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Name: James
Status: student
Grade: 9-12
Location: MN
Country: USA
Date: Fall 2013


Question:
What is it about wind energy that gives it such a relatively low output of electricity? If electricity is generated in the same way (electrical induction) as say, coal or hydroelectric power, why does it provide such little electricity in comparison? Is it just a matter of the rotor spinning faster, or is there something I'm missing? I understand that it is a lot less efficient in regards to actually harnessing energy from the wind, but why is the output so low? What am I missing about the process that differentiates these?



Replies:
Hi James,

I think the basic problem here is that you misunderstand what "efficiency" is! The fact that a coal or hydro powered generating plant outputs much more electrical energy than even a large wind turbine, has absolutely nothing whatsoever to do with their relative efficiency.

Efficiency of a power generating plant is simply defined as a ratio of electrical energy produced by the power generation system, divided by the mechanical energy used to operate the system.

In fact, coal powered generating plants actually have relatively low efficiency. They throw away more energy in the form of waste heat, than they produce in electrical power output.

On the other hand, a modern large wind turbine has surprisingly high efficiency; that is, most of the energy in the wind passing through the blades, is converted to electrical energy.

The main reason for the lower energy output of a wind turbine, as compared to a coal fired, or hydro powered generating system, is simply that there is far, far less energy contained in the wind that is captured by the wind turbine blades. Compare this relatively low energy in the wind moving through the turbine's blades, to the energy produced by the tons of coal burned per hour in a coal fired plant, or the energy of thousands of tons of water flowing through the turbines in a hydro dam, and it is obvious why a the electrical output of a wind turbine is so low as compared to a large coal-fired or hydro plant.

In short, the reason for a wind turbine's relatively lower energy output is simply that the wind passing through the turbine blades contains relatively low energy in the first place.\

Regards, Bob Wilson


Hi James

"Low output of electricity?", with respect to what? "Low" is a relative term.

Referring to this link: http://www1.eere.energy.gov/tribalenergy/guide/wind_turbines.html it is stated that "As wind speeds increase, the amount of electricity generated increases proportional to the cube (3rd Power) of the wind speed, so small increases in wind speed yield large increases in available power", but "Unlike traditional power sources, however, the wind is intermittent."

This article ( http://www1.eere.energy.gov/wind/wind_how.html) explains that there are "utility" scale wind turbines that range in size from 100s of KiloWatts (KW) to several Mega Watts (MW).

This article describes "small" scale wind turbines. http://www1.eere.energy.gov/wind/wind_dist_tech.html

" Distributed wind energy systems are commonly installed on, but are not limited to, residential, agricultural, commercial, industrial, and community sites, and can range in size from a 5 kilowatt turbine at a home to a multi-megawatt turbine at a manufacturing facility."

It is not a matter of turbine RPM. As explained in the video at the end of this article ( http://www1.eere.energy.gov/wind/wind_how.html), wind turbines rotate around a speed of 18 RPM (Revolutions Per Minute), but that is geared up to 1800 RPM.

Efficiency is the ration of the Output Power divided by the Input Power expressed as a percentage. Another measure is Dollars out divided by Dollars put in. This article calculates efficiency (capacity factor) by dividing amount of time producing power by the amount of time it does not produce power and comes to the number 30%. http://www.globalwindday.org/faq/how-efficient-are-wind-turbines/

But one must consider this is a zero emissions way of generating power. That is, we are not pumping carbon out of the ground and transferring it to the atmosphere. We are just taking atmospheric energy (wind power) and converting it to another form of energy

Sincere regards, Mike Stewart



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