The Greenhouse Implications of Replacing Potent GHG HFC's with HFE's in Refrigeration Systems.

While the Montreal Protocol was a tremendous (and all too rare) successful instance of public policy actually following scientific consensus and addressing the emergency of ozone depletion, the dirty little secret of that success is that the materials which replaced ozone depleting CFC's (chlorofluorocarbons) are HFC's (hydrofluorocarbons) very potent and very persistent greenhouse gases in their own right.

Many of these gases leaking out of all those 2005 Prius's air conditioners - the Prius user who thinks he or she is saving the world representing in my mind a case something like an alcoholic announcing that he is now no longer an alcoholic because he has switched from drinking Scotch to drinking beer - will still be present in earth's atmosphere 50,000 years from now, something like 5 times the distance, in time, between the period before writing was invented and today.

In terms of persistence, CFC's are not quite as bad as HFC's, but of course, the mechanism for the decomposition of CFC's destroys stratospheric ozone, whereas HFC's generally do not. The solution was to make something that doesn't, effectively, decompose at all. (Actually HFC's do decompose under irradiation, but with hardly the same alacrity as HFC's; they have long half-lives. The decomposition product of HFC's is the very toxic compound HF gas - also decomposition product with CFC's - and various acid fluorides, including fluorophosgene. Happily the concentration in the atmosphere is never very high, because the slow rate of formation allows HF to be neutralized by glass and carbonates before it can cause much harm.)

The global warming potential of one widely used HFC 1,1,1,2, tetrafluorethane, R-134a, is about 1600 times as high as carbon dioxide. This is actually not quite as bad as R-12, which has a global warming potential of 6700, which is hopefully irrelevant owning to the Montreal Protocol banning it.

Any substance that has a half-life will, no matter how quickly it is formed, will come into equilibrium, at which time it will be destroyed at the same rate that it is created. The position of the equilibrium has to do with the rates of destruction and formation. Since R-134a has a low rate of decomposition, one can have higher concentrations of it in the atmosphere than one can have of R-12, which has a shorter half-life, if a more problematic decomposition route.

It would be nice therefore to have compounds with a short half life and a relatively benign mechanism of decomposition.

Actually it now understood that such compounds exist. They are the HFE's, hydrofluoroethers, which are pretty good refrigerants, are non-flammable, and decompose with a short half-life with a mechanism similar to that of HFC's, generating HF and fluorophosgene that can be neutralized by silica (glass) and carbonates before accumulating in toxic concentrations.

It might seem like a slam dunk, but, um, not so fast. Although HFE's are effective, they are not as efficient. To wit, it takes more energy to make them work than it takes with HFC's.

This is discussed in the ASAP section of the American Chemical Society Journal Environmental Science and Technology in a paper by Blowers and Landsbury out of the University of Arizona, where they should care about things like refrigerants.

http://pubs.acs.org/doi/abs/10.1021/es9023354">Here is the abstract of the article, and a link for use of subscribers and people in good scientific libraries.

Here are some excerpts of the article from the text:



The bold is mine.

Note that the Joule-Thompson coefficient of all real gases, with the exception of hydrogen, helium and neon is positive, meaning that all real gases except these are potentially used as refrigerants. Indeed, liquid nitrogen is often made using the auto-refrigeration of compressed air being allowed to expand.

The problem is that using nitrogen to refrigerate itself involves an energy penalty. If energy is cheap, liquid nitrogen is cheap. If energy is expensive, liquid nitrogen is expensive.

These thermodynamic facts - like all thermodynamic facts - translate into environmental effects, since energy and the environment are inextricably connected, which is why energy storage always involves an environmental penalty.

To continue with excepting the paper, they consider - unlike an anti-nuke daydreaming about putative "someday" solar and wind systems with brazillions of battery packs - where energy comes from:



Note that coal use is likely to get worse not better although many attempts will be made to pretend otherwise, by building as a red herring one or two tiny coal gasification plants that could someday, possibly - if we really, really wanted to do it, even though we don't - be applied to sequestration, to distract attention from the huge new traditional coal plants. This is what I call the "Florida Power and Light Solution" which is marginally better than the German solution which is to announce that new coal plants don't count because they're, um, new. (It also helps to announce that you intend to use useless solar and wind power even as you build new coal and gas plants, another practice of the Germans who are in the process of replacing all of their nearly pollution free nuclear plants with "green" coal and gas plants.)

Now for the results of the paper:



The bold is, again, mine.

Note that HFE's become a reasonable and workable replacement for HFC's if and only if electricity is freely available without the generation of greenhouse gases.

This is entirely feasible with known technology, but regrettably, ignorance and mysticism prevent full application of the technology.

Of course driving around at high speeds in a tin can with ten or twenty gallons of gasoline aboard (some of it highly pressurized for the fuel injectors) isn't all that safe either. Air conditioners filled with flammable gases probably wouldn't increase the danger of automobiles all that much. Cars catch on fire all of the time. What's the big deal if a few more cars burn? We could save the environment twice: once with environmentally friendly refrigerants, twice by reducing the number of cars (and drivers!) on the road.

When discovered in the 1930's by Dupont, CFC's were considered a technological miracle. There were believed to be no drawbacks to their use, and, in fact, no drawbacks were discovered until the 1980's but when the drawback was discovered, the seriousness of the issue was recognized to be nearly catastrophic.

I would argue that the atuomobile - which was invented in part to deal with the environmental and health consequences of horse shit in the streets - was an even worse catastrophe with equally dire consequences.

For the record, I have often referred to the refrigeration power of DME, dimethyl ether, which I consider a wonder substance as a replacement for diesel fuel, for dangerous natural gas, and for propane. It is of course, much like propane inasmuch as it is flammable and is easily liquified. However its critical temperature is much higher, and it can be liquified under pressure at temperatures higher than the atmospheric pressure boiling point of water.

In theory, a car powered by DME - not that cars can ever actually be made "green" by use of any technology - a DME powered car could use its fuel as a refrigerant.

I don't like ammonia because it's very toxic and is fairly corrosive as well as, under the right circumstances flammable.

Propane is not really reassuring either.

Both propane and ammonia are fairly persistant greenhouse gases, the latter especially so because it will be converted to nitrous oxide environmentally, nitrous oxide being the third most important greenhouse gas there is.

Frankly we need to fix less nitrogen, not more. I freak out when ammonia is proposed as a motor fuel as a hydrogen carrier. It would be a horrible choice.

Carbon dioxide is better, I think, and I have argued for DME in automotive systems as both a fuel and a refrigerant, although truth be told, I'd rather just phase out the automobile all together.

I think that H₃COCF₃ has an acceptable profile. It shares DME's very short atmospheric half-life, on the order of days, as opposed to propane which is on the order of decades, and ammonia which, with its nitrogen cycle analogues, represents a huge environmental burden not only in the atmospheric sense, but in the eutrophication sense as well.

I think that humanity needs to get away from ammonia, although if you look carefully, you will see that a huge fraction of the protein now on earth has actually been made by Haber Chemistry.

I wrote about that point in a series I ultimately abandoned on another website:

http://www.dailykos.com/storyonly/2007/10/8/194846/997">Troll Rating Fritz Haber, Jimmy Kunstler, and The Oracle at Snowmass, Part 3.

One may argue that the HF decomposition route of H₃COCF₃ seems unpleasant, aesthetically, but HF has never been the problem really compared with other gases. If either CFC's or HFC's are to decompose, HF will be involved. The atmospheric concentrations of HF and fluorophosgene are measurable and have never approached toxicologically important levels. As an acidity issue, it's hardly the issue that nitric acid is, and nitric acid in earth's atmosphere is very much involved with ammonia as well as dangerous fossil fuels.

Carbon dioxide is also acceptable, and would in theory represent a de facto sequestration mechanism, albeit a trivial one.

Just because some, even many, fluorinated compounds have proved problematic, it is not appropriate to throw the baby out with the bath water.

But the elephant on the table, again, as the paper usefully points out is the energy involved and whence it comes. This is also true of water. Many of us here have probably made ice by pumping on it during playtime in the lab, but we should realize that there's a lot of electricity consumed in doing that.

It is, by the way, in my mind possible to accelerate the decomposition of all of the fluorinated problem child fluorine gases, in air and even include NF₃ from the semiconductor industry and SF₆ from the electrical transformer industry as a side product of certain kinds of putatively industrial processes, but I won't discuss that here.

Given enough energy we can do almost anything, including, in fact, the removal of atmospheric carbon dioxide, something I personally believe has now become urgent.

Go to any automotive air conditioning forum and behold the power of advertising, fear mongering, and dirty science.

But that's to be expected with anything so filthy as cars.