Energy storage in carbon nanotube super-springs

Abstract

A new technology is proposed for lightweight, high density energy storage. The objective of this thesis is to study the potential of storing energy in the elastic deformation of carbon nanotubes (CNTs). Prior experimental and modeling studies of the mechanical properties of CNTs have revealed nanoscale structures with a unique combination of high stiffness, strength and flexibility. With a Young's modulus of 1 TPa and the ability to sustain reversible tensile strains of 6% [1, 2] and potentially as high as 20% [3-5], mechanical springs based on these structures are likely to surpass the current energy storage capabilities of existing steel springs and provide a viable alternative to electrochemical batteries. Models were generated to estimate the strain energy of CNTs subject to axial tension, compression, bending and torsion. The obtainable energy density is predicted to be highest under tensile loading, with an energy density in the springs themselves about 2500 times greater than the maximum energy density that can be reached in steel springs, and ten times greater than the energy density of lithium-ion batteries. Practical systems will have lower overall stored energy density once the mass and volume of the spring's support structure and any additional extraction hardware are taken into account, with a maximum achievable stored energy density predicted to be comparable to lithium-ion batteries. Due to the poor load transfer between MWCNT shells and the radial deformation of larger SWCNTs, bundles of SWCNTs with diameters of 1 nm or smaller are identified as the best structure for high-performance springs. The conceptual design of a rechargeable portable power source is developed as a tool to study the performance and feasibility of building such a device.

(cont.) In this design, energy is stored in a grouping of denselypacked, aligned CNTs stretched in tension. The design includes an escapement mechanism to regulate the. energy release from the spring and a generator to convert the output work from the spring into the electrical domain. The results indicate that the performance of the power source scales well with size so there is flexibility in choosing the overall scale of the device. Achieving a high fraction of CNTs in the overall device proved to be challenging. Future work should concentrate on building and testing high-quality, densely-packed macroscale SWCNT assemblies that are expected to form the basis of super-springs for implementation into practical devices.