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Technical FAQ: Thermodynamics of recycling aluminum and carbon fiber structures

A refresher in Isaac Newton's laws of physics as they pertain to materials used in bicycle fabrication.

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Have a question for Lennard? Please email him at veloqna@comcast.net to be included in Technical FAQ.

Dear Lennard,
The Technical FAQ regarding repurposing carbon fiber\ included the following statement by Josh Poertner:

“The recycling of many materials such as aluminum actually requires more energy input than producing virgin material in the first place.”

I think he misremembers. Aluminum is among the materials for which the difference in the energy needed for extraction from ore versus recycling is largest – up to 20 to 1.
— Francisco

Dear Francisco,
That is correct when it comes to recycling aluminum cans. Here is Josh Poertner’s response to your question:

“I was able to get an answer from the guy I originally got that tidbit from 20 years ago. He’s now quite senior at Alcoa and had some interesting stuff to say but didn’t want to go on record. Long story short, for cans and basic aluminum, you are correct, aluminum is super efficient to recycle, so we will be changing the language on our site regarding this and happy to change that in my comments to VN regarding the larger recycling of aluminum.

The 10:1 number comes from the recycling of aircraft and apparently was a stat thrown about 20 years ago regarding why we would park planes at the boneyards and not recycle them as it took more energy to tear them down, separate the various grades of aluminum, strip the finishes and coatings, etc. and then melt it down. It seems that this is much improved these days, though there are apparently still issues with contamination, alloying, etc. that make the recycling of structural aluminum much more energy-intensive than something like cans. Alcoa has even opened a specific recycling facility to recycle aircraft, as the process is much more involved on the cleaning/stripping side of things and also more dangerous due to the use of modern lithium-aluminum which are both lighter and stiffer than traditional aluminum but pose some serious fire and explosion risks compared to the normal stuff!

We will be removing this from our site now that we know it’s both outdated and even then was overly limited in scope.

—Josh Poertner, Silca president

Thanks for your bringing that up, Francisco.
― Lennard

Dear Lennard,
The column about the environmental impact of micro-carbon particles is especially thought-provoking. While I find Josh Poertner’s response to your query interesting, and at the risk of seeming a hopeless pedant, I must take exception to his statement, “Once this process is started, it is completely energy-independent and can run off-grid, with each 100kg of composite producing enough energy to heat the next 100kg and so on.” To the best of my knowledge, this runs counter to physics as we currently understand it. (N.B., if Silca has come up with a perpetual energy formula, its potential application is vastly larger than the sealing of tubeless tires!) I’m not trying to give Josh a hard time; I simply think that if we are going to have a serious discussion around this huge issue in cycling, we should also be rigorous in our science.
— Loren

Dear Loren,
Thanks for shining a spotlight on this; it’s a useful thing to ponder. While physics tells us that there is no such thing as a free lunch, I’m not sure that applies here, as there is a lot of energy stored in the carbon matrix that is being released when breaking it down into fibers. Here is what Josh Poertner has to say about it.

“This isn’t my process, so I’m just using language similar to what I hear from our vendor who invented the process, which is definitely a mix of patents and trade secrets that we’re not privy to. However, my understanding is that the process involves heating the composite matrix to one temperature to break the constituent subcomponents apart from each other, and that some of those subcomponents are then capable of releasing much more energy as they burn at higher temperatures when used as a power source when fed back into the process itself.

In my mind, this process is more akin to something like extracting oil from shale, it takes energy to get the oil from the shale, but the energy potential of the oil itself is far greater than what’s required to free it in the first place.

—Josh Poertner, Silca president

While Newton’s Second Law of Thermodynamics says that not all heat energy can be converted into work in a cyclic process, we’re not actually talking about making a perpetual-motion machine here. If we were breaking down the carbon structure and then rebuilding that same structure again, then your statement is correct; there is no way you could do that without adding more energy. However, when breaking down a structure to a less complex state, taking some of the energy stored in the structure and then using it to break down the next structure and still have energy left over for other things is exactly what we do every day when eating. That’s what food is: something an organism consumes to obtain additional energy than is used in obtaining, chewing, and digesting the food.

Taking this analogy further, we humans were able to develop the large brains we have by developing methods to obtain more energy-rich foods, like the meat of large animals, than simple gathering could provide. Our primate ancestors had disproportionately larger digestive tracts required to break down less energy-dense foods; a higher percentage of the energy obtained from the food was consumed in breaking down the food, leaving less available for its brain. The amount of energy stored in food, in part, determines how much energy we will have leftover for external work after eating the food.

There is a lot of energy stored in a carbon structure. We can think of it as the food for this process; breaking down an energy-dense structure offers the potential for having energy left over after breaking it down. It makes sense to me that this process could release some of the energy stored in a given mass of the carbon matrix, use it to break down the next unit of like mass, and still have energy left over.
― Lennard

Dear Readers,
Regarding Josh Poertner, some of you may be interested in this podcast interview he did of me last fall for his excellent Marginal Gains podcast series. This segment covers my history and how my love affair with and understanding of bicycles developed over the years. I think you might find it informative.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of The Haywire Heart,” and author of many bicycle books including Zinn and the Art of Road Bike Maintenance,”DVD, as well as Zinn and the Art of Triathlon Bikesand Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.” He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn on Twitter.