A novel class of two-stage electrical energy generators is presented for rotary machinery and rocking platforms in which the input speed is low and varies significantly, even reversing. Applications include wind mills, turbo-machinery for harvesting tidal flows, floating platforms and the like. Current technology using rotary generators requires gearing or similar mechanisms to increase the input speed to make the generation cycle efficient. Variable speed-control mechanisms are also usually needed to achieve high mechanical to electrical energy conversion efficiency. In this paper, electrical energy generators are presented that can efficiently operate at very low and highly variable and even intermittent and reversing speeds without requiring gearing or other speed control mechanisms. The generators are very simple in design and can significantly reduce complexity and cost, especially those pertaining to maintenance and servicing. In addition, these new generators can expand the application of energy harvesting to much slower input speeds than current technology allows.
The primary novelty of this technology is the two-stage harvesting system. In these energy harvesting systems, input mechanical energy from the environment such as wind or ocean waves is stored in a primary sub-system (stage) as potential energy. When the level of potential energy reaches a certain predetermined level, it is released into a secondary sub-system (stage). The secondary sub-system converts the stored mechanical energy into electrical energy. The secondary sub-system is preferably designed as vibrating mass-spring type energy harvester to achieve relatively high and nearly constant natural frequency and use piezoelectric or magnet and coil type generators to convert stored mechanical energy of vibration to electrical energy.