Page 16 - PEN eBook October 2025

DESIGN DESIGN

applications, energy harvesting can be used through Many algorithms have been developed, each created source and the minimum operating voltage of the
increased storage element and harvester (for example, for specific applications, including ultra-high-accuracy chosen storage element. This is especially important
photovoltaic [PV] cell) size. systems based on artificial intelligence. However, when using supercapacitors, which can go down to
some algorithms can be energy-intensive to run, 0 V, and failure to introduce this margin can introduce
The first step, therefore, is determining if energy making them impractical for small energy harvesters. significant challenges, with the boost converter
harvesting is a suitable option, with the system power For energy harvesting, the key algorithm classes that potentially struggling to provide the necessary voltage
requirements considered against the energy availability. should be considered are constant voltage tracking; for charging.
Undertaking a power budget analysis will let you open-circuit ratio, which is also known as fractional
understand both the average and peak power demands open-circuit voltage; and constant impedance tracking. Determine the supply voltage and
to help you determine if it is feasible. current range
These will each favor specific energy sources. Energy harvesting can power a wide range of
There are several easily available guides that will talk Some PMICs (see below) will do the heavy lifting and applications, each with highly specific power supply
you through these steps. For example, e-peas has a incorporate these and define target applications. But if requirements. The two key parameters to meeting
free “How to analyze the power budget of an energy your selected PMIC does not, this sub-step is critical. these needs are voltage range and current range. Figure 3: Application circuit with energy harvesting and
harvesting system” walkthrough in the FAQ section of storage, connected to PMIC
its website. Select the storage It is therefore important to understand the
There are two main categories of storage element: minimum and maximum power supply voltages
These calculations can start from either the batteries (chemically stored energy) and capacitors for the application circuit and to identify voltages Optional functions include supercapacitor cell
load consumption, to calculate the power needed (electrostatically stored). From an environmental point that enable better performance or reduce power balancing, storage element thermal protection,
for the application, or from the source power, to of view, batteries pose a greater risk of damage, taking consumption. Similarly, it’s vital to determine if average power monitoring, and a 5-V charger
define power constraints before the application is over 100 years to decompose and leaching harmful the application requires a regulated output, as this capability.
developed. substances as they do. Supercapacitors, on the other will affect the PMIC selected, along with the ripple
hand, can be relatively easily recycled. voltage. Prototype the system
The calculations to do this are widely available At this point, it should be noted that several
(including on the e-peas website). However, the Batteries do have a higher energy density but cannot Likewise for supply current, it’s vital to determine evaluation boards are available that will assist in the
easiest way to undertake this analysis is through a be discharged below their overdischarge threshold the maximum peak current that the application will prototyping and testing of the system. These will
power profiling tool. Good examples include Silicon (typically 2 V). This is also true for hybrid lithium draw, with this having potential effects on the selected enable the testing of different MCUs, energy sources,
Labs’ Simplicity Studio Energy Profiler and Nordic capacitors, whereas supercapacitors can go down to regulated voltage output. Likewise, the idle current storage technologies, PMICs, data transmission
Semiconductor’s NRF-PPK2. zero. needs to be known, with special care needed to deliver protocols, and other on-board components.
high efficiencies in idle mode.
Select the optimal energy source Of course, the storage element will, in many cases, be REPLACING THE PRIMARY CELL IN A
As we touched on above, typical sources dictated by the specified ambient energy source, with Select the PMIC TEMPERATURE REGULATION SENSOR
of ambient energy that are suited to battery the energy source input voltage always needing to be A typical IoT system comprises a sensor or actuator, A good real-world example of switching out the
replacement applications include small PV cell lower than the storage element output. coupled with a low-power MCU, data transmission, primary battery is a temperature-sensing device for
arrays, thermoelectric generators, and piezoelectric and supporting elements such as memory, crystal regulating HVAC systems, with an IoT system using a
transducers that convert kinetic energy into Finally, it is good practice to ensure a sufficient oscillators for clocking, security features, and, of temperature sensor, Bluetooth LE wireless protocol,
electricity. voltage margin is created between the maximum course, power. energy-efficient MCU, and e-peas’s PMIC.
boosted output voltage range of the energy-harvesting
PV cells work well with bright sunlight but also IoT systems are typically already optimized for A power budget analysis of the proposed system has
under electric lighting. Thermoelectric generators efficiency, but this is even more vital in shown that a small, flexible, 44 × 30 × 0.15-mm PV
are also ideally suited for smart homes and use the energy-harvesting systems, and if upgrading from array weighing 0.2 g, alongside a 0.5-mm-thick lithium
stable temperature gradient found within them to batteries, all components within the system should be storage element, is enough to capture sufficient energy
be effective. Vibration transducers are a good option reassessed. from daylight to power it.
with moving machinery. And for applications such as
a wireless light switch, which needs power only when As per Figure 3, an energy-harvesting-based system In terms of size, the system, including PV array,
being used, a kinetic switch can be used. replaces the batteries with not only the energy source lithium battery, and circuit board with PMIC and
(e.g., PV cells) but an energy store (capacitors or application circuitry, can be brought together as a
Of course, ambient energy sources are typically rechargeable batteries), plus, crucially, a PMIC. stacked assembly to create a unit less than 4 mm high
not continuous, so it’s also vital to consider how to with the equivalent area of half a credit card.
buffer energy and allow for continuous usage. In an energy-harvesting system, the PMIC optimizes
interactions with the source and connects the energy In testing, once fully charged from PV and running
At this point, it should be mentioned that, in storage system and the main supply for powering with a 10-second sample rate, it was able to keep
tandem with selecting the energy source, a key the load. It also handles the monitoring and control running while kept in total darkness for over 12 days
sub-step is to select and implement the right circuitry to enable cold-start and wakeup as well (the logger’s memory became full after this point). We
maximum power-point tracking algorithms to ensure Figure 2: Example of a good and bad practice for selecting as cell balancing and storage element protection, also repeated this with a 10-minute sample rate, and
the source and storage combinations for a boost PMIC used
every possible joule is extracted from the source. in an ambient IoT system preventing overcharging/deep discharging. for this, it was still running after 24 days in the dark.

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