Selective Thermal Emitters for High-performance All-day Radiative Cooling

Passive radiative coolers (PRCs), which pump excess heat to cold exterior space via thermal radiation, have emerged as a promising energy-free technology in cooling buildings, thermal power plants, and photovoltaics. However, designing a ‘daytime’ PRC is challenging due to the simultaneous requirement of high reflectance in the solar spectral regime (0.3–2.5 μm) and high emissivity in the atmospheric transmittance window (8–13 μm). Here, we present a large-area compatible and lithography-free nanoscale multilayer design of daytime PRC based on two pairs of tandem silicon dioxide– aluminium nitride dielectric layer cascaded to a silver ground metal placed over a silicon substrate. We theoretically achieve near-perfect reflectance (97.3%) over the solar spectral regime while maintaining high emissivity (80%) in the atmospheric transmittance window. During the daytime under direct sunlight, the cooling power of the proposed structure is reported to be 115 Wm-2 with a temperature reduction up to 15 K below the ambient temperature, when the effect of convection and conductive heat transfer is considered. Our design is polarization-independent and angle-insensitive up to 70 degrees of angle of incidence. An excellent match between our theoretical and simulation results validates our findings. The fabrication tolerance study reveals that the cooling performance of our robust design is unlikely to degrade much during experimental realization. The figure of merit calculation indicates that our PRC can outperform recently reported daytime PRCs.

Surface Pattern over a Thick Silica Film to Realize Passive Radiative Cooling

Passive radiative cooling, which cools an item without any electrical input, has drawn much attention in recent years. In many radiative coolers, silica is widely used due to its high emissivity in the mid-infrared region. However, the performance of a bare silica film is poor due to the occurrence of an emitting dip (about 30% emissivity) in the atmospheric transparent window (8–13 μm). In this work, we demonstrate that the emissivity of silica film can be improved by sculpturing structures on its surface. According to our simulation, over 90% emissivity can be achieved at 8–13 μm when periodical silica deep grating is applied on a plane silica film. With the high emissivity at the atmospheric transparent window and the extremely low absorption in the solar spectrum, the structure has excellent cooling performance (about 100 W/m2). The enhancement is because of the coupling between the incident light with the surface modes. Compared with most present radiative coolers, the proposed cooler is much easier to be fabricated. However, 1-D gratings are sensitive to incident polarization, which leads to a degradation in cooling performance. To solve this problem, we further propose another radiative cooler based on a silica cylinder array. The new cooler’s insensitivity to polarization angle and its average emissivity in the atmospheric transparent window is about 98%. Near-unit emissivity and their simple structures enable the two coolers to be applied in real cooling systems.

Infrared and Terahertz Compatible Absorber Based on Multilayer Film

In this paper, a similar Fabry-Perot cavity structure utilizing a multilayer film structure consisting of an ultrathin metal film is demonstrated for absorbing the infrared ray. This structure has low emissivity in the atmospheric window (3–5 and 8–14 μm) and high emissivity in the nonatmospheric window (5–8 μm). These properties improved the stealth performance which causes the high emissivity in 5–8 μm to radiate more energy to reduce its temperature. Based on this, the periodic microstructures were added to the surface of the materials that enhanced the absorption of terahertz wave (0.1–2.7 THz). The absorber based on multilayer film has a simple structure and low manufacturing cost. This work may provide a new strategy for infrared and terahertz compatible stealth technology.

The Effect of Refractory Wall Emissivity on the Energy Efficiency of a Gas-Fired Steam Cracking Pilot Unit

The effect of high emissivity coatings on the radiative heat transfer in steam cracking furnaces is far from understood. To start, there is a lack of experimental data describing the emissive properties of the materials encountered in steam cracking furnaces. Therefore, spectral normal emissivity measurements are carried out, evaluating the emissive properties of refractory firebricks before and after applying a high emissivity coating at elevated temperatures. The emissive properties are enhanced significantly after applying a high emissivity coating. Pilot unit steam cracking experiments show a 5% reduction in fuel gas firing rate after applying a high emissivity coating on the refractory of the cracking cells. A parametric study, showing the effect of reactor coil and furnace wall emissive properties on the radiative heat transfer inside a tube-in-box geometry, confirms that a non-gray gas model is required to accurately model the behavior of high emissivity coatings. Even though a gray gas model suffices to capture the heat sink behavior of a reactor coil, a non-gray gas model that is able to account for the absorption and re-emission in specific bands is necessary to accurately model the benefits of applying a high emissivity coating on the furnace wall.