De Luca, Daniela (2022) Selective coatings for efficient solar energy conversion in flat panels. [Tesi di dottorato]

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The main limitation of solar power is linked to our ability to turn it into electricity or heating in an efficient and cost-effective way: the fraction of the produced renewable energy is still very low compared to the total demand, which makes even more interesting and indispensable the research in more efficient energy usage, generation, and conversion systems. To provide solutions to these problems, this thesis focuses on the description and development of new optical elements useful for improving the performance of solar devices, showing that controlling and tailoring the optical properties of such elements is crucial. In fact, this Ph.D. program is born as a part of the FSE-FESR National Operational Program "Ricerca e Innovazione 2014-2020" of the Italian Ministero dell’Istruzione, dell’Università e della Ricerca, and intends to support the promotion and strengthening of fostering collaboration between academic world and private industry in the field of scientific research. Therefore, in this context, I have been involved in the development and optimization of new products in collaboration with the company TVP Solar [11]. The first part of this Ph.D. program has been focused on the increase of the energy conversion efficiency in high-vacuum flat solar thermal panels through the development of a novel filter, the IR mirror, to be applied on the glass envelope of the HVFPs developed by the company TVP Solar. In this context, such mirror is useful for triggering the cold-side external photon mechanism in evacuated collectors, allowing for the recovery of the infrared radiation emitted by the hot absorber. However, even if it has been developed for a high-vacuum flat solar thermal panel, the approach is general enough that it could potentially be extended to any emissive solar thermal device. The second part of the Ph.D. program is focused on the development of selective emitters to be used in thermo-photovoltaic (TPV) technologies. In fact, the development and use of selective emitters could play a key role in increasing the efficiency of these devices and in reducing heat losses: they would act as a "filter" that lets only photons with energy above the bandgap of the photovoltaic cell pass through. The idea is to design easy-to-fabricate selective emitters based on multilayers, as the ones currently available involve mainly metamaterials, metasurfaces, plasmonics structures or nanofabrications, which could be difficult to scale at an industrial level. One of the main drawbacks in the development of both IR mirrors and selective emitters is the need for simple and cost-effective solutions, which must be easily reproducible on a large scale production and used on a daily basis in industrial applications. This brought with it many constraints, such as the use of deposition techniques that can be easily applied on an industrial level, the design of structures which avoid complex techniques (such as nanostructuring of surfaces, nanoparticles or ion implantation), the choice of materials inexpensive and easy to handle, etc. In fact, this work represents the first step towards the broader, industrial development of these elements, where all the aforementioned characteristics are fundamental.

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