Electricity generation - gear mechanics and speed

[Yoann]

I watched a documentary recently about electricity generation, and I've been trying to figure out whether an example they provided is actually possible or not in theory.

If a big wheel, much like a Ferris wheel, spins but slowly, very slowly (let's say 5 revolutions per minute), and you attach enough gears to it, could it make a turbine spin fast enough to generate electricity? I am aware that you would need a lot of energy to even make this wheel spin since the more gears there are, the more friction there is and thus the harder it is to spin, but I'm just wondering whether it is even possible to produce electricity from a slow movement, as long as there are enough gears (and provided you apply enough force to make it spin).

Another example would be with a wind turbine; if we added more gears to the turbine, would it still generate electricity if the blades spun at a lower speed than required in current wind turbines?

 

[cjl]

Absolutely. There wouldn't necessarily even be all that much friction. The torque required would be rather large, but there's no technical reason why it would pose a problem. In fact, some modern wind turbines do spin about that slowly (5-10 rpm), and then they have gearboxes to speed up the shaft speed to something that would be more useful to a generator.

 

[Yoann]

Oh great, thanks for the quick reply! Puts my mind to ease :)

 

[danny.mcshane]

I know this post has been dead a while but I have been searching the issue for a few days and his seemed relevant.

What kind of force are we talking to turn the wheel? The 8.6MW PMG generators for example, they spin at 16rpm. What kind of force (torque I think) is being generated by the blades to overcome the magnetic resistance? I was searching Nm's needed to rotate shafts and some hefty pieces of kit can be rotated without masses of energy being used.

 

[cjl]

For ~8.6MW and 16rpm, you're talking about 3.8 million foot pounds. I already calculated that for you in your other thread though...

https://www.physicsforums.com/showthread.php?t=734057#post4637063

 

[AlephZero]

I was searching Nm's needed to rotate shafts and some hefty pieces of kit can be rotated without masses of energy being used.

Sure, it doesn't take much energy to rotate a shaft so long as you are not taking any energy out of the system, e.g. by generating electricity.

You can turn the fan of a big jet engine slowly round with one finger. But it takes maybe 50 MW of power to spin it up to say 3000 RPM and produce its maximum thrust.

To put 3.8m foot pounds in some sort of perspective, suppose you made a crank handle 10 feet long: you would have to apply about 170 tons of force to the crank to turn the generator.

That's one reason why gearboxes are a good idea for smaller wind turbines (because a gearbox plus a small generator running fast "wins" over a big generator running slowly), but eventually the gearbox has to be be so big that it becomes impractical. Even with a 20 foot diameter gearbox, you still have than 170 tons of force trying to snap off the gear teeth. Scale that down to a say a 4 foot diameter gearbox, and now you have 850 tons of force not 170.

This is what a gearbox that handles 5MW looks like - compare its size (the three sections of blue casings at the left hand side of the picture) with the jet engine that drives it. That gearbox spins runs much faster than a wind turbine, so the loading on the gears is much lower.

[

( I can't find a good picture of what's inside the gearbox, but it looks more like "agricultural engineering" than "watchmaking".)

 

[cjl]

Even very large wind turbines use gearboxes, but the gearboxes for a multi-megawatt wind turbine are enormous and weigh as much as a large truck (tens of thousands of pounds), due to the enormous amount of torque they have to handle.

 

[danny.mcshane]

Thanks guys, and sorry for the repetition. Its the putting energy into get it out bit that gets me but it is sinking in slowly.

3,800,000 foot pounds of force is like 52MW of energy right? So I convert 52MW of energy from wind to produce 8.6MW of useable electricity. Assuming the calcs are right (which is risky) is that the best we can do at the moment?

Does hydroelectric convert more of the energy in water than these can from wind?

 

[danny.mcshane]

Another post explained something about blade length too. I was trying to learn about torque and lbs - ft or Nm but could someone please advise.

Say the force needed to rotate a nut is 3.8m Nm. If I attached a big wrench to the nut then the force required decrease every meter away from the nut I go (there is a formula I saw) so is this why blades on turbines are as long as possible.

 

[CWatters]

Torque (in Newton Meters) = Force (in Newtons) * Radius (in Meters)

So yes the longer the lever the less force is required to undo a nut.

However this is not the main reason why the blades on a wind turbine are so large. There are a number of reasons for that. The main two are...

1) Wind turbines extract energy from the wind so the more wind they can interact with ("catch") the more energy they can extract.
2) Wind speeds increase with height so the taller the better.

 

[danny.mcshane]

Oh god, that makes so much sense. I will crawl back in my hole.

 

[cjl]

Thanks guys, and sorry for the repetition. Its the putting energy into get it out bit that gets me but it is sinking in slowly.

3,800,000 foot pounds of force is like 52MW of energy right? So I convert 52MW of energy from wind to produce 8.6MW of useable electricity. Assuming the calcs are right (which is risky) is that the best we can do at the moment?

Does hydroelectric convert more of the energy in water than these can from wind?

This is one area where it's easy to get confused - the units for torque and energy seem the same, but you have to be careful here. 3.8 million foot pounds of torque applied at a rotational speed of 16rpm is 8.6MW of energy, and without knowing the rotational speed, you can't calculate a power. If you were to apply 3.8 million foot pounds of torque to a stationary shaft, the input power would be zero, and if you were to apply it to a shaft rotating at 160rpm, the power would be 86MW. Power is dependent both on the torque (or force) applied, and the rotational (or linear) speed at which it is applied.

 

[CWatters]

or..

Power (in Watts) = Torque (in Newton Meters) * Angular velocity (in Radians per second)

If you don't understand "Angular velocity"... it's essentially the same as RPM only different units.