Understanding how to calculate power from force

[pslarsen]

Hi all
I have been looking at understanding power for a simple energy generation concept, and I think that I originally made an error in my calculations because I wasn't considering the relative velocities. So I basically generated the force made by a kite in static conditions, but if I need to generate power the kite needs to move which reduces the velocity that the kite experiences and can use to generator force.

Using relative velocities I have made the calculations as per the attachment. Could someone please confirm that they are correct.

Kind Regards,
Peter

 

[anorlunda]

Good for you. Sitting on dry land, you have discovered a principle that sailors have been learning for 20000 years. When you sail into the wind, the apparent velocity is wind speed plus boat speed. When you sail directly away from the wind, the apparent velocity is wind speed minus boat speed; and if boat speed equals wind speed, there is no apparent wind at all.

You also want your power generation to be continuous. If you allow the kite to blow away from the wind making power, you come to the end of the rope. Then you must collapse the kite and reel in the rope to start again.

Your solution should be the same as sailors have used for 20000 years. Never sail directly into the wind or away from the wind, sail at an angle. That is continuous power as long as the wind blows. That is what wind turbines do. The motion of the wind turbine blade is at angle 90 degrees relative to the wind.

 

[pslarsen]

Yeah that's what I figured. That means that I need to move towards some type of kite design that loops in figures 8's as there are a ton of people who already does. I guess I can't use the simple drag equation to calculate the force made by a kite..

I need to think about this..

Thanks a lot
Br, Peter

 

[pslarsen]

So something that I am not sure I understand - and that the work performed...

If the kite moves nearly infinitely slow over 100 meter (case 0), then it will have performed 1007x100 Joule but if it moves at the speed of the wind it will have performed ZERO Joule due to the force being zero. Does this make sense?

Kind Regards,
Peter

 

[russ_watters]

If the kite moves nearly infinitely slow over 100 meter (case 0), then it will have performed 1007x100 Joule but if it moves at the speed of the wind it will have performed ZERO Joule due to the force being zero. Does this make sense?

Yes. No force, no work. Such a kite is more commonly referred to as a balloon (or perhaps a piece of paper blowing in the wind). It just drifts with the air, neither getting nor losing any work/energy from the wind.

 

[anorlunda]

So something that I am not sure I understand - and that the work performed...

If the kite moves nearly infinitely slow over 100 meter (case 0), then it will have performed 1007x100 Joule but if it moves at the speed of the wind it will have performed ZERO Joule due to the force being zero. Does this make sense?

Kind Regards,
Peter

Work is force times distance. We can call the work power. Energy is the integral of power.

When the kite sits still, there is no power (because distance=0)
When the kite moves at wind speed there is no power (because force=0)

 

[bob012345]

Work is force times distance. We can call the work power. Energy is the integral of power.

When the kite sits still, there is no power (because distance=0)
When the kike moves at wind speed there is no power (because force=0)

Did you mean we can call the rate of doing work power?

 

[anorlunda]

Did you mean we can call the rate of doing work power?

No, I said it wrong. force*distance=work=energy. force*speed=power.

Note my signature, "Power is to Energy as Speed is to Distance"

 

[Baluncore]

Sorry about this slight deflection of the topic, but there is a type of kite that employs rotors that makes it look like a helicopter. As air flows up through the rotors, they autorotate, so it is really a tethered form of autogyro.
https://en.wikipedia.org/wiki/Autogyro

Because they have no propulsion motor they are called a rotor-kite or gyrokite.
https://en.wikipedia.org/wiki/Rotor_kite

They work well as an observation post when flown from ships at sea, as they fly higher, with less drag than a normal kite. A good example is the German designed Wagtail;
https://en.wikipedia.org/wiki/Focke-Achgelis_Fa_330

Such a kite can generate power while it is flying, held by a tether that can also be used as the powerline.
http://skywindpower.com/#files_WWO__homepage_36_0

No pilots license is required to fly on a tethered kite, but there are national altitude restrictions on kites.

 

[Baluncore]

Kite power (without rotors) is advancing rapidly. Google “kite power generation”

See; “Power Maximization of a Closed-orbit Kite Generator System”. 2011.
https://hal.archives-ouvertes.fr/hal-00619320/document

 

[Randy Beikmann]

Did you mean we can call the rate of doing work power?

You are exactly right. That is in fact the most fundamental definition of power.