Solar Thermophotovoltaic System Research Articles

Thermally Stable Two-Dimensional Photonic Crystal for Selective Emitters

ABSTRACTA fundamental challenge in solar-thermal-electrical energy conversion is the thermal stability of materials and devices at high operational temperatures. This study focuses on the thermal stability of selective emitters for solar thermophotovoltaic (STPV) systems to enhance the conversion efficiency. 2-D photonic crystals are periodic micro/nano-scale structures that are designed to affect the motion of photons at certain wavelengths. The structured patterns, however, lose their structural integrity at high temperature, which disrupts the tight tolerances required for spectral control of the thermal emitters. Through analytical studies and experimental observations, the four major mechanisms of thermal degradation of 2-D photonic crystal are identified: oxidation, grain growth and re-crystallization, surface diffusion, and evaporation and re-condensation. In this work, the design of a flat surface photonic crystal (FSPC) is proposed and experimental validations are performed.

Modeling and Simulation of a Thermophotovoltaic System with NaF Heat Storage

This paper presents the modeling and simulations of a solar thermophotovoltaic (STPV) system with sodium fluoride (NaF) as thermal storage material. A Cassegrainian optical concentrator is modeled using Ray Tracing to determine its optical efficiency under realistic solar irradiance data. Also, a tungsten photonic crystal (W PhC) selective emitter with a Germanium (Ge) PV cell are simulated using PC1D software where the cell efficiency and power density are evaluated. Thermal analysis is carried out using COMSOL-Multiphysics to study the system thermal behavior and determine the overall system efficiency and power output. The results show that the designed system can reach 85.2% solar to thermal efficiency and a corresponding 11.9% efficiency for the Ge cell. We also concluded from the thermal analysis that NaF does not allow a uniform temperature distribution inside the system due to its low thermal conductivity.

Development and experimental evaluation of a complete solar thermophotovoltaic system

ABSTRACTWe present a practical implementation of a solar thermophotovoltaic (TPV) system. The system presented in this paper comprises a sunlight concentrator system, a cylindrical cup‐shaped absorber/emitter (made of tungsten coated with HfO2), and an hexagonal‐shaped water‐cooled TPV generator comprising 24 germanium TPV cells, which is surrounding the cylindrical absorber/emitter. This paper focuses on the development of shingled TPV cell arrays, the characterization of the sunlight concentrator system, the estimation of the temperature achieved by the cylindrical emitters operated under concentrated sunlight, and the evaluation of the full system performance under real outdoor irradiance conditions. From the system characterization, we have measured short‐circuit current densities up to 0.95 A/cm2, electric power densities of 67 mW/cm2, and a global conversion efficiency of about 0.8%. To our knowledge, this is the first overall solar‐to‐electricity efficiency reported for a complete solar thermophotovoltaic system. The very low efficiency is mainly due to the overheating of the cells (up to 120 °C) and to the high optical concentrator losses, which prevent the achievement of the optimum emitter temperature. The loss analysis shows that by improving both aspects, efficiencies above 5% could be achievable in the very short term and efficiencies above 10% could be achieved with further improvements. Copyright © 2012 John Wiley & Sons, Ltd.

Global optimization of solar thermophotovoltaic systems

ABSTRACTIn this paper, we present a theoretical model based on the detailed balance theory of solar thermophotovoltaic systems comprising multijunction photovoltaic cells, a sunlight concentrator and spectrally selective surfaces. The full system has been defined by means of 2n + 8 variables (being n the number of sub‐cells of the multijunction cell). These variables are as follows: the sunlight concentration factor, the absorber cut‐off energy, the emitter‐to‐absorber area ratio, the emitter cut‐off energy, the band‐gap energy(ies) and voltage(s) of the sub‐cells, the reflectivity of the cells' back‐side reflector, the emitter‐to‐cell and cell‐to‐cell view factors and the emitter‐to‐cell area ratio. We have used this model for carrying out a multi‐variable system optimization by means of a multidimensional direct‐search algorithm. This analysis allows to find the set of system variables whose combined effects results in the maximum overall system efficiency. From this analysis, we have seen that multijunction cells are excellent candidates to enhance the system efficiency and the electrical power density. Particularly, multijunction cells report great benefits for systems with a notable presence of optical losses, which are unavoidable in practical systems. Also, we have seen that the use of spectrally selective absorbers, rather than black‐body absorbers, allows to achieve higher system efficiencies for both lower concentration and lower emitter‐to‐absorber area ratio. Finally, we have seen that sun‐to‐electricity conversion efficiencies above 30% and electrical power densities above 50 W/cm2 are achievable for this kind of systems. Copyright © 2012 John Wiley & Sons, Ltd.

Metamaterial-based integrated plasmonic absorber/emitter for solar thermo-photovoltaicsystems

We present the concept of a solar thermo-photovoltaic (STPV) collection system based ona large-area, nanoimprint-patterned film of plasmonic structures acting as an integratedsolar absorber/narrow-band thermal emitter (SANTE). The SANTE film concept is basedon integrating broad-band solar radiation absorption with selective narrow-band thermalIR radiation which can be efficiently coupled to a photovoltaic (PV) cell for powergeneration. By employing a low reflectivity refractory metal (e.g., tungsten) as a plasmonicmaterial, we demonstrate that the absorption spectrum of the SANTE film can bedesigned to be broad-band in the visible range and narrow-band in the infraredrange. A detailed balance calculation demonstrates that the total STPV systemefficiency exceeds the Shockley–Queisser limit for emitter temperatures aboveTe = 1200 K, and achieves an efficiency as high as 41% forTe = 2300 K. Emitter temperatures in this range are shown to be achievable under modest sunconcentrations (less than 1000 suns) due to the thermal insulation provided by the SANTEfilm. An experimental demonstration of the wide-angle, frequency-selective absorptivity ispresented.

Design and global optimization of high-efficiency solar thermal systems with tungsten cermets

Solar thermal, thermoelectric, and thermophotovoltaic (TPV) systems have high maximum theoretical efficiencies; experimental systems fall short because of losses by selective solar absorbers and TPV selective emitters. To improve these critical components, we study a class of materials known as cermets. While our approach is completely general, the most promising cermet candidate combines nanoparticles of silica and tungsten. We find that 4-layer silica-tungsten cermet selective solar absorbers can achieve thermal transfer efficiencies of 84.3% at 400 K, and 75.59% at 1000 K, exceeding comparable literature values. Three layer silica-tungsten cermets can also be used as selective emitters for InGaAsSb-based thermophotovoltaic systems, with projected overall system energy conversion efficiencies of 10.66% at 1000 K using realistic design parameters. The marginal benefit of adding more than 4 cermet layers is small (less than 0.26%, relative).

Investigation of the entropy generation and efficiency of a solar thermophotovoltaic system

Based on the second law of thermodynamics, a theoretical model is established for efficiency analysis and optimization of a solar thermophotovoltaic (STPV) system. The entropy generation in the photo-electricity conversion process is numerically calculated and analyzed for the component modules involved in the whole system assembly. The effects of the emitter temperature, concentration ratio, energy bandgap, cell temperature and the transmissivity of the filter on the entropy and the efficiency are investigated. Meanwhile, the conversion efficiencies of the modules are discussed. The following results are obtained: at first the entropy generation of the system reduces but then increases with the rise of the emitter temperature. When the concentration ratio improves, the entropy generation of the system falls. The increase of the bandgap value and chemical potential of the cell also causes the reduction of the entropy generation. Along with the increase of the cell temperature, the entropy generation increases and the conversion efficiency of the STPV system decreases.

Design and analysis of solar thermophotovoltaic systems

Solar thermophotovoltaic (STPV) is a light-electricity conversion system consisting of a solar concentrator, an emitter, a filter, PV cells and the cooling system. This paper is aimed at establishing a method for both purposes of design and performance analysis of the STPV device, which consists of a set of governing equations integrating all these components. The effects of several structural and operational parameters on the performance of the STPV system are investigated. The concentrator performance corresponding to the non-parallelism of solar rays, different aperture ratios and tracking errors is analyzed. A 3-D model is constructed to investigate heat transfer process in the thermo-electric conversion system. The performance of the spectral filter is studied. In addition, the effect of the cooling subsystem on the STPV is analyzed. Numerical simulation suggests that the emitter temperature seriously affects the system performance. An optimized non-periodic filter will remarkably improve the spectral efficiency of the STPV. As an example, an optimized design and analysis of the whole system is carried out in this article.

Detailed balance analysis of solar thermophotovoltaic systems made up of single junction photovoltaic cells and broadband thermal emitters

This paper presents a detailed balance analysis of a solar thermophotovoltaic system comprising an optical concentrator, a cut-off broad band absorber and emitter, and single junction photovoltaic cells working at the radiative limit with an integrated back-side reflector in a configuration in which the cells enclose the emitter to form an optical cavity. The analysis includes the effect of multiple variables on the system performance (efficiency and electrical power density), such as the concentration factor, the emitter-to-absorber area ratio, the absorber and emitter cut-off energies, the semiconductor band-gap energy and the voltage of the cells. Furthermore, the effect of optical losses within the cavity such as those attributed to a back-side reflector with reflectivity lower than one or to a semi-open optical cavity is also included. One of our main conclusions is that for a planar system configuration (the emitter, the cells and the absorber have the same area) the combination of low concentration and a spectrally selective absorber provides the highest system efficiencies. The efficiency limit of this kind of systems is 45.3%, which exceeds the Shockley–Queisser limit of 40.8% (obtained for a single junction solar cell, directly illuminated by the sun, working under maximum concentration and with an optimized band-gap). This system also has the great benefit of requiring a very low concentration factor of 4.4 suns.

Evaluation of thermal radiation dependent performance of GaSb thermophotovoltaic cell based on an analytical absorption coefficient model

Applying the model dielectric function method, we have expressed the absorption coefficient of GaSb analytically at room temperature relating to the contribution of various critical points of its electronic band structure. The calculated absorption spectrum shows good agreement with the reported experimental data obtained by spectral ellipsometry on nominally undoped sample. Based on this analytical absorption spectrum, we have qualitatively evaluated the response of active absorbing layer structure and its photoelectric conversion properties of GaSb thermophotovoltaic device on the perturbation of external thermal radiation induced by the varying radiator temperature or emissivity. Our calculation has demonstrated that desirable thickness to achieve the maximum conversion efficiency should be decreased with the increment of radiator temperature and the performance degradation brought by any structure deviation from its optimal one would be stronger meanwhile. For the popular radiator temperature, no more than 1500 K in a real solar thermophotovoltaic system, and typical doping profile in GaSb cell, a reasonable absorbing layer structure parameter should be controlled within 100–300 nm for the emitter while 3000–5000 nm for the base.

Absorber and emitter for solar thermo-photovoltaic systems to achieve efficiency exceeding the Shockley-Queisser limit

We present theoretical considerations as well as detailed numerical design of absorber and emitter for Solar Thermophotovoltaics (STPV) applications. The absorber, consisting of an array of tungsten pyramids, was designed to provide near-unity absorptivity over all solar wavelengths for a wide angular range, enabling it to absorb light effectively from solar sources regardless of concentration. The emitter, a tungsten slab with Si/SiO(2) multilayer stack, provides a sharp emissivity peak at the solar cell band-gap while suppressing emission at lower frequencies. We show that, under a suitable light concentration condition, and with a reasonable area ratio between the emitter and absorber, a STPV system employing such absorber-emitter pair and a single-junction solar cell can attain efficiency that exceeds the Shockley-Queisser limit.

Thermal analysis and optimization of the emitter in the solar thermophotovoltaic (STPV)

A model for predicting the performance of the emitter in the solar thermophotovoltaic (STPV) system is presented in this article. The effect of non-parallelism of sun rays on concentration capability is numerically calculated, also the flow field in the emitter cavity and the temperature distribution of the emitter with different inlet conditions are compared. Numerical results show that free convection of the air inside the emitter cavity has great effect on the emitter temperature and may reduce the electricity output of the cells. At last, a new kind of selective film is put forward. Through optimizing the cut-off wavelength of a selective film, radiation loss is further reduced and system efficiency is improved.

Analytical model of solar thermophotovoltaic systems with cylindrical symmetry: Ray tracing approach

AbstractAn analytical model for a full solar thermophotovoltaic (STPV) system with cylindrical symmetry is presented. In contrast to the already published TPV models, this model considers both TPV optical cavity and absorber (or heating) systems. An analytical ray‐tracing method has been used to analyse the radiation interchange within a TPV optical cavity. It allows us to calculate the portion of the absorbed power in each component within the system. Using this model, a broad analysis of an STPV system has been carried out, finding the optimum configurations and presenting a detailed loss analysis. Copyright © 2009 John Wiley & Sons, Ltd.

Investigation on performance of a solar thermophotovoltaic system

In the light of the thermo-electric conversion principle, a model for predicting the performance of a solar thermophotovoltaic system is presented. The temperature distributions of the emitter for different concentrator ratios are numerically computed and the influence of the concentrator ratio on the system performance is analyzed. Numerical results show that the emitter temperature and the system efficiency increase with increase of the concentrator ratio. The effects of some other factors such as the spectral filter, cell temperature, and the reflectivity of the top and bottom surfaces of the emitter on the conversion performance of the STPV system are discussed.

Emitter Formation and Contact Realization by Diffusion for Germanium Photovoltaic Devices

Standalone germanium solar cells are under development for application in high-efficiency mechanically stacked multijunction solar cells and thermophotovoltaic systems. To realize a suitable device, a more fundamental research has been done on germanium doping, surface passivation, and contact formation. In this paper, emitter formation and contact realization are discussed in detail. Emitter formation is done by phosphorous diffusion from a spin-on dopant (SOD) source. Critical parameters are the diffusion time, diffusion temperature, and phosphorous content in the SOD. Front contact formation is done by an innovative method, where the contacting metal is diffused through the amorphous silicon passivation layer. The specific properties of the diffusing metal, diffusion temperature, and diffusion time are important. Using the developed process, a stand-alone germanium solar cell has been realized with a world-class AM1.5G energy conversion efficiency of 7.8%

Solar Thermophotovoltaic Converters Based on Tungsten Emitters

Results of a solar thermophotovoltaic (STPV) system study are reported. Modeling of the STPV module performance and the analysis of various parameters influencing the system are presented. The ways for the STPV system efficiency to increase and their magnitude are considered such as: improvement of the emitter radiation selectivity and application of selective filters for better matching the emitter radiation spectrum and cell photoresponse; application of the cells with a back side reflector for recycling the sub-band gap photons; and development of low-band gap tandem TPV cells for better utilization of the radiation spectrum. Sunlight concentrator and STPV modules were designed, fabricated, and tested under indoor and outdoor conditions. A cost-effective sunlight concentrator with Fresnel lens was developed as a primary concentrator and a secondary quartz meniscus lens ensured the high concentration ratio of ∼4000×, which is necessary for achieving the high efficiency of the concentrator–emitter system owing to trap escaping radiation. Several types of STPV modules have been developed and tested under concentrated sunlight. Photocurrent density of 4.5A∕cm2 was registered in a photoreceiver based on 1×1cm2GaSb cells under a solar powered tungsten emitter.

Development of GaSb Photoreceiver Arrays for Solar Thermophotovoltaic Systems

In comparison to conventional solar photovoltaics, where sun radiation is converted into electricity directly by solar cells, solar thermophotovoltaic (STPV) conversion has some specific advantages. These advantages come from the fact that in thermophotovoltaics the photon radiator is always inside the conversion system and near the photovoltaic cells. For these reasons we are developing small prototypes with sun heated emitters and photoreceiver arrays to be installed inside complete STPV systems. In order to achieve these complete STPV systems, the first step is to determine the optimum way of packaging the TPV cells into STPV arrays, choosing the best series/parallel configurations depending on the TPV cell band gap, the size of arrays, and the materials. This is the goal of this paper. To carry out the calculations, 18 and 24 cell arrays have been connected following different series and parallel configurations, using the PSPICE commercial circuit-simulation software. Each TPV cell is simulated as a block consisting of the well-known photogenerated current source, two dark diodes of ideality factors equal to one and two, and two resistances, one in parallel and the other in series. As a result, recommendations about the size and front grid design of the GaSb cells are obtained. When the optimally designed cells are connected to be included in two specific systems, recommendations about the best parallel/series connection are achieved. Evaluation on the performance of the arrays at nonuniform illumination is also carried out. The first photoreceiver arrays are being constructed and implemented in real STPV systems following these recommendations.

1515 周期的表面微細構造による高融点金属からの熱放射スペクトル制御

Control of optical properties of materials has been investigated for efficient thermal energy utilization. Applying the resonant effect between periodic microstructured surface structure and photons, the spectrally selective thermal radiation surface, this is so-called selective emitter, is made on Si and high temperature metals by means of micro-machining techniques. These selective emitters can be applied to Solar thermophotovoltaic systems selective solar absorber, and sky radiator etc. Thermal radiation spectra of the emitters are measured at high temperatures and selective emission due to the surface microstructures is clearly observed. Numerical calculations based on rigorous coupled-wave analysis (RCWA) are also performed to predict spectral properties of the emitters.

Large area solar cells : Elliott, James F., Meikleham, Mrs. V. F. (1290-60)

By converting broadband sunlight into narrowband thermal radiation matched to the bandgap of thermophotovoltaic (TPV) cells, solar thermophotovoltaic (STPV) systems could potentially reach a high conversion efficiency far exceeding the Shockley–Queisser limit. However, actual STPV systems exhibit much lower efficiency due to non-idealities in solar absorbers, thermal emitters and TPV cells. In this work, the STPV system with selective metamaterial solar absorber and emitter is investigated, whose conversion efficiency is between 8% and 10% with concentration factor varying between 20 and 200. This conversion efficiency is remarkably enhanced compared with the conversion efficiency of less than 2.5% for the STPV system employing black absorbers and emitters. The sidewall emission losses from the absorber–emitter module and the non-unity view factor between the thermal emitter and TPV cell will diminish the performance of the STPV system, whose effects are also quantitatively discussed in this work. Furthermore, the non-planar STPV systems with larger emitter–absorber area ratios are investigated, whose conversion efficiency can reach up to 12.6% under 200 suns when the emitter is four times as large as the absorber.