For years I have been wondering why wind turbines are designed with very skinny blades (photo http://en.wikipedia.org/wiki/File:Turbine_aalborg.jpg ). Most of the available wind passes right through the circular "reach" of the device without touching the blades. Wouldn't it be more efficient if the blades were shaped something like those of a typical electric fan (photo http://en.wikipedia.org/wiki/File:Electric_Fan_720x1070.jpg ), or a pinwheel toy (photo http://en.wikipedia.org/wiki/File:Green_pinwheel.jpg ) ? I would have assumed that the objective would be to maximize the surface area to capture as much wind as possible. I note that the sail of a sailboat is given a shape to produce a lot of surface area. I note that expressions for flux (light, electric, magnetic) are proportional to intensity multiplied by surface area. Can anyone please explain this design shape to me?