The proliferation of ultralow power wireless sensor nodes for measurement and control, combined with new energy harvesting technology, has made it possible to produce completely autonomous systems that are powered by local ambient energy instead of batteries. Powering a wireless sensor node from ambient or “free” energy is attractive because it can supplement or eliminate the need for batteries or wires. This is a clear benefit when battery replacement or servicing is inconvenient, costly or dangerous.
Many wireless sensor systems consume very low average power, making them prime candidates to be powered by energy harvesting techniques. Many sensor nodes are used to monitor physical quantities that change slowly. Measurements can therefore be taken and transmitted infrequently, resulting in a low duty cycle of operation and a correspondingly low average power requirement.
For example, if a sensor system requires 3.3V at 30mA (100mW) while awake, but is only active for 10ms out of every second, then the average power required is only 1mW, assuming the sensor system current is reduced to microamps during the inactive time between transmit bursts. If the same wireless sensor only samples and transmits once a minute instead of once a second, the average power plummets under 20µW. This difference is significant, because most forms of energy harvesting offer very little steady-state power; usually no more than a few milliwatts, and in some cases only microwatts. The less average power required by an application, the more likely it can be powered by harvested energy.
Energy harvesting sources
The most common sources of energy available for harvesting are vibration (or motion), light and heat. The transducers for all of these energy sources have three characteristics in common:
1) Their electrical output is unregulated and doesn’t lend itself to being used directly for powering electronic circuits
2) They may not provide a continuous, uninterrupted source of power
3) They generally produce very little average output power, usually in the range of 10µW to 10mW.
These characteristics demand judicious power management if the source is going to be useful in powering wireless sensors or other electronics.