The success of solar absorption surfaces lies in their efficiency. A major influencer on efficiency is the ability of the surface to minimise heat losses due to thermal radiation. The holy grail of efficiency therefore is to apply a coating that achieves high absorptance and low thermal emittance.

These days a range of solar selective coatings are available, including:

• intrinsic absorbers
• semiconductor–metal tandems
• multilayer tandems
• ceramic–metal composites (known as cermets)
• textured absorbers
• photonic crystalsⁱ

Many coatings are deposited from wet chemical routes, such as electrodeposition, however alternatives such as physical vapor deposition (PVD) have increased in popularity over the years, to overcome low thermal stability issues and increase efficiency.

Physical vapor deposited (PVD) coatings
PVD (or thin-film coating) takes a solid material and vaporises it in a vacuum to subsequently deposit it on a surface. The result is a very thin, bonded metal layer that helps improve the appearance, durability and importantly the function of the surface.

PVD deposited solar selective coatings have been shown to deliver high absorptance and low emittance and, since the 1990s, have been developed for both mid- and high-temperature applications (the former being ideal for solar hot water and industrial process heat applications, and the latter for concentrating solar power systems for solar thermal power generation).

Hydrophobic lacquer coatings
One feature that is popular with solar manufacturers is ensuring that the surface coating is hydrophobic. This is particularly important in rainier or dustier climates. Hydrophopic coatings are highly water repellent and offer long lasting performance. As well as having self-cleaning properties, such coatings make it difficult for pollution and other contaminants to stick to the product’s surface, promoting longevity, and reducing maintenance requirements.

Alanod – PVD and hydrophobic coatings for solar thermal systems
Alanod’s eta Plus^® and mirotherm^® coating systems are continuously vapour coated and sputtered in a worldwide, technologically unique air-vacuum-air process. This helps them achieve high absorption rates with a low emissivity, to maximise efficiency.

Also, Alanod’s mirosol^® TS – ideal for use in regions with high solar radiation – has a selective, hydrophobic lacquer applied to the aluminium in a specially developed process. This means that for the first time, in addition to PVD coatings, lacquers using the coil coating method are now also available for solar thermal collectors.

Eta Plus^® – solar absorption coating for absorbers in flat plate collectors, in tube collectors and in air and large-scale facade collectors

• Available on copper and aluminium
• High solar absorption up to 96%
• Low thermal emissivity of 4%
• Suitable for laser and/or ultrasonic welding
• PVD vacuum coated

mirotherm^® – ideal for solar thermal systems

• Diffuse and direct light solar absorption coating
• Proven quality with over 20 years experience
• Up to 96% solar absorption
• Reverse side treatment optimised for laser welding
• PVD vacuum coated

mirosol^® TS – ideal for solar thermal systems installed in regions with high solar radiation

• High corrosion resistance solar absorption coating
• Suitable for laser and/or ultrasonic welding
• Available on aluminium
• Hydrophobic lacquer is applied using coil coating method
• Not sensitive to fingerprints

To find out more about Alanod’s PVD and hydrophobic coating materials click here.