The masses of Earth’s largest bodies of water are pulled by the gravitational field of the moon as it rotates around the planet. Tidal energy is harvested from the rising and falling water through a variety of methods including in-stream devices, barrages, tidal pools and dynamic tidal power, all of which work by driving a turbine.
The latter three methods rely on blocking the flow of the tidal waters within an area. The blockage creates a height difference that exploits gravity to produce greater force. The down side to these three methods is environmental disruption to the surrounding bodies of water. Older types of barrage systems are known to be particularly detrimental to fish populations. (Of the three, dynamic tidal power is the least damaging, as it does not enclose waters completely.)
In-stream systems are becoming increasingly popular, largely because they have the least impact on the environment. Furthermore, the systems are easier to remove for upgrades and create less disruption for sea-going traffic.
Tidal power is one of the most stable, reliable and adjustable types of green energy. Water-driven turbines also pose less risk to aquatic life than wind turbines do to avian life because the denser fluid drives a higher resistance at a lower rate of speed.
Although tidal power is more expensive to produce than wind power or solar power, newer technologies are improving efficiencies and lessening environmental impacts. In Atlantic Canada, which has the world’s highest tides, it is estimated that tidal energy could generate billions of dollars of revenue annually.
See a simple video demonstration of how a water turbine works: