Thermal actuators have a few functions for solar thermal systems. In home systems, they typically act as a safety mechanism to prevent water freezing in the collectors or pipes. In utility-scale applications, actuators are also used in tracking systems to help the collectors accurately follow the sun across the sky and reset the collectors at night so the process can start over in the morning.
Actuators can be thermal or electrically driven. A thermal actuator responds to changes in temperature. It consists of a "cup" that holds a thermally-sensitive substance (air or liquid), a diaphragm, plug and piston. When the substance within the actuator is warm, it expands, putting outward pressure on the diaphragm and plug, which drives the piston and creates motion. When it cools, it contracts, allowing the piston to settle back into its original position. This thermally driven back-and-forth motion can be used to create energy, or as is the case in many solar thermal systems, act as a switch that enables water in the system to drain back into the insulated tanks, where it is safe from freezing.
Thermal Energy Pumps / Thermal Actuator / Thermal Expansion Tanks