What is a vacuum tube collector?

The vacuum tube collector is a solar collector type with a tubular shape, the absorber of which is inside the tube and is surrounded by a vacuum. It is a collector that is much more efficient in the winter than a collector plate, as the heat losses due to the ambient temperature are only minimal. Its disadvantage is the higher purchase price. In addition, in the event of a longer period of frost, frost is likely to be frozen on the collector, which can practically not be removed at temperatures below zero.