Michael Ghebrebrhan

Design and global optimization of high-efficiency thermophotovoltaic systems

Despite their great promise, small experimental thermophotovoltaic (TPV) systems at 1000 K generally exhibit extremely low power conversion efficiencies (approximately 1%), due to heat losses such as thermal emission of undesirable mid-wavelength infrared radiation. Photonic crystals (PhC) have the potential to strongly suppress such losses. However, PhC-based designs present a set of non-convex optimization problems requiring efficient objective function evaluation and global optimization algorithms. Both are applied to two example systems: improved micro-TPV generators and solar thermal TPV systems. Micro-TPV reactors experience up to a 27-fold increase in their efficiency and power output; solar thermal TPV systems see an even greater 45-fold increase in their efficiency (exceeding the Shockley-Quiesser limit for a single-junction photovoltaic cell).

Enabling high-temperature nanophotonics for energy applications

The nascent field of high-temperature nanophotonics could potentially enable many important solid-state energy conversion applications, such as thermophotovoltaic energy generation, selective solar absorption, and selective emission of light. However, special challenges arise when trying to design nanophotonic materials with precisely tailored optical properties that can operate at high-temperatures (> 1,100 K). These include proper material selection and purity to prevent melting, evaporation, or chemical reactions; severe minimization of any material interfaces to prevent thermomechanical problems such as delamination; robust performance in the presence of surface diffusion; and long-range geometric precision over large areas with severe minimization of very small feature sizes to maintain structural stability. Here we report an approach for high-temperature nanophotonics that surmounts all of these difficulties. It consists of an analytical and computationally guided design involving high-purity tungsten in a precisely fabricated photonic crystal slab geometry (specifically chosen to eliminate interfaces arising from layer-by-layer fabrication) optimized for high performance and robustness in the presence of roughness, fabrication errors, and surface diffusion. It offers near-ultimate short-wavelength emittance and low, ultra-broadband long-wavelength emittance, along with a sharp cutoff offering 4∶1 emittance contrast over 10% wavelength separation. This is achieved via Q-matching, whereby the absorptive and radiative rates of the photonic crystal’s cavity resonances are matched. Strong angular emission selectivity is also observed, with short-wavelength emission suppressed by 50% at 75° compared to normal incidence. Finally, a precise high-temperature measurement technique is developed to confirm that emission at 1,225 K can be primarily confined to wavelengths shorter than the cutoff wavelength.

Tailoring photonic metamaterial resonances for thermal radiation

AbstractSelective solar absorbers generally have limited effectiveness in unconcentrated sunlight, because of reradiation losses over a broad range of wavelengths and angles. However, metamaterials offer the potential to limit radiation exchange to a proscribed range of angles and wavelengths, which has the potential to dramatically boost performance. After globally optimizing one particular class of such designs, we find thermal transfer efficiencies of 78% at temperatures over 1,000°C, with overall system energy conversion efficiencies of 37%, exceeding the Shockley-Quiesser efficiency limit of 31% for photovoltaic conversion under unconcentrated sunlight. This represents a 250% increase in efficiency and 94% decrease in selective emitter area compared to a standard, angular-insensitive selective absorber. PACS: 42.70.Qs; 81.05.Xj; 78.67.Pt; 42.79.Ek

Tetrastack: Colloidal diamond-inspired structure with omnidirectional photonic band gap for low refractive index contrast

Omnidirectional photonic band gaps opening at low values of refractive index contrast have been found for a nonspherical colloid-based photonic crystal structure. A mechanically stable design is described for the diamondlike photonic crystal composed of colloidal tetrahedra. The proposed tetrastack structure displays omnidirectional 2–3 band gap over a large range of filling fractions, refractive index contrasts, and building block orientations. The threshold refractive index for the inverted tetrastack structure was 1.94. A gap width of 25.3% relative to the center frequency was obtained for an inverted tetrastack with a 0.21 filling fraction of silicon.

Dimer shape anisotropy: a nonspherical colloidal approach to omnidirectonal photonic band gaps.

Theoretical calculations of the photonic band gap forming properties are reported for a class of colloidal dimer-based structures with similarity to zinc blende and which map onto diamond or opalline face-centered cubic structures at the extrema in shape parameters. Inspired by the range of nonspherical building blocks for self-assembly synthesized using seeded emulsion polymerization and sol-gel techniques, we explore in particular the band structures as a function of dimer lobe symmetry and the degree of lobe interpenetration for tangent dimers. Complete photonic band gaps were observed between the second and third, fifth and sixth, or eighth and ninth bands for various shape classes. As well, select inverted and direct dimer-based structures showed two complete band gaps simultaneously.

Global optimization of silicon photovoltaic cell front coatings

The front-coating (FC) of a solar cell controls its efficiency, determining admission of light into the absorbing material and potentially trapping light to enhance thin absorbers. Single-layer FC designs are well known, especially for thick absorbers where their only purpose is to reduce reflections. Multilayer FCs could improve performance, but require global optimization to design. For narrow bandwidths, one can always achieve nearly 100% absorption. For the entire solar bandwidth, however, a second FC layer improves performance by 6.1% for 256 microm wafer-based cells, or by 3.6% for 2 microm thin-film cells, while additional layers yield rapidly diminishing returns.

Hafnia-plugged microcavities for thermal stability of selective emitters

Two-dimensional arrays of micro-cavities effectively control photon motion and selectively emit radiation tailored to the preferred bandgap of photovoltaic (PV) cells, thus enhancing the efficiency of thermophotovoltaic energy conversion. At the high operating temperatures, however, the micro- and nano-patterned structures of the selective emitters quickly lose their integrity––obliterating the tight tolerances required for precise spectral control. Even if oxidation, recrystallization, and grain growth could be avoided with single-crystal tungsten (W) selective emitters with vacuum packaging, surface diffusion, evaporation, and re-condensation are not avoidable in long-term operation at high temperatures. The concept of a planar array of plugged micro-cavities to suppress the curvature-dependent thermal degradation modes is proposed and tested. Based on scale-accelerated failure tests of silicon devices, the lifetime of W selective emitters operating at 1100 K is estimated to be at least 30 yr.

Textile Frequency Selective Surface

Frequency selective surfaces (FSSs) are ubiquitous on rigid substrates and increasingly on flexible polymeric substrates. Here, we developed a FSS on a textile which is neither rigid nor smooth. We fabricate a textile capable of rejecting the millimeter-wave radiation in a narrowband, while retaining desirable textile properties such as flexibility and breathability. The resonators and resonant wavelength are on the order of the weave pitch. Durability tests are performed and spectral response is measured.

Sputtered Tantalum Photonic Crystal Coatings for High-Temperature Energy Conversion Applications

Thick sputtered tantalum (Ta) photonic crystal (PhC) coatings on Inconel were investigated as a potential replacement for bulk refractory metal substrates used for high-temperature emitters and absorbers in thermophotovoltaic energy conversion applications, where high-temperature stability and high reflectance of the surface in the infrared wavelength range are critical in order to sustain high operational temperatures and reduce losses due to waste heat. A selective emitter and solar absorber 2D PhC were fabricated in 8 and 30 micron sputtered Ta coatings, respectively, using standard semiconductor processes as a proof of concept. The fabricated PhCs showed high spectral selectivity in good agreement with the numerical simulations. The PhCs, coated with a thin HfO2 protective layer, sustained one hour anneals at 700, 900, and 1100°C with very little structural degradation or change in their optical properties. This study presents a promising alternative to bulk substrates as a relatively low-cost and easily integrated platform for nano-structured devices for high-temperature applications.

Tunable millimeter and sub-millimeter spectral response of textile metamaterial via resonant states

We report on a new textile metamaterial created by adding metal wires directly into the polymer yarn. Split-ring resonator-like extended states are created. Simulations revealed that the extended states can be easily tuned via the geometry. Measurements of the transmittance spectrum as a function of the polarization angle in the low terahertz range were also performed and these peaks were ascribed to a polarization-dependent resonator model. The fabrics are viable candidates for flexible and deformable gigahertz and terahertz-enabled metamaterials.