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Devon
Solar & Damp Devon
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Technical Information
The main component of the solar water heating system is the evacuated tubes that are fitted to the roof of the building. These tubes are double walled vacuum glass tubes with a selective black coating to absorb the suns energy. The outer tube is made of extremely strong transparent borosilicate glass that is able to resist impact from hail up to 25mm in diameter (see Figure 1).
Why a vacuum glass tube?
The principle is the same as a thermos flask, a vacuum is an excellent insulator. The vacuum insulation properties are so good that while the inside of the tube may be 150°C, the outer tube is cold to touch. This means that evacuated tube solar collectors can perform even in cold weather.
Inside the glass tubes is a heat pipe (see Figure 2). The heat pipe is hollow with the space inside evacuated. Inside the heat pipe is a small quantity of purified water and some special additives.
Based on the 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 the solar collectors have a boiling point of only 30ºC. So when he heat pipe is heated above 30°C 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.
How much energy does the sun supply?
To the right (Figure 3) is a map showing the average solar radiation on a 30° incline facing due south (Solar trade association).
In practice the panels should supply up to 40 to 50% of your hot water needs over the year with in practices up to 90% in the summer.
Figure
1
Figure 2
Figure 3