SF₆ in Infrastructure

A Greenhouse Gas 23,500x More Potent Than CO2 May Lurk in Infrastructure Needed for Clean Energy Transition—But There are Alternatives and Regulatory Solutions

By Katie Waters, ReGrid Intern

June 2021

The federal government is projected to invest $100 billion in the next four years on power infrastructure. In building a leaner, cleaner electricity system, we must not only use more efficient technologies that result in lower energy loss, but also consider less harmful alternatives to the potent greenhouse gases commonly used in large-scale infrastructure today.

A key example is sulfur hexafluoride (SF₆), a synthetic gas used to insulate switchgear (GIS) of traditional power plants, wind turbines, and other electrical transmission and distribution equipment. SF₆ has a global warming potential (GWP) 23,500 times that of CO2, and persists in the atmosphere for 3,200 years. While the switchgear of a wind turbine could require as little as 3 kilograms of insulating gas, a substation for overhead power lines can require several tons of SF₆.

In 2019, SF₆ equipment users reporting to the EPA recorded a net quantity of 34 million MTCO2e of SF₆ in the US power system—an increase from 2018, which reported only 28 million MTCO2e. These are likely underestimates because not every facility that uses SF₆ must report. Existing data suggests that our current regulatory framework is not enough to reduce the environmental hazards of sulfur hexafluoride, and thus SF₆ merits further investigation as we look towards investing in clean energy and infrastructure.

Leakage potential

Poor gas seals, gas containment failures, and older equipment with a higher leakage threshold all contribute to SF₆ leakage into the atmosphere. While this leakage can be mitigated to a certain extent by upgrading to newer equipment, accidental leakage can still occur regardless of equipment due to regular maintenance, retiring equipment, and normal operation. In 2019, the EPA reported the nation’s total SF₆ emissions from the use of electrical equipment at close to 2.5 million MTCO2e of SF₆, equivalent to the emissions of 531,000 passenger vehicles a year. Atmospheric SF₆ levels continue to increase. NOAA reports that in October 2020, the global monthly mean of SF₆ is 10.36 parts per trillion—over double the atmospheric levels of SF₆ in 2000. Without regulation, California alone would have an uptake in emissions of 81,000 MTCO2e of SF₆ by 2036, or the greenhouse gas emissions of 17,500 passenger vehicles driven for a year. The Massachusetts Department of Environmental Protection considers reducing SF₆ used in gas-insulated electrical infrastructure switchgear to be among the most impactful strategies to decrease overall greenhouse gas (GHG) emissions.

Some potential alternatives to SF₆

Cost-effective solid, liquid, gas, and air insulated alternatives exist, but unlike SF₆, there is not a one-size-fits-all alternative and instead depends on application, ambient temperature, and voltage level of the equipment. More options for a variety of voltages are being developed. Critically, however, once SF₆-compatible equipment is installed, it cannot use alternatives, and this equipment is intended to last for decades. This “delayed emissions effect”—in which it could be decades until gas-insulated switchgear would need to be replaced—suggests that future atmospheric SF₆ levels could increase if regulations and alternatives to SF₆-insulated equipment are not established. Without regulations, equipment users may not face the appropriate incentives to properly dispose of SF₆ by recycling or destroying the gas, processes that would minimize the release of SF₆ into the atmosphere at the end of its life cycle.

Though some alternatives are already in production, an assessment by the European Commission notes that many alternatives lack not the technical feasibility, but the market to incentivize production. Regulations must work to incentivize equipment users to adopt alternatives that fit their individual needs in order to align the actions of the user to mitigate environmental harm.

Current regulations

A 2015 F-gas regulation in the EU limits the amount of SF₆ (which is included as a fluorinated, or F-gas) that can be sold from 2015 onwards, phasing down sales until they reach one-fifth of 2014 sales by 2030. In new equipment like refrigerators, air conditioning units, and aerosols, the use of F-gases like SF₆ are banned. However, this ban does not yet include the main driver of SF₆ usage and production--energy transmission equipment. These regulations also try to prevent emissions by requiring proper maintenance and end of life recovery.

Though the 2010 Waxman-Markey bill would have regulated SF₆ in the United States on a national level through a cap and trade program designed to reduce aggregate greenhouse gas emissions, the bill only passed the House. Currently in the US EPA only requires reporting of SF₆ emissions for owners and operators with a total capacity of SF₆ above a certain threshold of 7,838 kg.

Beyond reporting, two states utilize an annual emission rate limit to regulate SF₆. To mitigate leaked SF₆ in gas insulated switchgear, the Massachusetts Department of Environmental Protection (MassDEP) began reducing the annual emission rate of SF₆ in GIS equipment meeting the federal reporting threshold by 0.5% each year beginning in 2015, and require owners to report both the annual rate and mass of emissions to ensure they fall below limits. For all other GIS not subject to federal reporting requirements, any equipment purchased after January 2015 must have a maximum leak rate of 1% as ensured by the GIS’s manufacturer, and all owners must record the amount of SF₆ added to their active equipment annually.

California has also used a declining annual emission rate to regulate SF₆, and after a decade under this approach, the state is looking to phase out SF₆. Once approved by the Office of Administrative Law, the tiered phase out of the gas would begin in 2025 for SF₆ used in all switchgear under 145kV. By 2029, the state would phase out SF₆ in all switchgear between 145kV and 245kV, and then ultimately phase out all SF₆ by 2031. A report by the California Air Resources Board demonstrates that a current SF₆ user could save 122 million dollars in operation, maintenance, record keeping, and reporting costs by 2036 compared to the current regulatory mechanism, thus justifying the transition. The proposed regulation in California will result in the reduction of SF₆ emissions between 2020 and 2036 by 391,000 MTCO2e, or the emissions of 84,903 passenger vehicles driven for one year.

Other Potential Solutions

There are many potential policy mechanisms for addressing harmful greenhouse gas emissions, with various advantages and disadvantages that are beyond the scope of this piece to discuss. While several prior examples of SF₆ reform have been implemented on a statewide level, SF₆ emissions cannot be contained within state borders. Therefore, nationwide action is needed to truly transform our power system towards greater efficiency and lower emissions. Phasing out SF₆ on a national level is one option. A trade group, the SF₆ and Alternatives Coalition, offers a number of driving factors needed to successfully phase out SF₆ here.

Another option would be to institute a nationwide greenhouse gas (GHG) emissions tax on all such gases commensurate with their GWPs. Thus, the tax on SF₆ would be 23,500 times the tax on carbon dioxide emissions. Since SF₆ emissions are difficult to accurately measure, but the volume sold is a known quantity, a GHG tax on SF₆ could be imposed at the point of sale. As the equipment leaks SF₆, the owner would have to purchase more. Less gas leaked means that the owner would pay less tax. When it comes time to retire the equipment, the owner may extract the gas and resell it, thereby recouping the GHG tax associated with the reclaimed amount, which disincentivizes simply releasing the gas into the atmosphere. Thus, taxing the gas at the point of sale rather than its emissions takes into account these outcomes and eliminates the difficulty with emissions detection and enforcement.

This structure resembles the deposit refund systems imposed on recyclables or advance recovery fees imposed on hazardous electronic waste. This solution would give equipment owners flexibility to adopt the best SF₆ alternative to fit their needs, reduce leaks, and/or pay for the environmental externality. In the event that no gas is lost, the tax essentially becomes a deposit. What should be done with that “deposit” throughout the 30-50 year lifetime of gas insulated switchgear? In other GHG regulatory systems, the revenues from CO2 emissions may be used to support energy efficiency programs. Similarly, the SF₆ emissions tax and “deposit” could be used to fund research and development of alternatives, improved leak detection technology, and switchgear technology that further mitigates leakage.

While states can make an impact by implementing SF₆ regulations, the federal government should consider the impacts of a potent greenhouse gas like SF₆ and how to appropriately incentivize emissions reductions or abatement if it is looking to build a clean power system that can mitigate the worst impacts of climate change. By implementing regulations that incentivize uptake in SF₆ alternatives for varying applications and continuing to fund SF₆ alternatives research, we will be better positioned to build a more cost-effective and sustainable energy sector.