A wind energy extraction apparatus utilizing a separate surface enclosing the turbine rotor is disclosed. An embodiment of the present invention includes a slotted and un-slotted duct of specified geometry enclosing a wind turbine rotor in such a manner as to provide acceleration of the ambient air though the rotor at a velocity above that which an open rotor exposed to the freestream would experience, thereby resulting in an increased amount of energy extraction relative to a comparable open rotor. In one aspect, the wind turbine rotor is positioned in the duct at a location downstream of the smallest cross-sectional area of the duct as this will provide the maximum power output. According to another aspect, the rotor geometry is such as to incorporate the effect of the duct on the incoming wind velocity profile.