Tidal energy captures the rise and fall of ocean tides. The Moon's gravity pulls on the oceans, creating two high tides and two low tides each day. In some places, the difference between high and low water can reach 10 meters or more. That height difference stores energy.

Two main technologies exist. Tidal barrages are dams built across estuaries. Water flows through turbines as the tide comes in and goes out. Tidal stream generators look like underwater wind turbines. They sit in fast-moving currents and spin as the tide flows past. Stream generators have less environmental impact than barrages.

Tidal energy is predictable. Tides follow the Moon, so operators know years in advance when power will be available. That is a big advantage over wind and solar. The drawbacks are cost and location. Few sites have the right conditions. Saltwater corrodes equipment. Building in the ocean is expensive. Still, a few projects operate successfully in France, South Korea, and Canada.

Common technologies

• Tidal barrages
• Tidal stream turbines
• Tidal lagoons
• Dynamic tidal power
• Underwater kites