Due to their cylindrical shaped solar collectors, evacuated tube solar collectors are able to make efficient and maximum utilization of solar energy. What takes these devices a step further than their other counterparts, is their design. These appliances are manufactured with rows of parallel glass tubes for maximum efficiency. Industrial and commercial sectors utilize evacuated solar tube collectors. They are preferred for domestic heating as well.
In evacuated tube solar collectors, several rows of parallel glass tubes (solar tubes) are connected to a header pipe. Each tube is made of two layers of strong borosilicate glass. A vacuum layer is formed within the tubes by withdrawing (evacuated) the air from each tube. This enables prevention of heat loss through convection and radiation. As there is no air inside the tubes, thermal loss is minimum and impact of ambient temperatures is also low. The vacuum helps the tube to retain the amount of heat captured, thereby allowing maximum generation of heat. The sun's energy is captured by a special absorbing film placed in between the two layers of the glass. This film is able to absorb about 90-92% of the sun's ultraviolet rays. And the remaining amount of energy (10-8%) is exhausted into the atmosphere.
The amount of heat absorbed is then transferred to a flat or curved aluminum or copper fin inside the inner layer of glass. This fin is attached to a metal heat pipe that transfers the heat to a copper header at the top of the collector. A small amount of acetone is placed inside the copper heat pipe. When the heat pipe is heated, the acetone turns into steam and rises to the top of the pipe. This acetone steam is converted into its liquid form in a condenser. This liquid flows back to the heat pipe and the process is repeated.
Unlike other solar collectors, the evacuated tube ones can collect the sun's energy from multiple angles. This is possible because of their 360º tubular design. When the sun is at lower angles, it is almost impossible for typical solar collectors to trap the solar energy. This is not the case with evacuated solar collectors. They are built with a broader collector area so as to allow maximum utilization of the sun's energy.
Evacuated tube solar water heaters work more efficiently in mid morning and mid afternoon. For places with freezing climates, evacuated solar collectors are able to maintain a temperature in the range of 75ºC to 175ºC. Their operation is not affected by rising or falling temperatures. This property makes these solar heaters ideal for places with cold climate.
Comparatively, evacuated tube solar collectors require lesser space and the collectors are easy to install. Emission of carbon dioxide is also maintained at a lower level. Replacement of the damaged tubes is possible, lest there should be any flaw in the system. Due to the vacuum created, the tubes do not get hot. Evacuated tube solar water heater promises a durability of at least twenty years, with minimal maintenance.
A Few Considerations
As these solar appliances are capable of generating a large amount of heat, the water storage tanks may get over heated. To prevent this, the hot water must be regularly mixed with water at room-temperature (cold water). This is to keep the temperature in the hot water tanks, at a safe level. Also, the location of the appliances has to be such that they are less prone to damage by hail storm or falling branches.
The only disadvantage of evacuated tube solar collectors is their pricey tag. As mentioned earlier, they are ideal and best suited for places experiencing colder climates, like northern states. Places with warmer climates can go for standard flat plate collectors. The cost of these appliances vary depending upon the number of solar tubes they are built with. Different manufacturers may quote different prices.