Small buoys for energy harvesting: Experimental and numerical modeling studies

We present the development and application of small buoy systems for wave energy harvesting (free-floating or slackly moored), to produce about 1 KW per unit at full scale. These systems are targeted for powering distributed marine surveillance and instrumentation networks, and should be simple in concept, easily deployable, storm resilient, and low maintenance. Our work involved design, experiments (both laboratory and field testing), and numerical simulations in realistic irregular wave climates, of two new types of buoy systems equipped with an embedded Linear Electric Generator (LEG; made of a permanent magnet, suspended to a spring, oscillating within a (two-phase) coil), whose armature motion is excited essentially by the buoy\u27s wave-induced heave, with some effects of roll. The first design (DC2 buoy) has a spherical float, to which a cylindrical canister is rigidly attached, which houses the LEG. A rod, attached to the LEG magnetic armature, exits through the bottom of the canister and connects to a large submerged resistance platform (which also serves as ballast). The differential motion between the float and platform heave drives the LEG oscillations. By contrast, the second design (DC3) is a self-contained (water tight) resonating multiple-spar buoy (or Starspar), in which a longer central spar houses the LEG and is surrounded by shallower, satellite spars, providing both form stability and a reduced overall average draft (necessary to achieve a proper heave resonance period). The LEG, which has a large ballast simply attached to its bottom, oscillates as a result of buoy heave through coupled resonance. Hence, LEG oscillations are maximized by matching starspar heave and LEG natural periods, and both of these to the targeted sea state peak spectral period. For spar buoys, the former is simply controlled by buoy draft. Scale model experiments are performed to calibrate numerical model parameters (essentially viscous drag coefficients), and select buoy characteristics to maximize energy production. Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE)