Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States. A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below that of the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres. Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved because sky access during the day results in heating of the radiative cooler by the Sun. Here, we experimentally demonstrate radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight. Using a thermal photonic approach, we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO2 and SiO2 that reflects 97 per cent of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.

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/m(2) at ambient temperature. This cooling persists in the presence of significant convective/conductive heat exchange and nonideal atmospheric conditions.

Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n 2 ∕ sin 2 θ, where n is the refractive index of the active layer, and θ is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the conventional limit can be substantially surpassed when optical modes exhibit deep-subwavelength-scale field confinement, opening new avenues for highly efficient next-generation solar cells.T he ultimate success of photovoltaic (PV) cell technology requires great advancements in both cost reduction and efficiency improvement. An approach that simultaneously achieves these two objectives is to use light-trapping schemes. Light trapping allows cells to absorb sunlight using an active material layer that is much thinner than the material's intrinsic absorption length. This effect then reduces the amount of materials used in PV cells, which cuts cell cost in general, and moreover facilitates mass production of PV cells that are based on less abundant materials. In addition, light trapping can improve cell efficiency, because thinner cells provide better collection of photogenerated charge carriers, and potentially a higher open circuit voltage (1).The theory of light trapping was initially developed for conventional cells where the light-absorbing film is typically many wavelengths thick (2-4). From a ray-optics perspective, conventional light trapping exploits the effect of total internal reflection between the semiconductor material (such as silicon, with a refractive index n ∼ 3.5) and the surrounding medium (usually assumed to be air). By roughening the semiconductor-air interface (Fig. 1A), one randomizes the light propagation direction inside the material. The effect of total internal reflection results in a much longer propagation distance inside the material and hence a substantial absorption enhancement. For such light-trapping schemes, the standard theory shows that the absorption enhancement factor has an upper limit of 4n 2 ∕ sin 2 θ (2-4), where θ is the angle of the emission cone in the medium surrounding the cell. This limit of 4n 2 ∕ sin 2 θ will be referred to in this paper as the conventional limit. This form is in contrast to the 4n 2 limit, which strictly speaking is only applicable to cells with isotropic angular response, but is nevertheless quite commonly used in the literature.For nanoscale films with thicknesses comparable or even smaller than wavelength scale, some of the basic assumptions of the conventional theory are no longer applicable. Whether the conventional limit still holds thus becomes an important open question that is currently being pursu...

Radiative cooling technology utilizes the atmospheric transparency window (8–13 μm) to passively dissipate heat from Earth into outer space (3 K). This technology has attracted broad interests from both fundamental sciences and real world applications, ranging from passive building cooling, renewable energy harvesting and passive refrigeration in arid regions. However, the temperature reduction experimentally demonstrated, thus far, has been relatively modest. Here we theoretically show that ultra-large temperature reduction for as much as 60 °C from ambient is achievable by using a selective thermal emitter and by eliminating parasitic thermal load, and experimentally demonstrate a temperature reduction that far exceeds previous works. In a populous area at sea level, we have achieved an average temperature reduction of 37 °C from the ambient air temperature through a 24-h day–night cycle, with a maximal reduction of 42 °C that occurs when the experimental set-up enclosing the emitter is exposed to peak solar irradiance.

A solar absorber, under the sun, is heated up by sunlight. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic for either operational or aesthetic considerations, but the resulting heating is undesirable. Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties. Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal. When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13°C due to radiative cooling. Our work shows that the concept of radiative cooling can be used in combination with the utilization of sunlight, enabling new technological capabilities.radiative cooling | thermal radiation | photonic crystal | solar absorber T he universe, at a temperature of 3 K, represents a significant renewable thermodynamic resource: it is the ultimate heat sink. Over midinfrared wavelengths, in particular between 8 and 13 μm, Earth's atmosphere is remarkably transparent to electromagnetic radiation. This wavelength range coincides with the peak wavelength of thermal radiation from terrestrial structures at typical ambient temperatures. Thus, a sky-facing terrestrial object can have radiative access to the universe. Exploiting this radiative access has led to the demonstration of radiative cooling (1-7), as well as proposals for direct electric power generation from thermal radiation of terrestrial objects (8).Whereas historically radiative cooling was largely developed for night-time applications (1-6, 9-13), recent works have achieved daytime radiative cooling (7,14). In particular, it was shown that the radiative cooling to below ambient air temperature can be achieved (7), with a photonic structure that reflects almost all incident sunlight and simultaneously emits significant thermal radiation in the midinfrared. Such a structure, being a near-perfect solar reflector, makes no use of incident sunlight. On the other hand, in many applications, including solar cells (15) and outdoor structures (16), the utilization of sunlight through absorption is intrinsic for either operational or aesthetic considerations, but the heating associated with sunlight absorption is undesirable. For these applications, lowering operating temperatures via radiative cooling is only viable if one can simultaneously preserve the absorption of sunlight.Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal, using a thermophotonic approach (17-30). When placed on a silicon absorber under sunlight, such a blackbody preserves and even slightly enhances sunlight absorption, but reduces the temperature of the silicon absorber by as m...