BY KAI-UWE GRAW
An old dream of humanity is to make use of the almost immeasurable energy of ocean waves. Their destructive power has however up to now not permitted any economically reasonable design to survive for long, although there have been many promising attempts and approaches.
he reduction in carbon dioxide emissions, which is increasingly urgent and is a major goal of governments and societies, has conferred a new significance on wave energy – as on all renewable energy sources. Interest in wave energy power plants, which could make appreciable contributions to the world’s energy supply, is steadily growing. The use of wave energy for generating electric power has been under investigation for many decades. However, the countless, sometimes extremely naive suggestions for the application of wave energy have given this renewable energy source a dubious aftertaste in the public mind. But the long-term commtiment of a few research teams is now leading to a rethinking of this view.
The ocean waves contain inexhaustible reserves of energy. They are estimated to store around ten million ter – awatt hours of wave energy per year. This makes them, in principle, very attractive as an energy source. However, large waves can deploy a destructive force which makes huge demands on the stability of the wave power plant under load. It is thus particularly interesting to employ wave power plants precisely where the force of large waves must
in any case be broken: along coastal protection installations. Conventional breakwaters only reflect or dissipate the wave energy without making use of it. Wave power plants, in contrast, extract the energy and convert it into useful electric power. Furthermore, the use of a breakwater as the structure for a power plant reduces the cost of the plant which is integrated into it. Figure 1 shows a new wave power plant in the Spanish harbor Mutriku, whose construction was supported by the EU.
Renewable Energy. Edited by R. Wengenmayr, Th. Buhrke. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
surface tension of the water is also a very weak force. It is important only for waves that are shorter than about one centimeter: Such waves are flattened by the surface tension. In all other cases, gravity waves predominate. The force of gravity pulls the water in the wave crests down towards the troughs and thus tends to equalize the differences in height, acting as a restoring force. A simple introduction to the theory of waves is given in .
Water waves produced by the wind are generated mainly over deep water. Their form depends on the wind velocity, the duration of the wind, and the distance they have propagated since they were generated. The regions of the ocean surface with the most wave energy are therefore the open oceans, far from the equator (Figure 4). The wind is subject to friction with the surface of the ocean water; it pushes on individual water particles and thus accelerates the water layers near the surface. Turbulences in the airflow give rise to pressure differences between different parts of the water surface. To equalize these differences, the surface rises and sinks. This now-rough surface is subject to ever stronger pressure differences from the wind, which in turn increase the amplitude of the surface roughness. In this way, higher and higher quasi-periodic waves are formed.
Wave dynamics in the end limits further growth of the waves. The simple model of “linear wave theory“ already gives a realistic value for the maximum wave height. It is ca. 14 % of the wavelength. According to linear wave theory, the individual water particles in the wave attain speeds at the tops of the wave crests which are greater than the propagation velocity of the waves themselves. They practically “fall out“ of the wave in the direction in which it is moving. At this maximum height, the wave thus becomes unstable, and the wave crests form foamy whitecaps. Turbulence consumes part of the wave energy. When the waves have reached their maximum height and their period no longer changes, even if the wind continues to blow, the sea condition is called a “fully developed sea”. After the wind has died down, the waves can maintain their energy
over distances of many thousands of kilometers. They are then referred to as “groundswell“.
When the waves move into shallow water, their length and velocity decrease. Friction with the ocean bottom dissipates their energy and often changes their direction. When the wave velocity has dropped to a certain limiting value, the waves break and form a foamy surf. The breaking waves also lead to energy loss through turbulence. In planning coastal wave energy power plants, this process must be taken into account.