manufacturers. One manufacturer recently presented an all-glass evacuated tube
collector, which may be an important step to cost reduction and increase of lifetime.
Evacuated tube collectors have demonstrated that the combination of a selective
surface and an effective convection suppressor can result in good performances at
high temperatures. The vacuum envelope reduces convection and conduction losses,
so the cylinders can operate at higher temperatures than flat-plate collectors. Like
flat-plate collectors, they collect both direct and diffuse radiation. However, their
efficiencies are higher at low incidence angles. This effect tends to give evacuated
tube collectors an advantage over flat-plate collectors in day-long performance. The
characteristics of the evacuated tube collector considered in this study are shown in
Table 8