be strong enough to support the applied load (used to compress the CNTs) without reaching failure itself. Another point of consideration is that the energy density of the combined spring and supporting structure is always lower than the energy density of the spring alone. Energy density comparison A CNT spring made of bundles of densely packed 1 nm diameter SWCNTs stretched to a 10% strain is predicted to have an energy density of 3.4 kJ/m3. The energy density of CNT springs loaded in tension is higher than the energy density of CNT springs loaded in compression. Whereas the current maximum energy density of a carbon-steel watch spring is reported to be between 1080 kJ/m3 and 3000 kJ/m3. Calculations show that when a support structure made of single crystal silicon carbide is used the energy density of CNT springs reduces to 1 kJ/m3. Even after considering a support structure and other energy extraction hardware associated with a CNT spring energy harvesting device, its energy density is much greater than mechanical springs and is in approximately the same range as that of lithium-ion batteries. The energy density is much lower than the energy density of any hydrocarbon used in combustion processes. Failure processes that limit energy storage A large number of CNTs are needed to store a significant amount of energy that can be used for macroscopic processes. In order to achieve such a large amount of energy storage the CNT springs must maintain high stiffness and elasticity. It is in practice quite difficult to have such high stiffness and elastic strains in yarns or fibers made up of assemblies of CNTs as they seldom maintain mechanical properties of an individual SWCNT. This behavior occurs due to atomic defects and imperfect organisation. Elastic loading is the preferred loading mechanism for reversible energy storage, experiments have been performed that indicate that loading within the fibers deviates from purely elastic behavior. Only a portion of the CNTs contribute to the load bearing at a given strain. The unequal amount of slack within each CNT due to the presence of atomic defects and tangling causes different CNTs to fracture at different strains. When MWCNTs are loaded in tension it is difficult to grasp their inner shells. Tensile tests of MWCNTs attached to atomic force microscope (AFM) tips at both ends show that fracture occurs at the outer shell in a way such that majority loading