Wireless mesh networks such as ZigBee constitute a major enabler of small-scale energy harvesting. Last winter, I wrote about a European company in the utility submetering market called LEM (see “A Measure Of Opportunity Awaits In Electric Meters,”). Submetering products are purchased by companies looking for finer-grained information about their utility usage, which they get from their suppliers.
LEM’s original core competency is Hall-effect devices, an ideal entry to scavenging power from electrical cables, although this isn’t a case of pure energy harvesting because the power company meters the milliwatts consumed by the LEM system. What’s instructive from a design standpoint, however, is the company’s implementation of its Wi-LEM mesh network. A complete system consists of energy meter nodes (EMNs), mesh gates (MGs), and mesh nodes (MNs).
Each EMN attaches to the electrical wiring for the system or machine it’s monitoring with split-core transformers. The EMNs measure active and reactive energy, maximum current, and minimum voltage at 5- to 30-minute intervals. Going up a level, each MG is a standalone ZigBee gateway that manages its EMN network. Any MG can manage up to 240 EMNs while storing the latest data from its network.
Up another level, LEM offers MNs, simple repeaters that extend the network’s range beyond the maximum 25-m line-of-sight range that typifies communication between an EMN and MG. With a network consisting of those three components, companies can build systems that cover an entire manufacturing center or residential facility.
Because it’s ZigBee, the wireless network essentially configures itself. Assembling the rest of the Wi-LEM hardware involves little more effort than clamping current probes around power leads and mounting the boxes.