A selective coating is the optical surface applied to a solar absorber that absorbs 94–95% of incoming sunlight while re-emitting only 4–5% of it as thermal infrared radiation. By suppressing radiant losses it keeps the collector efficient as it runs hotter, and it is the single biggest difference between a modern collector and a black-painted one.
How does a selective coating work?
A selective coating exploits the fact that sunlight and heat radiation occupy different wavelengths. Solar radiation arrives mostly between 0.3 and 2.5 µm; a hot absorber re-radiates in the thermal infrared, roughly 2.5 to 50 µm. The coating is engineered to be nearly black in the solar band — absorptance (α) of 0.94–0.95 — and nearly a mirror in the infrared band — emittance (ε) of 0.04–0.05. The absorber therefore captures almost all incoming energy but radiates back only a twentieth of what an ordinary black surface would, which is what lets a glazed collector deliver 60–90 °C water at useful efficiency.
Selective coating vs black paint: what is the difference?
Black paint absorbs sunlight almost as well as a selective coating — and then throws the energy away again. Its infrared emittance of 0.85–0.90 means radiation losses grow steeply with temperature, capping a painted absorber at low operating temperatures.
| Surface | Absorptance α | Emittance ε | Typical use |
|---|---|---|---|
| Matt black paint | 0.90 – 0.95 | 0.85 – 0.90 | Pool absorbers, unglazed panels |
| Black chrome (electroplated) | 0.92 – 0.95 | 0.10 – 0.15 | Older-generation collectors |
| Sputtered cermet (TiNOx class) | 0.94 – 0.95 | 0.04 – 0.05 | Modern flat plates, evacuated tubes, PVT |
The practical result: at a working temperature 50 K above ambient, a selectively coated flat-plate collector keeps a large share of output where a black-painted equivalent has already fallen to marginal efficiency.
What are selective coatings made of and how are they applied?
Modern coatings are cermets — ceramic-metal composite layers a few hundred nanometres thick, deposited on copper or aluminium strip by magnetron sputtering (PVD) in continuous roll-to-roll vacuum lines. Titanium-based systems of the TiNOx type are the best known; an anti-reflection top layer maximises solar transmission into the stack. Earlier technologies — electroplated black chrome and nickel-pigmented anodised aluminium — achieved good absorptance but higher emittance, and sputtering has displaced them in quality collectors because it is more consistent, more durable and free of plating chemistry.
Do selective coatings degrade over time?
Quality coatings are engineered for the collector's full service life. The dedicated standard ISO 22975-3 subjects coating samples to accelerated ageing — elevated temperature, humidity and condensation cycles — to demonstrate 25 years of service without significant optical drift, and coating stability is stressed again in the stagnation and exposure tests of EN ISO 9806. Real fleets bear this out: quality flat-plate collectors routinely reach 25 years of service on their original absorber surfaces. What degrades a coating prematurely is not sunshine but moisture standing inside a poorly ventilated or damaged casing — one reason collector housings have defined breathing openings.
Which coating numbers should you check before buying?
Check absorptance, emittance and the resulting optical efficiency. Reputable datasheets state α and ε explicitly; the Solar Keymark datasheet then shows what they deliver as certified performance — optical efficiency η0 of 0.75–0.82 for a good flat plate, together with the heat loss coefficients. A collector whose α/ε pair is worse than roughly 0.94/0.05 gives away performance that costs nothing to specify correctly, so the coating class is one of the fastest quality checks a buyer can make.
Frequently asked questions
Why are solar absorbers blue and not black?
The dark blue tint is an optical interference effect of the sputtered cermet and anti-reflection layers. The surface is effectively black across the solar spectrum — it absorbs 94–95% of sunlight — even though the eye sees blue.
Does a selective coating help on cloudy days?
Yes. The coating absorbs diffuse radiation exactly as well as direct sunlight, and its low emittance matters even more when the collector runs warm in weak irradiance. The absorptance/emittance advantage applies in all weather.
Can a selective coating survive stagnation?
It must — EN ISO 9806 exposure and stagnation tests verify exactly this. Quality cermet coatings are stable well beyond flat-plate stagnation temperatures of 160–200 °C, and the tube-type variants withstand evacuated-tube stagnation approaching 300 °C.
What is TiNOx?
TiNOx is a widely used trade name for titanium-based sputtered cermet coatings with absorptance around 0.95 and emittance around 0.04–0.05. It has become shorthand for the whole class of modern PVD selective coatings.