1, Vacuum tube
Evacuated tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with a special selective coating which features excellent solar radiation absorption and minimal reflection properties. The top of the two tubes are fused together and the air contained in the space between the two layers of glass is pumped out while exposing the tube to high temperatures. This “evacuation” of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes. The vacuum in the evacuated glass tube is less than 5×10-3pa This can only be reached and maintained over a long period of time through a specialised evacuation process in production resulting in an almost total elimination of convection and conduction losses from the tube.
2, Heat pipe vacuum tube
The heat pipe is hollow with the space inside evacuated, much the same as the solar tube. In this case insulation is not the goal, but rather to alter the state of the liquid inside. Inside the heat pipe is a small quantity of purified water and some special additives. At sea level water boils at 100oC, but if you climb to the top of a mountain the boiling temperature will be less that 100oC. This is due to the difference in air pressure.
Based on this principle of water boiling at a lower temperature with decreased air pressure, by evacuating the heat pipe, we can achieve the same result. The heat pipes used in solar collectors have a boiling point of only 30oC. So when the heat pipe is heated above 30oC the water vaporizes. This vapour rapidly rises to the top of the heat pipe transferring heat. As the heat is lost at the condenser (top), the vapour condenses to form a liquid (water) and returns to the bottom of the heat pipe to once again repeat the process.
At room temperature the water forms a small ball, much like mercury does when poured out on a flat surface at room temperature. When the heat pipe is shaken, the ball of water can be heard rattling inside. Although it is just water, it sounds like a piece of metal rattling inside.
This explanation makes heat pipes sound very simple. A hollow copper pipe with a little bit of water inside, and the air sucked out! Correct, but in order to achieve this result more than 20 manufacturing procedures are required and with strict quality control.