Wind Energy FAQs: Carbon and GHG Payback Period
In this post we look at the energy and carbon payback periods, for offshore and onshore turbines, using data from Siemens Gamesa Renewable Energy (SGRE) and Vestas – the world’s two largest wind turbine manufacturers.
We show how long it takes a wind turbine to generate as much electricity as is used, and to avoid the release of as much carbon dioxide as is emitted, during the entire lifecycle of that turbine.
In short: typical wind turbines, of the types illustrated below, have both energy and carbon paybacks of less than one year. This compares extremely favorably with all other forms of generation (coal, gas, nuclear, hydro and solar).
For the detail behind these 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.
This is shown below for both an offshore and onshore turbine;
Before working out the energy payback, one obviously needs to know the energy consumption involved in the entire lifecycle of an offshore wind turbine. SGRE recently undertook this analysis in accordance with international standards (ISO 14021 Environmental labels and declarations – self-declared environmental claims – Type II).
That analysis was undertaken on their 8 MW offshore turbine which, by the way, would generate enough electricity to meet the needs of approximately 3,200 average American households. Their analysis showed the lifecycle energy consumption (including foundations, cable to grid and substations) is 20,900 megawatt hours (MWh). Since an 8 MW offshore turbine will generate approximately 34,000 MWh of electricity annually, it has an energy payback of 7.4 months.
By way of an independent check on SGRE’s results: in 2016 the paper ‘Life Cycle Assessment of onshore and offshore wind energy – from theory to practice‘ was accepted for publication by Applied Energy. It finds (Table 4) an energy payback for a 6 MW offshore turbine of 10 months: i.e. not significantly different than SGRE’s number.
The onshore turbine calculations have been conducted by Vestas in 2006. Their analysis was of a 2 MW turbine: an electrical capacity that is similar to the average turbine size in operation in the US today (2.15 MW) and is also similar to the 1.8 MW turbine shown in the photo at the top of this post.
By the way: a 2 MW turbine will generate enough electricity to meet the needs of about 650 average U.S. households.
The Vestas analysis demonstrates that the entire lifecycle consumption of energy for a 2 MW wind turbine is equivalent to 3,625 megawatt hours (MWh) of electricity: an amount of electricity sufficient to power 300 average American homes. Their analysis further noted that a standard 2 MW wind turbine would generate 5,650 MWh 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%. While this is more usual in the lower wind speed European environment; average U.S. capacity factors are higher. For turbines installed in the U.S. in 2014-2015, the average capacity factor achieved in 2016 was 42.5%. If one uses this higher number, a 2 MW machine would generate 7,450 MWh of electricity annually. In other words it would achieve energy payback in less than 6 months.
By way of an independent check on Vestas’ results: US researchers carried out an environmental lifecycle assessment of 2 MW wind turbine at a large wind farm in the Pacific Northwest. Writing in the International Journal of Sustainable Manufacturing, they conclude that a wind turbine will achieve energy payback within five to eight months of being brought online.
The carbon payback depends on the carbon intensity of the manufacturing process in addition to the carbon intensity of the electricity displaced by the operational wind turbines. These items vary enormously depending on where the turbine is made and also where it is used. For instance the highest GHG emitting state in the U.S. is Wyoming with 947 kilograms per MWh of generation (kg/MWh) and the lowest is Vermont at 6 kg/MWh. Wind turbines in Wyoming therefore have a very short carbon payback period. In Vermont, on the other hand, wind turbines will never pay back their carbon emissions.
To get around this problem we assume that wind turbines displace emissions at a rate corresponding to the U.S. average for the power generation sector which, in 2017, was 432 kg/MWh.
The aforementioned 2016 Applied Energy paper, assumes that a 6 MW offshore turbine will have lifecycle carbon emissions of 11 kg/MWh, will generate 31,045 MWh annually and have a life of 25 years: In other words lifecycle carbon emissions of 8,537 tonnes. Since the turbine will avoid GHG emissions of 421 (432 minus 11) kg/MWh, then it will need to generate 20,277 MWh to avoid the release of 8,537 tonnes of GHGs. In other words the turbine will have a carbon payback of 7.8 months
The aformentioned Vestas analysis shows that 3,295 MWh of energy is required to manufacture a 2 MW wind turbine. Their analysis does not show how much carbon is involved so let us assume a carbon intensity equivalent to the U.S. average i.e. 432 kg/MWh. This equates to lifecycle carbon emissions of 1,423 tonnes per turbine or (given annual energy production of 7,450 MWh, as noted earlier, and a 25 year life) 7.6 kg/MWh.
Since the turbine is generating 7,450 MWh annually and avoiding the emissions of 424.4 (432 minus 7.6) kg/MWh, then it will have ‘paid back’ its lifecycle GHG emissions in 0.45 of a year or 5.4 months.
…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 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 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“.
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.
The stark difference between the lifecycle emissions of thermal generators compared to renewables, is illustrated by this comparison from the United Nations’ Intergovernmental Panel on Climate Change.
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.