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Solar Vacuum Glass Tubes (SVGT) absorb
solar energy, converting it into heat for use in water and air heating
SGVT are also referred to as evacuated tubes, as the space between the two glass layers is evacuated to form a vacuum. SGVT have already been used for years in Germany, Canada, China and the UK. There are several types of solar tubes in use in the solar industry. The Solarwyse's SK-TF uses the most common "twin-glass tube" also known as the "Sydney Tube". This type of tube is chosen for its reliability, performance and low manufacturing cost.
Each solar tube consists of two glass tubes made from extremely strong borosilicate ("Pyrex") glass. The outer tube is transparent allowing light rays to pass through with minimal reflection. The inner tube is coated with two separate layers of special selective coating (Al-N/Al) 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 solar tubes. Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the solar tube absorbs the radiation from the sun and converts it to heat, we don't won't to lose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150oC / 304oF , the outer tube is cold to touch. This means that solar tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions). In order to maintain the vacuum between the two glass layers, a barium getter is used (the same as in television tubes). During manufacture of the solar tube this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2 , N2, O 2, H2O and H2 out-gassed from the solar tube during storage and operation, thus helping to maintaining the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition. See picture below.
Solar tubes are aligned in parallel, the angle of mounting depends upon the latitude of your location. In a North South orientation the tubes can passively track heat from the sun all day. In an East West orientation they can track the sun all year round. The efficiency of a solar water heater depends on a number of factors, inter-alia, the level of solar radiation (insolation) in your region. Solar Tube Basic Specifications
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