I would think that his generator would be in the form of a dynamo (as opposed to an alternator). A dynamo makes use of a commutator (just like a dc motor) to switch the direction of current so that it doesn't alternate.
Dynamos were commonly used in cars for battery-charging purposes, until they eventually got replaced by alternators (this change being made possible by the advent of cheap solid-state rectifiers).
Ah! Yep, I know about dynamos and commutators so that all makes perfect sense. Thanks for the info.
In the power station featured they moved the magnets and kept the winding stationary. That is a lot easier as you don't get loses on rotating contact resistance and sparks! And the AC they get out is what they want anyway.