Now this makes interesting reading!
“Comparative life cycle assessment of 2.0 MW wind turbines” by Karl R. Haapala and Preedanood Prempreeda. International Journal of Sustainable Manufacturing, 2014, Vol. 3, No. 2, pp. 170-185. http://www.inderscience.com/info/inarticle.php?artid=62496
This appears to be a fair analysis of the energy that goes into manufacturing, transporting, and erecting a large wind turbine, but the “payback” side of the equation is ridiculously without context. The authors simply compare the projected energy generation of the wind turbine against the energy expended to put and keep it there. They make no effort to consider the nature of the energy, that is, the CO₂ emissions that they tout wind energy to be free of in their first sentence. Because the reduction of CO₂ emissions is thus the driving justification for wind energy, this is quite a shortcoming; particularly in the hydropower-dominated northwest USA, the claim of “payback” in a mere 5 months is extremely misleading and irresponsible. On top of that, the authors assumed a 35% capacity factor, when the projection should be 25-26% according to Department of Energy data for wind turbines in Oregon.
In other words, it doesn’t matter how much energy a wind turbine produces. What matters is what emissions it displaces. It is clear from the NW USA grid data (http://transmission.bpa.gov/business/operations/Wind/baltwg.aspx) that generally when wind rises, it is hydro that gets ramped back in response. Hence: no CO₂ savings. Even in systems where fossil fuels are ramped, the CO₂ savings are not so clearcut, since quick-responding gas plants are much less efficient than they could be without having to fluctuate with the wind, and slower-responding plants are simply switched to standby, continuing to burn fuel to be ready to switch back to generation mode when the wind drops.
It is therefore doubtful that wind turbines can ever make up for the emissions from the energy expended on them.