The collectors, panels that collect the heat from the sun, come designed either as flat plate collector or evacuated tube. There are also different systems that are designed for delivery of the heat to a building such as a drain back system or a closed loop pressurized system. There are other systems as well but these two are the most used in the USA.
Flat panels are a metal box which has a glass front that allows the ultra violet rays of the sun to pass through. At the back of the box there is an absorbing material that collect the heat. In between the front and the back of the box, there are copper pipes that absorb the accumulated heat that is trapped in the box and transferred to the liquid that runs in these pipes.
Evacuated tubes do the same thing, but instead of a box, there are sets of glass tubes that trap the sun rays inside it and transfer it via a copper tube to the liquid that runs inside these copper tubes.
Either one of these methods of collecting heat from the sun might use a drain back system or a closed loop pressurized system.
The drain back system is a system that when there is no call for hot water, the liquid in the system drains back into a holding tank and will stay there until there until the next call for hot water. This type of system does not require a circulating pump for the liquid that runs through the collectors, but it does need a careful design with a continuous pitch of the delivering pipes in order to provide a gravity-based drain function.
The closed loop pressurized system uses a pump to circulate the heated liquid between the collectors and hot water holding tank inside a building. Such systems often require a design that provides for cooling liquid when it gets too hot.
In our colder climates, these systems will use glycol as its liquid, a non-toxic anti- freeze – while in warmer climates, water will be used as the liquid inside the heating loop.
No matter which system is in use, these systems are designed as a supplementary system to the conventional heating system in the house. A good design will allow for a 60 to 70 percent savings on heating bills.