Average emission rates fell again in 2020 throughout PJM’s footprint of 13 states and the District of Columbia, continuing their steady, multi-year decline.
To support the efforts of regulators, stakeholders and others working toward achieving environmental goals, PJM publishes an emission rates report (PDF) each spring featuring data from the previous calendar year on both average and marginal emission rates from electric generators in the PJM footprint.
PJM’s competitive wholesale electricity markets have encouraged the investment in, and entry of, new, more efficient technologies. New supply resources – including cleaner, more efficient generators, renewables, demand response and energy efficiency – are replacing older, less efficient resources.
This trend has contributed to the decline in emission rates over both the long-term as well as year-over-year.
The PJM system average annual value is a weighted average that accounts for higher loads during the summer and winter months. From 2019 to 2020, the average emission rates for carbon dioxide, nitrogen oxides and sulfur dioxide, measured in pounds per megawatt hour, declined in the PJM footprint.
- The carbon dioxide emission rate dropped 7%.
- The nitrogen oxides emission rate dropped 20%.
- The sulfur dioxide emission rate dropped nearly 22%.
Between 2005 and 2020, CO2 emission rates fell 39% across PJM’s footprint; emission rates for nitrogen oxides are down 86% and sulfur dioxide 95%. (Those figures reaching back to 2005 are not included in this report.)
The report also measures the emission rates of marginal units during peak and off-peak hours. (Peak periods are all non-holiday weekdays from 7 a.m. to 11 p.m., and off-peak periods are all other hours.)
A marginal unit is the final generation resource committed during a dispatch interval to maintain system reliability and match energy supply and demand. Variations in dispatch patterns – based on customer demand – may change the set of marginal units for a dispatch interval, which can shift marginal emission rates.
This data can help interested parties better understand the environmental impact of electricity use, which may influence decisions, for instance, of when to charge an electric vehicle.
In 2020, on average, combined cycle gas-fired generators were the marginal units 64.3% of the time, compared with 62.1% the previous year. Coal units, on average, were the marginal units 17.5% of the time, down from 24.4% the previous year. Wind made up 6.8% of marginal units, on average, up from 3.8% in 2019.
As a general matter, PJM has visibility only into generation resources that participate in the wholesale electricity market. Other generation sources, including small diesel and behind-the-meter generation, are not accounted for in this emission rates report.