Hexagonal array grating structured PDMS film for all-day radiative cooling system

Abstract

Radiative cooling is a technology that is able to cool down the cooler surface without any external power source and relieve the cooling load of building. It is possible by emitting thermal radiation into space through the atmosphere, while mostly reflecting the irradiation emitted from atmosphere and the sun. The net cooling power achievable can be maximized by enhancing radiative emission power while suppressing absorption from atmospheric and solar irradiation. For that purpose, it is beneficial to have high emissivity in the transparent regime of atmosphere, namely the sky-window spectrum, while having low emissivity and high reflectivity in the rest of the spectrum. Selection of materials with favorable inherent property and further spectral tailoring by building sub-wavelength structures are two principles for achieving such property. Multilayered film, gratings, cavities, particle added films or paints, porous or hierarchy structured polymer film are noticeable approaches, where they largely differ in cooling performance and fabrication difficulty. In this work, hexagonal array grating patterned polydimethylsiloxane (PDMS) film (PDMS_HGF) is proposed for all-day radiative cooling system. PDMS is inherently emissive (i.e. absorptive) in the mid infrared (MIR) sky-window regime and highly transparent in visible to near IR solar regime. MIR emissivity of PDMS film could be further enhanced using periodic 2D grating structure, where fundamental problem of angular deviation in optical property is mitigated by choosing hexagonal array and circular grating shape. With silver or aluminum back-reflector layered with the film, highly selective optical property is achieved. It is worth mentioning that the structure is simple enough to be mass-producible using roll-to-roll fabrication method. We optimize the grating dimension to have maximum all-day accumulated cooling power. To benefit the total harvested thermal load is beneficial than considering only instantaneous power with static solar irradiation. In the process, we develop robust and efficient reflectance simulation method which utilizes rigorous coupled wave analysis with finite difference time domain method both post-processed with spectral averaging techniques. After that, we develop artificial neural network model for instantly predicting cooling performance from the grating dimensional variables, to apply it in genetic algorithm for finding the optimal variable set. Here, the all-day accumulated (or average) cooling power, the optimization goal, is found regarding the time-varying solar zenith angle penetrating through atmospheric environments which was conducted for the first time. All-day optimum PDMS_HGF shows 0.999 sky-window emission with 93.6 % solar reflection and theoretical performance enhancement by 6.6 W/m2 in all-day cooling power, compared to flat PDMS film cooler. Simple lithography process is used for fabrication for analysis, where parasitic surface roughness and corner rounding problem was suppressed to produce the PDMS_HGF with 3.6 W/m2 cooling power margin. Another important function that radiative cooling system must have is a resilience against weathering environment such as contamination and raining. Here, self-cleaning function has gotten attention at which rain droplet rolls-off from the surface together with contaminants in the pathway. Our PDMS_HGF possess enhanced self-cleaning function as demonstrated in droplet bounce experiment and contact angle measurement, which shows that the grating structure contributed to both performance enhancement and resilience of the radiative cooling system. In the last section, an experimental method to comprehensively characterize the performance of radiative cooler exposed to varying environment is devised and demonstrated. The methodology developed will assist the design and optimization of radiative cooling systems regarding its continuous and environment responsive performance.