|Research Areas & Activities Solar Energy Rational Organic Semiconductor Material Design A Pathway Towards Breakthrough Performance in Solar Cells Design and Fabrication of the First All-Carbon-Based Solar Cell Upconverting Electrodes for Improved Solar Energy Conversion Advanced Electron Transport Materials for Application in Organic Photovoltaics (OPV) Ultra-High Efficiency Thermophotovoltaic Solar Cells Using Metallic Photonic Crystals as Intermediate Absorber and Emitter Nanostructured Materials for High-Efficiency Thin Film Solar Cells Photon Enhanced Thermionic Emission (PETE) for Solar Concentrator Systems Hot Carrier Solar Cell: Implementation of the Ultimate Photovoltaic Converter Plasmonic Photovoltaics Self-sorting of Metallic Carbon Nanotubes for High Performance Large Area Low Cost Transparent Electrodes Artificial Photosynthesis: Membrane-Supported Assemblies that Use Sunlight to Split Water Lateral Nanoconcentrator Nanowire Multijunction Photovoltaic Cells Molecular Solar Cells Advanced Materials and Devices for Low-Cost and High-Performance Organic Photovoltaic Cells Inorganic Nanocomposite Solar Cells by ALD Nanostructured Silicon-Based Tandem Solar Cells Photosynthetic Bioelectricity Nanostructured Metal-Organic Composite Solar Cells Ordered Bulk Heterojunction Photovoltaic Cells Biomass Energy Hydrogen Advanced Combustion CO2 Capture CO2 Storage Advanced Materials & Catalysts Advanced Coal Advanced Transportation Advanced Electric Grid Grid Storage Other Renewables Integrated Assessment Advanced Nuclear Energy Geoengineering Exploratory Efforts All Activities Analysis Activities Technical Reports||
Ultra-High Efficiency Thermophotovoltaic Solar Cells Using Metallic Photonic Crystals as Intermediate Absorber and Emitter
Start Date: September 2008
Shanhui Fan and Peter Peumans, Electrical Engineering, Stanford University; Paul Braun, Material Science and Engineering, University of Illinois at Urbana-Champaign
This research is investigating a novel design for thermophotovoltaic devices with the potential to boost their energy conversion efficiency beyond current efficiency records of about 30%. In the proposed design, the absorber and the emitter consist of tungsten photonic crystals whose properties can be tailored to provide broad-band absorption of light over the entire solar spectrum and to adjust the emission spectrum to match the absorption characteristics of a silicon photovoltaic cell. The absorber and the emitter are integrated within the intermediate to optimize thermal transfer.
The thermophotovoltaic (TPV) concept reduces the energy losses associated with the absorption of light in a conventional cell by tailoring the average energy of the photons absorbed by it. In this scheme (see Fig. 1), the solar cell is not directly exposed to the solar flux. Instead, an intermediate – consisting of two thermally connected layers, the absorber and the emitter – is positioned in front of the cell. The receiver heats up to a reasonably high temperature by absorbing sunlight. Subsequently, thermal energy is transferred to the emitter that illuminates the cell with narrow-band thermal radiation that better matches its absorption characteristics. Conventional TPV designs include a filter between the emitter and the cell to back-reflect the photons with energy lower than the cell bandgap. This way, their energy is recycled to maintain the temperature of the intermediate. The absorption and emission spectra of the intermediate can be manipulated to improve performance, limited by a theoretical efficiency maximum of 85% under direct illumination.
In the system studied in this project the receiver and the emitter are integrated within a single intermediate structure and consist of metallic photonic crystals that can be designed to tailor both the absorption and emission spectra as a function of the illumination conditions (direct or concentrated solar light) and of the bandgap of the PV cell, so that a filter is not required.
The tungsten photonic crystals forming the intermediate structure (see Fig. 2) are fabricated using low-cost techniques such as colloidal self-assembly and holographic patterning. Their structural parameters are optimized through extensive computational design in order to maximize optical absorption over the entire solar spectrum, narrow-band thermal emission, angular response, and thermal transfer between the absorber and the emitter.
Geometry also determines other critical properties such as the angular variability of absorptivity and emissivity, and the thermal transfer efficiency between the absorber and the emitter. While emission should be confined to a narrow angular range for maximal transmission to the silicon cell, the optimal angular behavior of the absorber depends on the operation conditions – high concentration or direct, non-concentrated sunlight. Initial prototypes are designed with omni-directional blackbody absorption characteristics to operate at high concentration. Finally, the efficiency of thermal transfer within the intermediate, which depends on the width ratio between the solar and the emission spectra, can be maximized by optimizing the absorber/emitter surface ratio.
Practical devices will require materials and structures capable of withstanding extended operation times at temperatures in excess of 2000oC and frequent thermal cycling. Tungsten and other high melting-point materials (such as tantalum, molybdenum, and silicon carbides) are investigated. Alloying and thin-coating with specific protective layers are also explored to increase the device stability in these highly demanding operation conditions.
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