Bottled-up energy

2015 was a massive disappointment. Which is a shame, because according to Back to the Future II, I should have my own hoverboard, some self-lacing Nikes, and a Mr. Fusion reactor that I could put on the back of my hovercar. As of 2016, hoverboards are mostly Segways without handlebars (and they’re facing a massive recall), self-lacing Nikes are just coming out and guaranteed to be expensive, and the smallest fusion reactor is still in development and is the size of a truck. And just how am I supposed to put that on my Malibu!? It already doesn’t hover!

The Mr. Fusion thing would have been an awesome idea as it was in Back to the Future II: load it up with ‘fuel’, which is really any kind of garbage. Doc Brown threw in some beer, the beer can, a banana peel, and there you go – fuel for a hovering DeLorean. I’m not entirely sure how that works, but that’s Hollywood science for you. Anyway, what about the garbage we have now? We can’t just throw it in your engine and drive off into the sunset. Some research gets us a little closer, though.

A team of Chinese and American researchers recently published some research on methods of degrading polyethylene (PE) plastic. This is a good material to deal with, as this makes up almost 97% of the 1.1 billion pounds of recycled plastic bottles recycled in the United States according to a 2015 report. PEs are long chains of mainly carbon and hydrogen that could, according to the researchers, could be broken down into shorter chains of carbon and hydrogen, like octane. This should sound familiar; it’s gasoline (unless you’re in Europe, then it’s petrol). Sounds good, but even the researchers admit that this isn’t a new process. So why do they want to do research on this chain-breaking anyway?

Turns out PE holds together pretty well, as one would expect since in the process of melting and forming bottles. Longer, heaver carbon-and-hydrogen chains like PE tend to be more stable and require more energy to break apart. On the other hand, shorter chains are easier to break apart – this is where we get combustion from (or in the case of your car, thousands of small, hopefully controlled explosions per minute). This means that if one wants to break down these long PE chains into smaller, less stable chains (like octane or even diesel fuel), it would require a lot of energy, somewhere in the neighborhood of >400°C. That’s quite hot, which means you need a lot of energy to get the melted PE soup that warm. Even making fuel requires energy, just like everything in the universe. The authors propose they can do it at 175°C (374°F), all they need are some lighter carbon-and-hydrogen chains, some catalysts (Wikipedia to the rescue!) to make the whole shebang run faster, and some PE scraps.

Did they get what they were aiming for? They sure did: most of the scrap PE that they use (it’s only 0.3 grams; a lot of materials science research starts on a small scale) ended up as smaller chains of carbon and hydrogen, along with some solid wax. So, using some chemicals that would’ve been burned up in the fuel distillation process anyway and using temperatures associated with baking cookies rather than melting lead, they can make fuel from PE bottles. How this will work scaled up for industry, that’s when research joins with engineering. For now, this means that there will be other uses for recycled PE plastics other than making new bottles.

As if we needed more reason to recycle our bottles? We can power our cars, at least until we can make solar power really work, anyway, but it makes for more efficient use of energy in energy research and fuel production. It’s going to be a while before Mr. Fusion will power our cars using garbage as fuel, but at least Mr. Coffee will power us through our 2:30 crashes. I like to use this as fuel.

Featured article: Jia X, Qin C, Friedberger T, Guan Z, Huang Z. (2016) Efficient and selective degradation of polyethylenes into liquid fuels and waxes under mild conditions. Science Advances 2(6), 10.1126/sciadv.1501591

Featured image credit: Wikimedia Commons (CC BY 3.0, user: Trounce)



  1. Hollywood has a lot of ridiculous “logic”, but it was very interesting to read about the science behind “bottled” energy! There simply aren’t enough well-researched blogs out there. Keep writing! 🙂

    Liked by 1 person

  2. BTTF 2 didn’t use Hollywood science. Don’t you know how Mr. Fusion works? Obviously, Mr. Fusion used the ethanol from the beer, ground up aluminum from the can, and some nice “stable” potassium from the banana peel as fuel. Not enough fuel you say? He wasn’t refueling from empty, just topping off his tank. Doc Brown is OCD like that.


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