While flat plate collectors are all essentially made the same way and perform the same way from one brand to another, evacuated tube collectors vary widely in their construction and operation. Evacuated tube collectors are constructed of a number of glass tubes. Each tube is of annealed glass and has an absorber plate within the tube. During the manufacturing process, a vacuum is created inside the glass tube. The absence of air in the tube creates excellent insulation, allowing higher temperatures to be achieved at the absorber plate. Their similarity, however, ends there.
There are potential problems with evacuated tube collectors that you should consider. First, these collectors get hotter than flat plate collectors and can generate temperatures above the boiling point of water. This can cause significant problems in a solar water heating system. It is therefore critically important to always make sure there is an adequate load on the system to keep the temperatures below ninety-eight degrees Celsius. Second, the tubes are fragile. They are constructed using annealed glass, which is much more delicate than tempered glass. Care must be taken when transporting and handling the glass tubes.
Another concern is that the collector performance relies on the vacuum inside the tube. Because a rod or pipe exits in the tube on one end or both, a seal must be maintained at this junction. Most manufactures of evacuated tube collectors offer a short time guarantee, if at all. If the seal is broken, the performance is no better than a flat plate collector’s, and probably worse.
Although evacuated tube systems may be well suited to certain climates, this is not necessarily the case in South African conditions. The South African annual solar radiation value is somewhere between 1 450 and 1 950 kWh/m2 per annum, compared to an annual average of 910kWh/m2 in Europe.
In the case of evacuated tube systems, especially in pre-feed systems where the hot water is pumped directly into an electric geyser, water that is too hot has the potential to cause damage due to the excessive pressure.