The Predictable Renewable
Tidal power is unique among renewables because it is perfectly predictable. Driven by the gravitational dance of the moon, sun, and Earth, tidal currents follow cycles that can be forecast with precision decades ahead. This predictability is enormously valuable for electricity grid operators who must balance supply and demand moment by moment. While the tides are intermittent, they are never uncertain.
Power from Moving Water
Like wind turbines, tidal stream turbines extract kinetic energy from a moving fluid. But water is roughly 800 times denser than air, so a tidal turbine encounters far greater forces per unit area. A 16-meter tidal turbine in a 2.5 m/s current generates comparable power to a 90-meter wind turbine. The Betz limit applies equally - no more than 59.3% of the kinetic energy can be extracted - but the higher density means smaller, more compact machines.
Tidal Cycles and Capacity Factor
This simulation shows power output over a complete tidal cycle of approximately 12.4 hours. Current speed varies sinusoidally from zero (slack tide) to peak flow (mid-tide), and since power scales with the cube of velocity, output is highly concentrated around peak flow. The capacity factor - typically 20-30% - reflects this variation. Spring tides (new and full moon) produce roughly double the current speed of neap tides.
Arrays and the Future
The future of tidal power lies in large arrays of turbines, analogous to wind farms. Optimizing array layout is critical: downstream turbines sit in the wake of upstream machines, receiving reduced flow. Staggered layouts, variable spacing, and bi-directional turbines that operate on both flood and ebb tides maximize energy capture. As costs decrease along the technology learning curve, tidal arrays may become competitive with offshore wind at the best sites.