Solar collectors don’t need to be black
In the R&D project WABASELCOAT we have used nanotechnology to develop multi-functional coatings for improving the performance and the visual aesthetics of solar collectors
People want to have a choice between different colors – also when it comes to solar collectors. The problem is that colored solar collectors are far from as effective as the black ones. One solution is to paint the collectors with selective coatings, but most existing coatings are quite expensive and not very environmentally friendly. In the R&D project WABASELCOAT (WAter BAsed SELective COATings for intelligent facade collectors), nanotechnology has been used to develop multi-functional coatings for improving the performance and the visual aesthetics of polymeric solar collectors. Sustainability and cost efficiency have been key requirements throughout the product development phase.
Solar collector facade (Inaventa Solar, Oslo) with black, red and blue TISS paint coatings
Illustration: Chr. Roecker, EPFL
Increased demands on aesthetics
Building integrated solar solutions are becoming more and more popular. However, the acceptance of solar collectors as integrated elements of the building’s envelope is to some extent limited by aesthetic considerations, and one aspect of importance is the color of the panels. Because solar thermal systems often are relatively large in relation to the building envelope, they have a major impact on the visual quality of the building.
Tailor made coatings
Inaventa Solar has developed a new solar collector technology, which is based on high-performance polymers (HPP) and designed for building integration. In our search for available solar collector coatings on the market, we faced three challenges:
- existing, multifunctional coatings were made of solvent-borne resins, causing high volatile organic solvent emissions in the production process,
- coatings were only offered as non-selective black paints without performance enhancing properties, and
- decrease in absorption efficiency over time, due to dust and dirt
These challenges were addressed directly in the WABASELCOAT project, which was initiated by the National Institute of Chemistry-Ljubljana (NIC). The main objective of the project has been to develop brand new, multi-functional, spectrally selective paint coatings in different colors that could keep, or even improve, the efficiency of the solar collector. The coatings also had to be cost-effective, environmentally friendly and suitable for automated application in an industrial production line.
The National Institute of Chemistry-Ljubljana has a leading position in research on environmentally friendly colored coatings that can increase the solar collector efficiency. In this project, targeted work has been done to develop thickness-insensitive, spectrally selective (TISS) paints for polymeric absorber surfaces with the following characteristics:
- A solar absorptance higher than 90%
- A thermal emittance lower than 35%
- Durability of more than 25 years
- Resilience against dry stagnation damages up to 150°C
- Meet or exceed safety standards (anti-frost, flame retardant)
Colored TISS paints on PPS absorber samples
TISS paints are tailor-made, multifunctional coatings based on a variety of organic macromolecules and functional and processing additives. An important prerequisite for the successful selection and use of pigments is the usage of high absorptivity pigments with their high loading in combination with the low thermal emitting binder.
The paint contains nanoparticles (carbon nano tubes) that increase solar absorptance (ability to absorb solar radiation). The absorbed solar radiation is transferred to heat in the solar collector. To further enhance the efficiency, we have added a substantial amount of tiny metal flakes, which reduce the thermal emittance (heat that disappears).
Nanotechnology can also be used to protect solar collectors from dust and dirt. A protective film, based on polyhedral oligomeric silsesquioxanes (POSS), is applied to the transparent cover plate of the solar collector to achieve a cleaning effect. The surface will then function in the same way as the leaves on the lotus plant (the lotus effect). This means that you get water, dust and dirt to collect in perfect balls that run off the surface instead of sticking. This ensures that solar radiation is not hindered or disturbed by a dirty solar collector surface and the solar collector can produce maximum energy yield.
The paint coatings developed in WABASELCOAT are able to withstand the extreme conditions in a solar collector. A TISS-paint coated full-sized absorber was integrated in a solar collector frame and exposed to dry-hot Mediterranean climate at the test facility of FOSS University of Cyprus. Tests revealed very promising results after one year of exposure.
Furthermore, on-going, accelerated service-life testing of the TISS paints applied on absorber substrate samples have not revealed mechanical or optical failures. These durability tests included direct exposure to UV radiation, dry heat, humidity, temperature cycling, wind and soiling.
Coloured TISS paints applied on full-size PPS absorbers (left) and exposed to dry-hot Mediterranean climate at the test facility of FOSS University of Cyprus (right)
The National Institute of Chemistry-Ljubljana has submitted a Slovenian patent application with the title "Water-based sickness insensitive spectrally selective paint coatings" during 2020, which protects their invention in WABASELCOAT.
Dr. Ivan Jerman from the National Institute of Chemistry-Lubljana (left) was leading the WABASELCOAT project. (photo: Izvozniki.si)
An R&D project within the M-ERA.NET program
WABASELCOAT is an R&D project within the M-ERA.NET program, an EU funded network which has been established to support and increase the coordination of European research programs and related funding in materials science and engineering. The project has been running since 2017 and is coordinated by the National Institute of Chemistry-Ljubljana (NIC). Other project partners are Chemcolor, FOSS University of Cyprus (UCY) and Inaventa Solar.