Modern twist on clockwork stores more energy

This is not quite the message that researchers at the University of Maryland Baltimore County are delivering, but it might as well be as a team from there is storing 2.1MJ/kg in twisted carbon nanotubes – more, weight-for-weight, than Li-ion chemistry can hold.

“They found that the best-performing ropes could store 15,000 times more energy per unit mass than steel springs, and about three times more energy than lithium-ion batteries,” according to the university. “The stored energy remains consistent and accessible at temperatures ranging from -60 to +100°C.”

Power output is ≤1.85MW/kg.

After some initial twists to condition the 30μm diameter multi-strand rope, which is made from single-wall carbon nanotubes (~1μm x 1.5nm), almost all energy that was twisted into the rope was available as it untwisted – this remained true through the 450 cycles tested.

The outcomes above were after heroic efforts of nano-scale rope making, using miniature equivalents of several techniques, sometimes adding inter-strand modifiers to help transfer force between adjacent nanotubes.

2.1MJ/kg came from a yarn-wound rope infused with the flexible polymer elastomer TPU (thermoplastic polyurethane), after microwaves were used to melt the polymer so that it soaked into the rope.

How this energy density might be usefully exploited is discussed at the end of a Nature Nanotechnology paper describing this research: ‘Giant nanomechanical energy storage capacity in twisted single-walled carbon nanotube ropes‘.

UMBC worked with Suwa University of Science (Chino, Japan), Shinshu University and the University of Johannesburg.