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One of the key benefits of renewable energy is emissions reduction. A kilowatt-hour of electricity generated by a wind turbine displaces a kilowatt-hour of electricity that would have been generated by another source—usually a fossil-fuel generator. The wind-generated kilowatt-hour therefore avoids the fuel consumption and emissions associated with the fossil-fueled kilowatt-hour.

However, the reserves (a type of system-wide power) required to address wind's variability consume fuel and release emissions, so the overall savings are reduced. But by how much? That depends on the quantity of reserves required. Numerous studies have found that the reserves required are only a fraction of the aggregate wind generation and vary with the level of wind output.

To address wind variability, fossil-fueled generation that provides reserves could be forced to operate less efficiently, resulting in increased fuel consumption and emissions. For example, compare these three situations:

  1. A block of energy is provided by a fossil-fueled plant.
  2. The same block of energy is provided by a wind plant that requires no reserves.
  3. The same block of energy is provided by a wind plant that does require reserves.

In Situation 1, an amount of fuel is burned to produce the block of energy. In Situation 2, the fuel is saved, and all the associated emissions are avoided. In Situation 3, assume that 3% of the fossil generation is needed to provide reserves and that this generation incurs a 25% efficiency penalty. The corresponding fuel consumption to provide the reserves is then 4% of the fuel required to generate the entire block of energy. The actual fuel and emissions savings in Situation 3 relative to Situation 1 is then 96% rather than 100%. The great majority of fuel savings does occur.

NREL is addressing emissions issues on the transmission system. For more information, see Impacts of Wind and Solar on Fossil-Fueled Generators.