Summary: Light flowed like water, driving a new form of energy production.
Photons trickled like water off the relativistic flywheel. The wheel itself was covered by angled serrations containing reflective material. At regular intervals around the wheel, lasers drove the wheel. The beams slowly and surely accelerated the wheel, itself magnetically suspended in a vacuum chamber. The momentum of light itself drove it to a relativistic velocity.
The wheel was connected an inductive motor, which turned the energy into an ever-flowing cascade of electricity. The energy produced rivaled atomic power, and it exceeded it on scalability and safety. Even fusion and fission reactors converted water to steam to convert conventional turbines. The photonic flywheel had no need of ancient steam engines. Yet it was not unlike an ancient watermill, only with light instead of water.
The lasers that drove the flywheel were themselves powered by molecular vibration, scaled graphene energy harvesting. Optical filters ensured smaller beams converged into a single, focused one able to impart slight energy to the flywheel. As such, there were also redundances built into the flywheel's housing. It was as close to a perpetual motion machine as known physics might allow, yet kin to humanity's earliest inventions.
Like those earlier innovations, it was nothing more complex than spinning a wheel.