By Diego De La Fuente
Student Research Assistant, Center for Energy Studies
Rachel A. Meidl, LP.D., CHMM
Fellow in Energy and Environment, Center for Energy Studies
and Michelle Michot Foss, Ph.D.
Fellow in Energy, Minerals & Materials, Center for Energy Studies
As interest grows in wind, solar and electric vehicles, we will see a concurrent shift towards electrification. Penetration of energy storage and greater reliance on electrification for industrial processes will supplement enlargement of power grids with high and ultra-high voltage transmission lines, substations, and other infrastructure. While the promise of electrification is in part climate related and to address energy poverty, the lack of data and uncertainty around life cycle impacts highlights the need for an honest accounting of systems-level vulnerabilities, trade-offs and risk-shifting along the value chain.
Here we focus on only one of many distinct mitigation challenges that such honest accounting will need to consider: a synthetic, relatively obscure, and odorless gas: Sulfur Hexafluoride (SF6).
In 1997, the Kyoto protocol identified SF6 as one of the six main greenhouse gases (GHG). Not without a good reason: SF6 is the most potent GHG known to humanity, with a warming potential 23,900 times that of carbon dioxide (CO2) and atmospheric residence of up to 3,200 years.
Should we worry?
Although SF6 contributes only around 0.8% of CO2-equivalent modeled global warming, its potency and atmospheric lifetime persistence of 3,200 years necessitates action. The atmospheric concentration of SF6 has increased rapidly over time, primarily driven by demand for gas-insulated electric switchgear in developing countries. The annual emissions rate rose from about 7.3 gigagrams or Gg estimated in 2008 to about 9.04 Gg in 2018, an increase of 24% over the course of the decade. For reference, 9 Gg of SF6 equates to GHG emissions of approximately 44 million passenger vehicles driven for one year, or 226 billion pounds of coal being burned. Similarly, the global mean concentration of SF6 has increased steadily since tracking began in 1995 from approximately 3.5 parts per trillion or ppt to 10.5 ppt in 2021 (to date), a three-fold increase over the course of 25 years. The largest year over year increase occurred in 2017-2018 and amounted to 0.35 ppt.
In addition, those numbers may be actually much higher. Even though United Nations Framework Convention on Climate Change (UNFCC) partners are expected to report their GHG emissions, countries such as China, India and South Korea have not reported emissions of SF6. China itself is believed to be responsible for 36% of global SF6 emissions. Even some developed countries, including the U.S. and UK, may grossly underestimate their output. In all, the discrepancy between the actual SF6 emissions and what is reported might range between 2.5 Gg to 5 Gg from 1990-2018.
The issues of growing SF6 emissions and their underreporting particularly problematic as countries at all levels of developments push for electrification of their economies.
How is SF6 related to electrification?
Unique chemical properties of SF6 make it a great electrical insulator. It is highly stable, non-toxic, non-flammable, electronegative, and has excellent arc-quenching properties. SF6 replaced polychlorinated biphenyls (PCBs), which governments phased out and eventually banned due to concerns about dioxins. SF6was widely adopted as an alternative to oil and air for insulating mid- and high-voltage electrical equipment. Around 80% of all SF6 produced worldwide is used in the electric power industry. SF6 is used in gas-insulated switchgear for wind turbines to prevent overloading and short-circuiting. SF6 also is found in gas-insulated transmission lines, local distribution systems for safe delivery of electricity and transformers, and in the manufacturing photovoltaic panels. In electronics, SF6 is used in semiconductor devices found in cell phones, computers, and batteries for electric vehicles.
Over time, SF6 can leak during manufacturing, shipping, and storage of SF6 equipment and cylinders and during installation, operation, maintenance, decommissioning, disposal or recycling of gas-insulated equipment.
What are existing rules?
As might be expected from lack of reporting, policy and regulatory approaches to SF6 management are not widespread. In the U.S., legislation does not cover all segments of the SF6 life cycle and few states have rules related to SF6 management. California discourages use of SF6 by requiring that emissions rates from equipment may not exceed 1% (reduced from 10% in 2011). Massachusetts state law limits the annual leakage rate of equipment to 1% while equipment owners must comply with pre-established maintenance and recording procedures. Washington, Oregon, and New Jersey have reporting requirements for SF6 that allows for stringent emissions tracking.
The U.S. Environmental Protection Agency (EPA) mandates SF6 reporting for equipment with nameplate insulation capacity of 17,820 pounds or more. In 1999, EPA established a voluntary partnership with the power industry to reduce SF6 emissions. The partnership has helped reduce SF6 emissions by 74% as of 2018.
The European Union has also taken steps to reduce SF6 emissions, acknowledging that the fluorinated gases (F-gases) group has the strongest potential greenhouse effect with emissions doubling from 1990 through 2014. Current legislation calls for overall F-gas emissions reductions of two-thirds, to 2014 levels, by 2030. SF6 is prohibited from use in magnesium die-casting alloys and during filling vehicle tires. In 2020, the EU released a report indicating intentions to phase out SF6 in electric power systems.
What are SF6 mitigation strategies?
Because SF6 emissions can occur throughout the entire electric power infrastructure life cycle and as electrification expands to new technologies and applications, both sources and diversity of emitters will increase. The strongest reductions come from improving management practices. Upgrading and modernizing existing protocols and standard operating procedures throughout the SF6 supply chain helps reduce emissions. Establishing a life cycle approach ensures gas inventory tracking and accounting; leak detection and repair; proper recovery, recycling and disposal of SF6 and equipment; management of SF6 acquisitions; equipment upgrades and replacement; and proper decommissioning. Instituting such practices through expanded regulatory partnerships could drastically reduce emissions and increase accountability.
What are the alternatives?
Although many gases have been explored as viable alternatives to SF6, there is no proven, commercial alternative yet. SF6 has unique attributes as an electrical insulator and substitutes must also be non-flammable, non-corrosive, readily available, safe to handle and non-toxic.
When considering potential viable alternatives to SF6, it is important to take a systems level approach and consider full life cycle effects in order to understand sources of risks, how risks will shift, and how to mitigate them.
Trifluoroiodomethane, CF3I, with similar dielectric properties, is one possibility. However, CF3I is carcinogenic, mutagenic, and reprotoxic. It also causes oxidation and corrosion to electrical equipment. CO2 by itself or mixed with oxygen, CO2/O2, is another. CO2/O2 has a much larger potential environmental footprint than equivalent SF6 units. Other alternatives used or attempted include mixtures of SF6 with nitrogen, SF6/N2 and different fluorinated gases but few meet the replacement criteria. Proprietary options are in development. General Electric offers “g3”, otherwise known as “green gas for grid.” g3 has similar performance metrics to SF6 and can function at high voltages (up to 420 kV). Currently, three utilities use g3: National Grid (UK), Scottish Power Energy Networks (UK), and Axpo (Switzerland). ABB has produced a five carbon, C5, fluoroketone/air gas compound with a global warming potential 99.99% lower than SF6. The company is phasing this product into its existing switchgear platforms. Thus far ABB’sABB innovation has been deployed by SUC Coburg (Switzerland), ENEL e-distribuzione (Italy), and Lyse Elnett (Switzerland) in medium voltage switchgear. Nuventura, Schneider, and Siemens have developed SF6 free switchgear either using alternative gases or vacuum technology.
What is SF6 systems-level accounting?
A fundamental question is how best to ensure honest accounting of systems-level impacts, risks, trade-offs, and vulnerabilities. Are manufacturers, shippers, owners, operators, and those involved in decommissioning, recycling and disposal incorporating mitigation techniques into their management plans? Development and deployment of substitutes will take time. Meanwhile, the push is on to accelerate access to electric power, in the quest to address climate issues and alleviate energy poverty. In light of those goals, it will be even more imperative that SF6 emissions or environmental consequences of substitutes are truly sustainable and balanced throughout the system. Additionally, a well-established regulatory framework that has consistent accountabilities across the global SF6 supply chain will be essential for a transparent system that will situate us on path towards sustainability.
This post originally appeared in the Forbes blog on March 25, 2021.