A multilayer photonic structure is described that strongly reflects incident sunlight while emitting heat selectively through an atmospheric transparency window to outer space; this leads to passive cooling under direct sunlight of 5 degrees Celsius below ambient air temperature, which has potential applications in air-conditioning and energy efficiency.

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Passive radiative cooling below ambient air temperature under direct sunlight

Major developments, as well as remaining challenges and the associated research opportunities, are evaluated for three technologically distinct approaches to solar energy utilization: solar electricity, solar thermal, and solar fuels technologies. Much progress has been made, but research opportunities are still present for all approaches. Both evolutionary and revolutionary technology development, involving foundational research, applied research, learning by doing, demonstration projects, and deployment at scale will be needed to continue this technology-innovation ecosystem. Most of the approaches still offer the potential to provide much higher efficiencies, much lower costs, improved scalability, and new functionality, relative to the embodiments of solar energy-conversion systems that have been developed to date.

http://science.sciencemag.org/content/sci/351/6271/aad1920.full.pdf

Research opportunities to advance solar energy utilization

As a ubiquitous solar-thermal energy conversion process, solar-driven evaporation has attracted tremendous research attention owing to its high conversion efficiency of solar energy and transformative industrial potential. In recent years, solar-driven interfacial evaporation by localization of solar-thermal energy conversion to the air/liquid interface has been proposed as a promising alternative to conventional bulk heating-based evaporation, potentially reducing thermal losses and improving energy conversion efficiency. In this Review, we discuss the development of the key components for achieving high-performance evaporation, including solar absorbers, evaporation structures, thermal insulators and thermal concentrators, and discuss how they improve the performance of the solar-driven interfacial evaporation system. We describe the possibilities for applying this efficient solar-driven interfacial evaporation process for energy conversion applications. The exciting opportunities and challenges in both fundamental research and practical implementation of the solar-driven interfacial evaporation process are also discussed. The thermal properties of solar energy can be exploited for many applications, including evaporation. Tao et al. review recent developments in the field of solar-driven interfacial evaporation, which have enabled higher-performance structures by localizing energy conversion to the air/liquid interface.

Solar-driven interfacial evaporation

The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as critical for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an average measured absorbance of ~99% across the wavelengths from 400 nm to 10 μm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber–based solar steam generation has over 90% efficiency under solar irradiation of only 4-sun intensity (4 kW m−2). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufacturing of other nanophotonic structures and devices.

Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation.

Solar energy can be used to evaporate water and generate steam, however this usually requires expensive optical concentrators. Ni et al. demonstrate a low-cost solar receiver based on thermal concentration that generates steam at 100 ∘C without the need for optical concentration.

/pdf/steam-generation-under-one-sun-enabled-by-a-floating-3f6wdw1zvd.pdf

Steam generation under one sun enabled by a floating structure with thermal concentration

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.

/pdf/a-nanophotonic-solar-thermophotovoltaic-device-3kcdiqjv2h.pdf

A nanophotonic solar thermophotovoltaic device

United States. Advanced Research Projects Agency-Energy (Awards DE-AR0000471 and DE-AR0000181)

Concentrating Solar Power.

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 system conditions of operating temperature and irradiance. In particular a PhC absorber with solar absorptance ̅ 0.86 and thermal emittance ̅ 0.26 at 1000 K, using high-temperature material properties, is achieved resulting in a thermal transfer efficiency more than 50 % higher than that of a blackbody absorber. Furthermore, an integrated double-sided 2D PhC absorber/emitter pair is demonstrated for a high-performance solar thermophotovoltaic (STPV) system. The 2D PhC absorber/emitter is fabricated on a double-side polished tantalum substrate, characterized, and tested in an experimental STPV setup along with a flat Ta absorber and a nearly-blackbody absorber composed of an array of multi-walled carbon nanotubes (MWNTs). At an irradiance of 130 kWm-2 the PhC absorber enables more than a twofold improvement in measured STPV system efficiency (3.74 %) relative to the nearly-blackbody absorber (1.60 %) and higher efficiencies are expected with increasing operating Submitted to 2 temperature. These experimental results show unprecedented high efficiency, demonstrating the importance of the high selectivity of the 2D PhC absorber and emitter for high-temperature energy conversion. A high-performance selective absorber is a critical component in energy conversion systems such as solar thermal, solar thermochemical, and solar thermophotovoltaics. There exist a range of solutions with high absorptivity for low and intermediate temperatures. [1-3] However, for many applications, high operating temperatures (>700 K) are advantageous to achieve higher system efficiencies. Conventional absorbers are unsuitable at these high operating temperatures since there are more considerations to be taken into account. [4] Firstly, the materials and structures need to be thermally stable and maintain their optical properties at these high temperatures. Refractory metals are most advantageous due to their high melting point and low vapor pressure. Secondly, it is crucial that the absorber exhibits spectrally selective absorptance; namely high absorptivity in the shorter wavelength range to absorb most of the solar spectrum and low absorptivity (i.e., emissivity) in the longer wavelength range to minimize losses due to re-emission. Furthermore, this selectivity, i.e., the spectral range of high and low absorptivity, has to be tailored for the specific system operating conditions to achieve maximum system efficiency. It is therefore advantageous to use PhCs which offer the possibility to tailor the spectral absorptance [5,6] and thus optimize system efficiency. Several absorbers based on 1D multilayer …

Metallic Photonic Crystal Absorber-Emitter for Efficient Spectral Control in High-Temperature Solar Thermophotovoltaics

Solar thermophotovoltaic devices have the potential to enhance the performance of solar energy harvesting by converting broadband sunlight to narrow-band thermal radiation tuned for a photovoltaic cell A direct comparison of the operation of a photovoltaic with and without a spectral converter is the most critical indicator of the promise of this technology Here, we demonstrate enhanced device performance through the suppression of 80% of unconvertible photons by pairing a one-dimensional photonic crystal selective emitter with a tandem plasma–interference optical filter We measured a solar-to-electrical conversion rate of 68%, exceeding the performance of the photovoltaic cell alone The device operates more efficiently while reducing the heat generation rates in the photovoltaic cell by a factor of two at matching output power densities We determined the theoretical limits, and discuss the implications of surpassing the Shockley–Queisser limit Improving the performance of an unaltered photovoltaic cell provides an important framework for the design of high-efficiency solar energy converters The ability of photovoltaic devices to harvest solar energy can be enhanced by tailoring the spectrum of incident light with thermophotovoltaic devices Bierman et al now show that one such device achieves a solar-to-electrical efficiency of 68%, exceeding that of the solar cell alone

Enhanced photovoltaic energy conversion using thermally based spectral shaping

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.

David M. Bierman

Papers

A nanophotonic solar thermophotovoltaic device

Concentrating Solar Power.

Metallic Photonic Crystal Absorber-Emitter for Efficient Spectral Control in High-Temperature Solar Thermophotovoltaics

Enhanced photovoltaic energy conversion using thermally based spectral shaping

Role of spectral non-idealities in the design of solar thermophotovoltaics