We are after about 1kW on average in summer and about 4kW average in winter. This is so we can use the surplus to provide winter heating.
The force produced by a stationary windmill goes up as the square of the wind speed, as described by Bernoulli's Equation. When the windmill starts moving, the difference between windspeed and blade speed becomes more significant. There is a best blade speed for each best wind speed, but the equation is still a cubic. For a simple windmill, like a Savonius, the theoretical best efficiency is obtained when blade speed is 1/3 of windspeed.
All we have for the location is average windspeed for the county. The average wind for the exposed location we're considering may be higher than suggested, and since wind energy goes up as the cube of windspeed, the more the windspeed varies the more energy is produced, compared to an average speed all the time. The average speed may be considered to be the minimum power.
The approximation we're using is to divide the weather into four: for 25% of the time there will be no wind; for 50% of the time there will be average wind; and for 25% of the time there will be double average wind.
For a fixed-pitch mill, this generates the average rating 50% of the time, and four times the average rating for 25% of the time. That gives a total of 1.5 times the rating at the average windspeed.
For a variable-pitch mill, this generates the average rating 50% of the time and eight times the average rating for 25% of the time. This gives a total of 2.5 times the rating at the average windspeed.
Summer windspeed averages maybe 4.2 m/s, which with an air density of 1.25kg per cubic metre gives a power of 90W per square metre. A fixed pitch mill that is perfectly efficient would therefore generate 135W per square metre obstructed, and a variable pitch mill that is perfectly efficient would generate 225W per square metre.
In winter the windspeed averages 5.3 m/s, so with an air density of 1.33 kg per cubic metre gives a power of 198W per square metre. So a perfect fixed pitch mill would generate 397W per square metre, and a perfect variable pitch mill would generate 495W per square metre.
Of course no mill can be more efficient than the Betz Limit, in practice a bit less than 60%, so their sizes would need to be scaled up by this factor.
But it is obvious that the winter requirement is more than double the summer requirement, which leads to the obvious notion of having two, and servicing them one at a time in summer.
Here is some meteorological data for London (where I live) broken down by month:
| Jan. | Feb. | Mar. | Apr. | May | Jun. | Jul. | Aug. | Sep. | Oct. | Nov. | Dec. | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Average Windspeed, km/h |
14 | 14 | 16 | 16 | 14 | 12 | 12 | 12 | 14 | 14 | 12 | 14 |
| Sunshine, Daily hrs |
2.75 | 4.66 | 5.72 | 6.60 | 6.74 | 8.33 | 6.81 | 7.71 | 7.20 | 5.05 | 2.66 | 2.10 |
| Solar (150% in June) | 50% | 84% | 103% | 119% | 121% | 150% | 123% | 139% | 130% | 91% | 48% | 38% |
| Wind (150% in Mar) | 100% | 100% | 150% | 150% | 100% | 63% | 63% | 63% | 100% | 100% | 63% | 100% |
| 150% | 184% | 253% | 269% | 222% | 213% | 186% | 202% | 230% | 192% | 111% | 138% |
London wind met data is source The Washington Post, and sunshine data from Roehampton.
Here is some meteorological data for Galway (where I'd like to live) broken down by month:
| Year | Jan. | Feb. | Mar. | Apr. | May | Jun. | Jul. | Aug. | Sep. | Oct. | Nov. | Dec. | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Average Windspeed, kts |
9.8 | 10.9 | 11.1 | 11.0 | 9.5 | 9.5 | 8.9 | 8.7 | 8.6 | 9.6 | 10.0 | 9.6 | 10.5 |
| Rainfall, mm |
926.8 | 97.2 | 72.1 | 71.8 | 55.5 | 60.1 | 62.4 | 57.1 | 82.3 | 81.8 | 92.4 | 94.7 | 99.6 |
| Sunshine, Daily hrs |
3.48 | 1.58 | 2.34 | 3.34 | 4.93 | 5.77 | 5.13 | 4.59 | 4.44 | 3.69 | 2.65 | 1.93 | 1.42 |
| Solar | (50% in May) | 14% | 20% | 29% | 43% | 50% | 44% | 40% | 38% | 32% | 23% | 17% | 12% |
| Wind | (200% in Feb) | 189% | 200% | 194% | 125% | 125% | 103% | 96% | 93% | 129% | 146% | 169% | 141% |
| Total | 203% | 220% | 224% | 168% | 175% | 148% | 136% | 131% | 161% | 169% | 146% | 182% |
The peak wind speed is gusts of 93kts and 10min averages of 60kts in September. Galway met data is source The Irish Met Office, data for Shannon Airport.
1kt is also 1.86km/hr or 0.517m/s.
Wind power is generated using windmills of one sort or another. In summer, we want the windmills to generate enough to provide our electricity needs, but in winter we need another 3kW or so to help with heating.
HAWT mills in the 600W range are available: seven of the wg500 turbines from Bullnet might do the trick. That's nearly three thousand notes, though; it involves towers, and all they entail; and it's a 36v system.
The work of people like Hugh Piggott has made it clear that making a HAWT at home is not impossible. His permanent-magnet wind turbines come in various sizes including a 4kW design that might well suit our purposes. This design also gives similar dimensions to the ones we've calculated, and the site has similar weather: so that's a source of confidence.
The problem they seem to have is with excessive power. I have a couple of governed designs in my mind - one a downwind design that cones the blades away from the wind, and the other a pitch-changing design that relies on the weight of the blade working against a spring to furl the whole mill. Either design would regulate to produce a 50Hz output, give or take a little, and would be safe in high winds since removing the electrical load would allow the blades to reach the maximum design speed and so become fully furled. In both cases, a spring failure causes the mill to furl.
A governed design also has the advantage that dump load failure is not tragic. If the batteries exceed the maximum charge voltage, a backup circuit could simply remove the windmill from the circuit. A fixed mill would then smash itself up, but a variable mill will reach the top of it's governed speed, and stay there, taking just enough energy to overcome the friction in the bearings, no matter how strongly the wind blows.
It would be nice to have two of these, "to be sure, to be sure", each generating 2kW or so. If each of them is a 5m diameter design, they'd both be monsters, but either one would generate enough power for the summer, so they could be serviced in turn.
The coning design would be a fixed-pitch design, so to generate 2kW average in winter at 50% efficiency, it'd need to be just over ten square metres. That gives a blade diameter of a bit less than 3.6 metres - about 11'6. In summer it'd generate about 700W, which is going to be sufficient for our electrical needs. The peak power is going to have to be more than 7.2kW.
To generate 2kW average in winter with the pitch-changing design, assuming 50% efficiency, requires an area of just over eight square metres, giving a blade diameter of 3.2 metres, or about 10'6. The summer average generation is likely to be 900W or so, which should be sufficient. The peak power is going to have to be more than 5.3kW.
I like VAWT - it's easy to build and easy to maintain, because the generator is at the bottom of the pole, and doesn't need to be turned to face the wind. I wonder if it would be possible to build a Gorlov type VAWT using rip-stop nylon and a wooden frame. Some kind of rigging and spring arrangement might be used to reduce torque in very high winds.
The efficiency of a Gorlov is similar to that of a conventional HAWT: so again, we might want two mills, each presenting 10 square metres to the wind. That's a substantial structure, though.
Once the windmills are turning, there needs to be a way to get the power down the pole and to the places where it is needed. The obvious way to do this is with powerful permanent magnets and coils arranged in a three-phase design. This produces a smooth torque loading on the mill and uses the copper conductors in an efficient way. It's also familiar to the average electrician.
The safety decision is based on voltage. A 55V system is probably safe for outdoor use, in that the voltage is too low to kill someone. If that's provided as three-phase-plus-neutral, the peak voltage is around 78v and the delta voltage is about 95V RMS (55V x tan(60o)). If a variable pitch mill delivers 7.5kW, it'll need to deliver about 25A per phase; if a fixed pitch mill delivers 6kW, it'll need to deliver about 17A per phase.
For safety reasons the centre tap will be connected to ground and to the metalwork of the tower and mill.
This page is some notes on Domestic Power from Renewable Sources, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.