# Self-Cleaning Solar Mirror Coatings: From the Laboratory Scale to Prototype Field Tests
## 1 Introduction  
Solar mirrors play a crucial role in concentrating sunlight for various applications, such as solar power generation and industrial processes [1]. However, the efficiency of these mirrors can be significantly compromised by the accumulation of dirt, dust and other contaminants on their surfaces, leading to reduced reflectivity and overall performance [2]. To address this challenge, the development of advanced surface coatings, known as self-cleaning, has gained prominence in the field of solar mirror technology [3]. A lot of materials specifically designed to modulate wettability (for reducing water consumption in washing maintenance operations) and ensure optimal reflectivity and sustained energy capture over time have been proposed on a laboratory scale or on an intermediate scale in the form of small prototypes [4]. None of these coatings have reached the commercial state for different reasons. Passing from a laboratory scale to an industrial one is not a simple task when there are strict optical requisites to be considered; for example, the external positioning of covered mirrors determines the degradation of the exposed layer, and above all, adding a further step to existing production lines of solar mirrors has to be evaluated in economic terms as trade-off between its cost and real benefits in terms of performance [5]. This last aspect of the economic evaluation of water saving in ordinary solar field maintenance operations is difficult because each site has its own peculiarities that make comparison and, in general, absolute evaluation difficult, but which nevertheless allow us to draw an indication of the fact that any water saving, especially in desert areas,is always profitable [6]. In a previous work, the feasibility of changing alumina, that is, the most diffuse last layer in all mirror architectures, with AlN compounds obtained by means of sputtering deposition was investigated [7]. Different auxetic AlN politypoids have been fabricated (from a 52\({}^{\circ}\) to 100\({}^{\circ}\) water contact angle, WCA, due to the major covalent character of the Al-N bond with respect to the Al-O one) with the purpose of tailoring wettability, preserving optical clarity in the desired solar range and granting durability over time considering the inorganic nature of AlN. Such promising results have been obtained on a laboratory scale but conceived for a subsequent scaling-up in consideration of the fact that they are produced with an intrinsically scalable technique. Although research products studied on a laboratory scale exhibit interesting properties, their effective utilization in the real world often requires a lot of applied research, devoted to verifying if the same properties obtained on the small scale can be preserved when dimensional and performance requirements must be satisfied. The first problem to solve is the selection of a fabrication technique that grants the same chemical-physical properties obtained in the lab on the small scale. For example, this occurs in the case of polymeric products, where the coating is obtained at the lab scale by means of spinning, which is clearly a nonscalable technique and, therefore, needs to be changed, e.g., with printing techniques. This work aims to fabricate inorganic self-cleaning auxetic and transparent aluminum nitride coatings, selected from the family of materials previously developed with sputtering processes on a pre-industrial scale for the purpose of uniformly covering 50 cm \(\times\) 40 cm prototypes, that can be tested as back surface mirrors, BSMs, in the demonstrative CSP Fresnel plant, named ENEASHIP and located in Casaccia (Rome) ENEA Research Center. In particular, the requirements of such coatings are as follows:  
* High transparency in the solar range, so as not to change the mirror's specular reflectance required for optimal operation in concentrating solar power plants. * Self-cleaning performance.