Air Emissions

Since 1993, ISO New England has produced an annual report on generator air emissions—a comprehensive analysis of the emissions produced by the region’s generation fleet and a review of relevant system conditions.

Significant Long-Term Reductions

Air emissions from regional generators have fallen dramatically over the last two decades. From 2001 to 2020, annual emissions for sulfur dioxide (SO2), nitrogen oxides (NOX), and carbon dioxide (CO2) declined by 99%, 78%, and 42%, respectively.

Fast Stats
  • 99%, 78%, and 42% decrease in annual regional emissions between 2001 and 2020 for sulfur dioxide (SO2), nitrogen oxides (NOX), and carbon dioxide (CO2), respectively
  • 67% decrease in coal-fired generation, between 2019 and 2020
  • 8% decline in oil-fired generation from 2019 to 2020
  • 9% increase in production from solar and wind resources, combined, between 2019 and 2020

New England Generator Air Emissions 2001 vs. 2020

generator air emissions

Source: ISO New England and the US Environmental Protection Agency’s Greenhouse Gas Equivalences Calculator

However, several factors—including increasing oil-fired generation during winter cold snaps due to natural gas pipeline constraints and the retirement of nuclear generation—have contributed to some slowdowns in declines in recent years and even some upticks. Learn about the changes between 2019 and 2020, for example, in the 2020 ISO New England Electric Generator Air Emissions Report.

The table below summarizes both long-term and year-over-year changes for total system emissions (the amount of system emissions) and emission rates (the pounds of emissions given off, on average, with every megawatt-hour of electricity produced). This is akin to comparing how many gallons of gasoline a car used versus its miles per gallon (MPG).

Average New England System Annual Emissions, 2011 to 2020 (Thousand Short Tons)

generator emissions

Source: 2020 ISO New England Electric Generator Air Emissions Report

The Drivers of Long-Term Emissions Reductions

Several factors have played a role in the overall reduction of generator air emissions:

  • Natural gas—The biggest contributor has been the region’s shift to lower-emitting, highly efficient natural-gas-fired generation. Natural gas-fired resources account for the vast majority of new generators built in New England since 1997, and they typically outcompete oil- and coal-fired generators in the marketplace to serve the region’s electricity needs.
  • Transmission—Improving weak spots and eliminating bottlenecks on the transmission system has allowed these new efficient, low-emitting generators to interconnect to the grid, run more often, and displace older, less efficient resources.
  • Tighter emissions controls—Implementation of emission controls, as required by federal regulations and stringent, leading-edge requirements set by the New England states, have helped reduce emission levels from coal-fired resources when they do run, contributing to the striking long-term decrease in SO2, in particular.
  • Renewables—The region’s increasing development of wind, solar, and other zero-emission resources will further contribute to reducing greenhouse gases.
  • Imported electricity—Since 2004, lower-priced electricity from outside New England has increasingly flowed in to serve regional demand, much of it from Canadian hydropower. This external generation doesn’t count toward regional air emissions.
  • Less demand—Since about 2005, annual demand for wholesale electricity from the regional power system has been declining, and with it, so has electricity generation. The Great Recession of the late 2000s and slow recovery helped dampen electricity consumption. Several long-term factors have also been at work to reduce the amount of power consumers pull from the grid, such as:
    • Energy-efficiency (EE) investment—The New England states are investing more than $1 billion annually and are national leaders in implementing EE measures, such as the use of more efficient lighting, appliances, cooling, and building operation.
    • Active demand resources—These power resources compete in the wholesale electricity markets by reducing the amount of power they’d normally pull from the grid, using practices like powering down machines or switching to an on-site generator.
    • Distributed generation—The growing numbers of small-scale solar power systems are one example of how more and more New Englanders are supplying some or all of their own power.

Year-Over-Year Variables

Two overarching factors are largely responsible for year-over-year changes in emissions:

  • The weather—Electricity demand is driven primarily by weather. Higher demand requires more electricity generation, which typically results in more emissions. Because of this, comparing annual cooling and heating degree days can provide some perspective. In 2020, there were 415 cooling degree days, which is 23.6% higher than the 20-year average. There were 5,513 heating degree days, which is 8.1% lower than the 20-year average. In other words, summer and winter were warmer than average. Overall, total energy generation declined by 3% from 2019 to 2020.

    Degree days are calculated by comparing a day’s mean temperature to the base point of 65°F. Each degree above 65°F is counted as one cooling degree day, while each degree below is one heating degree day. A day’s mean temperature of 90°F, for example, equals 25 cooling degree days, while a mean temperature of 45°F equals 20 heating degree days.
  • Power plant availability—New England’s power plant air emissions are also related to the specific units available and dispatched to serve electricity demand. The region’s higher-emitting generators tend to be dispatched when the weather drives up demand on very hot, humid summer days or when wintertime cold snaps lead to constraints on the interstate natural gas pipelines, which cause natural-gas-fired generation to become expensive or unavailable.

    Increasing amounts of installed wind power and other renewable power resources also play a role in seasonal emissions trends. In 2020, annual energy produced by non-emitting sources, such as nuclear, hydroelectric, solar, and wind generation, fell by 17%. Hydroelectric, solar, and wind generation exhibit seasonal differences in their output due to “fuel” availability—there’s less rain and onshore wind during the summer months. In winter, solar power doesn’t help meet the peak, which happens after sunset, and cannot produce power during cloudy conditions and during or soon after snowfall. Additionally, most regional solar power serves to reduce—not serve—electricity demand because it’s not connected to the regional transmission system.

2020 New England System Monthly Average NOx, SO2, and CO2 Emission Rates

air emissions

Source: 2020 ISO New England Electric Generator Air Emissions Report (May 2022)

Factors that Could Erode Emissions Reductions Going Forward

Some emerging factors could push the ISO to rely more on higher-emitting, less efficient resources to meet regional electricity demand:

  • Siting challenges are causing delays in building some of the region’s new power resources, particularly those running on natural gas.
  • New transmission lines needed to maintain reliability, as well as elective transmission projects that can connect to clean-energy resources, are also often met with opposition.
  • Some states are tightening emission limits for all generators—even state-of-the-art units running on relatively low-emitting natural gas. This could force the ISO to run higher-emitting generators in other parts of the region.
  • Any additional closures of regional nuclear facilities will remove major sources of zero-emission energy for New England.

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