In general vertical axis units tend to be lower efficiency than horizontal axis, but simpler to build & control. But even the best-case systems need the wind resource, and generally need to be mounted many 10s, if not 100s of feet above the ground to achieve reasonable (read "net present value greater than zero") performance, even in windy areas. Small units designed to be mounted on roofs/chimneys etc are basically house-ornaments- most would get better bang per buck out of investing it in photovoltaic (which also has a very long payback.) Wind doesn't scale to the micro-level as readily- there's a minimum swept area and windspeed factors to overcome. With photovoltaics you can always get SOMETHING out of even a single 200W collector, and it's as effective near the (unshaded) ground as it is mounted on a tower. |