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We present a radiative cooling material capable of enhancing albedo while reducing ground surface temperatures beneath fielded bifacial solar panels. Electrospinning a layer of polyacrylonitrile nanofibers, or nanoPAN, onto a polymer-coated silver mirror yields a total solar reflectance of 99 %, an albedo of 0.96, and a thermal emittance of 0.80. The combination of high albedo and high emittance is enabled by wavelength-selective scattering induced by the hierarchical morphology of nanoPAN, which includes both thin fibers and bead-like structures. During outdoor testing, the material outperforms the radiative cooling power of a state-of-the-art control by ∼20 W/m2 and boosts the photocurrent produced by a commercial silicon cell by up to 6.4 mA/cm2 compared to sand. These experiments validate essential characteristics of a high-albedo radiative-cooling reflector with promising potential applications in thermal and light management of fielded bifacial panels.

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]

We present a radiative cooling material capable of enhancing albedo while reducing ground surface temperatures beneath fielded bifacial solar panels. Electrospinning a layer of polyacrylonitrile nanofibers, or nanoPAN, onto a polymer-coated silver mirror yields a total solar reflectance of 99 %, an albedo of 0.96, and a thermal emittance of 0.80. The combination of high albedo and high emittance is enabled by wavelength-selective scattering induced by the hierarchical morphology of nanoPAN, which includes both thin fibers and bead-like structures. During outdoor testing, the material outperforms the radiative cooling power of a state-of-the-art control by ∼20 W/m2 and boosts the photocurrent produced by a commercial silicon cell by up to 6.4 mA/cm2 compared to sand. These experiments validate essential characteristics of a high-albedo radiative-cooling reflector with promising potential applications in thermal and light management of fielded bifacial panels. [ABSTRACT FROM AUTHOR]

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]

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]

Stabilizing high dispersions of catalytically active metals is integral to improving the lifetime, activity, and material utilization of catalysts that are periodically exposed to high temperatures during operation or maintenance. We have found that annealing palladium-based core@shell catalysts in air at elevated temperatures (800 °C) promotes the redispersion of active metals into highly dispersed sites, which we refer to as halo sites. Here, we examine the restructuring of Pd@SiO2 and Pd@CeO2 core@shell catalysts over successive 800 °C aging cycles to understand the formation, activity, nanoscale structure, and stability of these palladium halo sites. While encapsulation generally improves metal utilization by providing a physical barrier that promotes redispersion over agglomeration, our cycled aging experiments demonstrate that halo sites are not stable in all catalysts. Halo sites continue to migrate in Pd@SiO2 due to poor metal–support bonding, which leads to palladium agglomeration. In contrast, halo sites formed in Pd@CeO2 remain stable. The dispersed palladium also synergistically stabilizes the ceria from agglomerating. We attribute this stability, in addition to an observed improvement in catalytic activity, to the coordination between palladium and reducible ceria that arises during the formation of halo sites. We probe the importance of ceria oxidation state on the stability of halo sites by aging Pd@CeO2 after it has been reduced. While some halo sites agglomerate, we find that returning to air aging mitigates the loss of these sites and catalytic activity. Our findings illustrate how nanoscale catalyst structures can be designed to promote the formation of highly stable and dispersed metal sites. [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: 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]

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]

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]

Recent advances in thermophotovoltaic (TPV) power generation have produced notable gains in efficiency, particularly at very high emitter temperatures. However, there remains substantial room for improving TPV conversion of waste, solar, and nuclear heat streams at temperatures below 1,100°C. Here, we demonstrate the concept of transmissive spectral control that enables efficient recuperation of below-bandgap photons by allowing them to transmit through the cell to be absorbed by a secondary emitter. We fabricate a semitransparent TPV cell consisting of a thin InGaAs-InP heterojunction membrane supported by an infrared-transparent heat-conducting substrate. The device absorbs less than 1% of below-bandgap radiation, resulting in a TPV efficiency of 32.5% at an emitter temperature of 1,036°C. To our knowledge, this represents an 8% absolute improvement (~33% relative) in efficiency relative to the best TPV devices at such low temperatures. By enabling near-zero photon loss, the semitransparent architecture facilitates high TPV efficiencies over a wide range of applications.

With advances in thermophotovoltaic (TPV) cells enabling recycling of sub-bandgap photons, a key barrier to reaching high prototype efficiencies has become radiative losses to parasitic high-emissivity regions, such as heavily doped contact regions, defects in coatings, and inactive areas. Here, we examine the impact of such radiative losses on the performance of various candidate cell materials, including GaAs, Si, InGaAsP, InGaAs, GaSb, and InGaAsSb. The ability of a TPV design to resist this performance loss is termed "radiation-sink tolerance" (RST). We show that RST is directly proportional to the spectral overlap between the absorptance profile of the cell and the emission profile of the emitter, which can be improved by adding a lower-bandgap absorber, increasing the emitter temperature, and utilizing a selective emitter. Our RST expressions can be used to estimate the efficiency of a prototypical TPV generator based on a component-level measurement.

Rooftop solar thermal collectors have the potential to meet residential heating demands if deployed efficiently at low solar irradiance (i.e., 1 sun). The efficiency of solar thermal collectors depends on their ability to absorb incoming solar energy and minimize thermal losses. Most techniques utilize a vacuum gap between the solar absorber and the surroundings to eliminate conduction and convection losses, in combination with surface coatings to minimize reradiation losses. Here, we present an alternative approach that operates at atmospheric pressure with simple, black, absorbing surfaces. Silica based aerogels coated on black surfaces have the potential to act as simple and inexpensive solar thermal collectors because of their high transmission to solar radiation and low transmission to thermal radiation. To demonstrate their heat-trapping properties, we fabricated tetramethyl orthosilicate-based silica aerogels. A hydrophilic aerogel with a thickness of 1 cm exhibited a solar-averaged transmission of 76% and thermally averaged transmission of ≈1% (at 100 °C). To minimize unwanted solar absorption by O-H groups, we functionalized the aerogel to be hydrophobic, resulting in a solar-averaged transmission of 88%. To provide a deeper understanding of the link between aerogel properties and overall efficiency, we developed a coupled radiative-conductive heat transfer model and used it to predict solar thermal performance. Instantaneous solar thermal efficiencies approaching 55% at 1 sun and 80 °C were predicted. This study sheds light on the applicability of silica aerogels on black coatings for solar thermal collectors and offers design priorities for next-generation solar thermal aerogels.

Hematite holds promise for photoelectrochemical (PEC) water splitting due to its stability, low-cost, abundance and appropriate bandgap. However, it suffers from a mismatch between the hole diffusion length and light penetration length. We have theoretically designed and characterized an ultrathin planar hematite/silver nanohole array/silver substrate photoanode. Due to the supported destructive interference and surface plasmon resonance, photons are efficiently absorbed in an ultrathin hematite film. Compared with ultrathin hematite photoanodes with nanophotonic structures, this photoanode has comparable photon absorption but with intrinsically lower recombination losses due to its planar structure and promises to exceed the state-of-the-art photocurrent of hematite photoanodes.

To bridge the gap between theoretically predicted and experimentally demonstrated efficiencies of solar thermophotovoltaics (STPVs), we consider the impact of spectral non-idealities on the efficiency and the optimal design of STPVs over a range of PV bandgaps (0.45-0.80 eV) and optical concentrations (1-3,000x). On the emitter side, we show that suppressing or recycling sub-bandgap radiation is critical. On the absorber side, the relative importance of high solar absorptance versus low thermal emittance depends on the energy balance. Both results are well-described using dimensionless parameters weighting the relative power density above and below the cutoff wavelength. This framework can be used as a guide for materials selection and targeted spectral engineering in STPVs.

The design and simulation of a wide angle, spectrally selective absorber/emitter metallic photonic crystal (MPhC) is presented. By using dielectric filled cavities, the angular, spectrally selective absorption/emission of the MPhC is dramatically enhanced over an air filled design by minimizing diffraction losses. Theoretical analysis is performed and verified via rigorous coupled wave analysis (RCWA) based simulations. An efficiency comparison of the dielectric filled designs for solar thermophotovoltaic applications is performed for the absorber and emitter which yields a 7% and 15.7% efficiency improvement, respectively, compared to air filled designs. The converted power output density is also improved by 33.5%.