Compound Parabolic Collector (CPC) Vacuum Tube

This is a variation on the Sydney type where a curved reflector is added to the back of the collector. This shape of reflector is capable of reflecting the sun's rays onto the central tubular absorber, even as the sun tracks across the sky. The aperture area of the collector, is now the area of the reflector, but it can only lose heat through the vacuum tube and the manifold. For this reason, the thermal characteristics of this type of collector are excellent.

However while the collector may stay relatively clean for a number of years, dust and dirt tend to collect quite quickly on the back of the vacuum tubes and on the reflector. The collector will require regular (at least once per year) cleaning to maintain its output. One of the principal advantages of conventional vacuum tube collectors is that the wind can pass between the tubes, however with a reflector, increased wind loading is inevitable. It is also very important to verify that the reflectors are very tightly connected to minimise any rattle.

Maintaining the Vacuum

Glass (particularly borosilicate glass) is very slightly porous to air molecules, so in order to maintain the vacuum between the two glass layers, a barium "getter" is used (as in television tubes). During manufacture of the evacuated tubes this getter is exposed to high temperatures, which causes the bottom of the evacuated tube to be coated with a pure deposited layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 that passes through the glass wall thus maintaining the vacuum. The silver coloured barium layer will turn white if the vacuum is ever lost, making it easy to determine whether or not a tube is in good condition.