The What, Where and How of Wind Power

Choosing how to start something is often the most challenging part since the rest is usually about moving with the flow (turbomachinery pun intended). So, now that we got that out of the way let’s talk about our next topic after we do a quick flashback on the previous episodes of this Clean Energy series.

In the first post in this series, we discussed clean energy as a whole. After describing what it is and what it is not, we pointed out some of the energy sources we would analyze in subsequent articles.

The second post in this series took us on an extraterrestrial journey for two reasons: we looked at solar energy and we also went on a tangent about the rovers operating on planet Mars. I got so many “Likes” on these little droids that I figured I would keep going with them (that or I found a cool article that I’ll be sharing here) for this current post on one of the fastest-growing energy sources in the world: Wind Energy. What’s the link between Mars equipment and wind? See this recent discovery – https://www.space.com/41023-mars-wind-power-landers-experiment.html

Side note: ever wondered what would happen if the sun just blinked out? Check it out here – https://what-if.xkcd.com/49/

The wind we are looking at in today’s post is somewhere in between bovine flatulence and hurricanes in terms of intensity. Wind as we know it is created by air (or any fluid) moving from a zone of high pressure to one of low pressure. This high-to-low concentration migration might sound tricky, but it is easy to understand if you think of cars on a highway. It is more likely that cars stuck in a slow lane on the highway would move on to a lane with less traffic rather than the other way around.

Pressure varies with things like irregularities on the Earth surface, AKA altitude (“in case loss of cabin pressure occurs, oxygen masks will drop […]”), but also with temperature. This means that two people at the same altitude but in areas of different temperatures would experience different pressures. For example, think of standing at the North Pole vs. standing on a Caribbean beach vs. standing on a paddleboard in the Great Lakes. This example of standing at different places demonstrates the uneven heating of the Earth from the sun due to its shape (not flat), its rotation and its tilt, as we introduced in the previous post. But which location is under the most pressure? Colder temperature equals higher pressure.  Let me explain with another analogy, (even though this example has nothing to do with pressure, it will help the information stick).  When people get stressed, we say they are under pressure.  We can imagine somebody above the Arctic Circle is more stressed (cold, where to find food, shelter, etc.) than somebody enjoying a Mai Tai on the beach at an all-inclusive resort in Aruba. So here is your mnemonics; colder equals higher pressure.

Wind creation example
Figure 1 Wind creation example – http://www.ei.lehigh.edu/learners/energy/wind1.html 

Now that we have seen what wind was and the theory behind how it forms, we can start thinking about how to utilize this energy. Today we will talk about the aerodynamic aspect of wind turbines while in a future post we will be focusing on the assessment of such technology as wind power; pros, cons, where, what, etc.

We talked about wind being a moving fluid and we know we want to extract energy from it. This is done by slowing it down and converting the difference of velocity into mechanical work.
This is far from being a new technological concept. Indeed, in the first century CE the first wind wheel was invented by Hero of Alexandria in Greece to power a musical organ; he did also invent the concept of the coin-operated vending machine, but due to a lack of sodas and gelatinous candies at the time it provided a specific amount of holy water for each coin inserted instead.

Windmills are another more recent example of pre-wind turbines which were putting the wind to use. The concept was to grind grains therefore utilizing the kinetic energy available and converting it to mechanical energy through the panes/sails attached to the shaft and rotating a moving mill stone on top of a stationary stone. This type of wind-powered equipment traces its origin to the twelfth century in Europe with possible Middle-Eastern influences.

Although windmills do include a turbine, the terminology was only changed once the mechanical energy was transformed into electric energy making it easier to store, transport and includes a very wide range of applications. This last component of the energy conversion is done by a generator that sits at the top of our current wind turbines and is linked to the blades through a gearbox which is used to bring the turbine rotation speed of about 18 rpm to ~1800 rpm where it can be distributed. This means that each turbine tower is technically independent and self-sustainable to deliver electricity where needed.

The first wind turbines, as we recognize them today, were created at the end of the 19th century and in 1896 one of them was used for the first time to provide power to a Danish village.

Today’s wind turbines are actually very sophisticated machines that include more than 8,000 parts. We typically see the modern wind turbines as the horizontal-axis type, but vertical-axis types do exist with limited popularity.

Several important technological changes have allowed us to make even better use of this free energy than by using windmills. Examples include:

  1. 2 or 3 aerodynamically designed, propeller-like blades that increase the lift of the blades (similar concept as for airplane wings) due to the uneven pressure on either side of a blade (pressure and suction sides) resulting from its shape and curvature. Wind turbine blades these days include several 3D features that make them well optimized for their working conditions; twist, tapering, sweep, curvature of the camberline, etc.
  2. Turbine rotation along the tower axis which allows orienting the face of the turbine in the direction of the wind to extend its range of applicability as winds are not always coming from the same direction (Where I live, the wind is coming from: due South 17% of the time; 14% from the Southwest; 10% from the Southeast; and the 59% left is scattered around wind frequency rose which would make for significant losses/non-use of the wind turbine if it were fixed in one given direction).


    Wind frequency rose at home
    Figure 2 Wind frequency rose at home – https://globalwindatlas.info/
  3. Variable pitch rotor blades also allow extending the range of use and the performances of wind turbines by adjusting the blade angle of attack to provide better lift and therefore convert more energy.
  4. Taller towers give access to stronger winds and provide the opportunity to use longer blades and therefore sweep a bigger area while also being harder to rotate due to the added mass. The images below show the wind power density potential in the Northern hemisphere at altitudes of 100m vs 200m, respectively from top to bottom, using the same scale.

 

Wind power potential across the Northern hemisphere at one m altitude
Figure 3 Wind power potential across the Northern hemisphere at 100 m altitude
Wind power potential across the Northern hemisphere at two m altitude
Figure 4 Wind power potential across the Northern hemisphere at 200 m altitude

Despite the recent improvements in blade design, reaching new heights for the towers, using longer blades, etc. one must understand that converting 100% of the wind energy is unrealistic. Indeed, a theoretical limit to how much energy could be transformed was derived by Albert Betz in 1919 and this yielded the following efficiency based on the laws of conservation of mass and energy: 59.3%. With the current designs reaching between 70 and 80% of this theoretical, maximum value more efficient systems are possible for the future with a strong emphasis on wind turbine farms working smartly to control each turbine as part of a larger group instead of individually.

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