Passive directional sub-ambient daytime radiative cooling

Abstract

Demonstrations of passive daytime radiative cooling have primarily relied on complex and costly spectrally selective nanophotonic structures with high emissivity in the transparent atmospheric spectral window and high reflectivity in the solar spectrum. Here, we show a directional approach to passive radiative cooling that exploits the angular confinement of solar irradiation in the sky to achieve sub-ambient cooling during the day regardless of the emitter properties in the solar spectrum. We experimentally demonstrate this approach using a setup comprising a polished aluminum disk that reflects direct solar irradiation and a white infrared-transparent polyethylene convection cover that minimizes diffuse solar irradiation. Measurements performed around solar noon show a minimum temperature of 6 °C below ambient temperature and maximum cooling power of 45 W m–2. Our passive cooling approach, realized using commonly available low-cost materials, could improve the performance of existing cooling systems and enable next-generation thermal management and refrigeration solutions.

Colored and paintable bilayer coatings with high solar-infrared reflectance for efficient cooling

“…Because of its passive and eco-friendly operation and its net cooling effect (4-7), radiative cooling designs have been widely investigated. Examples of these designs include white paints (8)(9)(10), porous (11) or metallized (12)(13)(14) polymers, polymerdielectric composites (14)(15)(16)(17)(18), photonic architectures (19)(20)(21)(22), and natural materials (23)(24)(25). Usually, these designs maximize radiative cooling by using metal mirrors or white materials with high R solar .…”

Accelerating the discovery of multilayer nanostructures with analytic differentiation of the transfer matrix equations

“…As illustrative objective functions, consider two generic figures of merit for thermal radiation applications: the first (ρ) will be a dimensional figure of merit and will take the form of a density (for example, a power density for radiative cooling applications [18][19][20][21][22][23]), and the second (η) will be dimensionless figure of merit and will take the form of an efficiency (for example, the luminous efficiency for incandescent light sources [24][25][26][27]). The main ideas apply also to figures of merit which depend only upon optical properties; we introduce and optimize several such figures of merit in the Results section.…”

“…Designing materials on the nanoscale can have a profound impact on how optical energy flows through those materials, which can in turn dramatically improve the performance of nanostructured materials for energy-related applications including solar and (solar) thermophotovoltaic energy conversion [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17], radiative cooling [18][19][20][21][22][23], incandescent lighting [24][25][26][27], among others. Multilayer planar nanomaterials, stacks of flat materials with nanoscale thickness, have emerged as promising candidates for such applications because they present highly tunable optical and thermal emission properties and are relatively easy to fabricate.…”

“…Multilayer planar nanomaterials, stacks of flat materials with nanoscale thickness, have emerged as promising candidates for such applications because they present highly tunable optical and thermal emission properties and are relatively easy to fabricate. As seemingly simple as such structures are, the spectral shaping capabilities they offer have enabled passive cooling well below ambient temperature [19][20][21][22][23], and incandescent lighting with efficiencies meeting or exceeding LED bulbs [27]. Also important is that the optical and thermal emission properties of such structures can be efficiently simulated; for example, several of the authors have developed an open-source package called WPTherml [28] that is based on an efficient analytic method for solving Maxwell's equations for such structures known as the transfer-matrix method (TMM) [29].…”

An ode to polyethylene

“…Furthermore, properly sized pores in polyethylene films can achieve the same effect of scattering the visible light and letting the thermal radiation through as the properly sized fibers in woven, knitted, or non-woven fiber-based fabrics. 13,54 Figure 5(d) shows an infrared camera image, which illustrates how human body covered by visibly opaque white polyethylene fabric cools radiatively via transmission of the emitted body heat through the fabric. The ability to control the transmittance properties of the polyethylene fabrics in a broad spectral range by engineering the fiber/pore sizes offers a variety of useful applications in wearable technologies, bedding, bandages, tents, and apparel.…”