After discussing Hydropower and tidal energy

in the previous video it is now time to introduce you to fairly new and exciting ways of harvesting

the power in the ocean: Ocean Current and Wave Energy. The largest fraction of the earth surface

is covered by water: to be more precise 70% of our planet surface is covered by water. The wind causes waves to form at the surface

of the water. These can contain huge amounts of potential

and kinetic Energy. In addition, ocean currents continuously move

water all around the world due to temperature differences and other effects . All this movement of water contains vasts

amounts of energy as well. Wouldn’t it be amazing if we could harvest

a small part of this energy and provide the world with sustainable energy? Although engineers have been studying and

experimenting for many decades, these types of ocean energy technologies are still in

the experimental phase. Maybe the ocean floor at some places will

look like this in a few decades? To make a real estimation about its potential

we will look into the the principle of these types of ocean energy. For ocean current energy this is quite straightforward:

the kinetic energy of a moving fluid, ocean water in this case, will be converted to electrical

energy by a turbine. The formula’s for a rough estimation are

exactly the same as for wind turbines . The kinetic energy equals 0.5 times the mass

times the velocity of the water squared. Kinetic energy per second is the same thing

as power. The mass flow is expressed by the density

of the ocean water times the area of the turbine and the velocity of the water. -The area can be calculated with the diameter

of the turbine. We combine these three formulas with the power

coefficient of the turbine to find the effective power. The power factor defines the percentage of

the energy the turbine can harvest from the kinetic energy in the ocean current at certain

conditions. The big difference in outcome compared to

a wind turbine is caused by the difference in fluid density. Water is more than 800 times as heavy as air

so contains a lot more kinetic energy. Downsides of underwater turbines are corrosion

and algae growth. So how much is this for a turbine under water? Let’s do a small design calculation together. Let’s imagine a turbine with a diameter

of 20 meter, an ocean current velocity of 1.5 meters per second, which is below average

for the gulf stream. We take as power coefficient 0.3 which is

a fairly conservative estimation if we compare it to wind and other hydro turbines. Using these numbers we find a power of the

turbine of 0.4 MW. This might seem low compared to a much larger

wind turbine, however, a big advantage of ocean current energy is that is continuous. So it can provide a base load to the electricity

grid, where other types of energy are dependent on fluctuations of weather, seasons and tides. Note, that exactly the same turbines, but

much smaller, are currently used to operate in shallow waters to make use of tides in

stead of ocean currents. Let’s move on to wave energy. These snake-like machines are one of the many

different mechanical concepts that are currently being tested to harvest wave energy. In these animations you can find different

concepts, like on the left – the so-called attenuators concepts, in the middle the concept

based on oscillating wave surge convertors and on the right the concept based on point

absorbers. The physics behind wave energy is more complicated

than ocean current so we won’t go in to the details. This is because wave energy it is not only

composed of a kinetic energy but also by potential energy. In this graph you can see an representation

of a wave. The wave has a horizontal speed and the water

itself also has a velocity which is different at every position. The dark blue part is above the average surface

level and therefore has potential energy. This can be expressed as potential power,

per meter ocean perpendicular to the wave. In this formula only the density, the height

and speed of the wave need to be known. The formula of the kinetic energy, surprisingly,

is exactly the same. Making the total energy twice as high. The height of a wave depends on the duration

of the wind and the speed of the wind which scientists make visible in these kind of maps. This map shows a part of the atlantic ocean

between North America and Europe. Here we can see that around the North Sea

the highest waves occur. Let’s take a moderate estimate, a wavespeed

of 10 meters per second and a height of 1 meter. If we fill in these numbers in the formula

this results in 25 kW per meter. Researchers have calculated this for different

parts in the world and made this map. The areas between scotland and greenland,

the south coast of australia and south africa and the west coast of chile and Canada are

perfect for wave energy. Furthermore we can see our estimate of 25

kW/m is indeed pretty conservative.The total estimated potential for Wave energy at places

near the shore exceeds 1 TW. Considering the total Energy consumption of

the world of about 17 TW, wave energy can play a serious role in a sustainable energy

future. The oceans offer even more energy sources. Unfortunately we don’t have enough time

to talk about ocean thermal energy which uses temperature differences in different layers

of the sea and osmotic power which harvests energy out of the mixture of salt and fresh

water. A nice overview of energy sources the ocean’s

deliver can be found at the website oceanenergy.tudelft.nl. This video lecture is followed by some exercises

to get more feeling for the potential of these energy sources that are offered by our oceans.