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The Darrieus Self-Starting Problem
For a Darrieus wind turbine to be self starting, it must be capable of going from rest to a blade tip speed greater than that of the wind therefore utilizing lift forces. The Darrieus utilizes aerodynamic lift by allowing the rotating blades to create an angle of attack with the oncoming wind. This angle of attack creates a separate wind velocity on both sides of the blade and therefore a pressure differential. The side of the airfoil upwind carries a higher pressure gradient than the side downwind. The forces of the pressure differential accelerate the blade which spins the rotor and supplies torque. The reason the Darrieus is incapable of self starting is because there is an initial rotation velocity of the blades required to create the angle of attack. There are existing methods of correcting this problem like: Darrieus/Savonius hybrid, guide vanes, and folding airfoils.
The Hybrid Darrieus/ Savonius allow a smaller Savonius blade profile inside the outer Darrieus airfoils. The Savonius utilizes aerodynamic drag and is therefore capable of self starting. By rotating the shaft, it starts the rotation of the Darrieus airfoils. The problem with this hybrid method is that there are now torque requirements from two spinning blade sets for the shaft to support. The material selection for the shaft is now limited to extremely strong and expensive materials, failure to compensate for the combine torque of the Darrieus and Savonius spinning airfoils will result in premature failure. Also, because the Savonius operates on wind drag, it is incapable of speeds greater than that of the wind, the Darrieus by itself has no limitations but will now be limited with a Savonius attached. Lower RPM will result in lower power output.
Guide vanes direct the flow of the wind to create an initial angle of attack with blade profiles at rest, giving the Darrieus the means to self start. To successfully design these guide vanes computational work must be done on the dynamic stall and airflow separation of the blade profiles at rest. The blades can be at rest anywhere on the rotor between 0 and 360 degrees. The computational times required for every possible angle at rest is a major time requirement that our limited time schedule couldn’t accommodate.
Folding airfoils are a mechanically ingenious way of solving the Darrieus self start problem. These airfoils initially have an expansive surface area capable of utilizing aerodynamic drag and once a relatively sufficient velocity is achieved they will fold in on themselves to create an airfoil minimizing drag resistance and fully utilizing aerodynamic lift. Mechanical design complexity ruled out this option.
The Hybrid Darrieus/ Savonius allow a smaller Savonius blade profile inside the outer Darrieus airfoils. The Savonius utilizes aerodynamic drag and is therefore capable of self starting. By rotating the shaft, it starts the rotation of the Darrieus airfoils. The problem with this hybrid method is that there are now torque requirements from two spinning blade sets for the shaft to support. The material selection for the shaft is now limited to extremely strong and expensive materials, failure to compensate for the combine torque of the Darrieus and Savonius spinning airfoils will result in premature failure. Also, because the Savonius operates on wind drag, it is incapable of speeds greater than that of the wind, the Darrieus by itself has no limitations but will now be limited with a Savonius attached. Lower RPM will result in lower power output.
Guide vanes direct the flow of the wind to create an initial angle of attack with blade profiles at rest, giving the Darrieus the means to self start. To successfully design these guide vanes computational work must be done on the dynamic stall and airflow separation of the blade profiles at rest. The blades can be at rest anywhere on the rotor between 0 and 360 degrees. The computational times required for every possible angle at rest is a major time requirement that our limited time schedule couldn’t accommodate.
Folding airfoils are a mechanically ingenious way of solving the Darrieus self start problem. These airfoils initially have an expansive surface area capable of utilizing aerodynamic drag and once a relatively sufficient velocity is achieved they will fold in on themselves to create an airfoil minimizing drag resistance and fully utilizing aerodynamic lift. Mechanical design complexity ruled out this option.