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Wave Energy

Overview:

Wave energy refers to the segment of renewable energy dedicated to finding a solution to harnessing the power contained in ocean waves. Due to the way in which waves are formed, wave energy can be considered a tertiary form of solar energy. Unequal heating of the earth’s surface by the sun generates wind which, as it blows over water, creates waves. What makes this an excellent source for energy production is that waves are in abundant supply (almost 75% of the earth’s surface is water), they are constantly being formed and they are able to efficiently transport and retain energy for thousands of miles even after the winds that created them have stopped blowing.

As a side note, a distinction should be made between wave energy (described above) and tidal energy, which looks for solutions to harness the power contained in tidal currents created by the gravitational pull of the moon on the earth.

Potential:

The power of a wave is determined by several factors, including wave height, wave speed, wavelength and water density. Although estimates about possible electrical production capacity vary, according to a report by the European Commission in 1992, for Europe alone the total offshore energy potential is between 120-190 TWh/year and 34-46 TWh/year for areas near the shore.

As this study also indicates, the power from offshore waves is far greater that than those of coastal currents. The reason for this is that as waves approach the shore they are modified in different ways, such as a change in direction (refraction) or power (due to the losses created by friction with the sea floor and breakwater systems). These changes result in energy losses that could be as high as 50% as compared to offshore energy, depending on the continental shelf and the characteristics of the sea floor. Furthermore, it is more difficult to efficiently capture the energy near the coast.

For these reasons, offshore devices have become the main area of focus for their ability to generate significant amounts of electricity while shoreline systems, on the other had, are better used for supplying smaller amounts of energy for specific purposes.

Technology:

Despite its tremendous potential, capturing the energy contained in waves is far less developed and less clear when compared with solar and wind energy systems. Unlike large wind turbines, there is a wide variety of wave energy technologies resulting from the different ways in which energy can be absorbed from the waves and depending on the water depth and the location (shoreline, near-shore, offshore). Several methods have been proposed to classify wave energy systems, according to location, to working principle and to size (“point absorbers” versus “large” systems). The classification in Figure 1 is based mostly on working principle. The examples shown do not represent an exhaustive list and were chosen from the projects that have reached the prototype stage or at least were the object of an extensive development effort.

Classification of Wave Energy Converters based on working principle Source: IEA Ocean Energy Systems Annual Report 2008

Most of the first prototypes to be built and deployed in open coastal waters are or were located on the shoreline or near shore and are sometimes named “first generation” devices. In general they stand on the sea bottom or are fixed to a rocky cliff. Shoreline devices have the advantage of easier maintenance and installation and do not require deep-water moorings and long underwater electrical cables, but the less energetic wave climate at the shoreline reduces significantly their output. The typical first-generation device is the oscillating water column (OWC). Another example is the overtopping device Tapchan (Tapered Channel Wave Power Device).

Offshore devices are basically oscillating bodies, either floating or (more rarely) fully submerged. They exploit the more powerful waves available in deep water (typically at depths greater than 40 meters). Offshore wave energy converters are in general more complex compared with first-generation systems.

In the case of Wedge’s technology, it has been specially designed to be used in off-shore, point absorber systems.