Frequently asked Questions on Wind Turbine

In this article we are going to answer Frequently Asked Questions (FAQs) on wind turbine.

Do wind turbines produce much noise?

Contrary to the popular belief, wind turbines are not noisy. They do produce a swish sound which can be heard by a listener standing close to the base of the turbine. To put this into perspective, your air conditioner noise level is about 50 decibel. A wind turbine that is 300 meters away produces noise that is 43 decibels. It has to be remembered that turbines are located well away from the population centres both because of regulations and for their own effectiveness.a

Where is the world’s biggest wind farm?

The world’s biggest wind farm will be in Gansu Province,  China. It has a planned capacity of 20 GW. Almost 7 GW has already been completed.  Once completed, it would be one of the biggest power plant in the world alongside Three Gorges Dam, which is also in China and has a capacity of 22 GW.

Where is the Wind Industry heading?

Wind Industry has seen exponential growth in the last decade.  The industry having already plucked the low hanging fruit of on-shore winds is now moving towards off-shore wind energy resources. Floating wind turbine designs are being tested that can be installed in deep waters without the need for bottom mounted towers.  Larger wind turbines are being produced. The median of wind turbine production has shifted over time from 1.2 MW to 2.3 MW and now 3.5 MW turbines.

What is the advantage of larger turbines?

Having larger turbines is much more advantageous than several small turbines for several reasons. Although it is sometimes difficult to install and commission them in hilly locations  and transport then through narrow winding roads and dusty tracks.  The first advantage is that these large wind turbines intercept wind at a higher altitude. Wind quality improves as we move further up from the ground level. See earth boundary layer for more details. Larger wind turbines have a better RoI (Return on Investment) and can provide much more energy for the same footprint.

Why dont wind turbines move fast?

Modern Wind turbines unlike their counterparts from past are designed to rotate slow. The slow speeds lengthen the life of blades which face constant sheer force from wind. Furthermore, gearboxes face wear and tear due to constant movement. One has to remember these turbines are rotating 24/7. They may look to rotate slow, but the linear velocity at the tips can reach very high.

Why do most wind turbines have 3 blades?

Increasing the number of blades increases the efficiency. Two blades are better than one and likewise three blades are better than two. So why not four blades, five blades or more?

The answer is that the relative increase in efficiency with increasing number of blades keeps dropping following the law of diminishing returns. So while a fourth blade may add a bit of efficiency it just doesn’t justify the added cost & not to mention the added weight increase on the hub and nacelle.

It makes more sense to add more blades on a small turbine.

What is Betz limit?

In the wind energy physics, there is a limit called the Betz turbine limit. Wind cannot impart all of its energy to the blades. At most it would impart 59.3% of the total energy it carries. This number of 59.3% is the Betz limit.

Adding too many blades makes the wind see the wind turbine as an obstacle or a wall. This makes it more convenient for the wind to side step or by-pass the turbine rather than going through it. And it’s not just adding too many blades that makes the wind see the turbine as a wall or an obstacle. Even if there are very few blades but they are spinning very fast, the wind can see them as a wall. This brings us to the question, how fast the turbine blades should be moving with how many blades to gain the maximum energy out of wind and for the wind to still pass through them.

To learn this we introduce a new factor called tip speed ratio.

So what is Tip Speed Ratio or TSR?

Tip speed ratio is simply the linear velocity of turbine at the tip divided by the wind speed.

For different number of blades of turbines, different tip speeds have been specified for maximum energy extraction. For example  tip speeds ratio of 6 has been reported to be the optimum for a two bladed turbines while a TSR of 5 has been deemed optimum for a three bladed turbine.

Tips Speed Ratio for various turbines

Tips Speed Ratio for various turbines

So for example,  lets say there is a three bladed horizontal axis turbine. The speed of the wind is suppose 10 m/s. We should design the turbine such that its blade velocity at the tip do not exceed 50 m/s. This is because the optimal tip speed ratio for a three bladed turbine is around 5.

How are wind turbines positioned in a wind farm?

It is very important that wind turbines are not located in each others wake. Or in other words one wind turbine should not be located in the wind shadow of another turbine particularly in the direction of prevalent wind.

Ideally wind turbines should be at least 3 to 5 blade diameter lengths away from each other.  This alleviates the influence of one wind turbine over the other. It should be noted that as wind goes past the blades, its turbulence increases. This turbulent wind if hits the other turbine can cause a lot of vibration. Installing wind turbine further away from an upstream turbine irons out some of the turbulence in the air. Often distances are limited.   So in a wind farm, wind turbines can be installed in a staggered arrangement or in an “O” shape configuration.

This picture shows the influence of upstream wind turbines over down stream turbines

This picture shows the influence of upstream wind turbines over downstream turbines
 Image Courtesy: Vettenfall

What is capacity factor?

Capacity Factor  is a number that encompasses productivity of any power plant. In short it is the ratio of average power produced divided by the peak power capacity of the power plant. One has to remember, no power plant runs constantly at peak capacity. In fact even coal fired powered plants face down time because of maintenance.

Wind turbine blades similarly moves at different speeds throughout the year. At times the blades even come to a standstill.  And therefore wind turbine produces a fraction of the power over the year compared to the power it would have produced if it were running at peak capacity all the time. Normally a capacity factor above 25% is considered healthy.

Here is all the information in video format.

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