Turbines / conservation of energy

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Discussion Overview

The discussion revolves around the principles of energy conservation as applied to wind and hydroelectric turbines. Participants explore how turbines extract energy from fluid flows, the role of kinetic and potential energy, and the implications of these processes on broader concepts such as the Earth's rotation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the kinetic energy of the fluid remains the same before and after the turbine, raising the question of where the energy extracted by the turbine originates.
  • Another participant clarifies that power, rather than energy, is the relevant quantity, noting that the change in velocity of the fluid leads to a change in kinetic power.
  • A participant explains that in hydro turbines, the energy extracted comes from the potential energy of the water due to gravity, as the water falls through the turbine.
  • In the context of wind turbines, a participant describes the airflow dynamics and references Betz Law to model kinetic power extraction.
  • Another participant emphasizes that pressure changes across the turbine are crucial for understanding power generation.
  • One participant expresses satisfaction with the hydroelectric explanation but remains uncertain about the wind turbine mechanics, indicating a need for further contemplation.
  • There is a discussion about the potential impact of tidal energy schemes on the Earth's rotation, with differing views on the significance of such effects.
  • Some participants question the relationship between energy extraction from turbines and the rotational kinetic energy of the Earth, with one asserting that only mass movement can affect rotational speed.

Areas of Agreement / Disagreement

Participants express a mix of agreement and disagreement, particularly regarding the implications of energy extraction on the Earth's rotation and the mechanics of wind turbines. No consensus is reached on these points.

Contextual Notes

Some claims rely on specific assumptions about fluid dynamics and energy conservation that may not be universally accepted. The discussion includes references to models like Betz Law and the mechanics of reaction turbines, which may require further clarification.

Who May Find This Useful

This discussion may be of interest to those studying fluid dynamics, energy conservation principles, and the mechanics of renewable energy systems, particularly in the context of wind and hydroelectric power generation.

kalamater
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Wind turbines / hydroelectric turbines - I think the same fundamental Physics applies to my question. The fluid (water or air) prior to meeting the turbine has kinetic energy, after the turbine the flow must be the same (or there would be a 'build up' of fluid) - so it has the same KE as before ?, yet the turbine 'extracts' energy. Where has the energy which the turbine has produced come from ?
 
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The flow (kg/s) is constant. In the physical model we use power, not energy. To calculate energy, we just multiply power by time. The kinetic power is what changes, just because the velocity does, E = 1/2 G (V1-V2)^2, where G is the flow. The power that generate the turbine is just because the change of the velocity in that part of the fluid. The flow remains constant because the area of the section changes according with that. Was that your question?
 
In the case of a hydro turbine, the water is falling through the turbine - the elevation of the liquid surface upstream is higher than the elevation downstream. So the energy extracted into turning the shaft comes from the potential energy of the upstream liquid - in other words, gravity is powering the turbine.
 
In the case of a wind turbine, you can think of the flow following the bath of a diffuser. As the air passes through the wind turbine disc it slows down and expands out word because of its slower velocity. This is analogous to the down wash of a helicopter which the flow becomes more narrow do to an increase in velocity. The kinetic power extraction limit of a wind turbine is modeled using the Betz Law.

http://www.windpower.org/res/turbtubx.gif
 
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What changes from one side of the turbine to the other is pressure. Power in a turbine is mass flow rate times pressure.
 
Many thanks everyone, yes some good ideas to pick over here, I'll chew on the power and pressure ideas. The hydro argument over potential I like, guess if there was no turbine in the way, the ejected water would be at a greater velocity, all (or nearly all) the PE being converted to KE. I generally feel happy now about the hydro idea but will have to think on about the wind generators. Thanks

As an aside, I was told some time ago that the tidal barrage scheme in North France would result in the slowing of the Earth's rotation by some 0.25 seconds in 1 million years (I haven't actually bothered to do the calculation myself) - presumably as we move more and more to 'renewables' such as wind generators, we'll slow down the rotation a bit more - after all we don't get energy for nothing, it has to come from somewhere.
 
Thanks also for Betz law and the link - time to go away and do a bit of thinking!
 
kalamater said:
As an aside, I was told some time ago that the tidal barrage scheme in North France would result in the slowing of the Earth's rotation by some 0.25 seconds in 1 million years (I haven't actually bothered to do the calculation myself) - presumably as we move more and more to 'renewables' such as wind generators, we'll slow down the rotation a bit more - after all we don't get energy for nothing, it has to come from somewhere.

What!? How exactly do wind turbines or changes in tide affect the rotation of the earth? The only thing we can do to change its rotational speed is move mass outwards away from the surface of the earth. All the satellites and garbage in space will have a greater affect than anything France does.
 
Sorry, not accusing France of anything, but I was given to understand that any tidal energy schemes result in slowing of the Earth's rotation by reducing it's rotational KE, after all the energy has to come from somewhere - if not the Earth's rotational KE then where else ?

Betz law in Google seems to answer my original questions though - thanks again.
 
  • #10
kalamater said:
The fluid (water or air) prior to meeting the turbine has kinetic energy, after the turbine the flow must be the same (or there would be a 'build up' of fluid) - so it has the same KE as before.
Only the mass flow remains the same. In the case of a wind turbine, the air flow is diverted "outwards" by the turbine blades, slowing the air and changing it's direction.

http://en.wikipedia.org/wiki/Wind_turbine

Most water turbines are "reaction turbines". The water flow is contained in pipes so it won't expand under pressure. If the turbine's axis is horizontal, then pressure of water decreases as it flows through the turbine, converting pressure energy into mechancial energy. If the axis is vertical, then gravitational potential energy is also converted into mechanical energy.

http://en.wikipedia.org/wiki/Water_Turbine
 

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