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Feature: Energy harvesting


Figure 1: Comparison between data rate, bandwidth and power consumption of short-range and cellular radios. A wireless subsystem can consume from as little as 150µW, to 400mW [Courtesy of Voler Systems]


Simple power management Because environmental energy sources are not necessarily available when a wireless sensor or other IoT device is ready to transfer data, a rechargeable battery or capacitor is used as a buffer. An Energy Harvesting Power Management IC (EH PMIC) handles the energy from the harvesting subsystem, delivers the charge to the energy buffer, then powers the load when needed; see Figure 3. Thermoelectric harvesters are


low-impedance devices, producing continuous DC current at a low voltage. PV cells output low voltage DC too, but the current, and subsequently the impedance, varies with the incident light level. When used to harvest energy from vibration, piezoelectric devices deliver bursts of energy, potentially at higher voltages. Most EH PMICs are designed to


Figure 2: Solar (outdoor) photovoltaic outperforms every other energy source, but this energy source is not always available, and even if it is, systems may benefit from using complementary harvesters


integrated circuit. Te radio consumes the most energy in almost every instance.


Range, data rates and wireless protocols If wireless sensor modules are used close to aggregators or gateways, such as hubs or routers, short-range radio protocols like Bluetooth, Zigbee, Z-Wave or Wi-Fi are used. Te choice of protocol will be determined by the required data rate and range, and its cost; see Figure 1. Where sensor distribution covers a wide geographical area, an LPWAN or cellular connection may be needed, which is more power-hungry. To determine the extent to which a


wireless sensor module can be powered from harvested energy, the starting point is an analysis of the power profile of the module when used in the target application. Smart meters might send small packets of data infrequently; security cameras may send a lot of data continuously, making them far more


demanding from an energy perspective. Most wirelessly-connected IoT sensors


will have energy demands somewhere between these two extremes.


Energy harvesting for IoT sensors Te main micro energy harvesting technologies suited to powering sensors are photovoltaic (PV) cells, piezoelectric or electrostatic converters that harvest energy from vibration, and Peltier harvesters, which convert temperature gradients into electrical energy; see Figure 2. Harvesting RF energy is an option too, but it’s inefficient and suitable only for the most frugal applications. At 20% typical efficiency, PV cells can


generate about half a watt from an area of 35-40cm2


. Cells cost under $1 each when


purchased in medium to high quantities. Tey are less efficient indoors, but the latest PV harvesters may be sufficient for powering low-power radios. Piezoelectric harvesters are a tad more expensive, but generate substantially less energy.


operate with one harvester technology. This precludes using alternative harvester technologies if the chosen source cannot satisfy system demands. To adopt multiple harvesting technologies, a dedicated EH PMIC is needed for each, adding to system cost, size and power consumption, and making design work more time consuming and complicated. Often, external conditioning circuits are needed to make the energy harvester’s output compatible with the EH PMIC’s input. This problem can be addressed by


using recently-available smart EH PMICs. These manage the energy created by PV cells from ambient light, piezoelectric or electrodynamic devices that harvest energy from vibrations, or thermoelectric generators that convert a temperature difference into electricity. They work with harvesters that produce microwatts to milliwatts of power. Our UK company, Trameto, has


developed a family of smart EH PMICs called OptiJoule. Up to four harvesters of the same or mixed types can be connected in any combination to a four-input OptiJoule EH PMIC, without additional interface components.


www.electronicsworld.co.uk April 2023 19


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