A coolant loop for trough-reflector solar energy conversion systems has open coolant supply and discharge reservoirs. Coolant is driven by siphoning pressure through cooling channels which have attached solar cell arrays. The siphoning pressure is produced by maintaining the free surface of coolant in a coolant supply reservoir at a higher elevation than the free surface of coolant in a coolant discharge reservoir. The cooling channels have air evacuation and air inlet ports to facilitate initiation and termination of siphon-pressure-driven coolant flow. The cooling channels also have in-line flow control valves that respond to control signals generated by coolant temperature sensors.