A typical flat-plate collector is an insulated metal box with a glass or plastic cover and a dark-colored absorber plate. The flow tubes can be routed in parallel, as shown here, or in a serpentine pattern. 
An air collector is a simple flat-plate collector used mainly for space heating. Air flows through the collector by natural convection or when forced by a fan. 
Unglazed collectors are commonly used to heat swimming pool water. No glazing is needed because the system is used when both outdoor temperatures and solar radiation levels are high. 
A batch heater is a simple solar water heater that uses one or more black tanks filled with water and placed in an insulated, glazed box. 
An evacuated-tube collector generally consists of a fluid-filled absorber tube surrounded by a vacuum. 
A parabolic-trough collector can achieve higher temperatures than flat-plate or evacuated-tube collectors because it focuses sunlight collected over a large area onto a receiver located along a narrow focal line. 
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