Suitability of Anodic Porous Alumina as a Passive Radiative Cooler: An In-Depth Study

Abstract

Passive radiative cooling technology has the potential to revolutionize the way of cooling buildings and devices, while also helping to reduce the carbon footprint and energy consumption. Pioneer works involving anodic aluminum oxide (AAO) nanostructures showed controversial results. In this work, we clarify how the morphological properties and chemical structure of AAO−Al samples affect their optical properties and their cooling performance. Changes in thickness, interpore distance, and porosity of the alumina layer, as well as the used counterions, significantly affect the cooling ability of the AAO−Al structure. We measure a maximum temperature reduction, ΔT, of 8.0 °C under direct sunlight on a summer day in Spain, coinciding with a calculated peak cooling power, P cool , of 175 W/m 2 , using an AAO−Al sample anodized in sulfuric acid, with 12 μm of AAO thickness and 10% of porosity. These results represent a significant improvement over previous studies, demonstrating the potential of AAO nanostructures to be used in thermal management applications.

Metallic Coatings Boost the Cooling Power of Nanoporous Alumina

“…To enhance the radiative cooling of AAO nanostructures, they were coated with different metals. The AAO nanostructures were first prepared using a two-step anodization process. ,, The anodization was performed in 50 wt % ethylene glycol containing 10 wt % sulfuric acid, at 0 °C, under an applied voltage of 19 V. The first and second anodization times were 24 and 8 h, respectively. Next, an aqueous solution of CuCl 2 and HCl was used to chemically remove the Al substrate.…”

“…For a more reliable comparison, the Cu bulk was characterized in the absence of a sample: when the solar light is turned on, the Cu bulk has a temperature of 27.0 ± 0.8 °C, and when the solar light is turned off, it remains at 19.5 ± 0.8 °C. The field measurements were carried out on the building’s rooftop using an outdoor characterization setup . This setup consisted of multiple polystyrene foam blocks covered by Al foil and sealed by using low-density polyethylene (LDPE).…”

“…Calculations of cooling power density ( P cool ) were carried out using the experimentally obtained emissivity data of every cooler, at the ambient temperature recorded by the weather station, considering a heat-transfer coefficient, h CC , of 12 W/m 2 ·K as representative for the outdoor characterization setup . The equations can be found in the Supporting Information (eqs S1–S6).…”

“…Herein, a simple and low-cost method to enhance the PDRC performance of nanostructured anodic aluminum oxide (AAO) by applying metallic coatings is proposed. As with other porous materials, − AAO has been shown to exhibit good PDRC properties due to its high porosity and emissivity, with the advantage that it can be easily fabricated by anodizing aluminum foils. , Table summarizes some examples of works using alumina to develop passive radiative coolers.…”

“…There are very few works in which AAO nanostructures are combined with metals: Fu et al and Diaz-Lobo et al used an Al substrate, while Lee et al and Zhou et al used an Ag substrate. Researchers have tested how well AAO nanostructures cool Al and Ag substrates, but they have not investigated how the type and thickness of the metallic layer affect the cooler’s performance.…”