Estimated lifecycle emissions of wind energy are put at 34.1 gCO2e/kWh (Nugent and Sovacool, 2014).
Resource inputs and technology
Lifecycle CO2e emissions are significantly higher for geared onshore turbines than for gearless and offshore counterparts. Turbines with gearboxes require significantly more stainless steel and reinforced concrete than lighter synchronous models. Despite increased materials to reach the seabed and larger nature of offshore turbines, their emissions intensity is found to be lower. (Nugent and Sovacool, 2014).
An average wind speed of 7.5 m/s compared to 8.5 m/s is estimated to have a 4 gCO2e/kWh lifecycle emissions difference. Clearly not the most important factor when compared with sizing and longevity (Nugent and Sovacool, 2014).
The lifecycle emissions trend with respect to turbine life is approximately 40 gCO2e/kWH for a 20 year lifespan, 28.53 gCO2e/kWh for 25 years and 25.33 for 30 years (Nugent and Sovacool, 2014). It is important that any such figure accounts for the increase frequency of maintenance and grid curtailment of the turbine as its lifetime.
A recent study of UK wind farms found that the output decreased by 1.6% +/- 0.2% for every year of operation, meaning they could operate for up to 25 years and possibly beyond for modern machines (Staffell and Green, 2014).
Some studies have estimated transportation to be as high as 28% of the total lifecycle emissions of wind energy (Nugent and Sovacool, 2014). Very often getting the large components to site can be difficult – in some cases requiring helicopters to lift kit. However in the offshore industry manufacturing locations are increasingly being sited next to the coast enabling the large components to be taken straight to site.
As with PV and other renewable technologies that rely on hardware, the biggest factor in manufacturing emissions is the energy mix used. When it is a renewable energy mix, the emissions are very small. Using a Brazilian electricity mix (eight times lower than the global average at 64gCO2/kWh) gives manufacturing emissions of 7.1 gCO2e/kWh and using 566gCO2e/kWh gives 9-11 gCO2e/kWh (Nugent and Sovacool, 2014).
Sizing and Capacity
The laws of physics dictate that larger rotor diameters and taller hub heights (generally meaning higher wind speeds) will give exponentially higher power outputs than their smaller counterparts.
P = 1/2 ρ E A v3
P = power
ρ = density of air (kg/m3)
E = efficiency of the turbine (e.g. 20% is 0.2)
A = area wind passing through perpendicular to the wind (m2)
v = wind velocity (m/s)
Studies have found that 20 x 5kW turbines have lifecycle emissions of 42.7 gCO2e/kWh, 5 x 20kW 25.1gCO2e/kWh and a 100kW 17.8gCO2e/kWh.
The current record for size is the 8MW Vesta V164 turbine installed off the Danish coast, generating 192,000 kWh in a 24 period (Vestas, 2014).
Click here for a map of some of the largest wind installations in the world.
And information on how wind energy is forecast http://www.metoffice.gov.uk/media/pdf/n/7/12_0056_Informed_World_class_science_and_weather_forecasts_for_wind_farms_web.pdf