Re: Solar power

[LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>]

A message on wind energy (from another list) is posted below. Does anybody
have a clear idea why progress in that area of technology, and in attempts
of using other forms of solar energy, is so slow. Such sources of power 
are highly desirable in view of global warming.
                                                       Ludwik Kowalski
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Subj:	RE: wind energy

Thanks, Jean, for sharing your piece on wind energy; a good and useful
bridge from physics to a practical issue we all can benefit from. Let
me add some statistics. But first one question. Should the area of the
windmill be calculated from the area of all blades or as the area of
the entire circle?

Here are some numbers from a newspaper article:

1) Today close to 100,000 Danes own shares in the hundreds of small
cooperatives that opertate 4,700 windmills. And they are making profits.

2)  20,000 people are employed by the wind industry in Europe.

3) In wind power, Europe has already overaten the United States, which 
led the drive to wind in 1980's. Europe's capacity of 4100 MW now amounts 
to two and a half times that of the United States, with Germany alone
surpassing the capacity of American wind farms.

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Jean wrote:
>  Your student is right : wind is air in motion, and you can tap the
>  kinetic energy of the air with a windmill.
>
>  If you consider one square meter of vertical area normal to the wind, 
>  the amount of mass traversing it is:
           area * speed * density of air ( = 1.3 Kg/cubic meter)

To obtain the corresponding energy, m v^2 /2 you obtain:

                Energy = density * area * (speed)^3 /2

Where "area" is the area of the windmill.  You have to use an efficiency
factor, because downstream from the windmill the wind will still have
some velocity left.

        All this assumes the air acts as an incompressible fluid, which
is not a bad approximation here.  Otherwise, one needs the full
Bernouli equation.

                For v = 10m/sec, the energy density is

                1.3 * 1000/2 = 650 watt/m^2
> comparable to the solar constant at sea level ( = 1 KW/m^2), but of 
> course the transformation into usable form is different in both cases.
>
>                Jean OOSTENS
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