The tide moves a huge amount of water twice each day, and harnessing it could provide a great deal of energy. But only about 20 sites around the world are possible locations, and only 2 have been built so far (one in France, one in Canada).
Although the energy supply is reliable and plentiful, converting it into useful electrical power is not easy.
Underwater turbines work rather like windmills do on land.
Ocean waves are caused by the wind as it blows across the sea. Waves are a powerful source of energy.The problem is that it's not easy to harness this energy and convert it into electricity in large amounts. Thus, wave power stations are rare.
..................At a wave power station, the waves arriving cause the water in the chamber to rise and fall, which means that air is forced in and out of the hole in the top of the chamber.
We place a turbine in this hole, which is turned by the air rushing in and out. The turbine turns a generator. One big problem is that of building and anchoring something that can withstand the roughest conditions at sea, yet can generate a reasonable amount of power from small waves. It's not much use if it only works during storms!
ADVANTAGES
The advantages of tidal barrages are that once they are built, tidal power is free, and doesn't produce greenhouse gases, or waste. It needs no fuel, is not expensive to maintain, and is reliable and predictable. The tides always come in and out in a regular pattern.
Relying on wave energy, once again is free - no fuel is needed, and no waste is produced. It is not expensive to operate or maintain, and it is capable of producing a great deal of energy.
DISADVANTAGES
A tidal barrage across an estuary is very expensive to build, and affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. There are few suitable sites for tidal barrages. Also, it only provides power for around 10 hours each day, when the tide is actually moving in or out.
Waves can vary - so sometimes you'll get lots of energy, and sometimes nothing. So you need a suitable site where waves are consistently strong. What you build must be able to withstand very rough weather from storms. Also, some designs are very noisy.