Solar controller

In the context of solar hot water systems, a solar controller is a device which ensures that the system works correctly. Here is a summary of what a solar controller comprises, what it does and how some controllers can perform other functions as well.

What does a solar controller comprise? Its minimum specification is as a box with a microprocessor inside with four cables attached. Two of these cables have temperature sensors at their far ends, one for the hottest part of the solar panel and the other for the coolest part of the water store, which is at the bottom. The other two cables are power in and switched power out to the pump. The power can come in from a mains electric supply, in the case of low carbon solar technology. In the event of the latest innnovation, a zero carbon controller, power is sourced wholly from a photovoltaic (PV) module which also acts as a power supply.

Other components beyond the basics mentioned above include extra temperature sensors for higher up on the water store, because it gets hotter the higher you go up it, and an LCD or other display. This display can tell the user things such as whether the pump is switched on, what temperarure the water at the top of the store is at. This is the temperature of the water which will come out of the taps and therefore it is a useful temperature to know about.

What does a solar controller do? Its main function is to control when the solar panel's pump is switched on or off. The pump is usually switched off when the solar panel is colder than the bottom of the water store and it is switched on when the panel is hotter. Switching the pump on allows the heat in the panel to be transferred to the store. Switching it off prevents the export of bought-in energy. Every few seconds, the temperatures in the panel and the store are measured and compared in order to allow this on or off decison to happen.

In addition, certain fine tuning can take place, such as allowing an overrun time to ensure that heat energy is not left lying about underlivered in interconnecting pipes when the pump is turned from on to off. Another fine tuning is that of the on differential (which may be say 4-15 degrees C) and the off differential which is usllay a few degrees lower. The wider the difference between these differentials, the fewer pump on-off cycles will take place. These factors are usually set by the solar installer in relation to the particular installation. The controller may also control certain safety features such as by permitting heat export when the hot water exceed a preset temperature such as 65C. This is a process of allowing the solar panel to export excess heat if it is not being used when the panel is cool, when light levels fall towards the end of a sunny day and is used in a range of solar thermal technnologies such as solartwin.