A surface either absorbs sunlight and turns it into heat or reflects it away, and the two always add up to all of the light: matte black absorbs 90-95% and bright polished metal reflects 70-90%. Paint every absorber matte black and keep shiny metal for reflectors, because a shiny absorber throws most of the sun’s power back into the sky.
Part of Solar Energy Fundamentals
Light that strikes a surface is split into the part the surface absorbs, which becomes heat, and the part it reflects, which does not; collector design chooses surfaces by that split.
The Split Decides How Much Heat You Get
Whatever sunlight is not reflected (or passed through) is absorbed. Using the figures in the parent page, with 1,000 W/m² falling on a surface:
- Matte black absorbing 90-95%: 900-950 W/m² becomes heat.
- Polished metal reflecting 70-90%: it absorbs the remaining 10-30%, so only 100-300 W/m² becomes heat.
The same sunlight, and the black surface keeps several times as much of it (900-950 W/m² against 100-300 W/m²). A collector with a shiny pipe in place of a black one is a bad collector, however well built.
Glass in front reduces what arrives. The parent page gives glass as transmitting 85-90% of visible light; sunlight then meets the black surface. Taking the ends of both ranges together, 0.85 × 0.90 = 0.765 and 0.90 × 0.95 = 0.855, so about 765-855 W/m² becomes heat in a glazed black collector at the peak, before any heat is lost again.
Black Absorbs Heat In And Gives It Back Out
A surface that absorbs infrared well also emits infrared well. A matte black plate at 80°C radiates heat to the sky at a rate that rises steeply with temperature (the fourth power of the absolute temperature; The Greenhouse Effect in Collectors works the numbers). The glass over a collector blocks much of that outgoing infrared, which is why glazing is paired with the black surface.
Factory collectors use a coating that absorbs sunlight but emits little infrared, called a selective surface. Without access to one, matte black under glass is the practical choice.
What You Need
- Matte black finish — flat black spray paint, stove paint, or soot or lampblack bound with oil or another binder (Binders); paint rated for high temperature, since a collector can pass 100°C when it is not flowing
- A bright reflector where you want light redirected — polished aluminium, mirror glass or foil
- Two identical tins and a thermometer — for a quick test
Method: Test The Difference Yourself
Step 1: Fill two identical tins with the same volume of cool water. Paint one matte black. Leave the other bright metal.
Step 2: Stand both in direct sun for an hour, same angle, same shelter from wind.
Step 3: Stir and read both. The black tin is warmer, often by many degrees. If there is no difference, one tin is shaded or the black is glossy.
Step 4: Repeat with a pane of glass propped over both. The gap widens and the black tin gets hotter than in the open.
Common Mistakes
- Gloss black. A glossy surface reflects a visible fraction of the sun; use matte.
- Ordinary paint on a hot absorber. It blisters and flakes. Use heat-resistant paint, or soot and binder.
- Painting pipes and leaving bare metal between them. The bare strips reflect sunlight; blacken the whole plate.
Safety
A black surface in direct sun burns on contact
A black plate in full sun can pass 60°C, and skin contact at that temperature burns deeply in a few seconds. A bright reflector aimed at a person or at dry material also redirects the sun’s full power; do not leave reflectors where they can start a fire or flash into eyes. Treat burns with cool running water (Thermal Injury Care).
What’s Next
- Learn next: The Greenhouse Effect in Collectors — why glass keeps the heat in
- Combine with: Glass, Mirrors, Lights & Reflectors — where to find reflectors