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I was thinking about the limits of flywheel energy storage and wondered about carbon nanotubes. The amount of energy that can be stored per unit mass in a flywheel depends not on tensile strength but on the ratio of tensile strength to density. I can find sources which discuss CNT tensile strength (both actual tensile strength and upper bounds of what might be possible), and I can find sources giving density, but I haven't found one discussing both. Presumably I can't just take the largest value for the first and the least for the second, because the strongest is probably not the least dense.

So how high of a value can carbon nanotubes have for tensile strength/density? I'm primarily interested in the best case (what could be achieved by painstakingly placing atoms in the perfect place) but 'best in the lab' and 'best in production' would be useful too.

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  • $\begingroup$ en.wikipedia.org/wiki/Ultimate_tensile_strength $\endgroup$ – CuriousOne Apr 22 '16 at 22:44
  • $\begingroup$ As for technically achievable tensile strength, you may want to scale back from the literature value by a factor of two... it's very ugly when these things rupture... $\endgroup$ – CuriousOne Apr 22 '16 at 23:41
  • $\begingroup$ @CuriousOne: Well, en.wikipedia.org/wiki/Specific_strength is a bit closer to what I'm looking for; it gives (the remarkably precise) 59483 kN*m/kg for colossal carbon nanotubes. But I was hoping for something larger, more of "what is the most that this material could do, given lots of R&D". $\endgroup$ – Charles Apr 23 '16 at 3:21
  • $\begingroup$ Generally, it's unlikely that macroscopic pieces of these materials will be able to do anything even closer to what a microscopic sample promises. Another way to look at this is that the rim speed of a flywheel can't be higher than the speed of sound in the material, so you are looking for materials with high speed of sound. Diamond looks good (clearly above the metals), but I don't know how that translates to carbon nanotubes. $\endgroup$ – CuriousOne Apr 23 '16 at 5:24

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