The drainback system
There are 2 basic types of solar thermal system - Drainback systems and pressurised solar systems. If you are familiar with heating systems, a good way of comparing them is a drain back system is comparable to an open vent system, and a pressurised system is comparable to a sealed system.
A drainback solar thermal system features a drain back vessel. The fluid in the system drains back into this vessel when the system is not running.
The advantage of this system, is that there will never be non-moving fluid in the panels in winter, so there is no chance of it freezing. As a result, antifreeze is not necessary in a drainback system. There is also no requirement for an expansion vessel, saving space. The downside to these systems is that they tend to be noisy in comparison to pressurised systems, as there is obviously a lot of air present.
A sealed, pressurised solar system works in a similar way to a sealed central heating system. The entire system is filled with heat transfer fluid and then pressurised. The heat transfer fluid must contain the correct amount of antifreeze for frost protection in winter.
An expansion vessel is necessary to take up the expansion of the fluid as the temperature rises. It is important to remember that solar systems can achieve temperatures in excess of 200 degrees.
Some manufacturers will offer a protection vessel. These help prolong the life of the expansion vessel, as they protect the membrane within the expansion vessel from the intense heat that solar thermal systems can develop. The protection vessel is much smaller than the expansion vessel (usually about 5 litres on a domestic system). This vessel is filled with cold solar fluid, so that when the temperature (and therefore the pressure) in the system rises, and fluid is forced into the expansion vessel, the fluid that first goes into the expansion vessel isn't hot system fluid, but cold fluid from the protection vessel.
In both drainback and pressurised systems, the most common way of utilising the heat gained by the solar system is by installing a
twin coil cylinder
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