Energy from wave motion

Wave energy is a form of energy that is mainly caused by wind energy. Such waves contain both potential and kinetic energy. For ideal deep water waves, which are not subject to any ground friction, the total capacity of a standard wave of 1 m width is directly proportional to the product of the square of the wave height and the wave period.

An assumed so-called standardised spectrum based on the above relation between wave height and wave period allows for the determination of the wave power or energy in relation to height or frequency. For instance, wave heights of 1.5 m at an average wave period of 6.2 s are typical of the German North Sea coast and lead to a significant weight height of 2.11 m and a total wave power of approximately 14 kW/m wave front. If it was possible to exploit the whole energy of a wave front of the length of the German North Sea coast (about 250 km), according to our model, theoretically an approximate power of 3.6 GW could be generated (see /A-1/).

Because of its considerable energy potential, for several decades, wave energy has been investigated with regard to power generation. However, the multitude of more or less unrealistic proposals that have been elaborated discredited this type of renewable energy. Thanks to the tireless commitment of several research teams over many years, this view now changes gradually. The use of wave energy became a more and more serious and important option for electricity generation on a small and medium scale.

Systems for electricity generation from wave energy can also be used for the protection of the coast because systems using wave energy convert the energy from the sea into electricity and remove this energy therefore from the ocean. Under these circumstances the wave energy is not only reflected or dissipated. A good combination of power generation and coastline protection could thus also enhance the economic attractiveness of wave energy exploitation.

The principle of converting the motion of sea waves into mechanically useful motion is trivial. By "inverting" the principle of a piston engine the motion of a body floating on waves (replacing the piston) can make a shaft rotate by means of a rod drive, whereas the rotating shaft in turn drives a generator.

The feasibility of this basic concept has already been demonstrated /A-3/, /A-4/. The aim of conversion plants using wave power is thus not to demonstrate the technical feasibility of transforming wave power into electricity, but to enhance the technical reliability of providing electric power at reasonable costs. In reality, this goal can rarely be fulfilled due to a series of requirements, which are summarised as follows:

- Hydraulic optimisation is absolutely necessary to achieve high electrical efficiencies. If, for instance, only the upward and downward wave motions are exploited, 50 % of the energy contained inside the wave is wasted. However, design principles have been elaborated which allow using the entire energy of the waves.

- The power plant design also needs to be designed in order to withstand the "wave of the century". If a wave energy converter is designed for harnessing waves of a height of 1 m, it also needs to withstand waves of ten times this size. In our case this would be a 10 m wave containing 10 times the above wave energy. The precautions to be taken into consideration necessarily result in considerable additional design costs.

- The power has to be designed very reliably even under unfavourable operation conditions. During the period when wave energy is most effective (e.g. autumn storms) maintenance or repairs cannot be performed for weeks. If a system fails during this period, efficiency is tremendously reduced due to long downtimes.

The following sections contain a discussion of different wave energy exploitation systems. Please note that for this purpose no differentiation is made between breakers energy and wave energy. The former is assumed to be a form of the latter.

A.1.1 TAPCHAN system

Within a TAPCHAN (tapered channel wave energy conversion device) system water advancing over the beach by breakers or swells is conducted into a raised reservoir via a converging inclined channel (Fig. A.1). This tapered channel concentrates waves of different frequencies, coming from different directions and simultaneously converts the kinetic wave energy into potential energy. Within this channel the wave height is increased due to the decreasing width. This has the consequence that the water level raises and the seawater eventually spills over the narrow end of the channel into the reservoir whose water level is located several meters above the average sea level. From this storage reservoir, the seawater, accumulated at a higher energetic level due to the difference in height, can flow back to sea via a turbine.

Due to the storage reservoir this system requires more space than most other wave energy conversion systems. Because of inflow losses (including shallow water effects) only a limited amount of the original wave energy (of deep waters) can be used. However, due to the levelled drainage of the storage reservoir and the applied low-pressure turbine, which is state-of-the-art technology on the markets for power plant equipment, operation of this system is much easier than of most other breaker or wave powered energy exploitation systems. An additional advantage is that the system components applied within this power plant are not subject to open sea conditions, and thus offer a longer technical lifetime. Also the maintenance can be easily conducted. Furthermore, power plant components permanently in motion do not touch the waves, and conversion from kinetic into potential energy is performed by solid reinforced concrete elements. The plant thus also withstands bad weather conditions ("wave of the century"). It is moreover beneficial that such plants are easily accessible from the shore. As fresh seawater is continuously conducted into the storage reservoir the latter is also suitable for fish farm operation. When compared to a straight overflow edge, parallel to the wave crest, a considerable benefit of such wave or swell-powered generators provided with a tapered channel is that basically all waves reach the required height at some point in order to fill the raised reservoir over the narrow end of the channel.

Fig. A.1 Operating principle of a TAPCHAN system (according to /A-1/)

The operating principle has already been proven by the TAPCHAN demonstration plant built in Norway at Toftestallen, near Bergen, in 1986.

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