Every now and then you hear the argument made that wind energy should be rejected on the grounds that wind turbines have a negative net energy, meaning that it would cost more to build a turbine than it will ever return in energy terms. Additionally the claim is heard that fossil fuels are needed to build and maintain a wind turbine.
Here is a US government (DoE) document specifying a standard 5 MW offshore wind turbine (“NREL offshore 5-MW baseline wind turbine”).
In page 2, table 1.1 we find:
Rotor mass – 110,000 kg
Nacelle mass – 240,000 kg
Tower mass – 347,460 kg
Total steel mass – 700 ton
We are going to assume that the windturbine of the future is going to be produced with renewable electricity, where the steel will be made in an electric arc furnace:
That wikipedia article claims that the energy cost for one metric ton of steel is 440 kwh. Applying this to the data of the standard wind turbine mentioned above, we arrive at 440 * 700 kWh = 300 MWh. This is the equivalent to the power production of the same 5 MW standard wind turbine of 12 days full power.
Assume the requirement of 1000 ton of reinforced concrete for the foundation (obviously for onshore situation). From this source we learn that the energy cost 1 ton of reinforced concrete is 2.5 GJ. This comes down to ca. 6 days of windturbine operation at maximum power. That makes 18 days in total. Assume a more realistic load factor of 33%, we arrive at ca. 60 days of normal operation for the wind turbine to earn back the invested energy, after which the net energy harvesting starts. The tower will last centuries, blades and gearbox maybe 30 years. And again, the steel can be produced efficiently with electricity, no fossil fuel necessary. This calculation does not include gearbox and generator. Without these items, for a 30 years = 10,000 days, we arrive at an EROEI of 10k/60 = 160. Again, energy cost of gearbox and turbine are not included, as is road construction, transport and assembly. On the other hand the steel tower, representing half of the total steel mass of the turbine, is certainly not written off after 30 years (Eiffel tower was built in 1889 and is around already for 123 years, with no end in sight). It seems that the EROEI value of 20, mentioned in the 2006 theoildrum article (see below), maybe applies to smaller windturbines, but probably is too pessimistic for large offshore windturbines.
Wind energy: go for it.
[wikipedia] – Energy returned on energy invested
[theoildrum.com] – Energy from Wind: A Discussion of the EROI Research (2006)