My Kids ask - Does wind help reduce power consumption

[jeff m]

My kids have some question to ask so bear in mind that they are in grade school.

Hi,

We have been talking and learning about motion and studying examples to understand concepts. One discussion was about what happens when you take a fan and blow it into another fan. The first fan would cause the second fan to spin and it could create power from the second fan if it were set up to do so, but not enough power to run the fist fan. I understand this.

This got me to "daydream", which brought me to my questions. This is mostly about the concept so the exact details are less important.

You have a fan that required X amount of electricity to run it at a constant rpm which produces a wind velocity of 25 miles per hour. This fan is on a table in your back yard on a calm day.

Next, you have a "tunnel" (or pipe or tube slightly bigger than fan in above example). Through this tunnel is blowing a constant natural wind of 25 miles per hour. You place a similar fan (as the above example) inside this "tunnel" and turn it on. Given the fact that the tunnels natural wind speed is 25 mph and your fan will create a wind speed of 25 mph, I want to know what changes there might be to the required power used to operate the fan. Would the fan in the "tunnel" be using less electricity once it is at its operating speed? If not could changes be made to the fan such as using a much lower power consumption motor to run it. I wonder if it would be possible to remove the regular motor and put on a motor that would NOT be able to do the "work" required to get the fan started and up to the 25 mph speed (such as on the table in the backyard), but this "weaker" motor would be able to maintain the rpms at the 25 mph after "using" the tunnel wind to get it "up to speed".

You may think, "well, if the natural wind is 25 mph and you want to run the fan at 25 mph you could just disengage the fan motor and the natural wind would accomplish what you want at zero energy cost. This is beside the point of my question. All I want to know it the difference between the energy use in the first example and the second example, without the wind and with the wind.

But, to advance the conversation to make more sense, what if you wanted the fan to push the power 1 mph past the natural wind speed, from the 25 mph to 26 mph? Would this mean that now the 25 mph natural wind does nothing to help reduce the energy/electricity needed to operate the fan at 26 mph?

Thank you for your thoughts.

J

 

[Gaz]

Well I believe the main problem with free energy is friction but in your fans case it would probably suffer from a lack of friction between the two fans. It would probably be more efficient to just remove the fans and attach the two motors together but then your back to friction on the bushes and bearings of the motors stopping them from running constantly. The motor I guess you could try intermitting the power on and of to save some electricity. Maybe add a fly wheel but that would reduce any bonus from the wind. As for the wind it would depend on the size of your fan look at wind turbines if you can get the wind to turn your fan your creating free energy =). Try thinking about a water pump and a water wheel system maybe a few water wheels and dynamo's. Probably work better than your fans. I am no expert on this btw it's just my thoughts =) gl.

 

[russ_watters]

Welcome to PF!

My kids have some question to ask so bear in mind that they are in grade school.

But you aren't...

We have been talking and learning about motion and studying examples to understand concepts. One discussion was about what happens when you take a fan and blow it into another fan. The first fan would cause the second fan to spin and it could create power from the second fan if it were set up to do so, but not enough power to run the first fan.

Correct - good start.

Next, you have a "tunnel" (or pipe or tube slightly bigger than fan in above example). Through this tunnel is blowing a constant natural wind of 25 miles per hour. You place a similar fan (as the above example) inside this "tunnel" and turn it on. Given the fact that the tunnels natural wind speed is 25 mph and your fan will create a wind speed of 25 mph, I want to know what changes there might be to the required power used to operate the fan. Would the fan in the "tunnel" be using less electricity once it is at its operating speed?

Yes - your instincts are correct.

If not could changes be made to the fan such as using a much lower power consumption motor to run it. I wonder if it would be possible to remove the regular motor and put on a motor that would NOT be able to do the "work" required to get the fan started and up to the 25 mph speed (such as on the table in the backyard), but this "weaker" motor would be able to maintain the rpms at the 25 mph after "using" the tunnel wind to get it "up to speed".

Interactions like that can be pretty complicated, but for this case the wind may spin-up the fan partly and avoid the issue of starting it (and even if it doesn't, the wind is going in the right direction to help it start). Either way, since the kW required is lower, that means the motor size can be smaller.

But, to advance the conversation to make more sense, what if you wanted the fan to push the power 1 mph past the natural wind speed, from the 25 mph to 26 mph? Would this mean that now the 25 mph natural wind does nothing to help reduce the energy/electricity needed to operate the fan at 26 mph?

If the fan is using any power at all, it will do so because it is pushing on the air and making it go faster than 25 mph. If it only pushes the air up to 26 mph, it probably only uses a tiny fraction of the power requried when it was sitting on a table in your backyard. For a fan that generates essentially no pressure, the power required is basically just the momentum transfer to the air, which is a linear function of airspeed. So if you increase the air's speed by 1 mph instead of 25 mph, you use 1/25=4% of the power (assuming the same efficiency).

 

[pbuk]

If the fan is using any power at all, it will do so because it is pushing on the air and making it go faster than 25 mph.

No, there will be a lot of power used in heating up the fan motor, and in generating turbulence and sound waves in the air and heating it up.

 

[russ_watters]

No, there will be a lot of power used in heating up the fan motor, and in generating turbulence and sound waves in the air and heating it up.

I know fan/motor efficiencies: That's not what I was referring to. With the exception of a state where it is spinning at exactly the right speed to just keep-up with the air moving past it, either it is applying a force to the air or the air is applying a force to it. If it is applying a force to the air, it must be using electricity. If the air is applying a force to it, it must be generating electricity. That's what I was referring to.

 

[CWatters]

Next, you have a "tunnel" (or pipe or tube slightly bigger than fan in above example). Through this tunnel is blowing a constant natural wind of 25 miles per hour. You place a similar fan (as the above example) inside this "tunnel" and turn it on. Given the fact that the tunnels natural wind speed is 25 mph and your fan will create a wind speed of 25 mph, I want to know what changes there might be to the required power used to operate the fan. Would the fan in the "tunnel" be using less electricity once it is at its operating speed?

In general yes. Sometimes it's useful to think of a power supply pushing energy into a load, at other times it's more useful to think of the load drawing (sucking) energy from the power supply. This is a time when the latter approach makes things easier to understand.

Constant speed motors will draw more current from the supply if you increase the load on the output shaft. Likewise the current drawn will fall if the load is reduced.

By putting the fan in air already at 25mph the angle of attack of the blades is reduced and that makes them easier to turn. If the motor is a constant speed type then the effect is to reduce the load on the motor and the current drawn will fall.

If the motor wasn't a constant speed type reducing the load might allow it to spin even faster accelerating the air to say 35 or 40mph. Some motors should never be run without a load as they can self destruct.

 

[A.T.]

either it is applying a force to the air or the air is applying a force to it

Both happens at the same time per Newtons 3rd Law.