EEVblog Electronics Community Forum
Electronics => Power/Renewable Energy/EV's => Topic started by: DenzilPenberthy on November 14, 2019, 04:02:36 pm
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I just stumbled across this which I found interesting. There has been some increasing awareness recently of the environmental impact of SF6 leaks from HV switchgear. There was a thread here about it.
It looks like GE have developed an alternative gas which is much less damaging and they are rolling it out in to the field.
May be of interest to folks here?... It seems encouraging and looks like it passes the 'yeah nice but can I actually go and buy it yet' test.
https://www.gegridsolutions.com/press/gepress/Press_Release_g3%20Roadmap_Sept2019.htm (https://www.gegridsolutions.com/press/gepress/Press_Release_g3%20Roadmap_Sept2019.htm)
Product Roadmap here...
https://www.gegridsolutions.com/products/reference/g3_roadmap_2025-Brochure-EN-2019-10-Grid-GS-1668.pdf (https://www.gegridsolutions.com/products/reference/g3_roadmap_2025-Brochure-EN-2019-10-Grid-GS-1668.pdf)
There are some techical details buried away here...
https://think-grid.org/search-sf6-replacement (https://think-grid.org/search-sf6-replacement)
"Not a simple gas as SF6 is, but a gas mixture named g3 (green gas for grid) based on 3M™ Novec™ 4710 Dielectric Fluid from the fluoronitrile family specially developed by 3M for that purpose and used as an additive to a complementary gas. Among other candidates, fluoronitriles were studied extensively as the most promising chemical family. Among the different candidates tested, some have a high dielectric strength, but toxicity was an issue. So, new molecules were developed to resolve this problem, chemists and physicists manipulating molecular architecture to combine the advantages of previous candidates while eliminating drawbacks. Finally, one specific molecule, marketed as Novec™ 4710 fluid, was designed and selected as the best compromise for its excellent combination of properties:
significantly lower GWP compared with SF6;
boiling point: -4.7;
dielectric strength: 2.2 x that of SF6;
high thermal transfer capability;
low toxicity.
“However,” notes Kieffel, “Novec™ 4710 fluid cannot be used alone due to its liquefaction at low temperature. It was found that the best compromise is achieved by mixing it with CO2 (for its arc quenching capability) to create g3, a gas mixture suitable for disconnector and circuit breaker applications.” As a result, the GWP of the g3 mixture of fluoronitrile with CO2 can be brought down by well over 98 % compared with SF6."
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Am I missing something fundamental here?
SF6 has an awesome global warming potential, true, but it is also very, very dense. My recollection of SF6 is that it notoriously pools in the low parts of structures and in such spaces presents a danger of asphyxiation, ~6kg/m^3 at STP, where nitrogen is around 1kg/m^3 under the same conditions.
I am having some trouble with the notion of a gas that dense managing to get high enough in the atmosphere to matter. At street level, sure, but 10km up at the top of the troposphere? Got my doubts.
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I wouldn't claim to be an expert on the physics but yes, when the gasses mix, they will stay in solution. Otherwise the atmosphere would be stratified with the various gasses in layers according to their density.
I'm sure in a still column of gas, there will be some concentration gradient according to density but I suspect it's a very small effect in the atmosphere. Especially when the SF6 has tens of thousands of years of wind, convection, and Brownian motion to get up there.
Even if it stayed as a layer on the ground surely it would still contribute to warming? As long as it blocks infra-red from being radiated back in to space...
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Yeah, even under still conditions, diffusion will take it there eventually. And it's not like it's going anywhere down here, it's only broken down in the upper atmosphere as far as I know. Well, maybe a few molecules get chance encounters with lightning bolts. Actually over that time scale, maybe lightning is a notable contributor, no idea.
Relevant paper is awfully easy to find:
https://www.researchgate.net/publication/326961554_FluoronitrileCO_2_mixture_as_an_eco-friendly_alternative_to_SF_6_for_medium_voltage_switchgears (https://www.researchgate.net/publication/326961554_FluoronitrileCO_2_mixture_as_an_eco-friendly_alternative_to_SF_6_for_medium_voltage_switchgears)
The formula is (CF3)2CFCN, which can also be called perfluoroisopropyl cyanide. So it's a pretty heavy molecule, a CFC. Probably the biggest downside is the high boiling point, near that of butane for example. It will liquefy inside equipment, that's been turned off for a while in cold climates (like much of the US right now!).
Tim