Transparent energy-saving windows based on broadband directional thermal emission

Abstract

Passive radiative cooling has emerged as a sustainable energy-saving solution, characterized by its energy-free operation and absence of carbon emissions. Conventional radiative coolers are designed with a skyward orientation, allowing for efficient heat dissipation to the cold heat sink. However, this design feature presents challenges when installed on vertical surfaces, as nearby objects obstruct heat release by blocking the cooler’s skyward view. Here, we introduce a directional radiative cooling glass (DRCG) designed to facilitate efficient heat dissipation through angular selective emission. The DRCG is constructed as a multilayer structure incorporating epsilon-near-zero materials, specifically Si3N4 and Al2O3, layered on an indium-tin-oxide thermal reflector. This innovative design restricts thermal emission to specific angular ranges, known as the Berreman mode. Additionally, the transparent layers enable a visible transmittance exceeding 84 %. Theoretical simulations validate the enhanced cooling performance of the DRCG, exhibiting a temperature reduction of over 1.5 °C compared with conventional glass in hot urban environments characterized by a nearby object temperature exceeding 60 °C and a sky view factor of 0.25. Furthermore, outdoor experiments demonstrate that employing the DRCG as a window enhances space-cooling performance by ∼1.5 °C. These findings underscore the potential of transparent energy-saving windows in mitigating the urban heat island effect.

Ultrahigh Visible-Transparency, Submicrometer, and Polymer-Free Radiative Cooling Meta-Glass Coating for Building Energy Saving

“…The abrupt increase in reflectance beyond 70° can be attributed to the total reflection effect of light. This excellent angle-independence ensures that the meta-glass installed vertically on buildings exhibits stable cooling performance. , On the contrary, the R̅ NIR of PDMS-meta-glass exhibits significant angular dependence, decreasing as the angle increases (Figure S9). This impact of PDMS on the angular independence of the solar transmission (reflection) has been seldom discussed in previous studies (Figure S10).…”