Thermally Stable Two-Dimensional Photonic Crystal for Selective Emitters
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Thermally Stable Two-Dimensional Photonic Crystal for Selective Emitters Heon J. Lee1, Stephen P. Bathurst2 and Sang-Gook Kim3 1
Center for Computation Science, Korea Institute of Science and Technology, Seoul, 136-791, Korea. 2 Luxvue Technology Corporation, Mountain View, CA 94035, U.S.A. 3 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A. ABSTRACT A 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. INTRODUCTION The solar thermophotovoltaic (STPV) system is a novel approach to overcome ShockleyQueisser limit of photovoltaic (PV) systems through the control of absorption and emission spectra. Since STPV is not subject to the same limitations as conventional PV systems, STPV has the potential to greatly improve energy conversion efficiency in commercial products [1-2]. In STPV, a broad range of solar radiation can be absorbed with a selective absorber and the absorbed energy is converted to heat and transferred to the selective emitters. Selective emitters can selectively emit the wavelength shorter than the band-gap wavelength of PV diodes through a process called Q-matching [3]. Q-matching is achieved with periodic photonic crystal structures and selectivity is improved with increasing micro/nano-scale structure dimension. 2-D photonic crystals are most common choice for emitters based on their manufacturability, low cost, and Q-matching performance. By controlling the emission spectrum, the maximum theoretical conversion efficiency of 85.4% can be achieved with emitter temperature at 2271 oC [4]. The system’s conversion efficiency is determined by the cut-off wavelength of the selective emitter, band-gap of PV diode, and operating temperature of the selective emitters. After Q-matching, the conversion efficiency is only function of emitter’s temperature, and the efficiency proportionally increases with the 4th power of emitter’s operating temperature [5]. For increased efficiency, the emitter must operate at elevated temperatures. However, the micro/nano-scale structures used for controlling emission spectrum degrade quickly at temperatu
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