In the UK solar water heating can only deliver around 50-60% of year round water heating. In the summer months, 90% of the hot water needs of a typical home may be met by solar thermal collectors. However, this can fall to less than 20% in winter. The two standard collector panel types are “flat plate” and “evacuated tube”. Flat plate systems use a dark plate in an insulated box to transfer energy into the water system. Evacuated tube systems are more expensive and sophisticated, using metal strip collectors in vacuum tubes but have the advantage that smaller panels are needed.
There are three principle system types: Thermosiphon, Drain-back and Forced circulation.
Thermosiphon is used extensively in hotter climates. This type of system operates on gravity, requiring no pump. The water is heated and the hot water rises up into a water store fixed above the collector panel where it s kept until required. These systems are by their very nature heavy.
The Drain-back system on circulates water to the solar collector panel when there is a demand for hot water and only then if there is sufficient sunlight. When the system is off, the water in the collector drains into a small sealed vessel (or drain-back unit) by means of gravity siphon effect.
Forced circulation is the most common system used in the UK. The heat transfer fluid (normally a glycol water mix) is circulated through the collector panels by a pump. Sensors measure the panel temperature and water store temperature. As long as there is a 4-8°C temperature difference the pump will run and transfer heat from the panel to the store. These are sealed systems which can operate at high temperature (above 180°C).
A typical domestic system would require a panel approximately 3-4m2 in area at an installed cost of between £3,000 and £5,000.