Can energy harvesting be used to power edge IoT devices?
01 September 2022
Sensors, actuators, hubs & routers are common categories of IoT devices, sometimes installed where it’s easy to make wired connections to the internet. But many networks use a combination of wired & wireless technologies – and sensors are often installed where wired connections may be totally impractical...
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Wireless communications then require battery power – and while the batteries themselves don’t usually represent a significant system cost, the expense of sending someone out to change them every few weeks, months or years is far greater. As a result, Huw Davies, CEO & co-founder at energy harvesting PMIC expert, Trameto explains here, designers strive to create self-sustaining sensors that are either battery-less – or whose operating life exceeds that of their original battery. This cuts lifetime system costs and is better for the environment, because it reduces battery waste.
Powering wireless links is a key challenge. For most IoT devices, the primary contributor to power consumption is its wireless link, not the microcontroller (MCU) usually found at the heart of each device. The link’s power consumption will vary with the power levels needed to achieve the required data throughput rate and range, and how often the data is transmitted.
Sometimes only short hops are needed to connect devices at the IoT network edge to hubs or routers. In other applications, it may be necessary to transmit directly from the sensor to a cellular radio mast over much greater distances.
The choice of wireless protocol will be determined by such factors, and because manufacturers of radio chips and modules don’t always know which will be needed, many chips and modules are now configurable across a range of protocols. Some can even run different protocols simultaneously.
For example, Nordic Semiconductor makes switched multiprotocol chips that combine Bluetooth Mesh, Thread and Zigbee. Silicon Labs offers a particularly wide selection of multiprotocol chips and modules, and wireless modules from Murata and u-Blox offer IoT designers a choice of single and multiprotocol products for both short- and long-range communications.
Figure 1. Typical power consumption (mW) of wireless links in IoT applications vs. data rate & link range [Courtesy of Voler Systems]
Increasingly, the opportunity to use energy harvesting to power wireless links will be a consideration factor in the choice of wireless protocol. Figure 1 compares the typical power consumption in milliwatts between various wireless protocols at different data rates and link ranges. The power requirement is anything from 150 microwatts to 400 milliwatts.
Energy harvesting considerations: harvester types & power management
At lower powers, a variety of micro-energy harvesting technologies can be deployed.
Micro-energy harvesting technologies include solar or indoor light (photovoltaic – PV) vibration, temperature gradient and electromagnetic (wireless).
The energy sources may be transient and rarely available when the sensor or other IoT device needs to send or receive data. The design goal may therefore be to complement primary batteries, so they don’t need changing so often, to deliver charge to secondary batteries or to eliminate batteries, using capacitors or supercapacitors as energy buffers.
A power management integrated circuit (PMIC) is needed to process the energy from the harvester, manage the charge delivered to the buffer and deliver power to the load when it’s needed. A PMIC designed specifically for energy harvesters is called an EH PMIC.
Figure 2. Outdoor solar PV generators deliver the highest power density
The economics of energy harvesting
Economics are still the primary driver of many engineering design decisions. Many IoT devices can run for months or sometimes years on a coin cell that costs a few cents or pennies – not much, in the scheme of things. However, if you consider the cost of driving miles to change a 20-cent battery occasionally, the cost to run a system over its operating life looks much different.
In many applications, there may be more than one potential source of harvestable energy. A piece of industrial plant covered in sensors may be exposed to sunlight or artificial light some of the time. It will often produce kinetic energy through vibration, and there may be temperature differences where thermal gradient harvesters can be attached to the machine. Efficiency is optimised when energy from all available harvesting sources can be collected and managed. Not all sources are equal, as shown in Figure 2.
In terms of energy density, outdoor solar leads the way, but in industrial settings, harvesting thermal energy and kinetic energy shows clear potential. RF energy harvesting looks unlikely to be useful for anything except powering the tiniest of energy-frugal devices.
When considering costs, solar cells of 35-40 square centimetres deliver around 0.5 Watts (at <20% efficiency) and are available for less than 1 USD each in volume. Piezoelectric harvesters are typically at least an order of magnitude more expensive and produce far less energy but, in the appropriate application, may still save the considerably greater expense of callouts to change batteries.
Advances in all the technologies mentioned above, and others, mean that micro-energy harvesting is now economically viable for powering growing numbers of wireless IoT devices.
Figure 3. OptiJoule EH PMIC technology is like a ‘multiprotocol’ approach to power management of micro-energy harvesters
Design considerations for power management of harvested micro-energy
An energy harvesting power management integrated circuit – the EH PMIC – is an essential component in the design of a micro-energy harvesting system.
One design challenge is that each type of energy harvester has different electrical characteristics. Photovoltaic and thermoelectric harvesters produce a continuous trickle of direct current at low voltage. Thermoelectric harvesters are low impedance, delivering a continuous trickle of bi-polar DC at a low voltage. Photovoltaic harvesters also produce a DC voltage, but their output current and impedance vary with the level of incident light.
Most EH PMICs are designed to connect to a single type of harvester. Most are also limited by their architecture and have fixed input voltage ranges, so can’t work with all harvester types. These two factors alone may make some self-sustaining edge IoT sensors uneconomic. Readily available energy may be wasted when the energy harvesting system is designed for a single harvester type. What’s more, if interface components are needed, as they often are, the cost, size and complexity can become prohibitive. Using multiple EH PMICs to capture the energy from a variety of harvesting sources results in the same problem.
Trameto’s EH PMIC technology (called ‘OptiJoule’) offers one way of addressing the problem. Trameto EH PMICs have inputs that autonomously adapt to whatever harvester is connected to them – light, thermal gradient or movement harvesters – without the requirement for external interface circuits. Versions are available with up to four inputs and any combination of harvesters – of the same or different types – can be connected to each PMIC. The device automatically maximises the power delivered to the buffer in response to the energy available from each source.
In the same way that RF module manufacturers often adopt multiprotocol radios in their devices, by designing with OptiJoule EH PMICs, engineers can use a single PMIC for multiple applications. This is particularly valuable when the energy harvesting source is not yet known. It’s also useful in applications where multiple harvested energy sources are available. There’s no longer the need for a separate PMIC for each energy source, reducing cost, complexity and system size.
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