Energy Reserves from the Oceans

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 [2].

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.