Wave energy - a brief presentation [Norwegian]
Contents   Why ocean wave energy?   Some important terms and principles   Major challenges   A note on economy   Selected references   Appendices   Principles of energy extraction   Some types of wave energy converters    «The wind and waves are always on        the side of the ablest navigators.»        - Edward Gibbon
Why ocean wave energy? The ocean wave energy resource The global power potential of ocean waves is of the same order of magnitude as the present global energy flux through our communities. About 10% of the ocean wave energy reaches a coastline somewhere. This power potential is of the same order of magnitude as the total electric power production of the world. The vast energy potential is one of the reasons why energy from ocean waves is such an interesting possibility. On the northern hemisphere, the wave energy is maximum during the winter season. This is advantagous considering the seasonal variations of energy demands. Moreover, in countries depending on hydropower, such as Norway, the available energy is minimum during the winter. This means that an energy supplement from wave power to a energy system based mainly on hydropower, will help to reduce the need for storage capacity, and thereby reduce long term investment costs. Fields of utilization One advantage of wave energy is its versatility. So far, this resource has been utilized mostly in navigation buoys and water pumps. There are however several other possible applications, of which some are listed below: A wave-driven water pump may be used to supply water to desalination- or irrigation facilities close to a coast. Such constructions may be made both cheap and easy to maintain. This makes them especially useful in developing countries. Wave-driven pumps are also suitable devices for pumping clean sea water through fish farms, or through harbours and fjords, were the natural water circulation is poor. Another possible utilization is as a means of ship propulsion, by letting the waves move e.g. a so-called «whale-tail flap» driving forth the ship. Wave energy absorbing devices distributed along a coastline can be used to prevent erosion. In the future, wave power plants may also be used to produce hydrogen from sea water. The hydrogen may in turn be used as an energy carrier in vehicles, etc. The most obvious use of ocean wave energy, however, is as a means of electricity production. There are several ways to implement this. For instance, a wave driven pump as described above may be used to pump water from sea level into a basin a few meters higher up. By letting the water flow back through a conventional hydroturbin, the basin serves as an energy store. Most wave energy converters, however, are based on some turbine or piston system, directly utilizing the oscillatory motion created by the waves. Wave energy may prove to be of great practical importance as a means of providing electricity to isolated communities such as small islands, or to off-shore installations like fish farms. Commercial production of such wave energy devices, together with further research in this field, will cause the cost of wave energy to steadily decrease. This will in turn make it a probable future alternative as a general energy source in most countries with a coastline. It may be convenient to build a wave energy converter as part of a construction made for other purposes, thereby utilizing mutual technical and economical benefits. In Japan, wave energy converters of the oscillating water column (OWC) type have been built as a part of a breakwater. Similarly, a converter of the bouy type may be made part of a lightbuoy, hence providing energy for the buoy itself as well as for other purposes. One should also consider the possibility of combining wave energy and off-shore windmills, thereby taking advantage of common electric facilities as well as the possibility of a more even electric delivery to the grid.   Some important terms and principles Oscillations and waves An oscillation is a variation which repeats itself (exactly or approximately). The term usually describes the movement of a mechanical system (e.g. a pendulum, guitar string, spring, etc), or the currents and voltages of an eletric curcuit, but it can also describe somewhat more abstract relations, such as variation in populations, etc. A wave is an oscillation which, in addition to moving back and forth, also moves as a whole, originating from some kind of source. Light is an example of a wave, being electromagnetic oscillations travelling through space. Sound, which is pressure oscillations travelling through gasses or fluids, is another example. Water waves are yet another, being repetitive displacements of the water surface, travelling along the surface itself. Some waves move in a straight line, while others spread out as a circle or a sphere. The time elapsed between each repetition of the movement is called a period, T (see fig. 1). Fig. 1:   A wave is a function of time The physical distance between two subsequent maximum values (or two subsequent minimum values) of a wave is called the wavelength, L (see fig. 2). This quantity may be expressed as L = c*T, where c denotes the velocity with which the wave travels. Fig. 2:   A wave is a function of position Wave absorbator A wave absorbator is the part of a wave power plant which absorbs the energy from the waves, i.e. converts it into mechanical energy. A wave absorbator typically consists of some kind of body, e.g. a buoy or a pendulum, or a water column in a chamber. The body oscillates due to the waves, and the kinetic energy from this motion is converted by means of some pneumatic or hydraulic system utilizing pistons or turbines. Optimum amplitude The amplitude of an oscillation is its maximum value within a period. When oscillations or waves are not entirely periodic – that is, they do not repeat themselves exactly – the amplitude need to be computed by means of statistic relations. The waveheight of real water waves is typically determined in this fashion. The amount of energy which can be absorbed from water waves by an oscillating system depends on the damping of the system. An oscillation with no damping (e.g. a floating cork) cannot absorb energy at all. A device which is latched (infinite damping) cannot absorb energy, either. The optimum must consequently be some intermediate value. The amplitude of the system when this occurs, is called the optimum amplitude. Optimum phase The phase (or phase difference) of an oscillation or wave indicates its time shift relative to some reference (e.g. another oscillation or wave). The amount of energy absorbed by an oscillating system depends on the phase difference between the oscillation and the incoming wave, as well as on the amplitude. This phase difference is a function of the stiffness and inertia of the system. In order to obtain 100% energy absorption, these parameters needs to be adjusted to the wavelength. Since the wavelength varies with time, this adjustment must in principle be carried out continuously. Phase control In the case of small wave energy converters, optimum phase may be obtained for most wavelengths by means of so-called phase control. The simplest way to accomplish this is to restrain the movement of the system in parts of the oscillation period. It is crucial, however, that the motion is held back and released at the exact right moments. This in turn requires that the phase of the incoming wave can be determined accurately some time in advance, based on real-time measurement and computations. Much of the wave energy research during the last decade have been focused on this topic. A small wave energy absorbator which is sucessfully phase controlled is more cost efficient than a larger absorbator with no phase control (and which in principle cannot be phase controlled to any great degree). Good phase control (including measurements, theoretic foundation, real time computations and control equipment) may be just as important to the cost efficiency of a wave power plant as the particular physical mechanism chosen for energy conversion. Power Power means energy per time unit. Maximum energy absorbtion Disrecarding energy losses, a wave energy absorbator can absorb 100% of the energy from the incoming waves. (In fact, a small absorbator can absorb more energy from a plane wave than the portion which passes within the width of the absorbator. This phenomenon is due to interference between the incoming wave and the wave generated by the oscillation of the absorbator.) The theoretic maximum energy absorption can only be obtained if the absorbator oscillates with optimum amplitude and optimum phase (relative to the incoming wave). The energy absorption pr. volume of the absorbator has a certain theoretical maximum limit. From an economic point of view this limit is of much greater importance than the total energy output. The reason for this is that most of the investment costs depend directly on the amount of materials used in the construction, and hence, on the size of the power plant. The ratio of absorbed power to volume is greatest for small absorbators.   Major challenges Variation in wavelength The economy of an absorbator depends on its ability to absorb energy from a broad spectrum of waves. A large construction absorbs about equally well for all kinds of wavelengts, but the efficiency is typically rather low. Besides, such constructions will have high investment costs. Small constructions are cheap, and they absorb very efficiently when the wavelength approaches optimum (and the phase becomes optimum). For other wavelengths, however, the efficiency is very low. But this can be amended by means of phase control, as mentioned. Variation in wave height It is not economically profitable to dimension a wave power plant in order to utilize the largest waves. Such waves are sparse, and the power plant will be too big to utilize small waves. As mentioned, large size also means high investment costs. The economy will be substantially improved by dimensioning the power plant to the most common wave heights. The total power output will of course be lower, but the output can be increased simply by building several units, instead of one large structure. Small units cannot withstand extreme wave forces as well as larger structures. Consequently, some mechanism should be applied to minimize strain due to extreme condition, e.g. latching of moving parts in submerged position. If possible, the location of the power plant should be selected to ensure a small ratio of maximum to average wave height. Energy storage For any wave power plant, no matter how efficient and optimally located, there will be periods when the incoming wave energy is too small to produce any output. Thus, some kind of energy storage mechanism should be applied, evening out the energy output. It will also help to connect a large number of wave absorbators, dispersed in a wide area of the sea surface. However, if the electrical grid is adequately dimensioned, some variation in the output from individual power plants is usually tolerable. Energy losses Any convertion of energy entails loss. In the case of a wave power plant, energy losses is mainly due to friction. This friction may take place between the moving parts of the power plant, between the construction and the water, and in the water itself (viscous and turbulent losses). Roughly stated, more moving parts and sharper edges means more energy loss. The loss percentage also increases with the wave height.   A note on economy It's an interesting fact that the unit costs of a certain technology decreases as the accumulated number of produced units increases. Wave power plants are not profitable today. Subsidising such technology, and hence stimulating production of a sufficient number of units, will however make wave power plants more cost efficient in the long run. Estimates suggest that if a 5 MW power plant can produce energy to a cost of 0,20 dollars/kWh today, this cost is expected to drop to about 0,04 dollars/kWh before the total installed power capacity (in all the power plants together) reaches 250 MW.   Selected references Books and booklets Falnes, J.: Ocean Waves and Oscillating Systems. Cambridge University Press, 2002. ISBN 05-21-78211-2. Carmichael. A.D., Falnes, J.: State of the art in wave power recovery. In Ocean Energy Recovery, Seymour, R.J., editor. American Society of Civil Engineers, New York, 1992. ISBN 0-87262-894-9. Nye fornybare energikilder. 2nd edition. Norwegian Research Council (NFR) in cooperation with Norwegian Water Resources and Energy Directorate (NVE), 2001. ISBN 82-12-01621-8. (In Norwegian) Lennart Claeson m.fl: Energi från havets vågor. Technocean AB. Efn-rapport nr. 21, Energiforskningsnemnden, 1987 (order from; Allmänna Förlaget, Kundtjänst). ISBN 91-38-09691-9. (In Swedish) Ross, D.: Energy from the Waves. 2nd edition, Pergamon Press, 1981. McCormick, M.E.: Ocean Wave Energy Conversion. John Wiley & Sons, New York, 1981. Internet Books on wave power plant research. Oceanor, Norway. Wave energy research at NTNU. Wave energy at Wikipedia. World Energy Council – a survey of wave energy technology. Appendices   Principles of energy extraction   Some types of wave energy converters Dr.ing. Arne Brendmo Siste rev. 2010-01-19