1. Introduction
Energy consumption has been increasing worldwide due to modern society energy demands. In the European Union, 40% of the total energy consumption is in buildings. The Eurostat [
1] concludes that space heating represents 64.1% of the total consumption in buildings, domestic hot water (DHW) 14.8%, and space cooling 0.3%. A recent report by the International Energy Agency predicts also that refrigeration demands will triple worldwide by 2050 if no action is taken [
2]. Thus, there is a need for covering heating and cooling energy demands by using more efficient systems.
Solar thermal collectors are a mature and commercially implemented technology to produce hot water from renewable energy. Highly efficient solar collectors for exploiting solar irradiation in an optimum way have been developed in the last decades [
3]. Most current cooling systems run on compression cooling cycles, consuming high amounts of electricity, especially in the summer heat peaks. An alternative to produce cold is solar cooling, combining solar thermal collectors with an absorption heat pump. Absorption heat pumps reduce the electrical energy consumption but presents drawbacks such as low efficiency, the lack of small capacity units, and the need of high temperatures (>100 °C) to increase their efficiency [
4]. Moreover, auxiliary equipment is required, such as the absorption chiller and the cooling tower, which increases the cost of the installation and can result in health problems such as
Legionella. Another renewable alternative to produce cold is radiative cooling.
Radiative cooling [
5] is a phenomenon that has its origins in a physical principle discovered by the German mathematician and physicist Max Planck and it is based on the simple fact that all bodies radiate energy to the outside at a specific wavelength. This radiation depends on the body temperature and the low effective temperature of the space makes it possible to cool down below ambient temperature [
6,
7]. Therefore, when a body is at higher temperature than the space, energy is radiated to the space and, consequently, the body cools down. This cooling should be considered a renewable cooling, in the sense that no additional energy input is needed to induce this cooling to happen. In other words, heat transfer by cooling radiation occurs continuously and spontaneously in a spectral region of wavelengths, specifically at long-bandwidths between 7 and 14 μm (located in the so-called infrared region, and commonly known as the atmospheric window) where the atmosphere has a relatively low absorption. Since the atmosphere is mostly transparent at these wavelengths and does not absorb the energy emitted in this range, materials that emit radiation in the range of 7 to 14 μm send it directly into space.
Combining both traditional solar collection during the day and cooling production during night-time, using exclusively renewable energy, has been proposed by several authors [
8,
9,
10,
11,
12]. Vall et al. [
10] presented for the first time a single device called a radiative collector and emitter (RCE), which combines solar heating and radiative cooling functionalities using an adaptive cover. Optical characteristics of this adaptive cover are different (almost opposite) for solar collection than for radiative cooling. While the solar collection requires a cover with high transmittance of radiation in 0.2–4 µm wavelengths and a low transmittance for the rest of wavelengths, the radiative cooling surface requires a high transmittance in the wavelength range between 7–14 µm (to radiate to the outer space through the atmospheric window). Moreover, the absorber/emitter surface used for both heating and cooling purposes should have a high absorptivity/emissivity in the full solar and infrared (IR) spectrum (broad-band emitter), in contrast to the selective or narrow-band surfaces used in conventional solar collectors and radiative coolers [
13,
14]. More recently, other researchers have also proposed this idea of producing both heating and cooling, but with other solutions [
15,
16], especially using spectrally selective materials used as radiative cooler emitters. Other studies focused on the combination of both strategies, selective covers, and emitters to achieve solar collection and night-time radiative cooling [
17].
Solar collectors use a glass cover to act as a wind-shield, as it also provides a greenhouse effect. However, the use of glass is not compatible with radiative cooling, since it does not allow thermal radiation to pass through at the atmospheric infrared window, thus blocking radiative cooling. An adaptive cover is needed when hot water is produced during daytime (solar collector) and cold water is produced during night-time (radiative cooling). A traditional solar collector is modified to include a plastic film below the solar collector glass. This plastic film remains present in both modes (daytime solar collection and night-time radiative cooling). Several studies have demonstrated the importance of the use of a wind-shield for radiative cooling [
18,
19], in order to reduce the parasitic heat gains, and thus increase the cooling power. Although wind-shield materials for radiative cooling have been proposed, none of them fulfil all the requirements needed. Crystals, such as zinc crystals [
20,
21,
22], are good candidates for having high resistance to environmental conditions, but their high price and the impossibility of manufacturing them in large dimensions make them unfeasible. Chromic materials [
23,
24] are promising materials for radiative cooling but fundamental research is still needed, and they are also expensive to produce. Plastics are basically the materials used in radiative cooling applications because they are cheap and can be produced in adequate dimensions. However, a drawback is that they have less resistance to external environmental conditions.
To date, films of polyethylene (PE) have been widely used as wind-shields for their high average transmittance values in the atmospheric window. Average transmittance values for 50 μm low-density polyethylene (LDPE) films around 80% are found [
25,
26]. However, it is well known that polymers show degradation (thermal, oxidative, chemical, radiative and mechanical) when exposed to outdoor conditions. Abdelhafidi et al. [
27] reported that photodegradation of LDPE films is basically due to the ultraviolet (UV) radiation, which can be considered the most harmful factor of plastic degradation. Balocco et al. [
28] also demonstrated that photooxidation of polyethylene plastic films implies a process of bonds breaking, increasing the amount of low molecular weight material, as well as an increase of its hydrophilicity by the presence of carbonyl groups. Plus, an embrittlement of the material is also detected [
29]. Fourier transform infrared spectroscopy (FT-IR) and mechanical tests have been reported by different authors as suitable techniques to follow degradation of polymers [
30,
31,
32,
33,
34]. Ali et al. [
35] reported a substantial decrease of transmittance when exposing a 50 μm polyethylene film to outdoor conditions for 100 days, which led to a reduction of the radiative cooling system cooling power by 33.3%. Martorell et al. [
36] presented an experimental study where a decrease between 3.5% and 9% in the polyethylene average transmittances in the atmospheric window for three winter months of exposure to the environment was measured.
According to Zhang et al. [
19] the wind-shield needs to have high mechanical strength to withstand outdoor weather conditions, such as strong winds, strong rain, and even hail. Mechanical tests such as traction tests give insight in this issue. Some authors [
37,
38,
39] have focused research on finding the relationship between the change of the chemical structure and the change in mechanical properties. Carrasco et al. [
30] show that the replacement of the C–H bonds for C=O bonds turns into an increase of the Young’s modulus, producing a stiffness of the material.
Thus, it is important to find a material transparent to long-wave radiation (with high thermal transmittance in the wavelength range between 7–14 μm), highly resistant to abrasion and moisture, durable, with zero or very low degree of hygroscopicity, with a certain degree of hardness, certain tensile strength, and high degree of elasticity. As explained previously, polyethylene is widely used as a wind-shield for radiative cooling but it shows optical degradation when exposed outdoors and has poor mechanical performance. This work focuses on experimentally studying the optical and mechanical behavior of different high available and cheap plastic wind-shield candidates to find an alternative to polyethylene to be used as radiative cooling wind-shield for a solar collection and night-time radiative cooling applications.
4. Conclusions
This article presents a study of plastic films with thickness between 35–100 μm to determine their suitability to be used as wind-shields for radiative cooling applications. Plastic films samples covered with glass or uncovered and exposed to environmental conditions during 90 days have been studied. FT-IR spectra and traction mechanical tests have been used to study materials degradation.
A decrease in the average transmittance in the atmospheric window between 3.5% and 6.5% for LDPE-100 and LDPE-60, respectively and 9% for HDPE was calculated. PP-35 shows the lowest decrease in transmittance, with a value of 3%.
Polypropylene 35 μm (PP-35) does not show a significant aging process when covered with glass. When the plastic is exposed without glass, the decrease in the average transmittance is only 3%. In addition, FT-IR spectra only show a carbonyl absorption peak for the 90 days samples without glass, confirming a good aging behaviour.
Polypropylene (PP-35) is stiffer than polyethylene (higher Young’s modulus and maximum tensile strength). PP-35 after 90 days presents a low increase of Young’s modulus (of 7.99%), meaning that there is only a low stiffness of the sample. This result matches with the absence of double bonds and C–O groups in the FT-IR spectra, indicating the good aging behaviour of PP-35. Finally, an increase of 16.56% of the maximum tensile strength for 90 days was observed, indicating that no scission reactions occurred. This explains also while the elongation at break of the PP-35 increases after 90 days.
To sum up, polyethylene has been confirmed as a good candidate to be used as a wind-shield for radiative cooling. Polypropylene has been presented as an alternative because it is also transparent to long-wave radiation but presents better hardness, tensile strength, and elasticity than polyethylene.