Electric Power generated by the wind is a really great idea. It may be more romantic than practical. The energy density of wind is very low, so converting wind to do mechanical work isn’t as easy as it might appear. The DOE measures wind quality as Watts per square meter. Low quality wind is rated 100 Watts per square meter. So it would take a wind rotor 10 square meters in size to generate 1kW of electricity if the system were 100% efficient.
The Betz Limit for rotor aerodynamic efficiency is 59% for conversion of air flow, and the practical limit of propeller efficiency in horizontal wind turbines is about 44%. So roughly 20 square meters of wind rotor are needed to generate 1kW. Given that a typical residential electricity load might be in the range of 3kW per hour, you can see the problem. 60 square meters of rotor area would be needed and the wind would have to be blowing all the time.
This is a formula that just doesn’t work very well.
Can we make it better?
It is clear that if the wind quality goes up to 300 W/m2, the rotor requirement decreases to 1/3 the area. If the wind is 400W/m2, then smaller still. And there is a nice Department of Energy map of wind quality around the US that helps give us an idea where the wind is most suited to generate electricity.
Wind quality as defined by the DoE is somewhat of a “made up” term. So it can be a little misleading. The 2011 DoE map shows the best winds are off the shorelines of the US, on the Great Lakes and in the plains of Kansas and North Texas. And by the way, all that data is for 50 meters above ground. So to start off with, any machine design will have to be mounted 150 up. Based on the current solution, its like suspending a small bus weighing over 100,000+ pounds at 150 feet height, and higher. If that weren’t difficult enough, doing it on water is a serious mechatronic challenge.
Wind generated electricity comes from the wind turning a mechanical rotor which turns a generator. The efficiency gain of a few percentage points based on the rotor efficiency, the generator efficiency or the electrical conversion efficiency is very small compared to keeping the rotor turning more hours every day. As it turns out, wind availability is very much a function of where the generator is located and how high off the ground it is. At sufficient elevation there is prevailing wind, wind that blows all the time and wind that helps make airplanes fly.
Wind speed impacts the amount of electricity generated because power output increases as the cube of speed. This is known as the affinity law and it is the same phenomenon as observed in pumps and fans. So small increases in wind speed result in very significant increases in power output.
All that said, the question is whether wind power in its present form will deliver on its promise. This form of power has been attempted before, in the eighties, when federal subsidies were offered for those willing to take the risk. It didn’t work then, and it’s not working now. The difference is only a matter of scale. In those days it was 50 horsepower turbines and federal subsidies, now it’s 5 megawatt turbines and massive public funding that you and I didn’t get to vote for.
The fix is to start thinking about something different that has, at least, a chance to pay for itself without government subsidy and with a realistic shot at return on investment.