The underlaying reason is called "Kyoto" and "100 dollar oil prices"

The direct reason is "cost" and "energy-yield"

One rotor of 100 mtr diameter covers an (swept) area of 7854 sq. mtr. (= 100%)

5 rotors of 20 mtr diameter cover a combined (swept) area of 1571 sq. mtr (= 20%)


The energy yield is roughly proportional to the swept area; meaning that a large rotor can produce about 5 times as much energy then 5 smaller rotors using the same 100 mtr tall mast.

Note however that we are still mainly using about 75 mtr tall masts, so we can't fit 5 rotor of 20mtr in there while we can fit one 100 mtr rotor. On a 75 mtr tall mast you can only three 20 mtr rotors installed and have only 3/5*20% = 12 % of the energy yield which is peanuts. Actually the produced energy is even lower as a series of smaller rotors suffers much more from the windshadow of the tower then a single large rotor does.


Both Kyoto and other renewable energy agreements require nations to produce something like 20% of their anual electricity usage from C02 neutral means by 2020 if I remember correctly. That is a whole lot of installed power. It is much easier to achieve that using big rotors then small rotors.

Additionally, we have to get off-shore with these windpark in North-West Europe as we are too densely populated on land and our best wind in found on the seas. Putting down a foundation on the seabed to take the support tower and installing the tower is very large component of the total cost. Entlarging the rotor diameter isn't. That is why everybody is looking to make the largest wind turbines that we can. Because then the cost per produced KWh is the lowest. And of course KWh is what you are selling in the end. Meaning that when you have more of it, you can get more money back in return. One very important consideration to investors.

But we must also not forget that having 5 small rotors with their own drive shafts, bearing and generators is not cheaper then one large rotor with one big drive shaft and one big generator. I haven't seen such comparisons myself but I would be really surprised if 5 smaller rotor would come in cheaper then 1 large one. And that takes away the single argument that the small rotors had going for them.

So summerizing : Small rotors are just not economically attractive as the investments per installed KWh are too high, leading to far too long "return of investment" times. This is one area where indeed "advantages of scale" are to be found.

There are also several other more technical reasons but I won't bore you with those.

Wouter