Image: © RESOLThere are different types of a solar controller, which include differential controller, photovoltaic and timer solar controllers. A differential controller makes use of two sensors which are located at the hottest and coldest points of the solar hot water system. One sensor is situated at the top of the collector, where the hottest temperature is found, and one sensor is located at the bottom of the thermal storage tank, where the coldest temperature is found.
When a temperature difference of 5-20°F is recorded between the temperatures detected by the two sensors, the solar controller turns on the pump. When a temperature difference of 3-5°F is recorded, the pumps are turned off by the solar controller.
In a photovoltaic solar controller, a photovoltaic module is used to control the pump. The photovoltaic (PV) module generates direct current electricity which then powers the pump and turns it on. The PV module is situated right next to the solar collector, and generates electricity whenever there is sunlight. The pump turns on when there is sufficient solar energy. Sometimes solar hot water systems incorporate a pump activator to help get the pump turned on when there is sufficient solar energy.
The solar controller used here works quite efficiently, as the speed at which the pump circulates the water is in proportion to the amount of solar energy that is generated. The more solar energy there is, the more heat there is, and more water is circulated to get heated.
Another type of solar controller is a timer controller. This works by turning the pump on during the time of day that is guaranteed the most sunlight, for example during the hours of 9am to 4pm.