Wave Energy

The ocean is never still. Waves break and retreat every second of the day, all around the world – much to the delight of surfers and dolphins.

Did you know that waves transmit energy, not water? Energy is transferred in different ways through waves. For example, vibrations and magnetic fields in electromagnetic waves or vibrations of particles in sound. In water waves, the energy is transferred through vibration of water particles.

Wind blows over oceans, transferring its energy to the water and causing the particles to move in a circular motion. The rise and fall of water particles creates a wave, travelling in the direction of the wind. When we see waves coming into shore, it's easy to believe that the individual water particles are getting closer to us. However, the particles that move back and forth perpendicularly aren't actually moving significantly in terms of the wave's direction.

Have you ever done a Mexican wave at school, or a sports event? This works in a similar way. The wave moves around the whole area, but the people participating don't move at all.

Humans have been harnessing the power of the waves for centuries. The first patent for wave energy was filed in 1799, but it took many years to catch on. Let's sea what the technology is like nowadays…

New Wave Energy

Let's begin with a definition.

Wave energy is a new renewable energy technology.

Wave energy is mostly used for:

• Pumping water

• Electricity generation

• Desalination plants

Wave Energy: Technology and Diagrams

There are five main types of technology used to generate energy from waves.

• Absorbers: floating structures that absorb energy through its movements at the water surface. Once extracted, the energy is converted to electricity using a linear or rotary generator.

• Oscillating Water Columns (OWC): partially submerged enclosed structures. The upper part is filled with air, and remains above the surface of the water. Incoming waves are funnelled into the lower, submerged part of the structure. Waves entering causes the air in the top part to pressurise and depressurise. This pushes and pulls air through a connected air turbine above it, which drives a generator.

• Attenuators: floating devices with two 'arms' attached on a hinge. They operate parallel to the wave direction, effectively 'riding' the waves. Attenuators capture energy from the relative movement of the arms as the wave passes, converting it to electricity using a hydraulic pump.

• Overtopping devices: 'dams' that capture water as waves break into a reservoir. The water is then returned to the sea, passing through a turbine that powers a generator.

Overtopping devices are very similar to conventional hydroelectric dams.

• Oscillating Wave Surge Converters: hinged paddles on a mounted joint are pushed back and forth like a pendulum by the motion of the waves. The movement of the paddle drives a hydraulic pump that powers an electric generator.

Wave Energy: Locations

Wave power varies considerably around the world. It's only an economically feasible energy source in some regions of the world, depending on wind strength and access to coastlines.

• Regions between 30° to 60° N

• Southern latitudes that experience circumpolar storms

• Western coasts of temperate countries

Economically feasible areas for wave farms include Portugal, Australia, the US, and the UK.

Wave Energy in the UK

The windy coastlines of Scotland have excellent potential for wave energy generation. In fact, up to 21.5 GW of wave and tidal energy could be generated annually from the Scottish coastlines.

Scotland already produces around 10% of Europe's total wave energy – and the Orkney Islands are one of the world's leading areas for wave energy.

Worked Example: What's the best site for harnessing wave power?

The energy of a wave depends on its:

• Height

• Wavelength

• Breaking distance

The energy (E) of the wave per square metre is proportional to the square of the height (H):

E∝H²

For example, Wave A is twice as tall as Wave B. Wave A will have four times the energy per square metre of water as Wave B.

Fig. 3 - The stronger the wind, the taller the wave, so the more energy it has. Unsplash

During your A-Levels, you may be required to compare two potential sites for harnessing wave power, and work out which will generate more energy. This table shows wave height data from two different locations, Site A and Site B.

We can use a t-test to see if there is a statistically significant difference between the means of two sites. If the difference is significant, it's unlikely to have occurred by chance.

To use a t-test, we need to know the mean of each data set, the standard deviation of each data set, and the Sample Size of each data set.

Calculating Standard Deviation

• x̄: mean of the data set

• x: individual data measurement

• ∑: sum of

• n: Sample Size

• √: square root

$\frac{\sum {(x-x̄)}^2}{n-1}=s^2$

$\sqrt{s^2}=s\tan darddeviation$

Let's apply the information from Site A to this equation.

The mean of Site A is 4.58, and the sample size is 10.

$\frac{\sum {(x-4.58)}^2}{10-1}=0.602$

The standard deviation of Site A is 0.776.

Now let's do the same for Site B.

The mean of Site B is 2.58, and the sample size is 10.

$\frac{\sum {(x-2.58)}^2}{10-1}=0.806$

The standard deviation of Site B is 0.898.

Calculating t

Now that we've calculated the standard deviation, we're going to work out our t-value.

• x₁ is the mean of sample 1

• s₁ is the standard deviation of sample 1

• n₁ is the sample size of sample 1

• x₂ is the mean of sample 2

• s₂ is the standard deviation of sample 2

• n₂ is the sample size of sample 2

$t=\frac{(x_1-x_2)}{\sqrt{\frac{{\left(s_1\right)}^2}{n_1}}+\frac{{\left(s_2\right)}^2}{n_2}}$

Let's put this equation to use.

$5.329=\frac{(4.58-2.58)}{\sqrt{\frac{0.{776}^2}{10}}+\frac{0.{898}^2}{10}}$

So, our calculated t-value is 5.329. What's next?

Critical t-value

To see if our calculated t-value is significant, we need to find out the critical t-value.

We do this by working out our degrees of freedom (total sample size minus 2), which equals 18.

Then, we take our degrees of freedom and apply it to a significance table to find our the critical t-value. In environmental sciences, we use a significance level of 0.05. This means that there is a 95% chance that these results didn't occur by chance.

• If your calculated t-value is greater than the critical t-value, you can conclude that the difference between the means for the two groups is significantly different.

• If your calculated t-value is lower than the critical t-value you can conclude that the difference between the means for the two groups is not significantly different.

For 18 degrees of freedom, the critical T-value is 2.101. Our calculated t-value is 5.329, which is greater than 2.101. So, we can conclude that the difference between the means is significantly different.

Which site, A or B, is a better site for harnessing wave power?

Advantages of Wave Energy

What are the main advantages of wave energy?

• Renewable: waves are driven by wind, which move solar energy around the Earth. As long as the Sun is a part of our solar system, wave energy will be a renewable source of power.

• Reliable: waves are always in motion. They aren't dependent on the season or the weather.

• Accessible: approximately 72% of the Earth is covered by water – and 2.4 billion people live within 100 kilometres of the coastline. Wave energy has the potential to become an important energy resource for billions of people worldwide.

• High-energy: it's estimated that harnessing the movement of the oceans could produce up to 80,00 TWh (terawatt hours) of electricity – four times the world's current energy usage!

• Clean: wave energy produces no Greenhouse Gases or harmful Pollutants.

• No Land Damage: generating energy from waves has no impact on terrestrial ecosystems.

Disadvantages of Wave Energy

What are the main disadvantages of wave energy?

• Visually Unappealing: wave energy technology could be considered an eyesore, and may impact tourism in coastal areas, causing a knock-on effect on the local economy.

• Damage to Marine Life: wave energy technology is relatively new, so scientists are unsure of the impacts on marine life. Concerns include disturbance to the sea floor, damage to benthic habitats (affecting animals such as crabs and starfish), Noise Pollution, and a danger of toxic chemical leaks into the water.

• Ship Disturbance: wave technology may impact the passage of ships and other vessels.

• Initial Cost: most wave technology is still in the early stages of development, so it's expensive to build and install. However, it's expected that construction costs will fall as wave technology becomes more common.

• Maintenance: inspecting and repairing wave energy technology is difficult, time-consuming, and expensive.

I hope that this article has explained wave energy for you. To recap: it's a clean, carbon-free source of renewable energy. Harnessing wave energy is most suitable in windy coastal areas. The energy produced is proportional to the square of their height – the higher the wave, the more energy is produced.

^{1. Aberdeen Renewable Energy Group, Wave Energy, 2021}

^{2. Gretel von Bargen, Independent T-test, Biology for Life, 2022}

^{3. GRID-Arendal, Spots of potential for wave energy harvest, Green Economy in a Blue World - Full Report, 2013}

^{4. National Renewable Energy Laboratory, Wave Energy, 2015}

^{5. Renee Cho, Tapping into Ocean Power, Columbia Climate School, 2017}

^{6. United Nations, Factsheet: People and Oceans, The Ocean Conference, 2017}

^{7. University of Hawaii, Wave Energy and Wave Changes with Depth, Exploring Our Fluid Earth, 2022}