A fluid flow energy capture device for power generation has a rotor with a substantially vertical axis and a plurality of vanes. A mount for the rotor allows the rotor to rotate in response to fluid flow such as wind contacting the plurality of vanes, and the mount puts the rotor in rotationally driving communication with a generator. The rotor has an open center which allows fluid communication from spaces between the vanes to above the rotor. A plurality of guide surfaces define channels that bias fluid flow to be tangential to the rotor.