A solar heating panel (also called solar collector), is a dark coloured material that absorbs light and warms up, with a heat transfer fluid flowing through to transport the heat energy somewhere useful. In its simplest form this could be a black rubber bag laid out on the ground with swimming pool water flowing through it.
To reach useful temperatures for domestic hot water in the UK, the dark coloured surface needs to be thermally insulated to limit heat losses to the environment. A familiar example of collecting light energy in this way is the way a car dashboard heats up when the car is left parked in the sun.
The solar panel is located where it will get good light levels, often on the roof of the building, and a pump circulates heat transfer fluid (most commonly water mixed with antifreeze) through the panel. The heat from the panel is transferred to a storage cylinder as the heat transfer fluid flows through a coil of pipe inside the cylinder. By comparing temperatures at the storage cylinder with temperatures at the solar panel, the control system decides when the pump should run to gather heat energy.
An auxiliary heat source such as a gas boiler or electric immersion heater is required for days when light levels don't raise the water in the cylinder to the required temperature.
The upshot of this is that a proportion of the annual hot water requirement of the building is provided by the solar system, and a proportion is provided by the auxiliary heating system. The proportion of the total supplied by the solar heating system is termed the Solar Fraction, and this will depend upon the area of panels installed, the efficiency of the panels, and the demand pattern from the household.
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