Why wave energy?

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By now, most people will agree that the future energy supply should not be based on fossil fuels and that renewable sources of energy must gradually take over. But what does wave energy have to offer – what is its current status, and what are the advantages and disadvantages of this type of technology?

An overview of wave energy utilisation methods must be provided as background information. The motivation behind the utilisation of wave energy is very similar to that of other renewable sources of energy, i.e. factors such as CO2-based climate changes as well as the fact that those fossil fuels, which form the basis of our current civilisation, make up non-renewable resources.

So, what is the problem? The main problem is the price but also renewable energy variations make up a hurdle because we need electricity at all times – also on calm days. Therefore, in the long run we are going to need a mixture of various accessible and renewable sources of energy. As waves are generated by wind that creates energy across long distances, wave energy will be able to assist in levelling out the energy production within the system as a whole because it features a smoother production typically out of phase with regards to the wind. Furthermore, wave energy is easier to predict than wind energy, which is of significant importance when planning the rest of the network energy production.

Which other factors speak in favour of wave energy when compared to other renewable sources of energy? If we consider future large-scale utilisation of both wind and waves offshore, the factor of energy production per areal unit of ocean becomes a competitive parameter when comparing systems. Offshore wave energy plants in attractive locations facing the open ocean carry a substantial advantage compared to offshore wind mills. However, in certain situations the co-existence might turn out to be the best solution of all, both in terms of optimal area utilisation but also with regards to joint usage of infrastructure, services etc.  In terms of environmental impacts of these systems, the visual effects of wind mills make up one of the largest problems. For wave energy plants, this problem is of a much smaller scale.

So, how vast are these available wave energy resources actually? 5 TW (TW: 1012 W) seems a realistic bet based on the technologies available today. In 2005, the average global energy consumption level amounted to approximately 15 TW, of which electricity made up around 10%. In comparison, the amount of solar energy that hits the Earth amounts to some 120,000 TW or 8,000 times the amounts that we consume! Thus, on a global scale it is in fact possible to extract several times the global electricity consumption level from waves. The European coasts are generally hit by an amount of wave energy corresponding to the total electricity consumption in Europe. The Danish west coast also carries quite a good potential although it is in the lee of the British Isles. It is realistic to believe that wave energy can cover 5-10% of the Danish electricity consumption and that 30% is technically realisable. On the other side of the British Isles, the potential per meter of wave front is five times greater. In general, this shows that wave energy does indeed carry the potential to provide significant contributions to local, regional and global electricity production measures. From a Danish viewpoint, although this potential contribution to our own electricity production efforts is worth considering, the main motivation must be its export potential.

When extracting energy from waves, it is of utmost importance to understand certain ocean wave characteristics. When waves move across the ocean surface, wave energy is transmitted, and this process can take place out on the open ocean across vast distances without any energy loss worth mentioning. The wave movement of the water particles, however, does not move – similar to the way in which the spectators do not move but simply ”push” their neighbours a bit when creating a ”wave” in a stadium.  In this way, each individual water particle within the wave moves around in deep waters in circular paths, and the deeper down the column the water the particle gets, the smaller the circle radius. In more shallow waters, where the water particles hit the ocean floor, the vertical component is "densified” and the water particle paths turn into ellipses. In such situations, the energy loss is increased by the waves spreading through floor friction and wave refraction. Wave conditions change in accordance with weather and wind conditions.

This means that typical wave parameters are not constant over a certain period of time, so when designing a wave energy plant, one needs to have a detailed knowledge of the statistical distribution of these parameters in order to be able to adjust the plant to the location in question. Throughout the year, there will be significant variations in the amounts of energy available, whereby the highest amounts will be available in the winter and the lowest amounts in the summer (at our degree of latitude), which happens to fit the overall consumption pattern very nicely. In addition, waves are irregular and directionally dispersed, meaning that each individual wave within a given wave situation will feature a variable height, period and direction. These circumstances mean that the energy content out in the open ocean is much larger than in the shallow waters typically found closer to shore. Similarly, the ratio between force application during production and force application in extreme situations is much higher in the open ocean that in shallow waters. Based on this information, it is difficult to provide a clear-cut answer to the question of whether plants should be located in the open ocean or in shallower waters to provide the most optimal financial pay-offs, because whereas plants located in the open ocean would be expected to produce the largest amounts of energy, they must also be constructed in a way that allows them to resist much stronger forces out there. As a general rule of thumb, wave energy plants must be effective when it comes to absorbing wave energy stemming from smaller and more frequent waves but ineffective when it comes to big and rare waves. In such situations, it is more about survival than energy production.

So, what does such a wave energy plant look like? There is no clear-cut answer to that question. Resourcefulness is plentiful: this area has seen hundreds of concepts and just as many patents, however, until now no unequivocal answer has been identified. Most plants are classified as either "point absorbers", "terminators" or "attenuators". Point absorbers are floating objects with horizontal extensions that are only a fraction of the typical wavelength in which the objects have to perform. This means that the object is going to move up/down/forward/backwards during wave impact, and if this object is connected to a fixed or relative reference, energy can be extracted from that reference. Examples include AquaBuoy, WaveBob and many others. Terminators are plants, which are aligned parallel to the wave crests, thereby ”swallowing” the wave energy so to speak. Examples include coastal plants placed on the ocean floor such as OWCs and overtopping-based plants such as the Norwegian SSG; however, this category also includes plants such as the Danish Wave Dragon (see picture). Attenuators are plants aligned parallel to the wave crests. Examples include the Scottish concept Pelamis, which consists of four tubes linked by three hinged joints. Hydraulic rams extract energy from the relative movement within the tubes. Although the Danish plant Wave Star actually consists of a collection of point absorbers, one could include it in this category as well. Whether or not we will witness a move towards one single type of plant in the future – like it has happened within the windmill industry – is still unknown. Personally, I do not expect that to happen because of the vast number of parameters influencing the selection of a wave energy plant for a particular project locality; however, we also have to consider the fact that not all plants-to-be will ever reach a full scale stage.

So, which of these development stages are we facing today? As indicated above, wave energy is still in the very early development stages. Today, a handful of plants are being tested in what resembles full scale operation but only a few of them run for many hours and produce large amounts of energy.  There is a vast range of technologies available and the problem is how to determine which ones, if any, will in fact be able to generate sufficient amounts of electricity at competitive prices. At this point, realistic predictions on the amount of energy that a given plant would be able to produce in a given location can be made; however, it is still very difficult to define a realistic price. The first commercial installations available today indicate a price level of DKK 2-5/kWh. Although this is quite a high price, it is by no means higher than the price of electricity generated by solar power cells or windmills 20-30 years ago. People clearly expect mass production and optimisation to bring the prices down to the price level of offshore wind energy but this remains an undiscovered area. In order to get closer to a realistic price of electricity generated by wave energy, full scale demonstration plants must be established for longer periods of time in order to obtain a more detailed overview of construction, operational and maintenance costs. Such demonstration phases are rather expensive and time-consuming. They are, however, necessary in order to be able to make a reasonable decision about the involvement of wave energy in the long-term energy mix of the future.

This was written by Jens Peter Kofoed from Aalborg University