The most noticeable thing about the Flowtrack turbine is how simple it is! No pitch control mechanism, no yaw governing mechanism, no air brakes, no tilting head, no hydraulics and so on. Just two fixed blades, a turntable and tail to face the machine into the wind, and an emergency brake.
The answer is in the generator, and the blade profile. The Flowtrack machine is a variable speed, stall controlled design.
There is nothing new about stall governing - many first generation wind farm machines used this technique. Stall governing becomes a far more interesting possibility in a variable speed machine, not locked to the 50 cycle mains. The Flowtrack machine is conceived as either charging a battery or powering a DC distribution system, and although the generator is actually an AC machine, there is nothing restricting it to its design 50 cycles, and in the wind turbine it is used from 25 to 70 cycles, with commensurate speed variation.
It might seem obvious that stronger wind will make the wind turbine spin faster, but in fact the speed of the wind turbine can be made anything we like within limits, and that is exactly what the controller of the Flowtrack machine does. By varying the magnetic fields of the generator the controller can have the last say.
The importance of the blade profile is that the blades can be "hard stall". This allows the variation of generator speed to be a means of controlling the wind turbine.Small changes in speed can cause enormous changes in power developed.
It is not an exaggeration to say that the first and hardest job of designing a wind turbine is the business of preventing it blowing apart in high wind; the first problem is to PREVENT the machine generating power! Hard stall blades offer an alternative to turning the machine out of the wind or twisting the blades parallel to the wind. If they are running less than about five times as fast as the wind they generate very little torque. To work well the blades have to be running about ten times the wind speed. This behaviour is well displayed in the blade torque characteristics in the diagram following. It is clear that at any given windspeed there is a best rotation rate for the machine.
For low wind speeds it is desirable to follow these best rotation rates, but for high winds it is important to avoid them! The pink line represents the trajectory of our turbine as windspeeds vary.
Notice that the maximum torque line is not followed - Maximum power is to the right of maximum torque because power is torque times revs - just as in a car engine for example.
An advantage of stall control is that above about 10 m/s windspeed the machine can continue to operate at its rated power right through to 25 m/s. There is no cut-out above the rated windspeed, or foldback as is often seen in side facing machines.
Another important advantage is that if the machine is producing more power than is needed it can adopt an intentionally de-tuned trajectory such as the yellow one, and produce only the power that is needed. This avoids having to "dump" large amounts of unwanted electrical energy, avoids running all the parts of the machine harder than necessary, and very importantly, avoids making noise unnecessarily because contrary to what might be expected, the machine is very quiet when in deep stall.
Even more surprising is that the machine can run without a battery if it is provided with a modestly sized voltage clamp to absorb transients associated with wind gusts. The machine can be connected directly into a Microgrid with no intervening electronics past its controller box.
One disadvantage of the hard stall blades is that the starting torque is woeful. This makes such blades innappropriate to permanent magnet generators as their cogging torque would preclude starting until considerable windspeeds. A feature of the induction generator we use is that there is negligible magnetic field and thus drag until the generator excites at about twenty cycles/sec. At the other end of the scale the induction generator outperforms the permanent magnet generator also in that very high magnetic fluxes can be sustained in the iron when needed to decelerate the machine.
It has been suggested that three blades would help the starting performance which is admittedly marginal. It is infuriating to see the machine failing to get going until a 5 m/s gust on cold mornings sometimes when it could have been actually generating at 2 m/s! Three blades would also solve the demanding vibration problems encountered during yawing. Unfortunately the stall performance during gales would be compromised by three blades, both becasue of the setting angle, and becasue of greater delayed stall at the blade roots.
It is the combination of the hard stall blades and the induction generator with variable voltage and frequency which makes the machine practical and efficient.
See also PRECESSION IN WIND TURBINES