How evacuated tube collectors compare to flat-type solar collectors. Find out how they work, see their features and benefits. Who should consider buying tubular solar collectors?
One of the most popular and used evacuated tubes consists of many long parallel single or double-wall sealed tubes. There are usually 10 or more, typically made of glass or plastic and linked at the top by the insulated collector or manifold.
Inside every tube is the small copper pipe coated with a special material, which increases its ability to absorb solar energy. The copper pipe (heat pipe) is sealed and attached to a black copper fin called an absorber plate. The small amount of the heat transfer fluid (methanol or alcohol /water mix) is heated inside the heat pipe, and when it becomes a vapor, it rises to the top of the system, where it cools down, liquefies, and returns to the bottom of the pipe.
During this process, the latent heat is released, so the fluid that flows through the manifold (heat exchanger) gets the heat from the tubes and further transfers it to water stored in the water tank. When the state is changed, a large amount of energy is released, making this type of a collector very efficient.
In the case of the single-wall evacuated tube collectors, the absorber plate and the heat pipe are installed inside the vacuum, while in double-wall (twin) systems, the vacuum is between the layers of glass.
The selective surface material that absorbs the sun’s heat energy makes this system efficient as it can greatly increase water heater performance. As said before, these tubes are also known as vacuum tubes as the air is removed from the glass or plastic tube, and once the vacuum is created, it reduces the heat loss significantly. As the convection and conduction losses are reduced, the evacuated tube operates more efficiently and performs better at higher temperatures than flat-plate collectors.
Evacuated tubes perform better than the popular flat-type collectors, not only in colder but also in warmer climates. Because of their efficient heating, they are recommended for northern regions with colder and cloudy weather. As flat-type collectors, vacuum tubes collect both direct and diffuse radiation. Still, thanks to the cylindrical shape, they can increase efficiency by collecting the radiation when the sun is at low angles.
The vacuum does not help only retain the heat energy inside the tubes, but it also aids in reducing or eliminating condensation inside the collector, which was the case with the flat collectors, which prevents early failure. The absence of air in the tube creates excellent insulation, allowing higher temperatures to be achieved than other types of collectors. This design also eliminates water freezing problems during the cold winter days.
Evacuated tube collectors also have disadvantages. Since they are good heat insulators, the snow that accumulates on the tube does not melt; it sticks on the top and between the tubes, in some cases the whole wintertime, which reduces the efficiency and ability to attract the sunlight. This is why the tube installation at the recommended angle is important (some professionals recommend at least 25 degrees to ensure evaporation and condensation flow). In the case of the direct flow through the evacuated tube collectors, they can also be mounted horizontally on a flat roof.
Evacuated tubes are fragile and prone to more maintenance. They have thinner glass, so they are susceptible to breaks more often, but they can be replaced individually.
Another reason for tube replacement is when the vacuum seal is lost. The tubes can be installed individually, but it takes more time, while the flat plate collectors need more effort to hoist onto the roof, and no assembly is required.
Due to their cylindrical design, vacuum tubes take more space than flat plate collectors, which can be an important factor when space is limited. If there is no proper care, and since the high temperature can be achieved, the fluid might overheat and boil.