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Gradient epsilon-near-zero (ENZ) metamaterials offer broadband directional control over thermal emission. Implementing this approach using materials that remain stable in harsh thermo-chemical environments would allow it to be broadly deployed in thermal photonics. Our prior work showed that heterostructures of rock salt MgO and perovskite BaZr0.5Hf0.5O3 (BZHO) are stable up to 1100 °C in air, with no discernible intermixing. In this work, we design a gradient ENZ metamaterial made from three lattice-matched refractory oxides: MgO, BZHO, and NiO. The miscibility of MgO and NiO makes it possible to linearly vary the ENZ frequency of the metamaterial layers. BZHO is used as a thin, interlayer diffusion barrier. We model the emissivity of our gradient ENZ metamaterial at 25 and 1000 °C to demonstrate that the spectral bandwidth of directional emission is preserved at high temperatures despite changes in the optical properties of each material. Finally, we discuss practical fabrication challenges associated with the back reflector and offer potential solutions based on advancements in hetero-integration. Overall, this work shows a pathway toward gradient ENZ metamaterials with ultrahigh-temperature stability. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of molecular junctions are of considerable interest due to their promise for efficient energy conversion. While the dependence of thermoelectric properties of junctions on molecular structure has been recently studied, their temperature dependence remains unexplored. Using a custom built variable temperature scanning tunneling microscope, we measured the thermopower and electrical conductance of individual benzenedithiol junctions over a range of temperatures (100K–300K). We find that while the electrical conductance is independent of temperature, the thermopower increases linearly with temperature, confirming the predictions of the Landauer theory. [ABSTRACT FROM AUTHOR]

A high-temperature stable solar absorber based on a metallic 2D photonic crystal (PhC) with high and tunable spectral selectivity is demonstrated and optimized for a range of operating temperatures and irradiances. In particular, a PhC absorber with solar absorptance [ABSTRACT FROM AUTHOR]

The most common approaches to generating power from sunlight are either photovoltaic, in which sunlight directly excites electron-hole pairs in a semiconductor, or solar-thermal, in which sunlight drives a mechanical heat engine. Photovoltaic power generation is intermittent and typically only exploits a portion of the solar spectrum efficiently, whereas the intrinsic irreversibilities of small heat engines make the solar-thermal approach best suited for utility-scale power plants. There is, therefore, an increasing need for hybrid technologies for solar power generation. By converting sunlight into thermal emission tuned to energies directly above the photovoltaic bandgap using a hot absorber-emitter, solar thermophotovoltaics promise to leverage the benefits of both approaches: high efficiency, by harnessing the entire solar spectrum; scalability and compactness, because of their solid-state nature; and dispatchablility, owing to the ability to store energy using thermal or chemical means. However, efficient collection of sunlight in the absorber and spectral control in the emitter are particularly challenging at high operating temperatures. This drawback has limited previous experimental demonstrations of this approach to conversion efficiencies around or below 1% (refs 9, 10, 11). Here, we report on a full solar thermophotovoltaic device, which, thanks to the nanophotonic properties of the absorber-emitter surface, reaches experimental efficiencies of 3.2%. The device integrates a multiwalled carbon nanotube absorber and a one-dimensional Si/SiO2 photonic-crystal emitter on the same substrate, with the absorber-emitter areas optimized to tune the energy balance of the device. Our device is planar and compact and could become a viable option for high-performance solar thermophotovoltaic energy conversion. [ABSTRACT FROM AUTHOR]

Abstract: Phase change material (PCM) suspensions have received wide spread attention for increased thermal storage in various thermal systems such as heat sinks for electronics and solar thermal applications. To achieve further heat transfer enhancement, this paper investigates the effect of focusing micron-sized phase-change particles (PCMs) to a layer near the heated wall of a parallel plate channel. A numerical model for fully-developed laminar flow with a constant heat flux applied to one wall is developed. Melting of the focused PCMs is incorporated using a temperature-dependent effective heat capacity. The effect of channel height, height of the focused PCM stream, heat flux, and fluid properties on the peak local Nusselt number (Nu∗ ) and the averaged Nusselt number over the melting length (Numelt ) are investigated. Compared to the thermally-developed Nusselt number for this geometry (Nuo =5.385), Numelt and Nu∗ enhancements of 8% and 19% were determined, respectively. The local heat transfer performance is optimized when the PCMs are confined to within 30% of the channel height. The present work provides an extended understanding of local heat transfer characteristics during melting of flowing PCM suspensions, and offers a new method for enhancing heat transfer performance in various thermal-fluidic systems. [Copyright &y& Elsevier]

Abstract: The development of efficient solar thermal receivers has received significant interest for solar to electrical power conversion and heating applications. Volumetric flow receivers, where the incoming solar radiation is absorbed in the volume of a heat transfer fluid (HTF), promise reduced heat loss at the surface compared to surface absorbers. In order to efficiently store the thermal energy in the volume, nanoparticles can be suspended in the HTF to absorb the incoming radiation. In such systems, compact models are needed to design and optimize the performance. This paper presents an analytical model that investigates the effect of heat loss, particle loading, solar concentration and channel height on receiver efficiency. The analytical model was formulated by modeling the absorption of solar radiation by the suspended nanoparticles as a volumetric heat release inside the flowing HTF. The energy equation was solved with the surface heat losses modeled using a combined radiative and convective heat loss coefficient. The analytical solution provides a convenient tool for predicting the effect of different parameters, in terms of dimensionless numbers (Pe, NuE , , and θamb ), on two-dimensional temperature profiles and system performance. By combining the receiver efficiency with a power generation efficiency, idealized by the Carnot efficiency, an optimum receiver length where the total efficiency is maximized is determined. However, in practice, the maximum efficiency depends on the maximum allowable temperature of the working HTF. As a case study, predictions were made for Therminol® VP-1 with suspended graphite nanoparticles in a 1cm deep channel with a solar concentration of 10. The model predicts an optimum total system efficiency of 0.35 for a dimensionless receiver length of 0.86. Finally, the analytical model was used to estimate the optimum efficiency and the corresponding optimum receiver length for different design configurations with varying NuE and . The results from this paper will help guide experimental design of volumetric flow receivers for solar thermal based power systems. [Copyright &y& Elsevier]

Abstract: Improvements in solar-to-thermal energy conversion will accelerate the development of efficient concentrated solar power systems. Nanofluid volumetric receivers, where nanoparticles in a liquid medium directly absorb solar radiation, promise increased performance over surface receivers by minimizing temperature differences between the absorber and the fluid, which consequently reduces emissive losses. We present a combined modeling and experimental study to optimize the efficiency of liquid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A one-dimensional transient heat transfer model was developed to investigate the effect of solar concentration, nanofluid height, and optical thickness on receiver performance. Simultaneously, we experimentally investigated a cylindrical nanofluid volumetric receiver, and showed good agreement with the model for varying optical thicknesses of the nanofluid. Based on the model, the efficiency of nanofluid volumetric receivers increases with increasing solar concentration and nanofluid height. Receiver-side efficiencies are predicted to exceed 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This work provides insights as to how nanofluids can be best utilized as volumetric receivers in solar applications, such as receivers with integrated storage for beam-down CSP and future high concentration solar thermal energy conversion systems. [Copyright &y& Elsevier]

[Display omitted] • Pretreating catalysts at 800 °C in air impedes adverse sintering at 1000 °C. • Returning to 800 °C regenerates activity in sintered catalysts by redispersing Pd. • Mechanism for the sintering and redispersion of Pd explored. • Regeneration protocol can be repeatably used, due to persisting trap sites on CeO 2. • Regenerated catalysts effectively co-oxidize CO and C 3 H 8. Methods that can impede or reverse the effects of high-temperature sintering and deactivation are necessary to improve the utilization of catalytic materials. This work demonstrates that exposure to intermediate temperatures (800 °C) can effectively regenerate catalytic performance in sintered Pd-CeO 2 catalysts, through the disintegration and partial redispersion of agglomerated Pd, which increases the abundance of sites that interface strongly with the CeO 2 support. Pretreating the catalyst at 800 °C before sintering forms highly dispersed Pd species that slow the sintering of small, polycrystalline CeO 2 domains. This preserves the density of Ce3+ sites that trap mobile Pd, which improves the efficacy of regeneration. The regenerated catalyst is effective in the co-oxidation of CO and C 3 H 8 , two species which are known to competitively adsorb. The simplicity of this treatment makes it attractive for on-stream regeneration of catalyst performance. [ABSTRACT FROM AUTHOR]

Functional thin film superlattices with stability in extreme environments can lead to transformative performance in optical and thermal applications such as thermophotovoltaics. In this work, key issues associated with defects that prevent layer-by-layer growth in epitaxial, low-miscibility oxide superlattices are investigated. Layer protrusions, approximately 8 nm wide and 3 nm thick, arise from a strain relaxation mechanism in 8 nm bilayer superlattices of Ba(Zr0.5Hf0.5)O3/MgO and propagate through the subsequent superlattice layers forming an inverted pyramid structure that is spatially phase offset from the matrix. The density and size of these defects scales with the number of interfaces in the sample, indicating that surface roughness during growth is a significant factor in the formation of these defects. In situ high temperature transmission electron microscopy (1000 °C, in vacuo) measurement reveals that phase decomposition of Ba(Zr0.5Hf0.5)O3 and decoherence of the superlattice is nucleated by these defects. This work highlights that achieving optimum growth conditions is imperative to the synthesis of single-crystalline superlattices with sharp interfaces for optimized performance in extreme environments. [ABSTRACT FROM AUTHOR]

A correction to the article "A nanophotonic solar thermophotovoltaic device" that was published online in the January 19, 2014 issue is presented.

Abstract: Solar thermophotovoltaic (STPV) systems convert solar energy into electricity via thermally radiated photons at tailored wavelengths to increase energy conversion efficiency. In this work, we report the design and analysis of a STPV system with 2D photonic crystals (PhCs) using a high-fidelity thermal-electrical hybrid model that includes the thermal coupling between the absorber/emitter/PV cell and accounts for non-idealities such as temperature gradients and parasitic thermal losses. The desired radiative spectra of the absorber and emitter were achieved by utilizing an optimized two-dimensional periodic square array of cylindrical cavities on a tantalum (Ta) substrate. Various energy loss mechanisms including re-emission at the absorber, low energy emission at the emitter, and a decrease in the emittance due to the angular dependence of PhCs were investigated with varying irradiation flux onto the absorber and resulting operating temperature. The modeling results suggest that the absorber-to-electrical efficiency of a realistic planar STPV consisting of a 2D Ta PhC absorber/emitter and current state of the art InGaAsSb PV cell (whose efficiency is only ~50% of the thermodynamic limit) with a tandem filter can be as high as ~10% at an irradiation flux of ~130kW/m2 and emitter temperature ~1400K. The absorber-to-electrical STPV efficiency can be improved up to ~16% by eliminating optical and electrical non-idealities in the PV cell. The high spectral performance of the optimized 2D Ta PhCs allows a compact system design and operation of STPVs at a significantly lower optical concentration level compared with previous STPVs using macro-scale metallic cavity receivers. This work demonstrates the importance of photon engineering for the development of high efficiency STPVs and offers a framework to improve the performance of both PhC absorbers/emitters and overall STPV systems. [Copyright &y& Elsevier]

Although spectrally selective materials play a key role in existing and emerging solar thermal technologies, temperature‐related degradation currently limits their use to below 700 °C in vacuum and even lower temperatures in air. Here a solar‐transparent refractory aerogel that offers stable performance up to 800 °C in air is demonstrated, which is significantly greater than its silica counterpart. This improved stability is attributed to the formation of a refractory aluminum silicate phase, which is synthesized using a conformal single cycle of atomic layer deposition within the high‐aspect‐ratio pores of silica aerogels. Based on direct heat loss measurements, the transparent refractory aerogel achieves a receiver efficiency of 75% at 100 suns and an absorber temperature of 700 °C, which is a 5% improvement over the state of the art. Transparent refractory aerogels may find widespread applicability in solar thermal technologies by enabling the use of lower‐cost optical focusing systems and eliminating the need for highly evacuated receivers. In particular, a shift to higher operating temperatures while maintaining a high receiver efficiency can enable the use of advanced supercritical CO2 power cycles and ultimately translate to an ≈10% (absolute) improvement in solar‐to‐electrical conversion efficiency relative to existing linear concentrating systems. [ABSTRACT FROM AUTHOR]

Although spectrally selective materials play a key role in existing and emerging solar thermal technologies, temperature‐related degradation currently limits their use to below 700 °C in vacuum and even lower temperatures in air. Here a solar‐transparent refractory aerogel that offers stable performance up to 800 °C in air is demonstrated, which is significantly greater than its silica counterpart. This improved stability is attributed to the formation of a refractory aluminum silicate phase, which is synthesized using a conformal single cycle of atomic layer deposition within the high‐aspect‐ratio pores of silica aerogels. Based on direct heat loss measurements, the transparent refractory aerogel achieves a receiver efficiency of 75% at 100 suns and an absorber temperature of 700 °C, which is a 5% improvement over the state of the art. Transparent refractory aerogels may find widespread applicability in solar thermal technologies by enabling the use of lower‐cost optical focusing systems and eliminating the need for highly evacuated receivers. In particular, a shift to higher operating temperatures while maintaining a high receiver efficiency can enable the use of advanced supercritical CO2 power cycles and ultimately translate to an ≈10% (absolute) improvement in solar‐to‐electrical conversion efficiency relative to existing linear concentrating systems. [ABSTRACT FROM AUTHOR]

Recent work has demonstrated record-high thermophotovoltaic efficiency using thin-film InGaAs cells, but the power density of devices remains low. Elevated power densities are relevant to many thermophotovoltaic (TPV) applications, ranging from mobile generators to stationary energy storage of renewable electricity, and require effective management of heat and charge carriers. Here we investigate the use of single-junction InGaAs airbridge cells (ABCs) under such conditions. Experimental characterization of an InGaAs ABC with varying emitter and cell temperature is used to develop a predictive device model where carrier lifetimes and series resistances are the sole fitting parameters. The utility of this model is demonstrated through its use in identifying near-term opportunities for improving performance at elevated power densities, and for designing a thermal management strategy that maximizes overall power output. After accounting for the power necessary to cool the cells, this model shows that an InGaAs ABC with high material quality can reach a peak efficiency of ∼41% at 0.5 W/cm2, corresponding to an emitter temperature of 1070 °C, and sustain efficiencies above 36% up to 1.5 W/cm2. • Current airbridge TPV cells are limited by recombination, series resistance, and cell heating. • Improved material quality largely addresses recombination and series resistance losses. • An air-cooled design is proposed for thermal management. • A 41%-efficient TPV cell at 0.5 W/cm2 is predicted based on above improvements. [ABSTRACT FROM AUTHOR]