Harnessing Sunlight: How a Tiny Panel Woke Up the Cortex-M — Part 1

February 14, 2024

Blog

In a world where technology plays an increasingly important role, the search for sustainable and compact energy solutions becomes more vital. Here, we decided to explore one promising aspect of this quest: the potential of miniature solar panels to power Internet of Things (IoT) devices.

Currently, quite a few solutions on the market are positioned as compact IoT devices (which means they use only case-integrated solar panels, not external ones) capable of autonomously operating using only solar energy. However, the functional capabilities of such devices are often limited to operating primarily in power-saving mode with short data transmissions at long intervals in between. Also, such devices use specifically developed, non-standard solar panels to fit the form factor and power needs.

We decided to look a bit further and check whether the standard and mass-produced PV (photovoltaic) panels, already known for their size/efficiency ratio, could be the key to continuously powering more complex devices without overcomplicating the device development process.

Our findings were enlightening. The study showed that under the right conditions and with the proper setup of the IoT device operating algorithms, these panels have the potential to fully power such devices even in short-term high-performance modes. Moreover, the adaptability of the panels for indoor settings, where sunlight is limited, opened new doors for their application.

Approach to Experiments

We stuck to the basic idea of using one solar panel for one IoT device. Of course, more than one solar panel can be used in one device, or solar panels of different sizes can be externalized, and so on. However, our interest lies in ensuring that the device's casing is as minimalist as possible. In perfect conditions, the form factor of the device casing should be limited to the dimensions of the board on which the device operates.

This is why we focused our research on the capabilities of specific small-scale photovoltaic panels — Voltaic and Epishine — to find out if they could effectively power Cortex-M development kits, particularly how efficiently and extensively we could use the full specter of capabilities of such devices.

Selection of Test Components

Today's market offers a large number of PV panels for IoT needs. In our equipment choice, we followed the most straightforward principles - price, declared characteristics, and availability for purchase in large quantities. Simply because these are the main criteria by which equipment is usually chosen for manufacturing the MVP of a future IoT device.

PV Panels

In research, we focused on two distinct photovoltaic panels. Each brings unique attributes to the table: Epishine excels in low-light indoor settings, while Voltaic panels are more suited to outdoor environments. This comparison isn't about superiority, but rather showcasing how each panel's features contribute to efficient IoT operation in different scenarios.

Epishine Light Energy Harvesting Module:

The Epishine Light Energy Harvesting Module was chosen for our study due to its ability to collect energy efficiently in low-light indoor conditions, and we were interested in checking how these declared capabilities correspond to reality. The vendor describes this module as optimized for operation at illumination levels ranging from 20 to 1000 lux and offers adjustable output voltage ranging from 1.8V to 3.3V, allowing precise voltage adjustment to the requirements of a specific IoT device. The declared output current of up to 300mA should provide stable power even under high peak loads.

The module includes the GA230F 400mF CAP-XX supercapacitor, which stores electrical energy ranging from 1.9Ws at 3.3V output voltage to 3.4Ws at 1.8V, allowing the module to maintain IoT device operation during periods of low illumination. Interestingly, this module can be used independently or in combination with a primary battery voltage ranging from 1.2V to 5.5V, ensuring continuous operation of the device even when the energy stored in the supercapacitor is exhausted.

The integration of the energy harvester IC AEM10941 e-Peas is claimed to maximize the efficiency of light energy collection and distribution. At the same time, the output DC regulator TPS62740 should ensure the stability and reliability of the output voltage, which is exactly what we need for power-sensitive IoT devices with fluctuating energy consumption.

Voltaic PV Panels:

We selected the Voltaic P121 R1L and P122 R1J PV panels for their specific characteristics adapted to various lighting conditions. The P121 R1L panel has a maximum power of 0.3 W, a voltage of 5.9 V, and a current of 60 mA, making it effective for outdoor use, especially in intense sunlight. The P122 R1J model, with a maximum power of 0.32 W and a voltage of 2.3 V, was declared to provide a current of 150 mA.

To enhance the functionality of Voltaic panels, especially in comparison with the Epishine module that already includes a supercapacitor, we also included the Voltaic C116 Lithium Ion Capacitor Charger in our selection.

This device incorporates the Vinatech 250F VEL13353R8257G capacitor, providing an output from 2.5 to 3.8 V. It is equipped with a linear regulator and is designed for low-current consumption devices, making it suitable for working with Voltaic panels. This combination was chosen to create approximately equal experimental conditions for solutions from both vendors.

Development Kits

In our study, we examine the capabilities of photovoltaic panels using two distinct development kits: the Nordic NRF52832 and the Atmosic ATM3202. It's important to note that these boards are not intended to be compared directly against each other. Each board has its unique processor and architecture, with inherent strengths and weaknesses. The NRF52832 DK, with its ARM Cortex-M4 processor, contrasts with the ATM3202's advanced low-power design. Our goal is not to put these boards in competition but to evaluate the real possibilities and possible versatility of PV panels in powering diverse IoT devices.

Nordic NRF52832 DK:

We chose the Nordic NRF52832 DK development kit for our experiments for the following reasons:

• Energy Consumption: The NRF52832 DK is characterized by its low power consumption, making it ideal for IoT applications requiring extended autonomous operation. It consumes just a few microamperes in standby mode and up to 5.3 mA during active operation.
• Processor and Wireless Communication: With a 32-bit ARM Cortex-M4 processor and Bluetooth 5 support, it provides the necessary computational power and wireless communication for modern IoT devices, including automation systems, smart sensors, and more.
• Power Flexibility: The development kit supports a range of power supply voltages from 1.7 V to 3.6 V, allowing it to be used with various energy sources, such as solar panels and batteries.

These characteristics make the NRF52832 DK suitable for tour testing. Low power consumption, high computational power, and flexibility in power supply were key factors in our choice.

Atmosic ATM3202 Development Kit:

The Atmosic ATM3202 Development Kit was selected for our experiments due to its:

• Energy Efficiency: The ATM3202 stands out for its ultra-low power consumption, a key attribute for IoT devices in energy-scarce environments. It operates on minimal power, utilizing just a few nanoamperes in sleep mode and around 3 mA in active mode.
• Advanced Processor and Connectivity: Equipped with a high-performance processor and supporting Bluetooth 5.0, the ATM3202 offers good computational capabilities and wireless connectivity, suitable for diverse IoT applications like health monitors and smart home devices.
• Power Versatility: This development kit supports a wide voltage range from 1.8 V to 3.3 V, compatible with various power sources, including solar panels.

These attributes make the ATM3202 an ideal candidate for our research. Its exceptional energy efficiency and adaptable power requirements align perfectly with the demands of modern IoT devices powered by solar energy.