Written by Bruno Damien, Ecosystem Marketing Director at e-peas.
Many IoT devices still rely on disposable batteries, which often become a weak point in their design. This reliance on batteries leads to recurring costs, whether it’s consumers replacing coin cells or facilities teams having to service sensors across multiple locations.
The EU estimates that millions, potentially tens of millions, of batteries from IoT devices are discarded daily, despite efforts to facilitate their collection. Legislation and positioning papers are being developed to address this issue. Energy harvesting offers a solution by eliminating the need for batteries, but it does require changes to the power architecture. Here’s how you can design for it.
A typical IoT system consists of a sensor or actuator, a low-power MCU, data transmission components, and supporting elements like memory, crystal oscillators, security features, and power.
Transitioning from disposable batteries to energy harvesting involves minimal changes, with the addition of three specific components to the design: the harvester, an energy store, and a PMIC, as illustrated in Figure 1.
Advancements in solar technologies, PMICs, and MPPT tracking algorithms have improved conversion efficiency in recent years. Energy storage materials have also seen progress, with increased power density and recyclability. Flexible and ultra-thin capacitors and PV cells are enabling new form factors that don’t have to accommodate traditional battery compartments, making the transition to energy harvesting easier.
While each project may vary, implementing a design for energy harvesting involves six standard steps, which we will explore below.
Step 1 — Assess the variable power budget
Energy harvesting is not limited to low-power systems, as the size of the storage element and harvester can be increased to support higher-power applications.
The first step is to conduct a power budget analysis to determine the average and peak power demands. This analysis helps in determining the required storage and harvester size for the load, or setting power constraints early in the development process. Tools like SiLabs Simplicity Studio Energy Profiler, Nordic Semiconductor’s NRF-PPK2, Qoitec’s Otii, and Undalogic’s mini SMUs have simplified the task of analyzing the energy budget of applications.
Although IoT devices are optimized for efficiency, it’s essential to review each component for possible efficiency improvements before transitioning to energy harvesting designs. Correcting unexpected behaviors and making simple efficiency improvements can yield significant benefits.
Step 2 — Select the optimal energy source
IoT and consumer devices are often suitable for small photovoltaic cell arrays, but other options like thermoelectric generators and piezoelectric transducers are available.
PV cells can be used for both outdoor and indoor applications, with the latest generation capable of transitioning between different lighting conditions. Thermoelectric generators are ideal for smart home systems with a temperature gradient, while piezoelectric generators are suitable for moving machinery. Other options include kinetic switches, which provide energy only when needed, such as in wireless light switches.
Implementing various MPPT algorithms is necessary for optimal energy extraction, although advanced PMICs can now handle this task directly.
Step 3 — Select the storage
Energy from harvesters can be directed to storage in two main categories: chemical (batteries) and electrostatic (capacitors), with hybrid options like lithium capacitors also available.
The choice of storage depends on product capabilities. Supercapacitors are easier to recycle and offer excellent endurance in charge/discharge cycling and temperature variations, while batteries provide higher energy density. Sodium-ion batteries can reduce eco-toxicity. Considerations like overdischarge threshold are important, with supercapacitors capable of going down to zero compared to rechargeable batteries with a threshold of around 3V.
The choice of storage is influenced by the ambient energy source, with the source voltage needing to be different from the storage voltage range. Maintaining a margin of >0.3V is recommended to ensure the boost converter can always provide the required voltage for charging, as shown in Figure 2.
Step 4 — Determine the supply voltage and current range
Understanding the minimum, maximum, and optimal supply voltages for the application circuit is crucial in an energy harvesting system. It’s also important to determine if a regulated output is required, as this will impact the PMIC and ripple voltage.
For supply current, knowledge of both maximum peak and idle-mode currents is essential.
Step 5 — Select the PMIC
In an energy-harvesting system, the PMIC plays a vital role in optimizing interactions with the source, connecting the energy storage system to power the load, and handling monitoring and control circuitry for functions like cold-start, wake-up, cell balancing, and storage protection.
Optional PMIC functions include supercapacitor cell balancing, storage element thermal protection, average power monitoring, and a 5V charger capability.
A wide range of PMIC options exist, tailored for specific energy source types, power ranges, storage technologies, and more. Dual-source PMICs are available to accommodate multiple energy sources, while high dynamic range PMICs can handle the full output range of modern PV cells for indoor-outdoor use.
Step 6 — Prototype
The industry’s growing ecosystem of interconnected partners makes it easier to prototype with evaluation boards. This allows testing of various energy sources, storage technologies, PMICs, MCUs, data transmission protocols, and other components on-board.
Switching from disposable batteries to energy harvesting for IoT device power has been shown to reduce OPEX for facility managers by over 80% and TCO by 30-50% in different use cases. Sustainable products can also command a premium in both B2C and B2B markets.
About the author
Bruno Damien is the Ecosystem Marketing Director at e-peas, a leading developer of power management ICs and MCUs for energy harvesting applications.
Filed Under: Communications, Featured
