Marginal Emission Factors Considering Renewables: A Case Study of the U.S. Midcontinent Independent System Operator (MISO) System

Estimates of marginal emission factors (MEFs) for the electricity sector have focused on emitting sources only, assuming nonemitting renewables rarely contribute to marginal generation. However, with increased penetration and improved dispatch of renewables, this assumption may be outdated. Here, we improve the methodology to incorporate renewables in MEF estimates and demonstrate a case study for the Midcontinent Independent System Operator (MISO) system where wind has been commonly dispatched on the margin. We also illustrate spatiotemporal variations of MEFs and explore implications for energy storage technologies. Results show that because the share of renewables in MISO is still relatively low (6.34%), conventional MEFs focused on emitting sources can provide a good estimate in MISO overall, as well as in the Central and South subregions. However, in the MISO North subregion where wind provides 22.5% of grid generation, neglecting nonemitting sources can overestimate MEFs for CO₂, SO₂, and NO_x by about 30%. The application of expanded MEFs in this case also reveals heightened emission increases associated with load shifting of storage technologies. Our study highlights the importance of expanded MEFs in regions with high and growing renewables penetration, particularly as renewable energy policy seeks to incorporate demand-side technologies.