A radiative cooling device, based on a metamaterial able to mirror solar radiation and emit heat toward the universe by the transparency window of the atmosphere (8–13 µm), reaching and maintaining temperatures below ambient air, without any electricity input (passive), could have a significant impact on energy consumption of buildings and positive effects on the global warming prevention. A similar device is expected to properly work if exposed to the nocturnal sky, but during the daytime, its efficacy could be affected by its own heating under direct sunlight. In scientific literature, there are only few evidences of lab scale devices, acting as passive radiative cooling at daytime, and remaining few degrees below ambient air. This work describes the proof of concept of a daytime passive radiative cooler, entirely developed in ENEA labs, capable to reach well 12 °C under ambient temperature. In particular, the prototypal device is an acrylic box case, filled with noble gas, whose top face is a metamaterial deposited on a metal substrate covered with a transparent polymeric film. The metamaterial here tested, obtained by means of a semi-empirical approach, is a spectrally selective coating based on low cost materials, deposited as thin films by sputtering on the metallic substrate, that emits selectively in the 8–13 µm region, reflecting elsewhere UV_VIS_NIR_IR electromagnetic radiation. The prototype during the daytime sky could reach temperatures well beyond ambient temperature. However, the proof of concept experiment performed in a bright clear June day has evidenced some limitations. A critical analysis of the obtained experimental results has done, in order to individuate design revisions for the device and to identify future metamaterial improvements.

Soiling severely reduces solar mirror performance, requiring dispendious water consumption for cleaning operations and causing an increase in the levelized cost of energy (LCOE). An emerging technology for facing this problem consists of developing transparent self-cleaning coatings, able to be washed with a small amount of water by virtue of the modulation of surficial wetting properties. Nevertheless, the beneficial effects of coatings decrease in the first year, and coated mirrors show even higher soiling than non-coated ones. Moreover, it is important that coating production processes are economically convenient, consistent with the intended reduction of overall costs. The aim of this work is the research and development of a cheap and scalable solution, compatible with mirror fabrication steps and, in such a sense, economically advantageous. It involves the substitution of the alumina last layer of solar mirrors with more hydrophobic, potentially auxetic aluminum compounds, such as nitrides. In particular, 2D inorganic aluminum nitride thin films doped with metals (such as aluminum and silver) and non-metals have been fabricated by means of reactive sputtering deposition and characterized for the purpose of studying their self-cleaning behavior, finding a trade-off between wetting properties, optical clarity, and stability.

In this study, a low-cost, scalable and robust process is proposed as an innovative method for coating solar mirrors with a self-cleaning, transparent in the full solar range and versatile material based on auxetic aluminum nitrides, previously obtained at the laboratory scale. This work presents the scaling-up of the fabrication process from the laboratory to prototypal scale and the preliminary results of outdoor self-cleaning solar mirror field tests in the demonstrative concentrating solar power (CSP) plant ENEASHIP located in Casaccia (Rome) ENEA Research Center. Prototypes with a size of 50 × 40 cm have shown stability in external conditions: no coating degradation occurred during the test campaign. Their washing restores the initial reflectance affected by soiling and the self-cleaning performance allows for the utilization of a reduced quantity of water for cleaning operations with respect to the uncoated glass of back surface mirrors. A similar self-cleaning AlN coating could be utilized on other solar components affected by soiling, such as the glass envelopes in heat-collecting elements, PV panels and other parts where a self-cleaning performance combined with an optical one is required.

A low cost, scalable and robust process has been proposed as innovative method for coating solar mirrors with a self-cleaning, transparent in the full solar range and versatile material based on auxetic aluminium nitrides, previously obtained on the laboratory scale. This work presents the scaling-up of fabrication process from laboratory to prototypal scale and pre-liminary results of outdoor self-cleaning solar mirror prototypes testing in the demonstrative CSP plant ENEASHIP located in Casaccia (Rome) ENEA research centre. Prototypes 50x40cm have shown stability in external conditions: no degradation occurs during the test campaign. Their washing restores the initial reflectance affected by soiling and the self-cleaning performance allows the utilization of a reduced quantity of water for cleaning operations with respect to uncoated glass of back surface mirrors. A similar self-cleaning AlN coating could be utilized on other solar components affected by soiling, such as glass envelopes of heat collecting elements, PV panels and everywhere are required self-cleaning performance combined to optical ones.

Self-cleaning coatings for solar mirrors aim to reduce water usage for cleaning, cut down on maintenance costs for solar fields, and lower the overall electricity production costs in concentrated solar power (CSP) systems. Various approaches have been developed for mirrors with back surface (BSM) and front surface (FSM) architectures, all sharing the characteristic that the self-cleaning coating serves as the outermost layer, acting as a “skin” that protects against fouling. A recent trend in this field is to enhance this “skin” with sensing capabilities, allowing it to self-monitor its performance in terms of soiling or failure, contributing to the digitalization of solar fields and CSP technology. Building on previous work with auxetic aluminum nitrides and ZnO transparent composites, which were developed to replace alumina as the self-cleaning layer in BSMs, this study explores the potential of adding sensing properties to these coatings. The approach leverages the piezoelectric properties of the materials, which can be linked to dust accumulation and surface soiling, as well as their electrical resistive behavior, which can help monitor potential failures. The promising d33 values of sputtered piezoelectric AlN and the tailored electrical properties of ZnO composites, combined with their self-cleaning effects and optical clarity across the full solar spectrum, suggest that these coatings could serve as an intelligent, self-aware skin for solar mirrors.

Water scarcity in arid regions poses a significant challenge for the maintenance of Concentrated Solar Power (CSP) plants, where mirror cleaning consumes substantial resources. This study proposes a systematic methodological framework to bridge the gap between laboratory-scale surface characterization and engineering-scale water consumption. Through a gravimetric approach based on the physical principles of droplet retention (Furmidge theory), the water-saving potential of self-cleaning coatings has been quantified. Experimental results on 100 cm² specimens demonstrate that hydrophobic coatings can reduce residual water from 0.52 L/m² to approximately 0.24 L/m², achieving a water-saving potential of over 50%. The model incorporates a site-specific soiling factor (f_dirt) and was validated using field data from the ENEASHIP pilot plant. This approach provides a promising predictive tool for plant operators to optimize cleaning strategies and reduce operational costs, offering a scalable methodology for the solar thermal industry.