What is an Evacuated Tube?
Evacuated tubes absorb solar energy and convert it into heat for use in water heating. There are several types of evacuated tubes used in solar thermal collectors. Sunflower solar collectors use the most common "twin-glass tube". This type of tube is chosen for its reliability, performance and cost effectiveness.
Each evacuated tube consists of two glass tubes made from extremely strong borosilicate glass. The outer tube is transparent and allows sunlight to pass through with minimal reflection. The inner tube is coated with an aluminum nitride (Al-N/Al) coating. This selective surface is excellent at absorbing solar radiation with minimal reflection losses. During the manufacturing process, the air contained in the space between the two layers of glass is pumped out, while the top of the tubes are exposed to high temperatures. This fuses the two tubes together into a single evacuated tube. This "evacuation" of the gasses forms a vacuum, which is the most important factor in achieving the high performance of the evacuated tubes.
Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. The vacuum eliminates a physical connection between the two glass layers of the tube which means there is nothing to transfer thermal energy so the heat cannot escape! 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 remains within a few degrees of the ambient air temperature. This means that evacuated tube water heaters can perform well even in cold weather, when flat plate collectors perform poorly in these conditions due to heat loss.
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 evacuated tube this getter is exposed to high temperatures causing the bottom of the evacuated tube to be coated with a layer of pure barium. Called a "getter", this barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during it's lifetime and increases the longevity of the vacuum. This barium layer also provides a clear visual indication of the vacuum status; the silver-colored barium layer turns white if the vacuum is lost making it easy to identify.
Individual evacuated tubes are inserted into a manifold of either 10, 20 or 30 tubes in order to form a complete collector. When oriented toward True South, a passive tracking effect is achieved in the collector due to the round absorption surface, meaning the collector is absorbing the sun's radiant energy throughout the day and not just when the sun is directly overhead. In an East West orientation, a similar effect is achieved as the sun rises and falls across the sky over the course of the year.
The efficiency of an evacuated water heater is strongly related to the intensity of the solar radiation (known as "insolation") in your location. To learn more about insolation and the average values
click here.
