mast for Navitron 5KW turbine

[Ted]

In the WB 5000 curves, any idea whether the voltage parameter shown for each curve refers to DC input voltage or AC output voltage ? If the latter then presumeably a grid synchronised AC ouput would use the 250 V efficiency curve, n'est pas ?

For the WB 2500 curve, I guess its safe to assume that this refers to inverter efficiency at 250 VAC output ?

AIUI those are referring to the DC input voltage - which are supported in the range 246V-600V - so the efficiency is a 'band', rather than just a line, between the upper and lower limits shown for the WB5000.  For some reason SMA have not included a 600V line.

The output voltage will be between 198V and 260V for both models.

[Ted]

I don't know if it has been mentioned on here before by anyone but there is a piece of software available for download from NREL called HOMER that allows you to play around with different renewable scenarios. This, if fed the correct data, would be able to give an answer to the 2 small versus 1 big turbine question I think.

http://www.nrel.gov/homer/default.asp

[DiarmuidWrenne]

In  relation to the discussion on whether to use 2, 2kw or 1, 5 kw turbines.

 F828 was suggesting  that based on their anemometer readings 2, 2 kw would produce more daily power than 1, 5kw. This however does not take into account that the 5kw tower is 33% taller at 12 meters than the 2kw system tower at 9 meters and  the blades would extend another 1.2 meters higher again (12 + 6/2) -(9 +3.6/2) so that  the top of  the 5kw turbine blades is at 15 meters while the others are at 10.8.

Any way taking hub heights

12/9 = 1.333^(1/7) = 1.042.(4% faster winds).  1.042 ^3 = 1.13. So the wind energy at 12 meters is 13% more than that at 9 meters. This would be an underestimate as the 1/7 power rule is mainly applied to extrapolate speeds from 25 meters up and for grass country side . Below that height, significant ground turbulance rather than shear predominates. Also the blades are  significantly longer/higher in the 5kw. Taking max hieght, the figures work out at 15%

For wooded areas and small towns the power law is 1/3 resulting in increases in power output of 33% more power (between hub heights.  10% faster winds)

PS. I just read that line back to myself and had to do  a double take before I figured out what I meant. i.e. that if the existing anenometer readings are being taken in a 1/3 environment, then a 33 percent increase in hub height would lead to a 33 percent increase  in power  output. I don't mean that areas like that have more power than rolling fields.

Perhaps F828 could multiply the measures wind speeds by the speed increase factor above and use their software to calc the new energy output based on the curves.

Regards

Diarmuid

[Bargeman]

Hello Diarmuid,

Good point about differences in wind speed at the hub heights for the 2 and 5 kW turbines. However on my course we were specifically warned against using the simplified power law to estimate wind speeds at different heights. Apparently it is widely used to assess the peak wind load on tall buildings. However we were told that it's validity declines at lower wind speeds and there is no physical basis to the equation.

We were told to apply the log law to these kinds of estimates,

 U(Z) = U(Zref)*(ln(Z/Z0)/ln(Zref/Z0))

where Z is height to be assessed, Zref is a reference height, Z0 is surface roughness and U(Z) and U(Zref) are wind speeds at Z and Zref respectively.

Assuming our reference average wind speed to be 5 m/s at 10 metres height (a la NOABL) and surface roughness is 0.03 (worst case equivalent to a fallow field), then at 12m height we get a wind speed of 5*(ln(12/0.03)/ln(10/0.03)) = 5.157 m/s. At 9 metres the equivalent wind speed is 4.909 m/s. Using the log law method, the difference in wind speeds between a 9 and a 12 metres hub height is about 5%, where the turbines are located in a fallow field.

High grass has a surface roughness of 0.1, and the wind speeds change to 5.198 m/s at 12 metres and 4.886 m/s at 9 metres, i.e. a wind speed differential of 6.4%. A woodland or forest environment with a surface roughness of 1 produces a 9 vs 12 metre wind speed differential of 12.3%.

As you suggest it would be a good idea for F828 to modify his anemometer readings to take account of the impact of the height differentials. However I suggest doing so on the basis of modified wind speeds derived using the log law rather than the simplified power law. I'll have a go at doing this for the Weibull based wind speed distributions I used in my analysis.

Alternatively, just buy an extra 3 metre segment of tower and put the 2kW turbine on top of that.

regards

[suzdobson]

Is that possible Ivan?

Will another section of the mast you sell bolt on, or are they graduated and does that also mean one will need stronger guys and concrete base, can you still reach it with a scaffolding tower, or is it getting dangerous . 

One of the other attractions of the 2Kw mast over the 5 is that we can keep the costs down by putting it up ourselves with much less hassle.
Good stuff guys .........Merry Christmas 
Suz

[DiarmuidWrenne]

Hi Bargeman,

Well done.  The 1/7  law is a quick and dirty solution and indeed roughness indicators  are the best way to get the shear profile. However, you have to measure at multiple heights on the same  tower to calculate  this, using the reverse of your formula or use the values below.

Roughness heights http://www.windpower.org/en/stat/unitsw.htm#roughness

The thing about extrapolation (at all) is  that tweeking a figure here or there has a big impact.  For instance , the Zo is fairly subjective, so it happens that developers of a site will pick a high Zo which then shows that the wind resource at 50 meters is so much better than maybe it is. That way  they can sell the project or maybe get financing.

In reality, an extra tower section  would be great, but it will probably mean a extra set of (much longer) guy wires, and a much bigger ground plan. Also erecting an extra section might not be in the gin poles capability.

Is the 5kw a standalone tower  or is it stayed.

Happy Christmas (and midwinter to all the lurking celts out there)

Diarmuid

[Bargeman]

Hello,

Rerunning the 2 x 2kW vs 1 x 5kW analysis with the 2kW turbines at 9m hub height and the 5kW at 12 metre hub height produces the following result.

At a 5m/s average wind speed site the 2 x 2kW solution produces annual ouput of 8.84 MWh vs 8.0 MWh for the 1 x 5kWh, i.e. just over 10% better performance. So at a low wind speed site the balance remains in favour of the 2 small wind turbine solution, though the margin is reduced by taking into account differences in hub heights. The break-even between the two solutions occurs at an average wind speed of marginally less than 7 m/s, i.e. for sites with an average wind speed greater than 7 m/s the 5 kW turbine outperforms the 2 x 2kW solution. In my area (Kent) onshore sites with more than 7 m/s are few and far between, hence my interest in this analysis.

Main assumptions which underpin the analysis are,

1) surface roughness = 0.03, equivalent to open agricultural area without fences and hedgerows and very scattered buildings. Only softly rounded hills (thanks for the link Diarmuid).
2) Wind speed probability density function based on a Weibull distribution with k = 2 and C = 5.642 (at 5 m/s average wind speed).
3) Power curves transcribed from the Navitron web site (Ivan - once again I put out a plea for these to be made available in a numeric rather than just a graphical format).

This approach is only valid in the absence of specific wind speed measurement data for a site, and where an estimate of average wind speed is available. If you have reliable specific wind speed measurements for a site then clearly they should be applied.

Diarmuid - the 12 meter tower for the 5kW turbine is stayed, although Navitron are apparently fielding tilt-up towers in the nearish future ? The 4 guys have a 14m x 14m footprint. I would think that if one were to opt for a 2kW turbine atop of a 12 metre tower, the best approach might be to use one of those provided with the 5kW turbines, for which guys etc ought to be the right length. I would have thought Navitron would be able to put in an order for a 2kW turbine on a 5kW tower, providing the flange on which the generator is mounted is compatible, or could be easily made so.

regards