting.

The efficient absorption of solar spectrum radiation is the most critical step in solar thermal utilization. In this work, a near-perfect metamaterial solar absorber with broadband, wide angle, polarization insensitivity, and high-temperature resistance is proposed and investigated. The absorber takes advantage of the high melting point material, which consists of a TiN reflector, a SiO2 insulating layer, and a TiN ring array. In the spectral range of 300-2500 nm, an average absorption of 97.6% is achieved. The percentage of absorbed energy in the AM1.5 spectral radiation can reach 95.8%. The electric and magnetic field distributions show that the high absorption is attributed to the coupling effect of surface plasmon resonance, guided mode resonance and cavity resonance. Furthermore, the absorber is found to maintain high absorption performance at large angles of solar radiation and to be insensitive to polarization. The designed absorber maintains its broadband absorption performance well within certain geometric tolerances. This reduces the complexity and cost of manufacturing and facilitates practical applications. The research indicates that this work will benefit the design and application of solar thermal conversion and thermophotovoltaic systems.

Selective Emitter for Solar Thermophotovoltaic Applications

Selective Emitters (SEs) are the main components of solar thermophotovoltaic (STPV) systems; they act as intermediate thermal radiation emitters to shape the incident solar spectrum to match the wavelengths useful for the PV cell. In this work, we present the design, optimisation, fabrication, and characterisation of an SE based on a multilayer design made of SiNx, SiO2, and TiO2 layers. The SE is optimised to work with PV cells based on III-V semiconductors, such as GaSb, InGaAs, and InGaAsS, the bests suitable for SPTV applications. The fabricated SE shows an emitter efficiency (ηSE) of 50% if matched with a PV cell with an energy bandgap of 0.63 eV.

Broadband near unity absorption meta-structure for solar thermophotovoltaic systems and optical window applications.

Developing a meta-structure with near-unity absorbance in the visible and infrared spectra for solar energy harvesting, photodetection, thermal imaging, photo-trapping, and optical communications is a long-term research challenge. This research presents a four-layered (insulator-metal-insulator-metal) meta-structure unit cell that showed a peak absorbance of 99.99% at 288-300 nm and the average absorbance of 99.18% over the 250-2000 nm wavelength range in TE and TM modes, respectively. The symmetric pattern of the resonator layer shows polarization insensitivity with an average absorption of 99.18% in both TE and TM modes. Furthermore, the proposed design shows a wide incident angle stability up to ≤60 degrees in both TE and TM modes. The proposed structure also exhibits negative index properties at 288-300 nm and 1000-2000 nm, respectively. The negative index properties of the proposed design generate an anti-parallel surface current flow in the ground and resonator layers, which induces magnetic and electric field resonance and increases absorption. The performance of the proposed design is further validated by the interference theory model and a zero value for the polarization conversion ratio (PCR). The electric field E, magnetic field H, and current distribution are analyzed to explain the absorption mechanism of the proposed meta-structure unit cell. It also exhibits the highest photo-thermal conversion efficiency of 99.11%, demonstrating the viability of the proposed design as a solar absorber. The proposed design promises potentially valuable applications such as solar energy harvesting, photodetection, thermal imaging, photo-trapping, and optical communications because of its decent performance.

Broadband solar absorption by chromium metasurface for highly efficient solar thermophotovoltaic systems

We report a broadband metasurface solar absorber composed of refractory material chromium (Cr) for the intermediate structure of a solar thermophotovoltaic (STPV) system that can overcome the Shockley–Queisser limit for efficient solar energy harvesting. The metasurface absorber exhibits high broadband absorptance with an average higher than 90% for 300–1200 nm. The self-passivation property of chromium ensures resistance to oxidation and corrosion, besides the benefit of being a low-cost material having stability at higher temperatures. The proposed absorber forms one part of the intermediate structure, whereas the emitter forms its second part. The emitter shapes the incoming electromagnetic (EM) waves to have energies just above the PV cell bandgap to assist in efficient electron-hole pair generation. The proposed STPV system can achieve PV cell efficiency of 43.2% with an efficiency greater than 42% in a broad color temperature range of 1597–2573 K, which is comparable to the state-of-the-art. The innovation in our work comes from efficiency enhancement through the hybridization of selectivity and broadband response of the proposed absorbers and emitter. The designs are also subjected to detailed analyses to understand the mechanism behind their performance. Moreover, a robustness check is performed to know which design parameters are crucial to acquiring such results.

A multi-junction-based near-field solar thermophotovoltaic system with a graphite intermediate structure

A solar thermophotovoltaic (STPV) system can transform incident concentrated solar energy into electrical energy with an efficiency that could be higher than the Shockley–Queisser limit. Near-field thermophotovoltaic (NF-TPV) devices can generate larger electrical power output than traditional far-field TPV devices with the aid of photon tunneling. Moreover, multi-junction PV cells can boost the performance of TPV devices by effectively distributing the absorbed photon energy inside the PV cell. In this work, we design a multi-junction-based near-field STPV system with a practical and high-temperature stable graphite intermediate structure. To optimize the system configuration, we employ a genetic algorithm and a surrogate model based on an artificial neural network, which enables us to suggest a better design approach for the multi-junction-based NF-STPV system between the power output density and power conversion efficiency maximization scenarios. When the concentration factor of the incident solar energy is 5000 and the absorber-to-emitter area ratio is 3, we can achieve a system efficiency of 23%. By introducing a material whose emissivity is as high as a blackbody on the solar absorber, the system efficiency can be further enhanced up to 35%.

Construction and performance analysis of a solar thermophotovoltaic system targeting on the efficient utilization of AM0 space solar radiation

Solar thermophotovoltaic (STPV) has great potential as efficient power supply source for spacecraft to meet the demand of spacecraft miniaturization. In this work, a novel space STPV system is proposed to achieve the efficient utilization of the AM0 space solar radiation. Metamaterial structures were designed and FDTD method is used to analyze their radiation regulation mechanism. A multi-layer cylindrical periodic structure is used as the absorber which realizes a total absorptance of 0.9283 to AM0 radiation. A cylindrical periodic structure is used as the emitter to reshape the broadband thermal radiation as narrowband to match with the Si/InGaAsSb tandem cell, which realizes a highest TPV efficiency of 51.36%. System performance analysis is conducted and the system presents a highest STPV efficiency of 40.86% and good adaptability under wide range of operating parameters, which reveals its great potential to realize the efficient utilization of AM0 solar radiation for space power supply.

Nonreciprocal Solar Thermophotovoltaics

Compared to traditional solar cells, solar thermophotovoltaic (STPV) systems have the advantage of utilizing the full spectrum of solar radiation, but their thermodynamic efficiency is still far lower than the ultimate limit. This study shows that the efficiency deficit is caused by back emission from the intermediate layer toward the sun, resulting from the reciprocity of the system. The authors therefore propose using an intermediate layer with nonreciprocal radiative properties, which would suppress back emission and funnel more photons toward the cell, for a significant efficiency boost.

Highly confined spectrally selective absorber-emitter for effective solar thermophotovoltaics

A high efficiency in solar thermophotovoltaic (TPV) systems is achieved by recycling photons to the emitter in a cell with a high out-of-band reflectivity. Ideally, if all out-of-band photons can be recovered in a blackbody emitter, the effect would be equivalent to that when using a spectrally selective emitter. However, in practice, it is difficult to fully recover large number of out-of-band photons emitted from blackbody emitters. Meanwhile, spectrally selective emitters generally have lower thermal stability than blackbody emitters, resulting low in-band photon flux. Considering these practical constraints, our analysis demonstrates that the combination of spectrally selective absorbers and spectrally selective emitters has a higher system efficiency than the other cases using blackbody absorbers or emitters, even in systems that exhibit a high confinement performance at practical input densities of 200 W/cm2. A confined system with a high thermal radiation transport efficiency of 0.9 is constructed in which a tungsten cube coated with a hafnium oxide layer was used as a spectrally selective absorber and spectrally selective emitter. A total experimental system efficiency of 7.5% was achieved; using the model of the experimental system, we estimated a 13% system efficiency for a system containing a cell with Roob = 0.98, and a 19.5% system efficiency for a system with improved spectral selectivity of the absorber and emitter.