Freeze protection system for solar receiver

A solar receiver has an arrangement of heat transfer surfaces and a heat transfer fluid phase separator, such as a vertical steam/water separator, fluidly interconnected thereto. The receiver includes a plurality of heat transfer fluid filled components, and at least one alternate heat source. When various temperature measurements indicate freezing or solidification of the fluid is possible, the alternate heat source is activated to maintain a temperature of the fluid greater than the freezing/solidification point of the fluid. The application of the alternate heat source further induces natural circulation of the fluid within the components, further providing freeze/solidification protection to the receiver. A controller may be configured to receive sensed temperatures of the fluid, components, ambient air, etc., and use these temperatures relative to a threshold temperature to activate, vary output, and deactivate one or more alternate heat sources.