# Self-Cleaning Solar Mirror Coatings: From the Laboratory Scale to Prototype Field Tests
## 3 Results
### Field Positioning and Testing  
The ENEASHIP demonstrative CSP plant (see Figure 5) consists of a 36 m long linear solar collector, which, in turn, consists of 425 mirrors (62.5 \(\times\) 125 \(\times\) 4 cm), resulting in a total of about 330 m\({}^{2}\) of reflective surface. For testing self-cleaning mirror prototypes developed in this work and labeled as S_AIN (south AlN-coated mirror) and N_AIN (north AlN-coated mirror), some of the original mirrors were removed and then covered with the samples intended to be studied, indicated in the scheme with the light blue color to distinguish them from their substrate references, labeled as SB (south reference mirror) and SA (north reference mirror), colored in light green.  
Figure 4: Test sample transmittance in UV-VIS-NIR compared to the XG glass substrate.  
Figure 5: ENEASHIP Fresnel plant scheme with positions of prototypes and their references indicated in light blue and green (**a**) and photo of a particular view of the plant (**b**).  
Bearing in mind that the pre-industrial ENEA 2 sputtering equipment in the applied research laboratory allows for the processing of a 50 \(\times\) 40 cm substrate, to obtain a larger mirror, it is necessary to manufacture several identical sheets (using the robust and repeatable process described in the previous paragraph) and properly join them. An integration scheme useful for field tests was proposed, consisting of gluing the mirror panels with a size of 50 \(\times\) 40 cm obtained by sputtering onto the previously removed BSM solar mirror substrates, according to the diagram shown in Figure 6, where the entire surface of the BSM can be covered with three panels with a size of 50 \(\times\) 40 cm.  
The two identical mirror prototypes, S_AlN and N_AlN, fabricated as described, were placed in two different solar Fresnel field positions, as indicated in Figure 5. By means of a portable reflectometer, their reflectance was measured at discrete and fixed points indicated  
Figure 6: BSM commercial mirror (**a**) prototype scheme (**b**), photo of glued mirror panels in the mechanical workshop (**c**) and photo of fixed R% measurement points (**d**).  
in Figure 6d and compared with the value of adjacent uncovered mirror substrates SB and NB. The averaged initial reflectance values are reported in Table 2.  
Prototype S_AIN was exposed to the external conditions of the plant site, and the results for the first three months of the external measurement campaign are reported in Figure 7. The measurement campaign will continue in the following months.  
In Figure 8, the reflectance, R%, of two identical prototypes positioned to the south and north of the plant were compared with the uncovered substrate, taking into account the potentially different exposure.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample** & **S\_AIN** & **SB** & **N\_AIN** & **NB** \\ \hline
**R\%** & 92.99 \(\pm\) 0.12 & 95.57 \(\pm\) 0.07 & 93.38 \(\pm\) 0.19 & 95.24 \(\pm\) 0.08 \\ \hline \end{tabular}
\end{table}
Table 2: Prototypes’ averaged reflectance values.  
Figure 8: On-field reflectance comparison of the two prototypes with an uncovered mirror.  
Figure 7: S_AIN prototype reflectance field tests in external environment of Casaccia (Rome) ENEA-SHIP demonstrative plant site.  
Figure 9 shows a flow chart of the procedure for estimating the relationship between WCA and water consumption in the cleaning operation of the AlN auxetic coating deposited on glass compared to its substrate and Teflon samples.  
A retention of 42% of used water on the washed surface when WCA passes from 46.8\({}^{\circ}\) to 89.6\({}^{\circ}\) and of 58% when WCA passes from 46.8\({}^{\circ}\) to 126.1\({}^{\circ}\) can be observed.