The maximum working temperature in a heat-pipe solar collector is controlled by means of a memory metal spring which is positioned inside the heat-pipe's condenser.
The memory metal is programmed to change its shape at a pre-set temperature. This allows for the condenser fluid to be retained inside the condenser. When the programmed temperature has been achieved, the memory metal spring expands and pushes a plug against the neck of the heat pipe blocking the return of the condensed fluid and stopping heat transfer.
At temperatures below the maximum programmed limit, the spring contracts allowing the condensed fluid to return to the lower section of the heat pipe. It is than evaporated due to the heat from from the absorber plate, transferring thermal energy to the condenser.
The Temperature Limitiung feature protects a solar system during short term system component failure serving as its last safety level. It is NOT a design feature for seasonal (radiant heating) thermal application. A heat dump must be considered (space heating applications) when the collected energy exceeds load.