The main objective of the project wass to develop new spectrally selective paint coatings of nanomaterials for improving the performance and the visual aesthetics of building-integrated, polymeric solar collectors. The coating should also be cost-efficient, water-based and environmental-friendly, hence applicable for up-scaling in a solar collector production line.
The motivation was that solar building envelops do not always fulfil the demands of building designers, planners and owners. In this respect the development of coloured, water-borne, thickness-insensitive, spectrally selective (TISS) paint coatings with anti-soiling effect are a great step forward because we achieve high solar to thermal conversion efficiency and fulfil market needs as sustainability, longevity and aesthetic demands to the building envelope. These challenges were addressed in the M-ERA.NET project WABASELCOAT by project partners from research and industry in Slovenia, Cyprus and Norway.
The outcomes of WABASELCOAT were achieved in the following steps:
* Based on a market scan, the three basic colours were selected for further development, shades of red, dark blue and black. The basic findings are however not limited to these colors. The substrate for the paint coatings are extruded hollow-sheets of high-performance polymers with/without glass fibres content.
* Low thermal emittance of solar coatings in the thermal range of the electromagnetic spectrum reduce the heat losses of the solar collector. This was obtained by selecting waterborne binder and metallic/metallized flakes with lowest thermal emissivity. Among 32 binders, a polyurethane-based resin was chosen as the right candidate for our application after UV-VIS reflection measurements and IR spectroscopy. Various flakes suitable for waterborne and solvent-based coatings were studied. It turned out that the size of the scales and their surface area played a key role for the increased reflection in the IR range, which was evident from SEM images. Flakes from the Aquamet program were found suitable for waterborne systems and had the lowest thermal emittance (eT = 0.16).
* High solar absorptance of the coatings enhancing collector efficiency: Various multi-walled carbon nano tubes (MWCNT) were identified and incorporated in the binder. In the selection of the MWCNT, price and market availability were main criteria. We found that as received MWCNT needed to be shortened and their surface properties modified. This led to good distribution of the tubes in the binder and improved the solar absorptance to targeted 93% (black) and 90% (coloured coatings) under optimized lab conditions.
* The free surface energy of individual binders was investigated before and after curing in order to optimize the spreading of the coating on polymer substrates, which is very important for good adhesion of coatings. For the pre-treatment of the absorber substrate best adhesion was obtained with sand-blasting, which allowed for lower free surface energy, greater roughness, resulting in better coating flow, better adhesion and cohesion of the coating. For industrial use, this process is environmentally friendly and inexpensive, although applicability in a production line to be verified.
* The durability of the paint coatings was investigated. The paint coatings should sustain dry stagnation with temperatures up to 150°C if applied in a solar collector. For exposure to realistic, maximum thermal load, a full-sized, TISS-paint coated absorber was integrated in a solar collector casing and exposed to dry-hot Mediterranean climate (Cyprus). First stagnation tests passed satisfying after the inspection of exposure to one summer season.
Accelerated service-life testing of the TISS paints applied on absorber substrate samples did not reveal mechanical or optical failures until now. These durability tests included direct exposure to UV radiation, dry heat, humidity, temperature cycling, wind and soiling.
* Part of the work in the project was dedicated to the development of a hydrophobic and anti-soiling properties of the solar absorber coating. Various, so-called "POSS molecules" (Polyhedral oligomeric silsesquioxanes) were synthesized as additives for TISS paint coatings. Synthesizing can be done at room temperature, is scalable and transferrable to the industry. Different flame-retardant additives for polymer binders were evaluated. These are important for solar collector integrated in the building skin.
* A Slovenian patent application was filed during 2020, which protects the IPRs in WABASELCOAT. Five scientific publications on WABASELCOAT topics; 3 have been published and 2 are submitted to peer-reviewed journals during the project period.
The project results are planned to be used commercially after having passed online-production verification, secure supply chain and cost analysis.
Multifunctional paint coatings were developed for a new solar collector technology, which is by design a building envelop system, replacing conventional roof or facade covers. The coatings enhance the collector performance, allow architects and decision makers to choose among different colors in order to match the building's design. This will overcome limitations given by existing technologies and contribute to increase renewable heat production.
We focused on developing environmentally friendly, waterborne spray coatings. These can easily be applied, have a lower environmental impact during synthesis and application, and guarantee better working environment for the production staff. A reduced environmental impact will be positive to be included in product declaration and LCA figures of the new collector technology.
An important aspect was to build on easy-available paint components and obtain cost-competitive coatings for broad use and increase renewable energy production.
Solar absorbers incorporated in façades systems become a reality in the modern architecture, although they do not always fulfil the demands of building designers, planers and owners. In this respect coloured spectrally selective paint coatings are required. Water-borne thickness insensitive spectrally selective coatings with anti-soiling effect are one of the optimal choices since they offers the possibility of achieving high solar to thermal conversion efficiency, longevity and high aesthetic demands of architects. At present, the practical use is faced with three major problems:
i) non-selective coatings are used for increase the absorptivity,
ii) coatings are made by solvent-borne resins, high volatile organic solvents (VOC) emission and
iii) the dust and dirt are collected on the absorber surface resulting in decreased efficiency.
The efficiency can be improved using black or coloured spectrally selective inorganic and metallic pigments incorporated in water-borne resins with low thermal emittance resulting in minimizing VOC emissions. Anti-soiling properties will be incorporated by tailoring of new additives. Proposed coatings will be tested towards market needs.