Better dynamo's updates

Sucking it vacuum or putting in a gas lighter or heavier then air and sealing it air tight. The gas should be harmless.
Update: A lighter gas like helium might work very well
Update, problem: How to seal without causing drag

Heavier or lighter

Bigger or smaller

Extra:
More guidance of water
More pressure, from higher
Making it more durable and it should withstand
 Tungsten cadbide parts

Guidance of wind towards a cylindral and or axis windmill