Evacuated-tube collectors
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 are used.

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 collectors