A multifaceted polymer coating as thermal emitter for sub-ambient passive radiative cooling of buildings and photovoltaics

Kurzfassung

Passive radiative cooling technology offers viable solution for the thermal management of both buildings and photovoltaics (PV) sectors in terrestrial and extraterrestrial applications. Herein, we demonstrate and analyze the effectiveness of a simple solution-processed thin organopolysilazane polymer coating as thermal emitter for sub-ambient passive daytime radiative cooling (PDRC) of surface structures and solar cell devices. The 5µm thick Siliconoxycarbonitride (SiCNO) polymer emitter features spectral selective emissivity in Mid-Infrared spectrum due to the perfect overlap of the vibrational modes of Si-O-Si, Si-N-Si, Si-C bonds with the atmospheric transmittance window. Applying the SiCNO emitter on a metallic reflector substrate yields high reflection in the solar wavelength range (0.3-2.5µm), and high narrowband emissivity in the atmospheric transmittance window (8-13 μm). The thin PDRC device can cool down to 6.8°C below ambient corresponding to a net cooling power of 93.7 W/m2. The deep sub-ambient cooling performance of PDRC was experimentally demonstrated using a cryogenic indoor setup under vacuum conditions reaching a max ∆T of 33°C (at 10-3 Pa) below ambient. Using analytical modeling we studied the impact of individual parasitic heat losses on the PDRC under variable air pressure. We suggest a rational design of sub-components and operational pressure regimes for a vacuum-based cooler system in real-world applications. Moreover, the SiCNO coating is a suitable candidate as an encapsulating coating for solar modules due to the exceptionally high transmissivity in the solar spectrum range and high emissivity in the infrared region. Our thermal and electrical analysis demonstrated that the polymer emitter can solitarily cool down lightweight flexible solar modules in a lower earth orbit by 30°C without inducing efficiency losses from the device. The robustness of this hydrophobic protective polymer coating was proven through various accelerated degradation tests. The results from these investigations indicate that polysilazane polymer coating is an excellent thermal emitter material for both terrestrial and space applications.