The latest Gadget Freak column is, in my opinion, a real winner. Bo Andersson built a windmill to generate electricity in order to have a platform to experiment with. In the tradition of build-it-with-what-you’ve-got, he used surplus and recycled items in the construction of it.
Starting with the rotor, he built a 6 blade turbine with a 6 meter diameter. The blades are one of the few components not used or recycled. The rotor hub mounts to a rear end taken from a Chevrolet Suburban. In Andersson’s application the rotor connects to one axle, the other axle is fixed, and the driveshaft (which points downward) converts the horizontal rotation into a vertical rotation. The article doesn’t mention the gear ratio, but a truck like that likely has a rear end ratio from 3.5 to 4. With one axle locked the ratio is cut in half, and since this gear train is driven in reverse it’s a step up ratio, not down. Andersson probably ends up with the vertical shaft rotation about twice the speed of the horizontal shaft.
Since the rotor is mounted atop a 30 foot tower, Andersson has built a custom 22 foot driveshaft to carry the rotation down to ground level, where his generator is mounted. This arrangement is primarily to make the generator and related items easier to service. The generator is a 3 phase 1.5kW synchronous motor which he aquired from a surplus store.
As a horizontal axis rotor it needs to be positioned so that it always points into the wind. This is done with an anemometer and wind vane mounted on the tower. A Rabbit industrial control computer reads the wind direction and steers the rear end and attached rotor into the wind. This system uses a 1/3HP single phase motor with multiple step downs to achieve a final rotation speed of 1.8 rpm.
The connection to the grid is like the rest of the system: Less efficient than optimal, yet very simple. The generator generates A/C power at a frequency proportional to the speed at which it turns. The control system watches how fast the generator is turning, and when there is enough wind to turn the generator at a speed that produces a 60Hz output (1800 rpm shaft speed) then the generator is electrically connected directly to the grid. At exactly 1800 rpm the generator is neither producing nor consuming electricity. Any slower and it would turn back into a motor, consuming electricity and driving the rotor (and fanning Andersson’s back yard). Any faster than that and it acts as a generator, generating a back EMF larger than that present in the grid and slowing or possibly reversing the flow of electricity through Andersson’s electric meter.
In this way the need for AC-DC-AC converters is eliminated, at the expense of throwing away electricity that is generated below 60 Hz.
I once toured a power generation plant here in Austin TX and asked the chief engineer how a generator that is being brought online and has no phase relationship with the grid, is connected to the grid. The answer was simple: The utility control watches the frequency and phase of both the grid and the generator, and when they match they are electrically connected. Once the generator is connected to the grid there is no possibility (save complete mechanical destruction) of it ever becoming out of phase, and it will rotate in lockstep with every other generator connected to the grid.
Andersson’s article doesn’t mention phase detection, but his motor and the intertia of the connected components may be small enough that the jolt of connecting an out of phase generator directly to the grid may not be a big deal. I wonder how much noise is generated on the power line at the moment of connection?
Andersson has incorporated other features such as detection of loss of grid power, which will disconnect the generator from the grid, as without the load of the grid the system could over speed in high winds. In this condition the rotor is also turned 90 degrees to the wind to stop the rotor from spinning.
Questions and improvements
One reader posted a comment regarding the durability of the rear end in this application. I don’t see a concern there as the rear end was designed to handle 200 or more HP, and well over 1000 ft-lb of torque in its original application, and this system uses a 1.5HP generator with plans to upgrade to a 5HP generator in the future.
My question regards the connection of a 3 phase motor directly to the grid. Most residential properties only have 2 phase power, although a thinker and do-er like Andersson may well have a shop with 3 phase power. If not then there must be some phase conversion circuitry to adapt the 3 phase generator to a 2 phase grid. If this is the case then the design could be simplified with a 2 phase generator.
I would also suggest an additional safety measure that when the rotor is turned out of the wind for any reason, that the generator terminals also be shorted together. This will cause the generator to act as a brake on the rotor, something that the article notes is missing.
I did a quick read and found that the theoretical maximum power that can be generated by a windmill is approximately .5 * swept_area * air_density * velocity^3. However the Betz Limit says that no wind driven device can ever produce more than 59% of it’s theoretical maximum power. Further, typical system efficiencies are usually around 30%.
So how much power can Andersson’s setup generate? According to the above, with a swept area of 12.3m, density of 1.2kg/m^3, and velocity of 6.7m/s (15 mph) it will generate about half a kilowatt. How much is that? A house on average uses around 2-3 kilowatts. In a wind of 28mph his windmill will generate 3.5kW, enough to stop or reverse the electric meter of most houses.
This gadget freak gives Andersson two thumbs up for his design, and I encourage you to read his assembly notes for all of the technical details.
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