An evacuated collector tube catches sunlight and turns it into heat. Use the buttons to look inside, then follow the energy from the Sun to the hot fluid leaving the manifold.
A solar thermal collector is a machine that catches sunlight and turns it into heat for water or space heating. The evacuated tube type shown here is one of the most common designs: a row of glass tubes, each holding a dark absorber and a sealed heat pipe, feeding a manifold at the top.
From the outside the tube is just a clear glass cylinder. All the clever parts — the absorber coating, the vacuum, and the heat pipe — are hidden behind the glass, so cutting it open shows how light energy becomes hot water.
The outer glass tube is transparent, so most of the sunlight striking it passes straight through rather than bouncing off. That light carries radiant energy from the Sun toward the dark surface inside.
The selective absorber coating soaks up the light and converts it into heat. Because it is designed to give off very little heat back as radiation, it becomes much hotter than an ordinary dark surface would.
A sealed copper heat pipe runs through the absorber. The liquid inside boils where it is heated, and the hot vapour rushes to the cooler end. There it condenses, releasing the heat it carried, and flows back to be heated again.
The space between the glass and the absorber is pumped down to a vacuum. With almost no air, there is very little conduction or convection to carry the captured heat back out through the glass, so the tube stays efficient on cold, windy days.
Each tube's heat pipe ends in a condenser inside the insulated manifold. Cold fluid enters, is warmed by the condensers, and leaves as hot fluid that can be stored or pumped into a heating system.
Air carries heat away by conduction and convection. Pumping the space between the glass and the absorber down to a vacuum removes almost all of it, so the heat captured by the absorber stays inside instead of leaking back out.
It is a selective absorber. It soaks up sunlight very well but radiates little heat back, so it gets much hotter than plain glass or metal would. Its dark colour is why it is good at grabbing radiant energy.
A sealed heat pipe moves heat very fast and can tolerate freezing and overheating far better than a water-filled tube. It also keeps the fluid loop separate, so the collector can work as a self-contained unit.
The heat pipe's vapour rises to a condenser inside the insulated manifold. There it condenses and releases its heat to the fluid flowing through, which carries it away to a tank or heating loop.
Less well. They use direct sunlight, so clouds and shade cut their output sharply. Diffuse light still warms them a little, but strong direct sun is what makes them effective.