Tailored Indoor Setup for Characterization of Passive Daytime Cooler

Passive daytime cooling, which conveys heat from material to outer space through “atmosphere window” (8 – 13 µm) without external energy consumption, has emerged as a strong candidate to alleviate global climate change.¹ For an ideal passive daytime cooler, low absorption in solar range (0.3 – 2.5 µm) and high emissivity over the middle-infrared (MIR) region are stringently required. A significant amount of metamaterials with remarkable passive cooling capacity have been explored in recent years based on advanced fabrication techniques.² In order to evaluate the passive cooling capacity of a material, two essential techniques are typically involved. First is the spectroscopy investigation on material optical properties, which determines the spectral absorption in both solar and MIR regions. Integrating with the equation that comprises each energic term, one material's net passive cooling power can be theoretically calculated. The other technique is the experimental outdoor measurement, from that subambient steady-state temperature of passive cooler, and the cooling power that emitter can achieve at ambient temperature could be practically obtained. However, the outdoor measurement is impressionable and uncontrollable. The outcome depends strongly on atmospheric conditions, e.g., geographical location, solar intensity, ambient temperature, humidity, wind speed, air pressure. The ambiguous and unsteady outdoor measurement conditions limit the comprehensive characterization of passive cooler, on the one hand, and replication of measurement on the other hand. Additionally, because of that challenge, distinct materials can barely be compared reasonably.<br/>In this work, we proposed a tailored indoor setup for comprehensively characterizing the performance of daytime passive cooling materials. By using a liquid nitrogen-cooled hemisphere aluminum dome as the heat sink and AM 1.5 solar simulator to provide solar light, outdoor characterization for passive cooling materials can not only be imitated for nighttime but also daytime measurement. Interrogating the qualification of the setup with three reference materials, i.e., Ag mirror, PDMS film, and graphite coating, outstanding repeatability were demonstrated, and the emissivity dependence of sample temperature is observed in accordance with the expectations. Besides, with the proposed setup, the impact of variables, i.e., ambient temperature and solar irradiation intensity, on material cooling performance can be experimentally determined, which is unfeasible for outside measurement, on account of the uncontrollable measurement condition. Furthermore, our indoor setup is simple to recreate, which opens a pathway to compare passive cooling materials designed in different research groups.<br/><br/>1. Hossain, M. M.; Gu, M., Radiative Cooling: Principles, Progress, and Potentials. <i>Adv. Sci. </i><b>2016,</b> <i>3</i> (7), 1500360.<br/>2. Li, W.; Li, Y.; Shah, K. W., A materials perspective on radiative cooling structures for buildings. <i>Sol. Energy </i><b>2020,</b> <i>207</i>, 247-269.