Читать книгу Solar Water Heating--Revised & Expanded Edition - Bob Ramlow - Страница 22

When you look at a flat plate collector you will see that the sides and back are well insulated. However, there is no way to insulate that large pane of glass because it would block all the solar radiation. This obvious limitation has led to the development of evacuated tube collectors. Inventors sought a way to permit the transmittance of solar radiation while still insulating.

Evacuated tube collectors are constructed of a series of glass tubes. Each tube is made of annealed borosilicate (Pyrex) 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 by minimizing heat losses. Air is the medium in which convective heat is transferred. If all the air is removed from the tube, this method of heat movement is interrupted.

Because evacuated tube collectors are able to minimize heat losses they are able to achieve higher temperatures than other collector types. This can be an advantage or a disadvantage depending on how the system has been designed and/or what climate the collector is installed in. For applications that require high temperatures, such as solar cooling, evacuated tubes are the most common choice. However, care must be taken when planning and installing these collectors to ensure that the fluid does not overheat and boil during periods of stagnation or when the load on the system is low. Whenever the sun is out, you need to ensure that a fluid is flowing through the system to prevent overheating.

Evacuated tube collectors vary widely in how they are constructed and how they heat a fluid. The principal distinctions are how many layers of glass they have and whether they heat the solar fluid directly or use a heat pipe.

Figure 3.2: Evacuated tube