Photonic Crystal Selective Structures for Solar Thermophotovoltaics
Solar thermophotovoltaic (STPV) systems convert sunlight into electricity via thermal radiation. The efficiency of this process depends critically on both the selective absorber and the selective emitter, which are controlled by both the materials and the photonic design. For high concentration solar TPV applications, 2D photonic crystals (PhCs) made of refractory metals such as tungsten have demonstrated promising results. For even higher performance, we propose two photonic crystal based designs to both collect solar heat and reradiate above gap photons. First, a PhC selective structure (IPSS) which combines 2D photonic crystals and filters into a single device. Second, an Er Yb Tm co doped fused silica coated with a 17 bilayer structure also offers significant selectivity with greater ease of fabrication. Finite difference time domain (FDTD) and rigorous coupled wave analysis (RCWA) simulations show that both can significantly suppress sub bandgap photons. This increases sunlight to electricity conversion for photonic crystal based emitters above 24.3% at 100 suns concentration or 27% at 1000 suns concentration using a Ga0.42In0.58As PV diode with a bandgap of 0.7 eV (nearly lattice matched to InP) INTRODUCTION Solar thermophotovoltaic (STPV) systems convert sunlight into electricity via thermal radiation. The efficiency of STPV depends on the product of the selective absorber thermal transfer efficiency and the TPV system, which are controlled by both the materials and the photonic design. Metal dielectric composites like cermets and semiconductor metal tandem are strong candidates for operation of conditions such as 100 suns concentration and 1000 K. It is also demonstrated that with proper design and fabrication, single wall carbon nanotubes exhibit selective absorption as well. For artificial photonic structures, plasmonic selective absorbers were proposed to benefit from surface plasmon enhanced absorption. Photonic crystals made of refractory metals were also proposed to be highly efficient selective solar absorbers under high temperature 1000 K) and high concentrations 1000 suns) Nevertheless, it can be more challenging to design a selective solar absorber at higher temperatures 1500 K) and moderate solar concentrations 100 suns) for the highest performance and compatibility with the overall STPV system. One key challenge is that blackbody radiation at such high temperatures has strong overlap with the solar irradiance spectrum, making it extremely challenging to suppress thermal re radiation while maintaining solar absorption. From a materials perspective, system integration may be facilitated by the use of similar materials and structures in both the selective solar absorber and the selective emitter. Given that the spectral requirement for selective solar absorbers and selective emitters are different, achieving this property together with the prior two can be even harder. To address the challenges discussed above, we propose two different selective solar absorber and emitter designs based on the selective emitter designs proposed in The first one is a rare earth based selective solar absorber, while the second one is an integrated photonic crystal absorber (IPSA) For the first design, the naturally sharp transitions of rare earth elements constitute a sharp cutoff for the solar absorber, and the cutoff wavelength will be governed by the longest wavelength transition of the rare earth dopant. Typically, rare earth ions are doped in host materials with sufficiently low absorption. Commonly used host materials are glasses and ceramics, especially for lasers, amplifier fibers, and thermal emitters. We show that selecting the suitable dopants as well as a low loss host material is the key to design an efficient solar absorber without the need to include filters on top of the structure. We propose two rare earth designs with chirped quarter wave stack back reflectors suitable for operation under 100 suns and 1000 suns concentrations. For the second design, the IPSA combines a 2D photonic crystal with a chirped rugate filter into a single device. By directly stacking the rugate filter on top of the 2D PhC, the subbandgap photon recycling shifts to a non perturbative regime instead of a perturbative one. The net effect is a strong selectivity in the absorbance spectrum. We show that when the proposed IPSA is combined with an integrated photonic crystal selective emitter (IPSE) the STPV conversion efficiency can be as high as 24.3% RARE EARTH BASED SELECTIVE SOLAR ABSORBER Rare earth elements, such as Erbium, Thulium, Holmium and Ytterbium are known to have distinct atomic transitions identified in their absorption spectra. This property makes rare earths good candidates to provide sharp cut off in the solar absorber spectrum. In designing a rare earth based solar absorber, the selection of the host material, as well as the dopant element is crucial. First, dopants with atomic resonance wavelengths below the design cut off wavelength should be selected. Among all rare earths, Erbium, Ytterbium, Thulium and Holmium exhibit absorption peaks, mostly in the visible and near infrared that makes them good candidates for dopant materials. More specifically, co doping of Er and Yb will show a cutoff wavelength at 1.65 mm, while co doping Er, Yb, and Tm will show a cutoff wavelength at 1.9 mm. It is also important to increase the dopant concentration to maintain sufficient absorption for high energies using thick substrates, while keeping a minimal absorption at lower energies. Moreover, because the host material is naturally transparent, a back reflector is necessary for maximal absorption. A typical host material should exhibit sufficiently low absorption over a wide range of wavelengths extending in the mid infrared. A promising candidate is fluoride glass, such as CaF2 and BaF2, with transmission bands between 0.16 mm and 10 mm, and base absorption coefficient as low as 7.8e 4 cm with a sufficiently high melting point. Two designs of solar absorbers are presented here, suited for 100 suns and 1000 suns, respectively. Both designs utilize a 17 bilayer chirped Bragg mirror of indices 2.3 and 1.48, which reflects over a broad range of absorbed wavelengths to reduce transmission losses. The first design is a thick Er Yb doped CaF2 substrate of thickness 20 mm, reflective from 0.3 mm to 1.7 mm. The second design is an Er Yb Tm co doped fused silica, of thickness 8 mm, with reflection extending from 0.3 mm to 2 mm. Emissivity spectra are calculated based on extracted material optical parameters for rare earth transitions from, and obtained using the simulation tool: "Thermophotonic selective emitter simulations," available on nanoHUB.org, which simulates emissivity spectrum based on an open source rigorous coupled wave analysis (RCWA) simulation code 18 for multi layer rare earth based selective emitters/absorbers. The emissivity spectra for both designs are depicted in figure 1, along with the AM1.5D solar spectrum. Since the rare earth doped fused silica exhibits significant absorption starting at 4 mm, it will pose a limitation on the solar concentration requirement at 1500 K. Hence, greater solar concentration is required to overcome the significant spectral overlap. Note also that neither Er nor Yb show absorption peaks at 1.2 mm this gap reduces the maximum attainable thermal transfer efficiency. To assess the optimal operation conditions for the two given designs, a contour plot of the thermal transfer efficiency is shown for each design in Figure 2. Note that the thermal transfer efficiency is low in general with this design, since the coverage of the visible/near IR spectrum is not complete, and due to significant emission past 7 mm. Higher efficiencies are obtained with the second design, as in Figure 2(b) for high concentrations and low temperatures. Finally, the STPV efficiency is computed for both designs, using a Sm doped glass design of the emitter, similar to assuming an ideal solar concentration efficiency. Figure 3(a) shows the STPV efficiency for the given designs. We note that for lower temperatures, the TPV conversion efficiency is low, while for higher temperatures, the thermal transfer efficiency is low, giving rise to an optimal efficiency point. For the first design under illumination of 100 suns, the optimal temperature is 1350 K with maximal efficiency of 12.9% For the second design, under 1000 suns, the optimal temperature is 1500 K with maximal efficiency of 27% A contour plot of the STPV efficiency for the Er Yb doped CaF2 solar absorber is plotted in figure 3(b) The contour plot also shows a maximum efficiency of 19.1% achievable at 1650 K. The small remaining gaps in the visible/near IR absorption spectrum could be addressed through further optimization of the dopant profiles and substrate thickness. Figure 1 The emissivity spectrum of Er Yb Tm doped fused silica absorber (blue) and the emissivity spectrum of Er Yb doped CaF2 (green) The AM1.5D solar spectrum is plotted in red. Fused silica based absorber shows IR absorption edge at 3.7 mm, while the CaF2 based solar absorber shows extended flat minimal absorption band with an IR absorption edge at 7.5 mm that makes it a good candidate for low solar concentration (100 suns) The occupation of the spectrum also depends on the dopants, hence the cutoff for Er Yb (green) is at 1.65 mm, while the cutoff of Er Tm Yb (blue) is at 1.9 mm. Integrated Photonic Crystal Selective Structure Another approach that is easier to engineer is photonic design. It has been shown that an integrated photonic crystal is strongly selective in its emittance spectrum. Furthermore, its cutoff is in principle tunable with great flexibility. In order to achieve even higher STPV conversion efficiencies, we investigate an integrated photonic crystal structure (IPSS) that combines a selective absorber with a selective emitter as a sin