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Emissions

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. Many of the effects of renewables energy integration with relation to emissions were studied in the Western Wind and Solar Integration Study Phase 2 (WWSIS-2). WWSIS-2 analyzed the wear-and-tear costs and emissions impacts of cycling and simulated grid operations to investigate the detailed impacts of wind and solar power on the fossil-fueled fleet in the West. A key finding was that the negative impact of cycling on overall plant emissions is relatively small. The increase in plant emissions from cycling to accommodate variable renewables is more than offset by the overall reduction in CO2, NOx, and SO2. In the high wind and solar scenario, net carbon emissions were reduced by one-third.

Three-column chart titled "Emission Impacts of Cycling Are Relatively Small Compared to Emissions Reductions Due to Renewables." The first column has three rows: CO2, NOx, and SO2. The second column shows the emissions reduction due to renewables for each row: 29%-34% for CO2, 16%-22% for NOx, and 14%-22% for SO2. The third column shows the cycling impact for each row: negligible for CO2, 3-4 million pounds for NOx, and 3-4 million pounds for SO2.

Emissions reductions due to renewables and cycling impact.

Starts, ramping, and part-loading also have impacts on emissions. To address the lack of emissions data from cycling, NREL analyzed unit-specific measured emissions from the U.S. Environmental Protection Agency Continuous Emissions Monitoring dataset to develop refined emissions rates for CO2, NOx, and SO2 for most units in the U.S. portion of the Western Interconnection. Heat rates and emissions rates were calculated for part-load. In addition, unit-specific incremental emissions from starts and ramps were calculated using these measured emissions data. Part-loading generally results in a higher overall emissions rate, except for NOx emissions rates, which decrease for coal and gas steam units. Compliance with existing or proposed emissions regulations was not analyzed.

Chart showing a heat-rate curve for a typical gas combined-cycle unit. The generation in megawatts is plotted against heat rate in MMBtu/MWh. From 100 to 250 megawatts, the generation heat rate decreases from 9 MMBtu/MWh to 7 MMBtu/MWh.

Heat-rate curve for a typical gas combined-cycle unit.

For additional information on emissions, see Impacts of Wind and Solar on Fossil-Fueled Generators.