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

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.

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A nanophotonic solar thermophotovoltaic device

If properly designed, terrestrial structures can passively cool themselves through radiative emission of heat to outer space. For the first time, we present a metal-dielectric photonic structure capable of radiative cooling in daytime outdoor conditions. The structure behaves as a broadband mirror for solar light, while simultaneously emitting strongly in the mid-IR within the atmospheric transparency window, achieving a net cooling power in excess of 100 W/m2 at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions.

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Ultrabroadband Photonic Structures To Achieve High-Performance Daytime Radiative Cooling

Thermal radiation is one of the most universal physical phenomena, and its study has played a key role in the history of modern physics. Our understanding of this subject has been traditionally bas...

Radiative Heat Transfer

Nanophotonic systems, including photonic crystal microcavities and plasmonic metal nanoparticles, that are capable of changing the rate of spontaneous emission are reviewed and compared.

Modified spontaneous emission in nanophotonic structures

Converting from a broadband to a narrowband thermal emission spectrum with minimal loss of energy is important in the creation of efficient environmental sensors and biosensors1,2 as well as thermo-photovoltaic power generation systems3,4. Here, we demonstrate such thermal emission control by manipulating photonic modes with photonic crystals as well as material absorption with quantum-well intersubband transitions. We show that the emission peak intensity for our device can be more than four times greater than that of a blackbody sample under the same input power and thermal management conditions due to an increase in the temperature compared to the blackbody reference, and the emission bandwidth and angular spread are narrowed by a factor of 30 and 8, respectively. These results indicate that the energy saved by thermal emission control can be recycled and concentrated to enhance the narrow peak emission intensity. By constructing a thermal emission control device based on a multiple-quantum-well layer embedded in a two-dimensional photonic crystal, researchers demonstrate that they can convert a broadband thermal emission spectrum into a narrowband spectrum with minimal loss of energy.

Conversion of broadband to narrowband thermal emission through energy recycling

Thermal emission in the infrared range is important in various fields of research, including chemistry, medicine and atmospheric science. Recently, the possibility of controlling thermal emission based on wavelength-scale optical structures has been intensively investigated with a view towards a new generation of thermal emission devices. However, all demonstrations so far have involved the 'static' control of thermal emission; high-speed modulation of thermal emission has proved difficult to achieve because the intensity of thermal emission from an object is usually determined by its temperature, and the frequency of temperature modulation is limited to 10-100 Hz even when the thermal mass of the object is small. Here, we experimentally demonstrate the dynamic control of thermal emission via the control of emissivity (absorptivity), at a speed four orders of magnitude faster than is possible using the conventional temperature-modulation method. Our approach is based on the dynamic control of intersubband absorption in n-type quantum wells, which is enhanced by an optical resonant mode in a photonic crystal slab. The extraction of electrical carriers from the quantum wells leads to an immediate change in emissivity from 0.74 to 0.24 at the resonant wavelength while maintaining much lower emissivity at all other wavelengths.

Realization of dynamic thermal emission control

The control of thermal emission spectra using optical resonances has been attracting increased attention both with respect to fundamental science and for various applications, including infrared sensing, thermal imaging, and thermophotovoltaics. In this mini-review, we describe the recent experimental demonstrations of narrowband thermal emission with optical nanostructures, including metallic cavities, metamaterials, and all-dielectric photonic crystals. The spectral features of the controlled thermal emission (e.g., wavelength, linewidth, peak emissivity, and angular characteristics) are strongly dependent on the choice of both materials and structures of the emitters. Through the appropriate design of optical nanostructures, arbitrary shaping of thermal emission spectra, from single-peak ultra-narrowband (Q>100) emission for midinfrared sensing to a stepwise emissivity spectrum for thermophotovoltaics, has been successfully realized.

Realization of narrowband thermal emission with optical nanostructures

Thermal radiation transfer between two objects separated by a subwavelength gap (near-field thermal radiation transfer) can be orders of magnitude larger than that in free space, which is attractin...

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

We experimentally demonstrate single-peak narrow-bandwidth thermal emission with a quality factor (Q factor) of more than 100 at a wavelength of 9.1 μm. The emission is significantly suppressed at all other wavelengths. Our emitter is based on an intersubband transition in a multiple quantum well structure combined with a single high-Q resonant mode in a two-dimensional photonic crystal slab, which allows strong light-matter interaction only at a specific wavelength. Strong thermal emission is exhibited only in a limited angular range (∼20°) from the normal direction. Our results have potential applications in bio- and environmental sensors.

Takuya Inoue

Papers

Conversion of broadband to narrowband thermal emission through energy recycling

Realization of dynamic thermal emission control

Realization of narrowband thermal emission with optical nanostructures

One-Chip Near-Field Thermophotovoltaic Device Integrating a Thin-Film Thermal Emitter and Photovoltaic Cell.

Single-peak narrow-bandwidth mid-infrared thermal emitters based on quantum wells and photonic crystals