Stupid question about wind turbines

[BlackSabb]

Hi everyone. I have a stupid question about those giant 3 fan blade wind turbines. Before I start, I'm not an engineer or anything like that. I work in the health field actually, but I had a thought that has been bugging me for ages.

 

I heard someone say that one of those giant wind turbines can generate electrical power to supply 1000 homes. Now let for argument sake, we say that each wind turbine generates 1 million watts (1 megawatt). Please excuse me if that figure is way off, (it most probably is) but it's an easy round number for the purposes of this argument.

 

What I was thinking was this. How much power would it take for an electrical motor to turn the wind turbine? I'm sure you're thinking "well what's the point of that?". What I was thinking is that it would not take the combined electrical output of 1000 homes to turn that wind turbine. In other words, even though a wind turbine can produce 1 million watts, it wouldn't take that much energy to turn it electrically (instead of the wind). Let's say for argument that it takes 200 000 watts of energy to turn it.

 

So in other words, you could get far more energy than you put in. I drive by these windmills all the time and as I gaze upon it, I can't imagine that it takes the energy of 1000 homes to turn one.

 

So what is wrong with this theory? As I said, I'm not an engineer so I'm sure I've stated some obvious blunder. But this has been bugging me for ages, so I'm just dying to know.

 

Thank you.

[Welt]

You can not produce more energy from a system then it would thermodynamically allow. The energy you input in to work the generator would not break even, let alone produce extra.

 

If you are generating excess power then you either store or sell the extra depending on your needs.

[swansont]

If the turbine produces a MegaWatt of electricity, then you know for sure it takes more than a MegaWatt to turn it (under whatever set of conditions you have).

[BlackSabb]

If the turbine produces a MegaWatt of electricity, then you know for sure it takes more than a MegaWatt to turn it (under whatever set of conditions you have).

 

 

See, that's what I thought. And yet to me it doesn't add up. Again, let's say that each wind turbine produces enough energy for 1000 homes. I find it hard to believe that the bit of wind that causes a turbine to spin is the equivalent enery of 1000 homes electrical output. It doesn't take much wind for one of those turbines to spin. I'm just having a hard time imagining that the wind energy acting on those blades is the equivalent of so much power (1000 homes).

 

What I'm saying is this. That I can't believe that the wind energy acting on the turbine is the combined output of 1000 stoves and ovens, 1000 hot water heaters, 1000 air conditioners and heaters etc. In other words, tens of thousands of electrical appliances all running together is the same amount of gentle wind energy to turn a wind turbine.

 

I'm theorising it but not imagining it.

[CaptainPanic]

Hi everyone. I have a stupid question about those giant 3 fan blade wind turbines. Before I start, I'm not an engineer or anything like that. I work in the health field actually, but I had a thought that has been bugging me for ages.

 

I heard someone say that one of those giant wind turbines can generate electrical power to supply 1000 homes. Now let for argument sake, we say that each wind turbine generates 1 million watts (1 megawatt). Please excuse me if that figure is way off, (it most probably is) but it's an easy round number for the purposes of this argument.

Modern wind turbines produce pretty much exactly 1 Megawatt (MW). The biggest can produce up to 3 MW, and plans exist to go for even bigger.

 

Note that this is the maximum power output, not the average. Without wind, they produce a lot less obviously.

 

What I was thinking was this. How much power would it take for an electrical motor to turn the wind turbine? I'm sure you're thinking "well what's the point of that?". What I was thinking is that it would not take the combined electrical output of 1000 homes to turn that wind turbine. In other words, even though a wind turbine can produce 1 million watts, it wouldn't take that much energy to turn it electrically (instead of the wind). Let's say for argument that it takes 200 000 watts of energy to turn it.

You're wrong in your assumption.

 

If you don't re-build the turbine, it would take the energy of 1000 homes to turn it. Remember that they can turn a lot faster, but there is a big gearbox and generator inside the top of the turbine that slow it down (that "extract" the energy).

 

A wind turbine that doesn't have the breaks on can destroy itself... because it can turn that fast. Here's a youtube video of a wind turbine of about 1 MW that goes "top speed".

 

If you just want to turn those blades.... sure... you can do that easily. If you want to overcome the generation of 1 MW of electricity, then you really have to supply more than 1 MW of electricity to turn it.

 

Some big machines just use/generate a lot of energy! And a wind turbine is a pretty big one, and also: your house just doesn't use very much.

 

So in other words, you could get far more energy than you put in. I drive by these windmills all the time and as I gaze upon it, I can't imagine that it takes the energy of 1000 homes to turn one.

 

So what is wrong with this theory? As I said, I'm not an engineer so I'm sure I've stated some obvious blunder. But this has been bugging me for ages, so I'm just dying to know.

 

Thank you.

You propose a perpetuum motion device, which not only keeps moving forever, but also generates energy. Simply put: that's impossible.

Edited October 19, 2009 by CaptainPanic

[Sisyphus]

I'm theorising it but not imagining it.

 

Yup. Overestimating how much energy homes use and/or underestimating how much is in that big a cross-section of breeze. The circle described by 40 meter turbine blades has an area of about 55,000 square feet. Your typical swivelling fan moves air in a gentle breeze in a cross section of about 1 square foot.

 

And also remember, "1000 homes" is just approximate. If everybody turns everything on in their homes at once and it's only slightly windy, it will only be able to supply fewer.

[swansont]

Yup. Overestimating how much energy homes use and/or underestimating how much is in that big a cross-section of breeze. The circle described by 40 meter turbine blades has an area of about 55,000 square feet. Your typical swivelling fan moves air in a gentle breeze in a cross section of about 1 square foot.

 

And also remember, "1000 homes" is just approximate. If everybody turns everything on in their homes at once and it's only slightly windy, it will only be able to supply fewer.

 

And if we put some numbers to this: air moving at 10 m/s (~22 mph), with an area of about 5,000 m^2

 

[math]P= \frac{1}{2}\rho v^3[/math]

 

The density of air is a little more than 1 kg/m^3, so we have around 2.5 MW of kinetic energy in this wind

 

Average US household energy use is about a kW (somewhat less in Europe)

[Sisyphus]

Trivia: The current world's largest model of wind turbine is the Enercon E-126, which is 198m tall with a rotor diameter of 126m. It is rated at 6 MW, but is expected to usually generate more than 7. It's still in the prototype stage, but they're building some in Germany right now. Cool.

 

http://www.treehugger.com/files/2008/02/enercon_e126_largest_wind_turbine.php

[JillSwift]

Trivia: The current world's largest model of wind turbine is the Enercon E-126, which is 198m tall with a rotor diameter of 126m. It is rated at 6 MW, but is expected to usually generate more than 7. It's still in the prototype stage, but they're building some in Germany right now. Cool.

 

http://www.treehugger.com/files/2008/02/enercon_e126_largest_wind_turbine.php

Holeh cwap that thing is huge.

 

I had no idea they could produce so much power. Now I want 'em even more.