Image: © RESOLThe solar controller is a special electronic device used to control the circulation of the pump in a solar water heating system so as to harvest maximum heat while protecting against overheating. The solar controller protects the system by turning off the pump when water in the storage tank reaches maximum temperature. There are two main types of solar controllers in the market: PV-controlled pumps and differential controllers.
This type of solar controller uses a solar panel connected directly to the pump. The pump operates when there is sunshine and its speed will depend on the intensity of the sunlight.
A differential controller is a special type of thermostat that is used to measure two different temperatures simultaneously. The term ‘differential’ comes from the fact that the thermostat measures the temperature difference between two locations. A differential controller is a kind of solar controller that turns the solar collector on or off depending on the changes in temperature at the two locations. The controller monitors the thermostat's two sensors on a constant basis. One sensor in the differential controller is placed on the piping as close to the solar collector’s outlet as possible, while the other sensor is located close to the storage tank’s bottom.
The collector heats up rapidly when the sun rises, which makes the sensor change its resistance. This activates a relay in the solar controller and turns on the pump. When the storage tank reaches its maximum temperature or the sun cools down, the ‘differential’ between the sensors becomes minimal, prompting the thermostat to turn the pump off. A differential controller may either work with the factory pre-sets or user-defined settings. It measures resistances of the two sensors and then turns the pump either on or off depending on the difference between the two.
A solar controller is not required in either a batch heater system or thermosyphon system, as they are not systems with an active circulation which would need to be controlled. In the former system, water pressure is used to move energy while the latter system uses the natural rise of heat.