I have a darrieus that has a high torque at low rpm
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How is that possible? The blades of a normal Darrieus rotor stall at the front and at the back side if it runs at a low tip speed ratio. A stalling symmetrical airfoil has a very high drag coefficient and this causes a negative torque coefficient at low tip speed ratios. Normally this effect is compensated by using a small Savonius rotor at the top and the bottum of the Darrieus rotor. I have described another methode in my report KD 601.
You can get a small positive torque coefficient at lambda = 0 if you give the blades a small positive blade angle. However, a positive blade angle when the blade is at the front side means a negative blade angle when the blade is at the back side for a fixed blade with a symmetrical airfoil. As the wind speed at the front side will be a little higher than at the back side, this can result in a small positive starting torque coefficient. However, if the blade is running at its optimum tip speed ratio, this positive blade angle at the front side results in decrease of the angle of attack and so in decrease of the lift coefficient. Decrease of the lift coefficient results in decrease of the generated torque if the blade is at the front side. A negative blade angle at the back side side results in increase of the angle of attack. This increase of the angle of attack at the back side may result in stalling and this results in increase of the drag coefficient. So both effects together will result in reduction of the maximum Cp. A normal Darrieus rotor therefore uses a symmetrical airfoil and a blade angle of 0°. So if you have a high torque at low rpm, you don't have a normal Darrieus rotor.
But whatever you may have done to get a positive starting torque coefficient at low tip speed ratios, if you really have it, you can't trust on the braking torque of a PM-generator. So in this case you need a mechanical brake but this requires an automatic braking procedure if the rotational speed becomes too high. Manual activation of the brake is too dangerous.