Patent ID: 7780416

Claim:
A vertical axis windmill comprising a set of frames attached via horizontal bars to a vertical axis of rotation, each frame comprising a plurality of upper horizontal bars, a plurality of matching lower horizontal bars, and a plurality of sheets of lightweight material wherein the upper edge of each sheet of lightweight material is fixed at one of said upper horizontal bars, each of said sheets of lightweight material is allowed to move by pivoting or bending relative to said one of said upper horizontal bars, and the remaining edges of said sheets of lightweight materials are not attached to any structural support; wherein said lower horizontal bars are positioned to stop said sheets of lightweight material from being blown to one side of said lower horizontal bars, said one side being on the same side of each frame relative to the direction of rotation of said vertical axis windmill; wherein said lower horizontal bars are movable toward said one side of said frame such that there is a variable gap to allow part of the air to pass through in the downwind direction; wherein each of said movable lower horizontal bars is attached to side arms pivotably connected to said frame and wherein spiral springs are attached to said side arms to control the size of said gap.