Ocean current and Wave Energy Potential – Sustainable Energy – TU Delft

Ocean current and Wave Energy Potential – Sustainable Energy – TU Delft


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.

Leave a Reply

Your email address will not be published. Required fields are marked *