Harvesting energy into lithium-ion batteries

Even if the harvester has low efficiency, tapping into the energy of the surrounding environment is attractive because it is, for all practical purposes, an infinite source. Ambient solar, kinetic (vibrations), and thermal energy can be harnessed on-chip with photovoltaic cells and micro-electromechanical systems (MEMS) generators [2-3]. The amounts of energy and power levels that can be achieved, however, depend on the conditions surrounding the application and the compatibility of the available technologies. Relatively low-frequency ambient vibrations from engines, flowing water, gusting winds, moving people, and others, however, are abundant, stable, and predictable [4].

A Self-Sustaining System
Energy from ambient mechanical vibrations can be harvested by means of a magnetic field, an electric field, or a strain on a piezoelectric material [3-4]. Electromagnetic and piezoelectric scavengers, however, are less CMOS-compatible and less scalable. Electrostatic harvesters, on the other hand, are fully compatible with MEMS technologies and capable of generating moderate power levels without the use of exotic materials or obscure process steps. The foregoing scheme therefore harvests energy from an electrostatic generator and stores it in a thin-film polymer lithium-ion (Li-Ion) battery, which in turn powers the system, as illustrated in Figure 1. The electrostatic harvester does not convert energy continuously so an intermittent battery charger is used. To ensure the system is fully operational and self-sustaining, power-intensive tasks such as data transmission and reception are constrained to low duty-cycle operation, in other words, operate only when there is sufficient energy in the system to do so. Sensing and other low power functions may have longer duty-cycles, but for the sake of energy, they are also limited, unless they perform indispensable functions in the system.

Figure 1. Self-sustaining micro-system