Evacuated Tube Insulation

One of the genius elements to the evacuated tube design, used in solar thermal heat collection, is that the pipes exposed to the sun, never lose heat. That’s because the vacuum around the metal pipe has infinite R-value. In addition, a vacuum or air itself is very affordable. The tricky part is the structural dynamics required to create a vacuum, or to trap the air without allowing it to bleed out of the seams. If glass tubes can be evacuated, so can low-cost PVC tubes.