A small circulating pump is needed to move the heat transfer fluid through the system. These pumps are the only moving part of the system, and can be powered by either DC (direct current) or AC (alternating current) electricity. A photovoltaic (PV) panel can be used to power a DC pump, thus producing a self-powered and self controlled system These systems run when it is sunny and the fluid in the panels is hot. They shut off when it is dark or cloudy, therefore leaving the cooler fluid outside in the panels. If the system is powered by standard AC electricity a controller is needed to turn the pump on and off at appropriate times.
