Niall McMahon

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A Lonely Small Wind Turbine


Niall McMahon

There's a Marlec Rutland Furlmatic 910-3 Windcharger installed with a solar photovotaic panel at the roadside not far from where I live. The machine does not appear to operate all that much, at least not at the times of day that I pass by. The location is far from ideal, with a short tower, about six metres tall, and nearby hedgerows.

A quick calculation: relatively nearby Dublin Airport has an average wind speed at 10 m of about 5.25 m/s (very approximately). Guessing an efficiency of about 35%, again very approximate, and knowing that this Windcharger has a rotor diameter of 0.96 m, we can say that, at Dublin Airport, at 10 m, such a machine would have an average power output over a long period of time, say a year, of about 25 watts - you can use the expression Pavg = Cp*ρ*(((2/3)*D)2)*(Uavg)3, where Pavg is the average power output, ρ is the air density, D is the rotor diameter and Uavg is the average windspeed at the site. This is fine, depending on the application and need for remote power.

However, at 6 m, and in a shaded location, it seems likely that the average wind speed will be a lot less. Ground effects, even on a flat plain, will reduce the wind speed from 5.25 m/s to much less than 5 m/s. I sometimes see the machine turning at maybe 2 revolutions per second, or so, or 120 RPM. The tip speed ratio is the ratio of the machine's tip speed at its design point to the wind velocity. For a multi-blade machine, this ratio is maybe 3 or so. For a diameter of 0.96 m, this means that the wind speed, at these times, will be U = 2*π*0.48[m]*2[/s]/3= 2 m/s. If the average wind speed is as low as 2 m/s, then the average power will be 4 watts.

This is maybe not so surprising: the predominant winds come from the south-east to east. There is a ditch immediately below the tower, to the east; this may disrupt the already diminished winds rolling over the small fields and hedgerows. To the west, heavy traffic may interfere. To say nothing of a house, and large trees, across the road.

At the same time, maybe an average rate of energy transfer of 4 watts is what's needed at the site. Maybe there are no power lines nearby and the turbine's installation cost is cheaper than running a cable or maintaining a battery. This seems very unlikely in metro Ireland but it could be the case in a truly remote location. The point is that it all depends! It depends on how good the machine is (good in this case), what the resource is (poor in this case), what the load requirement is (unknown, in this case) and what the local infrastructure is like (good in this case). This machine, where it is, is almost certainly a poor solution.