Evacuated-tube collectors are typically more efficient at higher temperatures
than flat-plate collectors. In an evacuated-tube collector, sunlight enters
through the outer glass tube and strikes the absorber, where the energy
is converted to heat. The heat is transferred to the liquid flowing through
the absorber. The collector consists of rows of parallel transparent glass
tubes, each of which contains an absorber covered with a selective coating.
The absorber typically has fin-tube design (fins increase the absorber
surface and the heat-transfer rate), although cylindrical absorbers also
When evacuated tubes are manufactured, air is evacuated from the space
between the two tubes, forming a vacuum. Convective and conductive heat
losses are eliminated because there is no air to convect or conduct heat,
so evacuated-tube collectors are efficient at higher temperatures and
perform well in both direct and diffuse solar radiation. Evacuated-tube
collectors are more appropriate for most commercial and industrial applications
because they can achieve extremely high temperatures (170°F to 350°F).
However, evacuated-tube collectors are more expensive than flat-plate