The most obvious problem with flat plate collectors is the relatively large sheet of glass taking up one whole side of the construction – glass that can't be insulated without stopping the sunlight from heating the absorber plate. To solve this problem, evacuated tube collectors were invented.

How do you let enough sunlight in through a transparent medium, to warm an absorber, without letting the heat it generates escape through the same medium? Make that medium a vacuum! Heat doesn't travel well through a vacuum because there's nothing in it to heat, which is one reason why outer space is so cold.

Evacuated tube collectors replace the single, large, flat plate absorber with a series of smaller flat absorbers, each of which is contained inside a transparent tube (made of Pyrex, or annealed borosilicate if you prefer the scientific term). When the tubes are manufactured, the air is pumped out, causing a near-vacuum.

The collector works very much like a flat plate setup: sunlight warms the absorber plates and the heat is transferred out through a heating medium to where it's needed. But there's better heat retention in the absorber plates because they're surrounded by a vacuum. This can be a good thing or a bad thing: the collector plates can heat up to extremely high temperatures, which is really useful for solar cooling but which can be catastrophic in a system where the heat transfer fluid lies idle.

Evacuated tube collectors are built in a number of different ways and with many variations, so you'll need to look into the parts and how they work together. The biggest distinctions between models are the number of layers of glass and whether they use a heat pipe to transfer energy to the solar fluid or pass it directly.