Plastic films to be used in a night-time radiative cooling technology: Optical and mechanical study for a RCE device

Current energy situation is not at all promising for the future, due to the fact that the demand for energy increases annually (according to ICAEN data, more than 40% comes from buildings). Additionally, energy main resources have non-renewable origin. Thus, one of the measures implemented by the European Union is the long-term Decarbonization Plan. Consequently, renewable energies must play a priority role to reduce fossil fuels consumption. SEMB research group of the University of Lleida, due to the fact that the cold production requires a large consumption of electricity (wide use of compression systems) and solar collectors only work during the day (partial use), have devised a unique device, based on a modified solar flat collector, to combine the production of hot water (DHW) and cold water by means of radiative cooling in a renewable way. This system is called Radiative Collector and Emitter (RCE). This innovative system makes possible to produce heat during the day and cold during the night through an adaptative cover, obtaining temperatures below ambient. However, the main problem is that solar collection materials and radiative cooling materials have different properties. In addition, the fact of being exposed to the weather causes the materials to suffer from the aging phenomenon. It is widely spread in the literature the used of polyethylene in radiative cooling based on its great transmittance in the atmospheric window (7-14 µm). However, it is a material that degrades relatively easily in mechanical terms. Consequently, it was decided to carry out a study of the optical and mechanical properties of 5 types of plastics in order to choose the best candidate for the self-adaptive cover of the RCE. Specifically, in this paper average transmission values, aging evolution and mechanical properties are studied for five plastic films between 35 and 100 mm (two low density polyethylenes, LDPE-60 and LDPE-100, one high density polyethylene, HDPE-60, one polypropylene, PP-35 and one fluorinated ethylene propylene, FEP-50) exposed to environmental conditions during three months, using a Jasco FT-IR 6300 (for optical properties) and ZwickRoell BZ1-MMZ2.5.ZW01 (for the mechanical ones) series equipment. It should be noted that there are two types of experimental conditions for the different plastics studied: glass-covered samples and uncovered samples (design must be as close as possible to the RCE real conditions). Results obtained confirm the aforementioned: polyethylene is the material that shows better optical behavior in terms of transmission to the atmospheric window, but it has significant loss of mechanical properties over time. On the other hand, polypropylene presents an average transmittance only 3% lower than polyethylene but shows a great resistance to the aging phenomenon. In addition, mechanical properties are an order of magnitude greater than polyethylene. Consequently, it is concluded that polypropylene is a suitable candidate for this type of technology. However, more efforts must be made in the area of smart materials that have dual functionality (heat and cold).

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