Do Windmills Violate the Second Law of Thermodynamics?

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Discussion Overview

The discussion revolves around whether windmills violate the second law of thermodynamics, particularly in relation to the kinetic energy of air molecules and temperature changes. Participants explore the concepts of energy conversion, temperature, and the behavior of air molecules in the context of wind energy generation.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants assert that windmills convert the kinetic energy of moving air into useful energy without violating the second law of thermodynamics.
  • Others argue that the air actually heats up slightly rather than cools down when passing through a windmill, challenging the initial claim of cooling.
  • One participant questions their understanding of temperature and kinetic energy, suggesting that a drop in kinetic energy would lead to a drop in temperature.
  • Another participant clarifies that temperature is not solely dependent on kinetic energy and introduces the concept of frame dependence in kinetic energy.
  • There are discussions about the average velocities of air molecules and how they relate to the wind speed and the operation of windmills, with some participants providing calculations and corrections to earlier claims.
  • One participant mentions that even if there were a significant pressure drop across the turbine, it would not necessarily indicate a drop in system entropy, emphasizing the need to consider the entire system.

Areas of Agreement / Disagreement

Participants express differing views on the relationship between wind speed, kinetic energy, and temperature changes, indicating that multiple competing perspectives remain unresolved. There is no consensus on whether windmills violate the second law of thermodynamics.

Contextual Notes

Participants highlight the complexity of the concepts involved, including the definitions of temperature and kinetic energy, as well as the need to consider the entire system when discussing entropy. Some assumptions about the behavior of air molecules and their velocities are also noted as potentially unclear.

Papatom
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Hi,

From the movement of air molecules electricity is generated. The kinetic energy of the air molecules is converted into usefull energy and the air is cooled down.

Why do windmills not violate the second law of thermodynamics?
 
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The air is not cooled down. It actually heats up a little bit. Windmills use wind, the ordered motion of air, not heat, the unordered motion of the molecules.
 
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Thank you for your reply, I still do not understand this fully. Air molecules travel about 500 m/s. Assume the wind is 5 m/s. Then they have an average velocity of 505 m/s. You put a windmill somewhere and the molcules slow down to 404 m/s. Less kinetic energy so a drop in temperature. Where is my mistake?
 
Papatom said:
Less kinetic energy so a drop in temperature. Where is my mistake?

That's not what temperature is.

Replace wind with baseballs. Clearer?
 
Papatom said:
Air molecules travel about 500 m/s. Assume the wind is 5 m/s. Then they have an average velocity of 505 m/s.
No, this is not correct. If the wind is 5 m/s then the average velocity is 5 m/s. The average speed would be something like 500.03 m/s (I could be wrong on that, I think the average speed would be sqrt(500^2 + 5^2). After the windmill the average velocity will drop, but the average speed will change very little.
 
Papatom said:
Less kinetic energy so a drop in temperature. Where is my mistake?
Kinetic energy is frame dependent. Temperature is related to the mean kinetic energy in the frame, where the mean velocity (bulk movement) is zero.
 
I think I understand: A plane can travel 50 m/s but the temperature indide does not increase significantly or drop after slowing down.
 
Papatom said:
Thank you for your reply, I still do not understand this fully. Air molecules travel about 500 m/s. Assume the wind is 5 m/s. Then they have an average velocity of 505 m/s. You put a windmill somewhere and the molcules slow down to 404 m/s. Less kinetic energy so a drop in temperature. Where is my mistake?

Air molecules have an average velocity of around 5m/s in a 5m/s wind. In the wake of the windmill, this will be around 3m/s (velocity is directional, so the average will only be the wind speed, and will not include the thermal component). The average thermal speed of the air molecules will be basically unchanged by the windmill, so you end up with a reduction in bulk velocity and nearly zero impact on thermal speed.

EDIT: Although, from a practical standpoint, 5m/s is barely enough to get any power anyways - you'll need more like 8-10m/s before you can really do much with the wind.
 
Also, even if the wind speed were high enough that there was a significant pressure and thus temperature drop across the turbine, that still wouldn't be a drop in system entropy; you aren't looking at the whole system!
 

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