Evacuated tubes are the fundamental functional parts for every solar collector, used to absorb and convert sunlight into usable energy. Evacuated tube basically consists of two glass tubes (twin glass tubes) fused at both ends. There is a vacuum layer (space) between them, with the inner tube consisting of a solar absorbing coating. The evacuated layer provides an outstanding insulation that helps in retaining over 92% of the solar energy.
Design Features of Evacuated Tube
Each evacuated tube is made up of strong and transparent Borosilicate glass, a material also used in the manufacture of different kitchen glassware. Its thickened glass wall offer excellent durability and strength to resist breakage, as well as cost effective.
Between the twin glass tubes, the space is evacuated by expelling out the air to form a vacuum that provides an excellent insulating property against energy lose to the surrounding. Evacuated tubes are perfect for high hot water application especially in areas experiencing cold winters.
Barium getter is responsible for increasing the lifespan of the evacuated tube by ensuring the vacuum space remains efficient over the designated period, usually a period not less than 15 years. Its efficiency is maintained by the barium getter, which sucks any remaining air in the evacuated layer, as well as gases such as nitrogen that may diffuse from outside through the outer glass over time. The silver barium layer turns white whenever the vacuum is lost, making it easy to monitor the vacuum status.
Solar Absorber Coating
Every evacuated tube has a special base layer aluminum coating on the surface of the inner glass tube, which absorbs as much of solar energy as possible. Interestingly, this coating is applied through a very reliable process known as dc reactive sputtering. The stainless steel reflector surface prevents heat lose through radiation.
Vacuum insulation is highly effective when it comes to preventing heat lose. By use of absorber coating a lot of energy is captured into the inner glass tube, while the vacuum layer prevents the heat lose to the surrounding, ensuring the outer glass remains colder to touch. It is this combination that makes the evacuated tube so efficient in a solar water heater. During a strong sunlight the absorber coating can retain heat of about 200 degrees, implying that every evacuated tube can provide over 60 Watts energy output.
There are various ways of extracting heat produced in an evacuated tube. Common methods include; heat pipe, U tube pipe, direct water flow, or pipe-in-pipe. The most efficient method is the use of heat pipe, and then the heat is transferred to the heat exchanger bulb or the heat exchanger component of the collector.
Evacuated tube can be of different designs. There are single walled and double walled tubes, but both designs have different efficiencies. The most preferred are the twin wall tubes because they are more efficient in a solar water heater.