Hello, this is question really lot of people asks me and it is one of those questions that is very easy to answer, because these tubes works really very simply. It is in fact almost as simple as when some piece of black fabric is heating.
You know that anything on direct sunlight is heating and in the same way works these panels. There is really very dark black layer inside the tube that is called absorber. As you can see on the name, it absorbs the heat. Well, here is major problem of most older heating systems that uses sunlight. It has never been problem to get the heat from sun, the problem always was to preserve that heat. If you want to preserve heat, you of course need some insulator. Well, most of insulators have one thing in common and that is that it is not only insulates the heat, but also sunlight. Well, there is one insulator, that insulates heat almost perfectly and also does not affect sunlight at all. In fact it is even better than air and this insulator is NOTHING! Yes this is right, the best insulator for this purpose is NOTHING or vacuum if you want. This is the reason why solar vacuum tubes are using this. Vacuum lets the sunlight reach the absorber, but then insulates the absorber, so it does not lose any heat. Really absolutely genius idea, isn't it?
well then the heat from absorber goes to some medium that transfers it to the place we need it, usually some tank with water and here you go, free hot water from sunlight! Isn't that pretty amazing? More info about solar tubes you can also find here.