Bioinspired and Sustainable Materials Concepts for Passive Daytime Cooling

Passive daytime cooling is an advanced technology that uses radiative cooling principles to remove heat from surfaces and release it into outer space. This is achieved without requiring external energy inputs. The process involves using materials with specific optical properties to reflect sunlight and emit thermal radiation within the atmospheric window (8-13 microns). As a result, it can cool objects below ambient temperatures, even when in direct sunlight. Passive daytime cooling is important because it has the potential to significantly reduce energy consumption for air conditioning, lower carbon emissions, and provide sustainable cooling solutions in various climates.<br/>In this contribution we outline three concepts with a focus on sustainable or circular materials usage to realize scalable passive cooling materials.<br/>Firstly, we take inspiration from nature, where passive cooling materials based on chitin are used by the silver Sahara ant to maintain comfortable temperatures. We will, therefore, outline the potential of chitosan and chitin as polymer matrix materials for passive cooling composites. We demonstrate the fundamental passive cooling properties of this ubiquitous, biogenic polymer as a thermal emitter.¹ Nevertheless, this approach still requires a metallic support structure to effectively scatter solar radiance.<br/>Secondly, we outline how to repurpose post-consumer aluminium-plastic laminate (APL) waste, specifically from chip bags, into passive daytime cooling foils. APL waste poses significant recycling challenges due to its complex material composition. However, its mirror-like surface with high solar reflectance makes it well-suited for passive cooling applications. We have developed a flexible, low-cost cooling foil by coating APL waste with high-emissive polydimethylsiloxane (PDMS) or laminating pouch layer.^{2, 3} This innovative approach demonstrates the potential of converting waste materials into valuable resources for sustainable cooling technologies.<br/>Thirdly, we turn our focus to biocompatible, strongly scattering composite materials. Both the polymer and filler material are ubiquitously available and food-grade. Their combination results in strong solar scattering and broadband thermal emission. This approach even empowers non-experts in remote areas to fabricate paint-like passive cooling formulations that can be applied to various surfaces.<br/><br/><br/>(1) Lauster, T.; Mauel, A.; Herrmann, K.; Veitengruber, V.; Song, Q.; Senker, J.; Retsch, M. From Chitosan to Chitin: Bio-Inspired Thin Films for Passive Daytime Radiative Cooling. Adv Sci (Weinh) 2023, 10 (11), e2206616. DOI: 10.1002/advs.202206616<br/>(2) Song, Q.; Retsch, M. Passive Daytime Cooling Foils for Everyone: A Scalable Lamination Process Based on Upcycling Aluminum-Coated Chips Bags. ACS Sustain Chem Eng 2023, 11 (29), 10631-10639. DOI: 10.1021/acssuschemeng.3c00683<br/>(3) Song, Q.; Tran, T.; Herrmann, K.; Schmalz, H.; Retsch, M. Upcycling Chips-Bags for Passive Daytime Cooling. Advanced Materials Technologies 2023, 8 (18), 2300444. DOI: 10.1002/admt.202300444.