A solar vacuum tube collector works by transferring the heat energy from the sun to a copper heat pipe located inside the vacuum tube. This copper heat pipe contains a small amount of distilled water inside the tube. The tube is create like the glass evacuated tubes under a vacuum condition. This vacuum is the key to the solar water heater. You see at sea level water boils at 100 C. However as you reduce the pressure the boiling point of water changes. In fact water boils at the top of Mount Everest at only 35 C. This is because there is less pressure. Because the water inside the copper heat pipe is in a vacuum (reduced pressure) it boils at a much lower temperature of only 30 C.
The boiling water creates steam vapour that rises to the top of the heat pipe into the expansion bulb. This bulb is inserted into a copper heat exchanger and the hot steam loses its temperature and pressure and returns back to water. This cycle is continuous and involves no moving parts. This means the life of a solar vacuum tube is 20 plus years!