The Oil Drum, in a guest post by Kris de Decker, has put up the results of real-world tests of small wind turbines in Netherlands and the UK. The fundamental problems appear intractable:

Firstly, the energy production of a wind turbine declines more than proportionately to the rotor diameter, and secondly, the energy carried by wind declines more than proportionately to the wind speed. More specifically, power rises as the square of the rotor diameter, and as the cube of the wind speed. As altitude above ground is highly correlated with wind speed increases for any given location, the height of the pole independently affects wind speed and thus it is just as critical as the rotor diameter in deriving the expected power output. A good rule of thumb is that wind velocity increases as the one seventh power of distance from the earth’s surface, which works out to a factor of about 1.9 for an altitude of 80 metres.

So, when we multiply the rotor area power factor (square law) and wind speed power factor (cube law) together, we can see why the concept of a small “inexpensive” wind turbine at low altitude is well and truly financially dead on arrival. Take any given small wind turbine at low altitude; spend a little extra to double the rotor diameter and power increases by 4 = 2 x 2; and then spend some more to raise it on a higher pole at about 80m above ground so as to double the average wind speed it would be exposed to, increasing power again by 8 = 2 x 2 x 2; you just increased the energy output of your wind turbine by a factor of at least 32 = 4 x 8, for a cost increase that is an order of magnitude less than that. This is why small wind turbines mounted at low altitude are fundamentally flawed.

Don’t just take the laws of physics’ word for it, follow the link above and read the results of the real world tests of several small wind turbine installations in urban settings in the UK and Netherlands. Here is a very revealing snip:

Dutch wind energy expert Jaap Langenbach notes that close to the test site stands a (relatively) large wind turbine with a rotor diameter of 18 metres. It delivers 143,000 kWh per year, or an average power output of 16,324 watts. It can power 42 Dutch households. This large wind turbine costs only slightly more than all small wind turbines combined (17 percent more, to be exact, or 190,000 euro), but it delivers almost 20 times as much energy. This brings the cost down to 4,523 euro per household, which is 8 times more economical than the best performing small wind turbine (and 45 times cheaper than the worst performing small wind turbine).

Note for purists: air density falls linearly with altitude above ground, but wind power increases as the cube of windspeed. So the increased windspeed more than makes up for the tiny reduction in air density, which is in any case negligible over the 2 – 100m altitudes we are talking about here.