How long does it take a wind turbine to generate as much electricity as is used and carbon dioxide as is emitted, during the entire lifecycle of that turbine?
The short answer is that a typical wind turbine, of the type shown, will have an energy payback of less than 6 months and a carbon dioxide payback of around 6 months. For the detail behind those numbers - read on...
First some definitions. If the following three terms are already self-explanatory then so much the better;
Energy Payback: the period of time for which a wind turbine needs to be in operation before it has generated as much electricity as it consumes in its lifecycle (see below for 'lifecycle' definition).
Carbon Payback: the period of time for which a wind turbine needs to be in operation before it has, by displacing generation from fossil-fueled power stations, avoided as much carbon dioxide as was released in its lifecycle.
Lifecycle refers to the entire production cycle of a wind turbine: the extraction and manufacturing of raw materials and the subsequent manufacture of wind turbines, their blades and towers together with their transportation, erection, operation, maintenance, dismantling and disposal. In considering this one has to be aware that 80 percent of a wind turbine can be recycled.
The figure below illustrates the lifecycle of a wind turbine.
Before working out the energy payback one obviously needs to know the energy consumption involved in the entire lifecycle of a wind turbine. Vestas, the world's largest manufacturer of wind turbines, undertook such an analysis, in 2006, of a 2 megawatt turbine. This is an excellent size to use since it is the same as the average turbine size in operation in the US today and is similar to each of the two turbines shown in the photo at the top of this post.
FYI a two megawatt turbine will generate enough electricity to meet the needs of about 800 average Saskatchewan households.
The Vestas analysis demonstrates that the entire lifecycle consumption of energy for a 2 megawatt wind turbine is equivalent to 3,295 megawatt hours of electricity - this amount of electricity would be enough to power a single average Saskatchewan home for 375 years. Their analysis further noted that a standard 2 megawatt wind turbine would generate 5,100 megawatt hours of electricity annually and consequently would have an energy payback of 7.7 months.
However their calculations assumed a relatively low wind speed - specifically an average turbine capacity factor of 29 percent. The reality is that a wind turbine, operating with Saskatchewan's world-class wind resource, would have a capacity factor closer to 40 percent. Consequently a 2 megawatt machine would generate 7,000 megawatt hours of electricity annually or 3,295 in less than 6 months.
By way of an independent check on Vestas' results: US researchers carried out an environmental lifecycle assessment of 2 megawatt wind turbines at a large wind farm in the US Pacific Northwest. Writing in the International Journal of Sustainable Manufacturing, they conclude that in terms of cumulative energy payback, or the time to produce the amount of energy required in the production and installation of the turbine, a wind turbine with a working life of 20 years will offer a net benefit within five to eight months of being brought online.
The carbon emissions associated with the energy required to manufacture the 2 megawatt turbine will depend on the carbon intensity of the fuels used to generate that electricity. One might argue that Saskatchewan, which relies on coal for just under half of its electricity generation, has higher carbon emissions than the global average. Nonetheless and for the purpose of this analysis, we'll use Saskatchewan's average carbon emissions. In any event and as we'll show, it doesn't make much difference what average carbon emissions are used as the figure anyway nets out in the carbon payback calculations: nonetheless..
According to SaskPower's 2014 Annual Report (page 49) - average, system-wide, carbon dioxide emissions were 660 kilograms per megawatt hour. In other words the 3,295 megawatt hours of energy input to a 2 megawatt wind turbine would result in the release of 2,175 tonnes of carbon dioxide if the turbine was manufactured entirely using Saskatchewan-generated electricity.
To 'offset' the emission of 2,175 tonnes of carbon dioxide requires, given average Saskatchewan emissions of 660 kilograms per megawatt hour, the generation of 3,295 megawatt hours of electricity from an emission free source such as a wind turbine. This may, given the material in the previous paragraph, be stating the obvious - but there you go anyway.
As noted: a 2 megawatt wind turbine, operating with a capacity factor of 40 percent in Saskatchewan, would annually generate 7,000 megawatt hours of electricity. From this it is an easy step to see that the same turbine would need less than 6 months to generate 3,295 megawatt hours of emission-free electricity thereby avoiding 2,175 tonnes of carbon dioxide emissions.
...but what about emissions associated with thermal generators in 'standby' for backup?
There is an argument that, because wind turbines need backup generation for when there is no wind, thermal generators must be kept in continuous standby. According to the same logic the emissions from those standby generators must also be included.
One could write a PhD thesis by way of response but the short answer is that all forms of generation (coal, gas, nuclear, wind and solar) are supported by other generators and the backup needs of all are very similar. As a result the additional 'standby emissions' associated with wind turbines (and solar panels for that matter) are minimal.
The actual amount has been calculated by various bodies and most notably by GE for the Western Wind and Solar Integration Study. Those studies have found that additional emissions, caused by cycling thermal plant in standby mode, are "negligible".
But let's be generous - to the doubters out there - and assume that emissions are increased by 10 percent or to a total of 2,392 tonnes. To displace this volume of emissions will require 3,625 megawatt hours of output from the wind turbine or just over 6 months of generation.
Of course it must be noted that a coal- or gas-fired power station converts the energy, in the coal or gas, to electricity with an efficiency of 40 percent or less. What this means is that these thermal generators are always using significantly more energy than they generate as electricity. In other words they will NEVER pay back their lifecycle energy use or carbon emissions.
Now there's something worth bearing in mind the next time you get asked about lifecycle energy use and/or emissions of a wind turbine.
So there you have it. Not too shabby either!