A flat plate collector is a type of solar collector that has been around since the 1950s. It works by converting solar energy into heat energy. A flat plate collector can be configured to directly heat potable water or to transfer the energy from a transport liquid to the final target. The transport liquid is often a mix of water and antifreeze.

Roof-integrated flat plate collectors,
Figure: Viessmann WerkeA flat plate collector consists of a clear cover, usually tempered glass, that allows sunlight to pass through while controling the loss of heat. A dark plate, for example, absorbs the sunlight and converts it into heat, then absorber piping carries the heat away from the plate through an insulated backing. The frame is usually aluminum.

Careful consideration is required when a flat plate collector is to be installed in an area where freezing is likely. If pure water is used in the piping, rather than solar antifreeze or a water/antifreeze mix, damage is likely to occur if it is inadequately insulated.

Flat plate collectors have historically been made with metal plates, but special polymers are now also available. One of the best metals to use in a flat plate collector is copper.  Because of this, however, the cost can be highly variable depending on world markets. In addition, metal versions of flat plate collectors are more prone to freezing damage than the polymers. On the other hand, extreme heat can actually melt the polymer versions. Available sizes range from 24 to 48 square feet.

A common alternative to the flat plate collector type of solar thermal panel is the evacuated tube collector. This latter type of collector was first introduced in the 1970s and uses a series of large vacuum tubes with internal absorber plates to gather and convert solar energy.

The Flat Plate Collector - Optimal for Domestic Solar Water Heating

A flat plate collector is generally less expensive to purchase than an  evacuated tube collector, but the latter is overall more efficient in operation (Solar Collector Efficiency). The performance difference is particularly notable at lower temperatures. A flat plate collector, however, is better suited to regions that receive large quantities of snow. In these regions, even the smallest amount of light that reaches the collector will help heat the unit and melt the snow. Each collector type can have its own maintenance issues with no clear advantage of one over the other.