Why is the development of wave energy converters so challenging? Part 1: The nature of waves

The Wave Energy Prize is not concerned about just developing a wave energy converter (WEC) that can generate power; that’s the easy part (as outlandish as that may seem). Rather, the goal of the Prize is to identify WEC concepts that have the prospect for becoming commercially competitive with existing means of power generation, without unsustainable public subsidies, in the next fifteen or twenty years. While that is an aggressive goal, the Wave Energy Prize is designed to make it achievable. This blog post and the next will describe why developing WECs is so challenging. First, a primer on the nature of waves themselves.

"Deep water wave" by Kraaiennest - Own work. Licensed under GFDL via Commons
“Deep water wave” by Kraaiennest – Own work. Licensed under GFDL via Commons

Real ocean waves are complex, dynamic, and cannot be precisely predicted. They are not neat and well-organized sinusoidal waves with well characterized amplitudes, frequencies, wave lengths and phases that create predictable patterns on the surface of the water through interference, diffraction, and refraction.

Over the deep ocean, winds blowing over many hundreds of kilometers lead to the creation of sea states, which when analyzed spectrally (or, broken down by defining characteristics like frequency, amplitude, and phase), show the superposition of waves of varying frequency, amplitude, and phase. In fact, the spectral distribution almost shows the history of the winds experienced by the sea over the entire journey of the wave.

At a given site in the ocean, the sea state changes over a time span of thirty minutes to one hour. Over the course of a year, a site’s wave energy resource is typically with about 1,000 sea states. Each sea state has different spectral distributions with unique probabilities of occurring – this is the so-called wave scatter diagram.

But the complexity does not end there. Local wind-driven waves can add another “peak” (or high probability of waves of certain characteristics) to the wave spectrum, and the spectral characteristics of the waves change as they move from the deep ocean to shallower depths due to shoaling and focusing effects. Ultimately the waves crest and break as they approach the shoreline.

The Wave Energy Prize is deliberately focusing on deep water applications. This isn’t to avoid the added complexity of shoaling and breaking waves. Rather, it is because it is in the deep water where the wave energy resource is strongest. By the time the waves reach the shore, all the wave energy has dissipated due to friction with the sea bed, resulting in the waves’ energy equaling zero at the shoreline.

It is important to understand how water moves in wave motions to understand how to extract energy from waves (more on this in the next blog). Deep ocean waves are a combination of sinusoidal and longitudinal waves. With wave energy being zero at the sea bed in the deep ocean due to friction, the actual water particle motion varies as a function of water depth, with the water tracing approximately circular paths near the surface, as can be seen in the video below. These paths become more elliptical and eventually linear oscillations as depth increases. The oscillations have smaller and smaller amplitudes as depth increases, as well. The next time you go to the beach, stand in the ocean near the coastline, beyond the breaking waves. You’ll experience your upper body bring pushed to move in circular paths, while your feet don’t really move that much at all.

Thus WECs need to extract the energy from circular, elliptical, or linear fluid oscillations created by waves passing through water, waves that have a large number of spectral distributions as described above, and convert this energy into other usable forms of energy. This is no mean feat, one that the Qualified Teams of the Wave Energy Prize are hoping to achieve; more on this in the next blog post. Stay tuned!

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