Evacuation and deterioration of all-glass tubular solar thermal collectors

Abstract

Two evacuation and degassing processes for all-glass tubular evacuated solar collectors are described, the total process times being ≈ 50 min and ≈ 10 min. Collectors incorporating a sputtered graded metal—carbon on copper selective surface have been evacuated usther process, then aged using various procedures, at temperatures of 300 to 375°C for up to 5000 h. Deterioration of vacuum in each collector was studied by measuring thermal conduction between the absorber tube and the collector envelope. A significant proportion of the small amounts of gas detected is probably due to permeation of atmospheric helium through the borosilicate glass absorber tubes of the collectors. No deterioration of the properties of the selective surface was observed after ageing.