Solar energy is practically inexhaustible. It is the cleanest form of energy on earth and is composed of light, radio and other radiation from the Sun. Huge quantities of this energy are behind almost all natural processes on Earth. However, it is quite difficult to capture and store it in some form for later use.
Solar energy may be stored in batteries in the form of electricity, or transformed to thermal energy (heat). Solar heat can be used to prepare domestic hot water, there is even an air conditioning powered by sun. Using solar energy globally is probably the most effective way to bring the heat in homes. Generally, the amount of solar radiation that falls on the roof of a house is greater than the total energy consumed in this house.
The main types of solar collectors used in common applications are flat collectors and vacuum tubes. Flat collectors have been widely used due to lower prices. Evacuated tube solar water heater collectors are more expensive but much more efficient and being used for more than 20 years already. Lately prices have become more affordable due to large series production and better manufacturing technologies. Originally evacuated tubes were developed by the University of Sydney, Australia, this system is currently widely used in Germany, Canada, Britain and China because of high performance and low price.
Evacuated tubes are spherical, which allows passive tracking of the sun from morning till night, thus accumulating 40% more heat than flat collectors (which have maximum efficiency only at noon). Space between the two layers of the tube is vacuum which keeps the heat loss to a minimum. In addition, vacuum tubes have a selective coating, which allows tubes to absorb more than 90% of solar radiation falling on them.
Each tube consists of two concentric tubes of borosilicate glass (very strong and with a high degree of transparency), welded together. The gap between the two tubes is vacuum, inner surface of inner tube is coated with excellent selective absorbing coating. Vacuum makes a perfect insulation, so although the temperature inside could reach up to 150 C the outer tube remains cool. This property allows to use evacuated tube solar water heater technology in cold climates and this is also why tube collectors are more efficient than traditional flat solar collectors.