Most small and midscale wind turbines show low electricity production efficiency, here is one of the reasons:
Giant wind turbines are efficient because of the very large size of the blade near the centre, and the high speed of the blade (200 Km/h or more) at the end of the turbine.
Obviously, a 2 or 3 meters wind turbine cannot have 2 m blades, so the system must be set to turn fast enough to generate the require lift effect on the blade. This is the only way to withdraw an efficient part of the wind energy.
As most small and midscale wind turbines are direct drive engine, this increases the problem. For the Wind-Do 2.5 m diameter turbine for example, the best rotation speed to harness energy of a 3 m/s wind is between 8 and 16 rad./s (similar to 80 – 160 RPM). Bellow and above that, the efficiency drops rapidly. For the same turbine, the good rotation speed for a 7.5 m/s wind will be between 20 and 30 rad/sec.
The problem is that a 7.5 m/s wind will produce about 160 watts of kinetic energy per square meter with appropriate rotation speed, beside 8 watts for a 3 m/s wind. This is a 20 to 1 ratio for the energy production beside a 2 to 1 one for the rotation speed. A direct drive engine cannot support efficiently those two common wind speeds without special features, and this is only a part of the wind spectrum,
Lets consider now the following graphic:
This is the reality of wind speed in nature; measurements come from one of our field session.If your midscale Darrieus wind turbine is set by a processor that compute wind speed, your motor / generator system will probably switch with a 4.5 m/s wind. This mean you turbine rotation speed will have to be multiply by 4 in few seconds to reach the power generated by a wind gust of 7 m/s, or by six to reach the 8 m/s gust. Not much probable for a structure that harness 25 or 100 sq.m. of wind.
If your turbine rotation speed at 4.5 m/s is compatible with the one require for the 7 or 8 m/s wind gust, you may not loose too much energy in the transition, as your turbine will also act as a flywheel and store some kinetic energy. Still you produce energy only 50% of the time (when wind is above 4.5 m/s), and you loose useful energy below your wind speed set point.
If your wind turbine has a fix setting, it will most probably be set for wind of 3 or 3.5 m/s, the idea being to produce energy the most often possible. With the above wind profile, the set rotation speed will not be compatible with 7 or 8 m/s wind gust. This not means only that turbine rotation must be multiply by 10 in few seconds, but that the efficiency of the turbine will most probably be at 10 or 20% of its potential at the beginning of the gust. As an example, a direct drive Wind-Do turbine with appropriate rotation speed for a 3.5 m/s wind will have a negative energy production with a gust of 8 m/s. Overall efficiency of this kind of system will be very poor, and much bellow of what you will expect from measures in a wind tunnel.
One solution is to have a complex gearbox drive by a computer, which is not anymore a direct drive wind turbine.
The Wind-Do solution for this specific problem is call permanent modulated stimulation of wind turbine. The demonstration needs few pages of text and drawing that you can download here:
/Presentation_pemanent_stimulation.pdf
This provisional patent application is one of the numerous innovations proposed by Wind-Do with his wind turbine. If you are in the wind industry and you think this concept can be useful for your products, please send a written request of utilisation and we will deliver it free of charges or royalties.
WIND CAN DO IT !
Le mode opératoire des fermes d’éoliennes de Wind-Do diffère des autres sources d’électricité; en particulier, un certain surplus d’énergie qui ne peut pas être vendu au réseau est produit. Cette énergie est gratuite et elle peut entre autre être utilisée pour chauffer une serre de culture. Cette production est difficilement rentable dans les zones tempérées ou froides à cause des coûts de l’énergie requise, nous corrigeons ce problème.
