The battery life on edge devices is one of the limiting factors in IoT device lifecycle management. One solution is to harvest energy from the environment around the device and recharge the batteries or provide power directly.
One fundamental problem is that energy harvesting technology really can’t supply much energy with current technology. Even solar and wind are only sustainable in prohibitively large form factors and aren’t very usable when shrunk to IoT device sizes. What this means, practically, is that devices need to use very low amounts of energy, which has driven many of the IoT hardware design decisions over the past few years.
How to Harvest Energy
There are several well-established methods for energy harvesting, some of which might be new to you. The most common energy sources are light, heat, vibration, and radiofrequency.
Polycrystalline silicon or thin-film solar cells can convert photons to silicon electrons more efficiently than thin-film cells. Think about a small calculator. Solar is best used for battery charging, not direct power.
Thermoelectric harvesters collect heat from the environment to exploit the “Seebeck effect” that generates energy when two different metals are placed close together but at different temperatures. The size of the generator determines the output, and they are best used, as you’d expect, in already hot environments like industrial heating systems.
Piezoelectric transducers use vibration to generate electricity which is why they are often used to detect motor bearing noise, the vibration of aircraft wings, and on other parts. The output here is sufficient to power a device or charge batteries.
Some radio frequency receivers can convert low-frequency RF signals to a reasonably significant voltage output. It can also be paired with low-power processors, sensors, and radio modules to deploy power-independent and battery-free edge nodes.
Energy Harvesting Use cases
Of course, there are many IoT use cases for low-power, independent power sensors, and other edge devices. These include industrial monitoring, building automation, smart grid, agriculture, and defense applications.
Let’s look more closely at a few others.
One of the most promising Consumer-facing uses for piezoelectric energy harvesting is in wearables. Researchers at the University of Michigan reportedly developed a device that harvests energy from heartbeats and uses that energy to run a pacemaker or an implanted defibrillator, an excellent application for IoT Healthcare. Radiofrequency conversion is also being researched in healthcare wearables, mainly to recharge the batteries in pacemakers and transcutaneous electrical nerve stimulation (TENS) devices. Wireless patient charging stations in every walk-in!
A sensor in the experimental stages at MIT would harvest sound waves to power biological status sensors on people.
The cases in HVAC and Smart Building are pretty limitless. Solar panels can live large on building roofs to power lots of IoT systems, vibration and motion-driven generators on doors and floors can power occupancy sensors and other people-tracking devices.
In one experimental case, Oak Ridge National Laboratories reportedly has developed a pyroelectric generator that uses a bimetal cantilever that moves between hot and cold surfaces and could cool electronic devices and systems of all kinds while still generating energy.
Finally, the most needed and promising energy harvesting deployment area is remote and mobile industrial cases. Far-flung Smart Farms, with hundreds of sensors spread across thousands of acres, miles from any power source, need to be able to rely on those sensors to stay powered indefinitely because they cannot reasonably afford to send an endless stream of batteries to the field.
Likewise, the mobile supply chain is, by definition, in motion, and those sensors need to survive and stay powered for months on the road from the mine to the refinery to the factory to warehouse to end-users. To solve this, some makers are using piezoelectric sensors to harness the ocean’s motion, rail, and truck. Likewise, solar charging is a possible solution.
In any case, more energy is lost from the world’s system than we could ever harness. But we can do better.