Evacuated Tube
Evacuated tubes are the absorber of the solar water heater. They absorb solar energy converting it into heat for use in water heating. Evacuated tubes have already been used for years in Germany, Canada, China and the UK. There are several types of evacuated tubes in use in the solar industry. Unitech Solar collectors use the most common "twin-glass tube". This type of tube is chosen for its reliability, performance and low manufacturing cost.
The Evacuated tube is the outer tube which insulates the absorber (Heat-Pipe). This allows for better insulation as well as more efficient energy absorption compared to flat-plate collectors. Temperatures over 300 degrees F can be reached, which can be appropriate for steam and other high-temperature heating applications. Evacuated tube collectors also produce a useful amount of heat in partly cloudy and very cold conditions, making them a suitable choice for all climates in which these conditions are common. They are also used in industrial applications such as steam process heating and district hydronic 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 (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 evacuated tubes.
Cold Weather? 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 evacuated tube absorbs the radiation from the sun and converts it to heat, we don't want 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 evacuated tube water heaters can perform well even in cold weather when flat plate collectors perform poorly due to heat loss.