• David Armes

What is wave power?

Wave power for electricity generation involves capturing the energy of waves to produce power. The machine that does this is called a wave energy converter (WEC), and there are many designs for these converters. Wave power has lagged behind other renewable energy resources, but is starting to catch up with additional investment in renewable power generation methods. Let’s take a look at how wave power works, where it came from, and some of the potential methods of capturing it.


How does wave power work?


You may have heard of tidal power, which is the capture of energy from the Sun and Moon’s gravitational pull on the Earth. Wave power captures the energy of the waves themselves, generally near or on the surface of the water, where the forces are strongest. If the wind is faster than the waves, the wind transfers energy to the water. The height of the waves depends on a few factors, including duration of the wind, the distance the wind travels over the waves, the distance to the seafloor, and the topography at the seabed. The distance to and topography of the seafloor can help to focus the wave energy or disperse it. Deeper waters create a more circular motion of objects in the waves, while shallower waters create a more elliptical motion. For energy generation, larger waves are desirable, and those usually occur at the surface where the wind has the greatest influence, creating oscillatory motion. According to a summary of the physical forces of waves from Wikipedia, “In general, larger waves are more powerful but wave power is also determined by wave speed, wavelength, and water density.” A coast that reflects the waves produces pressure oscillations, which make near or on-shore wave energy converters popular.


“Motion of a particle in an ocean wave.

A = At deep water. The elliptical motion of fluid particles decreases rapidly with increasing depth below the surface.

B = At shallow water (ocean floor is now at B). The elliptical movement of a fluid particle flattens with decreasing depth.

1 = Propagation direction.

2 = Wave crest.

3 = Wave trough.”


By Original uploader was Vargklo at en.wikipedia - modified from Image:Wave motion-i18n.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3520814.


When was wave power first conceived?


The first patent for energy generation from waves was filed in 1799 by Pierre-Simon Girard and his son. The earliest application of this technology was in 1910, when a device was built to power his home in France. Through the 1800s and up to 1973, hundreds of patents for wave power devices were filed just in the UK. In 1940, Japanese Naval Commander Yoshio Masuda tested his concepts on wave power at sea to power navigation lights with an articulated raft design. These concepts didn’t really catch on until the oil crisis in 1973 spurred interest in wave power. In 1974 Stephen Salter invented the Salter’s duck or Edinburgh Duck, which was able to use a cam-like body to capture 90% of wave motion, converting 90% to electricity, for an overall efficiency of 81%. It showed promise, but when the price of oil dropped in 1980, the project was no longer funded. A recent, renewed interest in clean energy has brought wave power back to the forefront.


“A visual representative on how Salter's duck works.” Image from OpenEI.org.


What types of wave energy converters (WEC) are there?


Categories of wave power devices depend on the method of capturing wave energy, where they are located, and how the energy is transferred. According to Wikipedia, the four most common types are “point absorber buoys, surface attenuators, oscillating water columns, and overtopping devices.” You can see an example of each of those as well as some other devices in the graphic below.


1 The Point absorber buoy sits on the surface, connected with cables to the sea floor. They rise and fall with the waves, generating electricity in various ways.


2 Surface attenuators are similar to point absorber buoys, but use multiple floating segments to capture the flexing motion of the waves.


3 Oscillating wave surge converter is a device that is fixed on one end to the shore or sea bed while the other end moves with a float, flap, or membrane.


4 Oscillating water columns use waves to compress air through a turbine, and can generate high levels of noise during operation.


5 An Overtopping device uses wave speed to fill a reservoir, and the energy is captured by low-head turbines.


6 Submerged pressure differential WECs are a newer technology that uses membranes to produce pressure in a closed fluid system, which then powers a turbine. They are flexible and low mass, and can be tuned to different conditions.


7 Floating in-air converters are a new concept that captures energy from compartments containing sloshing water that powers turbines or a pendulum system.


By Hmarcollo - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=92510975.


Waves can also be focused into a channel to spin a turbine, similar to hydroelectric power, as seen below.


“Source: Adapted from National Energy Education Development Project (NEED) (public domain)”. Image from U.S. Energy Information Administration.


What is the potential of wave power?


According to the U.S. Energy Information Administration: “The theoretical annual energy potential of waves off the coasts of the United States is estimated to be as much as 2.64 trillion kilowatthours, or the equivalent of about 64% of U.S. electricity generation in 2019.” That’s a measurement of potential power if a lot of wave energy was captured. However, even with smaller installations wave power is a good supplemental renewable source of power and is highly efficient. Areas of interest are the west coast of the United States (including Alaska) and Europe, as well as Japan and New Zealand. So while wave power is not a panacea for renewable energy, in the right location it can be a very viable source.


“A major wave power installation off Australia's coast will use this type of power-generating buoy. OCEAN POWER TECHNOLOGIES, INC.” Image from Yale Environment 360.


Why is wave power lagging behind other renewable electricity sources?


There are now some large-scale wave power projects in the works, but one of the biggest hurdles is the complexity of harnessing the power and minimizing the negative effects of the devices. Compared to other renewable methods, it can be complicated to harness the power of waves as they vary in size, depth, and frequency. Wind power captures the power of the wind directly, and advancements in turbines mean big power from one installation. Research in wind has already produced the most beneficial turbine designs, which is why they all look identical with three thin blades. Many of the advancements are in creating larger turbines and bigger wind farms. Another big issue holding back wave power is that the investment in wind and solar has been substantial. We have seen the benefits of that investment as these power generation methods have far exceeded their projected timelines, with the cost of wind power now lower than most other forms of non-renewable sources. Comparatively, wave power has not received anywhere near the same level of investment. With a renewed focus on green energy, investment in wave power continues to rise and we will likely see more wave power devices in Pacific Northwest and Alaska, as well as the countries mentioned above. There’s no way to know if wave power will catch up to solar and wind power’s head start, but there are many companies working on a lot of different devices to harness the power of the sea.


If you'd like to learn more about electricity and the differences between AC and DC power, check out What’s the difference between AC and DC power?


Happy Fun Fact Friday!



Sources:

https://en.wikipedia.org/wiki/Wave_power

https://www.eia.gov/energyexplained/hydropower/wave-power.php

https://openei.org/wiki/Wave_Energy

https://e360.yale.edu/features/why_wave_power_has_lagged_far_behind_as_energy_source


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