Abstract: An overheat protection device (OPD) is implemented independently of the circulating system of a solar thermal collector and isolated from the environment. The system facilitates operation of a solar thermal collector at elevated internal temperatures (versus reducing or ceasing operation above a critical temperature). OPD includes a heat pipe filled with at least two fluids. In a non-heat conducting state, a temperature at an evaporator portion of the heat pipe is below a transition temperature and the dual-fluid includes at least one fluid in a liquid state and at least one fluid in a gaseous state. When the temperature at the evaporator is above the pre-defined transition temperature the OPD undergoes an abrupt transition to a heat conducting state, whereby the dual-fluid transfers heat from the evaporator area to the condenser area, thus transitioning from a state of thermal isolation from an environment to one of strong thermal coupling.