Micro generation meets micro consumption - Energy Scavenging Wireless Networks

 In a previous post I wrote a bit about wireless mesh networks as a means to cost effectively retrofit sensors into a building to allow feedback control and continuous monitoring of environmental conditions to reduce energy usage.

One problem with wireless: batteries.
While they may last for years, they do need to be replaced eventually. If you have truly adopted a full building installation of sensors, that would be hundreds of nodes and batteries tucked into all kinds of hard to access locations where the HVAC, power distribution, and other systems are located or routed.
Replacing all those batteries in all those places: not trivial.

Energy scavenging to the rescue!
Using versions of the technology that power your kinetic watch (mechanical micro-generators) or that can be stuffed into a shoe (piezoelectric), enough power can be generated from ambient vibrations to power the sensors. It's not a lot of energy, but it's probably enough for low power network devices:

• MCU + Transceiver for ZigBee (IEEE 802.15.4) draws somewhere between ~20mA and <200mA (say 50mA) (at ~3.3V) when transmitting with <1 uA in sleep mode
• Transmissions last a few tens of milliseconds (~15ms - 48ms) and sensors may transmit ~1/min.
• Energy required is, therefore ~7.9 mJ per minute (for 48ms transmissions).
• Micro-mechanical energy generators produce on the order of a few (1-5-ish... say 2) mW of power. This could yield up to ~120 mJ of energy per minute (depending on the consistency of vibrations).

So, even with a smaller or less efficient generator than I found in a few minutes of searching, with a small battery or capacitor to store the micro-generator's energy you're in business without battery changes.

image credit: inhabitat