Marginal Emissions Factors for the U.S. Electricity System

Abstract

There is growing interest in reducing emissions from electricity generation in the United States (U.S.). Renewable energy, energy efficiency, and energy conservation are all commonly suggested solutions. Both supply- and demand-side interventions will displace energy-and emissions-from conventional generators. Marginal emissions factors (MEFs) give a consistent metric for assessing the avoided emissions resulting from such interventions. This paper presents the first systematic calculation of MEFs for the U.S. electricity system. Using regressions of hourly generation and emissions data from 2006 through 2011, we estimate regional MEFs for CO(2), NO(x), and SO(2), as well as the share of marginal generation from coal-, gas-, and oil-fired generators. Trends in MEFs with respect to system load, time of day, and month are explored. We compare marginal and average emissions factors (AEFs), finding that AEFs may grossly misestimate the avoided emissions resulting from an intervention. We find significant regional differences in the emissions benefits of avoiding one megawatt-hour of electricity: compared to the West, an equivalent energy efficiency measure in the Midwest is expected to avoid roughly 70% more CO(2), 12 times more SO(2), and 3 times more NO(x) emissions.

Regional variations in the health, environmental, and climate benefits of wind and solar generation

“…Because nuclear provides base-load power and hydroelectric has a very low marginal cost, neither generation source is likely to be displaced by wind or solar. This assumption is discussed further by Siler-Evans et al (15).…”

“…On a per-megawatt-hour basis, wind and solar energy may result in different social benefits because of temporal differences. Wind output tends to be highest late at night, when demand is low and coal is more often on the margin (15). Solar output peaks midday, when demand is high and gas is more often on the margin.…”

“…To account for temporal variations, data are binned according to the level of system demand, which is a strong predictor of the marginal emissions rates of an electricity system (15,30). We used total fossil generation (based on CEMS data) as a proxy for system demand.…”

“…During low-demand hours (bottom 5%), displacing a megawatt-hour of electricity is expected to reduce $19 in damages from SO 2 emissions. Displacing electricity has a negligible effect on SO 2 during highdemand hours (top 5%), when gas-fired generators are on the margin (15). For each pollutant, separate regressions are used to calculate marginal damages using data within each bin.…”

Implications of integrating electricity supply dynamics into life cycle assessment: A case study of renewable distributed generation

“…The few identified papers integrating temporal aspects of electricity supply possess different limitations, such as approximating the dispatch order of power plants [17] or not accounting for the energy flows between electricity markets [18,19].…”