Читать книгу Solar Water Heating--Revised & Expanded Edition - Bob Ramlow - Страница 23

The collectors can be constructed of either a single or double tube of glass. In single-tube versions, the absorber plate and riser tube rest directly inside the vacuum. The configuration means that the solar radiation has to pass through only one layer of glass, but it requires a seal where the riser tube leaves the vacuum. This seal needs to be able to withstand the high temperatures experienced within the vacuum tube, but it must also remain pliable enough to withstand the expansion and contraction of the copper pipe extending out the end as it heats and cools.

Double-tube versions go by many names, Sydney tubes, Dewar tubes or Twin tubes, but all versions are principally the same. A tube is placed inside a larger tube, and the two are sealed together at the ends. The vacuum is then drawn into the space between the layers of glass. The absorber plate is either coated on the underside of the inside layer glass or placed within the vacuumed space. In the latter case, the heat will need to transfer through the inner layer of glass. Having the double tube eliminates the metal/glass connection, which may extend the longevity of the vacuum but requires energy transfer through two layers of glass.