Definition
Tidal power is a way to make electricity from the height difference in seawater between high tide and low tide, much like the rush of water when you pull the plug on a full bathtub. The moon's and sun's gravity cause tides that usually come twice a day, and this clean energy method turns that steady rhythm of the sea into power.
A giant sea dam that opens twice a day
Korea's coasts usually see two high tides and two low tides a day, when the water rises and then drains away. Along the west coast, the gap between high and low tide is big. Build a huge seawall (a barrier) with gates there, and the incoming tide can pour into the area behind it.
Then you close the gates and hold the water in, like a full lake. When the outside sea drops at low tide, you swing the gates wide open. The trapped water rushes out and spins giant underwater propellers (turbines).
The spinning turbines run a generator and make a lot of electricity. The basic idea is the same as hydropower, which uses falling water. The key difference is that tidal power uses the regular rise and fall of seawater, not river water.
To be precise: tidal power and tidal stream power are different
Many people lump all ocean electricity together. But tidal power, tidal stream power, and wave power each get energy in a completely different way.
Tidal power blocks off part of the sea with a dam and uses the height difference, which is the force of falling water. Tidal stream power builds no dam at all. It puts propeller-style turbines in places like Uldolmok, the strait made famous by Admiral Yi Sun-sin's Battle of Myeongnyang, and uses the speed of fast-moving seawater itself.
Wave power is different again. It captures the up-and-down motion of wind-driven waves on the surface, not the tides. Of these, the Sihwa Lake Tidal Power Station is a tidal power plant: it uses the drop between a lake blocked off by a seawall and the sea. Unlike the example above, Sihwa generates power from water flowing from the sea into the lake at high tide.
Clean energy that ignores the weather, with an environmental catch
Solar panels struggle at night or in the rain, and wind turbines stop when the wind dies down. Tidal power relies on tides, which come from the pull of the moon and sun, so it works in any weather. Its big advantage is that you can predict exactly when the power will be available.
It burns no fossil fuels, so it gives off no greenhouse gases or fine dust. It is a nearly limitless clean energy source. As long as the Earth and moon exist, the tides will come about twice a day, at times we can calculate ahead. The timing shifts about 50 minutes later each day, but it can be worked out accurately years in advance.
Still, building a huge seawall in the ocean has a cost. Tidal flats (mudflats) shrink, and the paths of water through the marine ecosystem change. If seawater can't flow in and out naturally, sea life can lose its home. That's why researchers keep working on ways to get this energy with less damage to the environment.
Common misconceptions
Tidal power makes more electricity when the wind is strong and the waves are high.
Using the motion of waves is called wave power. Tidal power doesn't depend on waves. It uses the height difference between high and low tide, which is caused by the moon's and sun's gravity.
Where you meet it
The Sihwa Lake Tidal Power Station in Ansan, Gyeonggi Province, is a leading example of tidal power and ranks among the largest in the world by output.
The Rance Tidal Power Station in France was completed in 1966 and has supplied steady electricity for more than half a century.
In one sentence
Tidal power is a green way to make electricity by spinning turbines with the height difference of seawater between high and low tide.