Investigation of Passive Radiative Cooling Using Biopolymers

Passive thermal radiative cooling (PTRC) has drawn massive attention in the past few years due to its advantages, including excellent cooling potential, no emission of greenhouse gases, silent operation, low maintenance, and off-grid operation. PTRC has been successfully demonstrated to reduce the electricity consumption required for cooling and ventilation of buildings. Several radiative emitters have been studied in the literature, such as pigmented paints, nanoparticle-based coatings, photonic crystals, metamaterials, and polymers. Among them, polymers have proven to be inherently strong infrared (IR) emitters, scalable, low-cost, flexible, easy to apply, and durable candidates. In addition to these features, biopolymers are eco-friendly and currently abundant in the market. Despite their significant advantages, there have been limited studies on the applications of biopolymers for radiative cooling. In this study, we report promising performances from a commercially available, affordable, and applicable biopolymer, cellulose, as a PTRC emitter. We fabricated several cellulose films with various structural characteristics and thicknesses. The emissivity and reflectivity of these emitter surfaces were measured for the desired wavelengths and direction. The obtained measurements reveal relatively high magnitudes of diffuse emissivity in the atmospheric window and high reflectivity in the solar spectrum range. Using the materials’ reflectivity and emissivity data, we theoretically calculated the net cooling power and the expected temperature drop. Each emitter demonstrated high cooling power and considerable temperature reduction based on the average recorded weather conditions in Lincoln, NE.