# From Self-Cleaning to Self-Aware Solar Mirror Skin
## 1 Introduction  
Significant efforts are being made to reduce the operating expenses (OPEX) of concentrated solar power (CSP) systems [1]. One key challenge is the performance of solar reflectors/mirrors, which is compromised by soiling [2, 3]. Restoring the specular reflectivity of these mirrors requires large amounts of water [4], and the cost of cleaning procedures directly impacts the levelized cost of energy (LCOE) produced by CSP plants [5]. Among the various strategies to address this issue, the most effective involves the development of solar mirrors coated with thin, transparent, self-cleaning films. These coatings allow for significantly reduced water consumption during cleaning, leading to considerable cost savings, particularly in regions with high labor or resource costs, such as desert areas [6]. Additionally, self-cleaning coatings play a crucial role in maintaining the cleanliness of all types of solar mirrors while resisting environmental degradation over time, without compromising optical transparency or reflectivity [7]. Consequently, an emerging trend forensuring the long-term performance of solar mirrors is the integration of autonomous and intelligent operational quality control systems, like those already being implemented in the photovoltaic sector [8; 9; 10]. Providing a solar field with intelligence and automating O&M (Operations and Maintenance) processes requires the ability to monitor the dirt level or detect failures in individual mirrors (or even the entire field), enabling on-demand cleaning or component replacement [11]. Practically, this could be achieved by adding commercial sensors to the mirrors. However, this approach would be costly and inefficient, as there are no commercial sensors currently suitable for use on solar mirrors. It is important to remember that solar mirrors have strict optical requirements, are exposed to outdoor conditions, and, even in medium-sized solar plants, are found in large quantities [12]. To date, no commercial sensor-equipped mirrors exist, only patented prototypes that incorporate scattering sensors into the mirror design [13]. If a layer of the existing solar mirrors--such as the exposed layer--were made sensitive to soiling (or other specific parameters), it would allow for the monitoring of their performance. This has motivated us to explore the integration of sensing thin films directly into the mirror structure. In our previous work [14], we demonstrated that the wetting properties of solar mirrors can be modified by applying transparent auxetic metamaterials to reflector surfaces using scalable processes, with the goal of reducing water consumption in cleaning procedures. Auxetic aluminum nitrides, deposited via sputtering on metallized low-iron glass, have been proposed as an ideal self-cleaning solution for back-surface mirrors. These coatings are transparent, hydrophobic, versatile, and can be applied to large substrates using cost-effective and scalable techniques. But can these coatings be further engineered to meet additional functional requirements? This study seeks to answer that question. Our objective is to develop self-cleaning metamaterials that can also function as sensors to detect mirror performance failures caused by dust accumulation, excessive humidity, or other critical factors such as structural damage, aging, corrosion, and erosion. Through a dedicated electronic interface and IoT integration, these materials could provide real-time data on soiling levels and the overall functionality of individual components or the entire solar field. Regarding aluminum nitrides, it is well known that thin films exhibit piezoelectric behavior when they crystallize in the hexagonal phase and grow perpendicularly to the substrate along the c-axis [15]. Piezoelectric phases develop an electric dipole when the mechanical strain is applied to them.