The water-in-glass evacuated tubular solar water heater
(Fig. 1) is the most widely used form of evacuated tube collector
because it has higher thermal efficiency than other
evacuated tube collectors employing metal-in-glass manifolds
and has simpler construction requirements and lower
manufacturing costs. A water-in-glass evacuated tube solar
water heater typically consists of 15–40 flooded singleended
tubes in direct connection to a horizontal tank.
The solar absorber tube consists of two concentric glass
tubes sealed at one end with an annular vacuum space
and a selective surface absorber on the outer surface (vacuum
side) of the inner tube. The heat transfer in this collector
is driven purely by natural circulation of water through
the single-ended tubes. Water in the tubes is heated by solar
radiation, rises to the storage tank and is replaced by colder
water from the tank.