While going for a stroll along the seaside, many enjoy the magnificent sight of waves hitting the shore. Few will realize the unique opportunity to visualize the enormous amount of energy that waves carry. Is this the next big source of renewable energy?
The Pelamis project demonstrated the viability of deploying single wave units for energy production. However, in order to be competitive, the industry needs to bring wave energy to the next level. Thus, the true question is how wave energy can be generated in large scale installations.
What is Wave Energy?
Wave power is occasionally confused with tidal energy, but they are very distinct. While tidal energy captures the ocean’s motions due to the change in position of celestial bodies like the moon, wave power is produced from ocean surface waves induced by the wind. Wave formation will depend on several factors, such as the duration of the wind, the wind speed, the distance over which the wind blows or the depth of the water. There is a certain randomness involved in the formation of wind waves, since consecutive waves will vary in wave height, length and travel with different speeds.
It is therefore difficult to give exact predictions and the industry generally works within ranges and probabilities. Although waves have different features, all linear monochromatic waves are characterized by an oscillatory motion that decreases exponentially with increased water depth. This becomes apparent when you follow a specific particle in a wave that follows a perfect circle in deeper waters. It ends up having a more elliptical shape in shallower waters. Waves propagate along the ocean surface and it is this transport of energy that wave devices try to capture.
The Technological Challenge
One of the main technological challenges of wave energy devices is, unfortunately, inherent to their concept. Traditionally, the design of an offshore structure is optimized by minimizing its exposure to external forces such as waves, as is evidenced by the tubular structures of jackets, for example. By reducing the external loads, lighter structures can achieve the same design objectives, while at the same time being cheaper. However, the waves that apply these external loads are exactly what the devices are trying to capture to maximize their energy production. The high variability and unpredictability of wave energy forms a second technical challenge for wave devices. While their design should be able to resist the most extreme load cases, too much contingency will only lead to excess capacity and drive up the costs. This is why surviving a winter storm is a crucial step for wave energy capture pilot projects.
In the meantime, it seems that the industry has not yet come to a clear consensus on the optimal way to capture wave energy. It continues exploring an incredible variety of concepts that have very distinctive shapes, as a simple Google image search will demonstrate.
The alternatives vary from a series of semi-submerged units that move along the ocean surface, to a mechanical flap moving upwards with every wave that passes by, to an offshore reservoir capturing water using gravity to return the water while it passes by some hydroelectric generators. The fact that wave energy can be extracted in a variety of environments only encourages this variety. Obviously, the wave dynamics or mooring possibilities in shallow water differ from those in deep water. Although today we can roughly divide the wave energy converters into a couple of main categories, the concepts are also very distinct within these categories. Additional confusion can arise from the research’s iterative process, as wave technology developers generally plan to test prototypes at different scales. It is therefore hard for potential investors to define the stage of technological development of a particular project. This makes it even more challenging for investors to compare the performances of different technologies and select the device they want to champion. Especially today, as harsh economic conditions reduce the investors’ appetite for risk, this lack of transparency can have a negative impact on the availability of financial support. Ironically, it is this same support that is crucial to develop pilot projects that reduce these technological uncertainties.
Large-scale wave energy
Narrowing the broad variety of proposals to a few winning concepts does not automatically provide the industry with the optimal solution for large-scale wave energy production. It is easy to imagine that a large unit is more attractive in terms of operation and maintenance cost (as opposed to a large amount of single units). It would be interesting for the industry to compare the performance of a large number of single wave energy converter units with that of a single large sized unit. Interestingly, due to the fact that full-size prototypes are more difficult to finance for large sized units, we currently see that the technology of small-scale units is maturing more rapidly. It is to be seen if this will influence the development of large-scale wave energy generation.
Wave energy has all the characteristics of an emerging technology and although there are successful wave energy converter units available, the technology still has a long way to go before maturing into a significant part of any national energy strategy.
Image: Wave Energy via Shutterstock