  Many technologies based on fluid-structure interaction mechanisms are being
developed to harvest energy from geophysical flows. The velocity of such flows
is low, and so is their energy density. Large systems are therefore required to
extract a significant amount of energy. The question of the efficiency of
energy harvesting using vortex-induced vibrations (VIV) of cables is addressed
in this paper, through two reference configurations: (i) a long tensioned cable
with periodically-distributed harvesters and (ii) a hanging cable with a single
harvester at its upper extremity. After validation against either direct
numerical simulations or experiments, an appropriate reduced-order wake-
oscillator model is used to perform parametric studies of the impact of the
harvesting parameters on the efficiency. For both configurations, an optimal
set of parameters is identified and it is shown that the maximum efficiency is
close to the value reached with an elastically-mounted rigid cylinder. The
variability of the efficiency is studied in light of the fundamental properties
of each configuration, i.e. body flexibility and gravity-induced spatial
variation of the tension. In the periodically-distributed harvester
configuration, it is found that the standing-wave nature of the vibration and
structural mode selection play a central role in energy extraction. In
contrast, the efficiency of the hanging cable is essentially driven by the
occurrence of traveling wave vibrations.
