Wind Generators

[empip]

Wind Turbine Power:

From .... www.awea.org/faq/windpower.html

P = 0.5 x rho x A x Cp x V³ x Ng x Nb
where:
P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
rho = air density (about 1.225 kg/m3 at sea level, less higher up)
A = rotor swept area, exposed to the wind (m²)
Cp = Coefficient of performance (.59 {Betz limit} is the maximum thoretically possible, .35 for a good design)
V = wind speed in meters/sec (20 mph = 9 m/s)
Ng = generator efficiency (50% for car alternator, 80% or possibly more for a permanent magnet generator or grid-connected induction generator)
Nb = gearbox/bearings efficiency (depends, could be as high as 95% if good)

Beware the velocity cubed !!

'Force 6' wind vel 10.8 - 13.8 m/s say 12.3 m/s ... 12.3³ = 1861
'Force 4' wind vel 5.5 - 7.9 m/s say 6.7 m/s............6.7³ = 301
That knocks a large dent in the performance..

...I went down to our local harbour and was amazed to see some of the yacht turbines spinning with hardly any wind, but then I saw that many others weren't, and realised that the ones that were cheerfully spinning weren't charging, they just hadn't been left connected to anything. I guess that large windmills (which keep their speed roughly constant by adjusting the pitch angle of the blades) are also often turning without generating. I started to wonder if there was less power in the wind than I had intuitively imagined....

I submit, anyone thinking of buying and installing a turbine should firstly invest in a wind speed monitor - Anemometer - with data logging capabilities... Use this to survey your own site ... then do the calcs...

www.reuk.co.uk/Survey-Wind-Speed-at-Turbine-Site.htm

Most folk do not get their car serviced properly never mind a blooming lump of metal hanging off the gable with scything blades vibrating with squeaking bearings -- potential liability !

 

[bernardgreen]

The liability for damage following mechanical failure is often over looked.


On the grander scale

http://www.burnham-on-sea.com/news/2006/wind-farm-27-02-06.shtml

A fire at the top cannot be dealt with so has to be allowed to burn out. Bits drop off. burning lubricating oil and/or hydraulic fluid can reach ground level etc etc

And if anyone burnt that much resin composite material at ground level the local authority would throw the book at them for the pollution and toxic fumes created.

And the bigs ones throw lumps of ice from the blades.

 

[Taylortwocities]

From that article

....With the Nissan fire, it was electricians who inadvertently started the fire............

 

[JohnD]

typical electricians

 

[Stoday]

I'd just love it if you walked us through your working.

I hope eveyone else would too. Here goes then, without maths formulæ:

1000 VA Knock off 200 VA for losses and power factor, leaves max output of 800W for max wind speed, = say, 2*W

Suppose the average wind speed is half the maximum (that's generous - it's likely to be lower). That would be W. (see that's why I made the max 2*W).

So (2*W) cubed = 800W; What does W cubed equal? Why, 100W.

OK?

 

[bernardgreen]

From that article

....With the Nissan fire, it was electricians who inadvertently started the fire............

But most others have been failures in the machinery or the generator.

There was a holiday caravan park which fitted wind turbines to several of the caravans which were not on mains electricity. They were so noisy that holiday makers tied string to the vanes to stop them spinning.

 

[Softus]

Here goes then, without maths formulæ:

empip's answer was characteristically full, precise, and clear. Yours was, well, different.

So (2*W) cubed = 800W; What does W cubed equal? Why, 100W.

OK?

Leaving aside the hidden and fantasmagorical equation you've used to compute the answer of "100", you seem to have used W to mean two entirely different things - any particular reason why?

 

[davelx]

Here goes then, without maths formulæ:

empip's answer was characteristically full, precise, and clear. Yours was, well, different.

So (2*W) cubed = 800W; What does W cubed equal? Why, 100W.

OK?

Leaving aside the hidden and fantasmagorical equation you've used to compute the answer of "100", you seem to have used W to mean two entirely different things - any particular reason why?

The figures given are correct. It is not even GCSE level maths:

Let's take the 800W max output as correct, and lets assume that the output power does indeed depend on the cube of the wind speed.

We can state:

Output power P = CW^3

Where C is some constant that depends on the design of the wind generator and W is the windspeed.

Now, we don't really care about the units here, other than the output power. So we can arbitrarily assign C a value of 1 to make the calculations simple.

We now have P=W^3.

ie 800 = W^3

W^3 = 800
W= 9.283

Let's halve the wind speed:

w = W/2 = 4.64

P = w^3 = 100.

Or, to put it another way : 2^3=8, so if you double the wind speed you get 8 times the power, and vice-versa. Compare this with electrical power where power increases with square of the current (P=I^2R). It's the same calculation with a different exponent, the constant R being the equivalent of C above.

 

[Softus]

The figures given are correct.

I'm not aware of ever saying that the figures were wrong.

It is not even GCSE level maths.

Quite so, and yet Stoday managed to make it appear to be kindergarten level plasticine modelling.

 

[davelx]

The figures given are correct.

I'm not aware of ever saying that the figures were wrong.

Technically true. But my perception/interpretation of your use of the quote marks around the 100 was that you were questioning the figure. I now know I was wrong.

However, one can only ever go by what one perceives, as it is impossible to know the true intent of the words without being able to read the poster's mind.

 

[Softus]

On a slightly mischievous point...

What does W cubed equal? Why, 100W.

This is only true if the value of W is 100^-2, i.e. 10.

 

[DESL]

And to cap it all our local authority has blocked most planning applications for them

 

[Stoday]

I tried to make it as simple as I could. It never occured to me that anyone would take 100W to mean 100 * W instead of 100Watt.

As a businessman said referring to gormless TV adverts "you can never underestimate the audience's intellegence"



Thanx davelx for the clarification.

 

[Softus]

I tried to make it as simple as I could.

Oh dear.

It never occured to me that anyone would take 100W to mean 100 * W instead of 100Watt.

Then your vocation must be one in which ambiguity is both rife and of no consequence. Christmas cracker joke writing, perhaps?

As a businessman said referring to gormless TV adverts "you can never underestimate the audience's intellegence"

I think the boot is on the other foot in this instance.