1) Flat plate collectors
Flat-plate collectors are commonly used in solar water- heating systems in residential, commercial and industrial projects. A flat-plate collector consists basically of an insulated metal box with a glazed glass cover and a dark-colored absorber plate. Heat from the sun strikes the absorber plate and is transferred to a fluid that circulates through the collector in tubes.
The collector housing can be made of plastic, metal or wood, and the glass front cover must be sealed so that heat does not escape, and dirt, insects or humidity do not get into the collector itself.
Many collectors also have controlled ventilation, so as to avoid condensation inside the glass front cover. The collector housing is highly insulated at the back and sides, keeping heat losses low. However, there are still some collector heat losses, mainly due to the temperature difference between the absorber and ambient air, and these are subdivided into convection and radiation losses. The former are caused by air movements, while the latter are caused by exchange of heat by radiation between the absorber and the environment.
Flat collectors can be mounted in a variety of ways, depending on the type of building, application, and size of collector. Options include mounting on a roof, in the roof itself, or free-standing. The type of thermal system, drain-back or closed loop, can also have a big impact on how the panels are mounted. A drain-back system must ensure that all water is removed from the panels and tubing, if these systems are used in climates where freezing can occur.
2) Evacuated tubes collectors
Evacuated tube solar collectors are very efficient and can achieve very high temperatures. An evacuated-tube collector contains several rows of glass tubes connected to a header pipe, or directly to the hot water tank. Each tube has the air removed from it (evacuated) to eliminate heat loss through convection and radiation. Inside the glass tube, a flat or curved aluminum or copper fin is attached to a metal pipe. The fin is covered with a selective coating that transfers heat to the fluid that is circulating through the pipe. There are two main types of evacuated tube collectors:
Heat pipe evacuated-tube collectors
U pipe evacuated-tube collectors
Low pressure collectors
Evacuated-tube collectors can get very hot, exceeding the boiling point of water and can cause significant issues in an existing domestic solar water system. You need to use your hot water every day to ensure the temperature doesn’t overheat in the tanks.