Energy in an ocean current vs energy in an air current

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Discussion Overview

The discussion revolves around comparing the energy contained in ocean currents to that in air currents, particularly in the context of wind and ocean turbines. Participants explore the relationships between velocity, density, and power output in both mediums, examining theoretical calculations and empirical data.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes a formula for power produced by turbines, suggesting that the energy in ocean and air currents can be equated under certain assumptions about density and turbine radius.
  • Another participant agrees with the initial calculations regarding mass flow rate and specific kinetic energy, but questions what is considered "low" in terms of energy comparison.
  • A third participant references external data claiming that an 8 knot tidal current is equivalent to a 390 km/h wind, citing the density difference between seawater and air.
  • One participant cautions against confusing energy with power, indicating a need for clarity in definitions.
  • Another participant suggests an alternative relationship between velocity and density, proposing that using different assumptions leads to a wind speed estimate of 426 km/h for the same energy output.

Areas of Agreement / Disagreement

Participants express differing views on the validity of the initial calculations and the interpretation of external data. There is no consensus on the equivalence of energy in ocean and air currents, and the discussion remains unresolved regarding the accuracy of the proposed wind speed estimates.

Contextual Notes

Participants rely on specific assumptions about turbine radius and density, and there are unresolved mathematical steps in the calculations presented. The discussion also highlights the complexity of comparing energy and power in different mediums.

Who May Find This Useful

Individuals interested in renewable energy, particularly in the context of wind and ocean energy conversion, as well as those studying fluid dynamics and energy transfer in different mediums.

R0man
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This isn't actually a homework question, but it does relate to a research project I'm doing relating to wind/ocean turbines. What I'd like to be able to do is state that:
There is as much energy in an ocean current moving at "X" knots as there is in an air current moving at "Y" kph.
I tried using P=1/2(ryo)(v^3)(pi)(r^2), where P is the power produced by the turbine, ryo is the density, v is the velocity, and r is the radius of the turbine.
Assuming P is equal in either case, I set the equations (one of water, one for air) equal to each other. I also assumed r was equal in either case, so r, pi, and 1/2 cancel, leaving:
(ryo_water)(v_water^3)=(ryo_air)(v_air^3)
Using:
ryo_air = 1.275kg/m^3
ryo_water = 1025kg/m^3 (seawater)
I end up with v_water = 0.1075(v_air)
So if I have my conversions right an 8 knot current would have as much energy as a 137.8kph wind. This seems reasonable to me, but it is very low compared to data I can find on the net.
Do anyone have any insight on this? Thanks in advance!
 
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Considering mass flow rate as [itex]\rho[/itex]VA and the specific kinetic energy as V2/2, and the same turbine area, the numbers seem about right.

What number is considered low?
 
I think it's low because of info found http://www.bluenergy.com/technology.html".
To quote them: "Sea water is 832 times denser than air and a non-compressible medium, an 8 knot tidal current is the equivalent of a 390 km/hr wind."
 
Last edited by a moderator:
Don't confuse energy with power.
 
well if one uses the relationship - (ryo_water)(v_water^2)=(ryo_air)(v_air^2), then one would get closer to 390 km/h.

However, using 8 knots = 14.8 km/h and the ratio of densities of seawater to air = 832, I get more like 426 km/h for the wind speed.

I'd have to give this more thought.
 

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