# Self-Cleaning Solar Mirror Coatings: From the Laboratory Scale to Prototype Field Tests
## 4 Discussion  
Fabricating new materials at the laboratory scale requires the synthesis and characterization of various samples to obtain the desired physical-chemical properties. As already explained in a previous work, in the case of auxetic AlN, this has led to a careful definition of different experimental conditions. To move toward large-area fabrication of the material, it is necessary to select among these conditions the most profitably scalable. The scaling-up of the sputtering process of auxetic AlN deposition from a laboratory scale to a pre-industrial scale with the proprietary ENEA2 multicathode sputtering apparatus was made possible by changing the tubular sample holder (utilized for obtaining materials at the lab scale) to a planar holder, as shown in Figure 2. With this arrangement, rotation is not possible, so the goal was instead to change the process parameters to obtain the desired film property in a frontal geometry (in terms of the trade-off between optical clarity and WCA). This was made possible, after a long experimental work, by changing the power supplied to the target (from 1500 W to 1800 W) and the total pressure of the process (from 0.1 to 3 Pa). Clearly, on substrates of dimensions larger than the sputtering target, it is important to verify the homogeneity of fabricated coating properties. For this reason, a series of testing substrates were positioned at different heights in the frame of the 50 \(\times\) 40 cm holder, that is, the maximum area obtainable with such apparatus, as shown in Figure 3. It was observed that only the sample 1 has a slightly little thickness for borders effects influence, while the other samples are comparable in terms of thicknesses and optical transmittance (reported in Figure 4). For the general purpose of adding one layer to existing metallized glasses of BSM solar reflectors, the observed inhomogeneity (<10 nm) can be accepted, especially considering that, when positioned on-field, such mirrors can have a tolerance of several hundreds of microns. The WCA reported in Table 1 differs from the medium value of 87\({}^{\circ}\) by a small amount due to thickness. So, the selected process conditions can be considered useful to produce prototypes for the desired application. Different samples with big dimensions were fabricated by means of the same process, glued onto commercial BSMs of the demonstrative plant substrates, according to the scheme in Figure 6, and positioned in the solar field of ENEASHIP Fresnel plant in two  
Figure 9: Flow chart of the water retention estimation procedure. homologous rows to the north and south, as indicated in Figure 5, next to reference mirrors obtained by gluing the same number of uncoated panels to their surfaces. Their reflectance was measured by means of a portable reflectometer following the measurement protocol described in session 2 for a preliminary three-month period. The initial values are reported in Table 2, and it can be noticed that there is only a little difference (R% \(-\) Rsub%% \(\times\) 5) with respect to the uncovered solar mirror substrate, thanks to the research work on maximizing the optical transmittance of coatings. The comparison between the north- and south-positioned prototypes' reflectance in the first 50 days shows a small difference in their soiling, suggesting that each mirror in a field can be subject to this discrete phenomenon to a different extent. At the same time, the external positioning does not deteriorate the coatings (Figure 7 shows reflectance vs. days for the entire exposure time), even in the presence of Scirocco rain events. Idem washing procedures consisting of spraying distilled water do not degrade self-cleaning layers, and they are able to restore the initial reflectance. These simple observations are not at all obvious, as many self-cleaning coatings, when positioned outdoors, delaminate from the substrate, especially if they are nano-micro-structured, and suffer erosive damage, which removes the self-cleaning performance.