Energy of the sea
The world’s seas and oceans may one day be able to supply us with all the energy we need to live and work. Today, there are very few power plants that use sea energy, and the existing power plants are mostly small. There are three basic ways to harness the energy of the sea: the use of wave energy, the use of tidal energy, and the use of temperature differences in water.
Energy of the waves
Wave energy, mechanical energy, is transformed by solar energy. Namely, waves are caused by winds, which occur as a result of differences in air pressure, and these differences are caused by different heating of individual parts of the Earth’s surface. Constant (planetary) winds cause constant ripples in certain areas and these are the places where it is possible to use their energy to start the turbine favorably. One way is for the wave to enter the room and expel air from it. That air drives the turbine which can then drive the generator. When a wave leaves the room, air enters the room through a passage that is otherwise closed. Another way is to use the vertical movement of the waves (up – down) to move the piston inside the cylinder. That piston can also run a generator.
Most systems that use wave energy are low power, but can be used to, for example, power a signal buoy or a smaller lighthouse.
Energy of the tide
The energy of the tides occurs due to the gravitational force (attraction) of the Moon and the Sun that acts on the water in the oceans. The use of tidal energy is similar to the use of river watercourse energy: the energy of water drives a turbine, which drives a generator, and so electricity is produced. For energy exploitation of tides, it is necessary to choose a suitable place on the coast, where the tide is high, with the possibility – by building a barrier (dam) – to insulate part of the sea surface to create an accumulation basin.
During high tide, namely, water enters the reservoir, which is then closed by a dam. With the onset of low tide, due to the height difference between the water levels in the pool and the sea (water in the pool remains at maximum elevation, and the sea level decreases), the potential energy of water in the pool can be used as in a conventional hydroelectric power plant: into the sea at a drop equal to the difference between the water level in the pool and the sea level. Leaking water through the turbines lowers the water level in the pool until the level difference reaches a minimum at which the turbine can still operate. When this minimum is reached, the turbine is stopped – until the next tide. If the turbines, however, are two-way (turbines through which water can flow both in one and in the other, opposite direction), a similar procedure is performed when approaching (raising) the sea during high tide: in this case the dam prevents water from entering the pool until the moment of the height difference between the sea water level and the one in the pool. After that, water from the sea is allowed to flow through the turbines into the pool.
In order to pay off the use of tidal energy, at least 5 meters of height difference between tides is needed. There are few places in the world where the difference between high tide and low tide is so great. Some power plants that use that energy are already in operation. It is the largest in France, the La Mance power plant, with a capacity of 240 MW.