Indicator for Estimation Vacuum Level in Transparent Insulation of Solar Collectors
Abstract
The major part of heat loss in solar collectors (up to 90 %) occur through transparent cover, thus the main problem in solar collector design is to chose transparent cover with high thermal resistance. As a rule a single glazing is used for transparent insulation of solar collectors. A glazing can be covered with a selective coating that reduces heat loss: selective coatings have low-emissivity properties in infrared wave range. Thermal resistance of a single glazing with selective coating is equal to 0,24 m2ÂK/W and without selective coating is equal to 0,13 m2ÂK/W at the following operating conditions: thermal receiver temperature is 100oC, environment temperature is –20oC, heat transfer coefficient from glazing to the environment is 20 W/(m2ÂK). It is possible to reduce heat loss of solar collector using vacuum (evacuated) glazing as transparent insulation (fig. 1). Vacuum glazing consists of two glass sheets of 2,5…4 mm thick with a gap of 0,2 mm between them. Glass sheets are sealed along the perimeter; the gap is evacuated and fixed by glass-ceramic support pillars (fig. 2).