Many embedded systems that cannot connect to the main power supply typically rely on battery power. However, when the battery power runs out, replacing the batteries can be relatively costly and cause considerable inconvenience. By using energy harvesting technology to provide perpetual power to the system, this issue can be resolved. This article will introduce how to use energy harvesting technology to establish permanently operating embedded systems and the related solutions offered by Silicon Labs.
Pursuing the achievement of perpetual operation in energy harvesting systems
Energy harvesting technology is rapidly becoming a viable power option for embedded system designers, enabling wireless sensors to be used in applications where traditional battery-powered designs were previously unfeasible. For example, energy harvesting power sources allow system designers to easily construct ultra-slim wireless sensors with a range of over 100 meters and a lifespan exceeding 20 years.
The ultimate goal of energy harvesting systems is to achieve perpetual operation. An energy harvesting system can achieve perpetual operation by ensuring that the energy harvested meets or exceeds the energy consumed by the system during operation. Energy management is a critical aspect of designing an energy harvesting system. The first step is to determine the available power output of the harvester. Energy harvesters can convert solar, mechanical, or thermal energy into electrical energy. Solar harvesters have the highest power density, capable of harvesting 15 mW/cm² of surface area. Maximizing the power output of the energy harvester is crucial to building a robust energy harvesting system.
The most important aspect of designing an energy harvesting system is to provide sufficient functionality while minimizing the power consumption of the embedded system. By selecting components with low leakage specifications and using ultra-low-power microcontrollers (MCUs), such as Silicon Labs' Si10xx wireless MCUs, low power consumption can be achieved. Most of the techniques used to achieve low-power operation in battery-powered systems can be applied to minimize power consumption in energy harvesting systems.
Let’s consider an example of a solar-powered wireless sensor node that transmits data every 20 minutes at an average current of 10 µA. This system is equipped with a solar panel that can provide a continuous current of 50 µA during daylight hours. The net current available to charge the battery during the day is 40 µA, and at night, the battery discharges at a rate of 10 µA. As long as the system is exposed to at least 4.8 hours of sunlight each day, the energy harvesting system can achieve perpetual operation.
Balancing the average power of thin-film batteries in energy harvesting and consumption
There are two types of energy harvesting systems capable of achieving perpetual operation, each with different energy storage mechanisms. The first type requires long periods to harvests and accumulates energy, using low-leakage, high-capacity energy containers such as thin-film batteries. Perpetual operation is achieved by balancing the average harvested energy with the average power consumption. These energy harvesting systems are the most flexible and typically experience short bursts of high-power consumption. They remain in low-power sleep mode most of the time, always powered and continuously harvesting energy. An example of this type of system is a solar-powered wireless sensor node.
The second type of energy harvesting system remains unpowered until it detects an energy pulse, harvested the energy, and stored it in a low-impedance energy container (such as a capacitor). After briefly power-on reset, the system uses the limited energy collected from the pulse to perform necessary system functions. Perpetual operation is achieved by balancing the total energy consumed during task execution with the energy harvested from a single pulse. An example of this type of system is a wireless light switch, which uses energy generated by a mechanical switch to transmit an RF signal to a receiver located at the light fixture.
Conventional batteries, such as coin cells, AA lithium batteries, and lithium-thionyl chloride batteries, have been used for years in embedded systems requiring long lifespans. The introduction of thin-film batteries offers system designers a new option to balance cost, size, and safety. As developers continually face pressure to reduce system costs, economical coin cells may seem like the best solution for lowering manufacturing costs and bringing products to market quickly. However, replacing coin cells incurs hidden costs. If you consider that the total lifetime energy storage capacity of a thin-film battery exceeds that of thirty CR2032 coin cells, you quickly conclude that the initial cost of a thin-film battery is negligible compared to the cost of replacing a coin cells thirty times and exceeds the embedded system's lifecycle several times over.
When considering battery size, thin-film batteries have the thinnest profile of all battery types (as small as 0.17 mm). The total lifetime capacity of a thin-film battery is equivalent to four lithium "AA" batteries or a single "C" size lithium-thionyl chloride battery, making thin-film batteries ideal for space-constrained embedded systems requiring an ultra-thin profile and long battery life.
Additionally, thin-film batteries do not present the safety hazards associated with larger conventional batteries, such as flammability and explosion risks. Because thin-film batteries are rechargeable, they store only a portion of their total lifetime capacity at any given time, making them safer in the event of an accidental short circuit or exposure to extreme heat or an open flame. Thin-film batteries also produce significantly less waste than larger conventional batteries, which often end up in landfills rather than being recycled.
Energy harvesting reference design accelerates product development
Power consumption has always been a critical issue affecting the operation of battery-powered IoT devices. Various organizations behind wireless standards are dedicated to helping meet consumer expectations for reducing the power consumption of devices in this field. Zigbee Green Power is a great example of considering energy harvesting in wireless communication design.
Silicon Labs and Arrow Electronics have jointly developed an energy harvesting reference design based on Silicon Labs' EFR32MG22 System-on-Chip (SoC). This design pairs a Zigbee Green Power light switch with energy harvesting power management. The MG22 is designed for the Zigbee protocol, is compact in size, and features advanced security functions, making it an ideal choice for ultra-low power end devices. Silicon Labs also offers energy-efficient power management ICs, such as the EFP0111, to provide better power management capabilities. Additionally, Silicon Labs provides MCUs, wireless starter kits, and Simplicity Studio, a powerful development and debugging environment, to help customers quickly develop energy harvesting systems.
The core element of this design is the energy harvesting generator, and this reference design uses ZF's monostable generator module. This is a bi-directional switch generator, meaning that energy is generated both when the switch is pushed down and released. The switch has a magnet with two poles, and pushing down on the switch generates a magnetic field that passes through the core and back to the other pole. Then, when the user releases the switch, the magnetic field changes and passes through the core in the opposite direction. This changing magnetic field generates a current, which is the energy that can be harvested. When the ZF generator is pressed or released, it generates an AC voltage, and the system can use this mechanical energy to turn on a light. The ultimate goal is to be able to switch the light on and off without wiring between the switch and the light fixture.
Powering IoT devices is an energy-intensive task. Innovating new methods of powering devices without batteries will simplify development and help create a cleaner environment. For example, the energy required to make an LED blink once is enough to transmit multiple RF signals. The combination of low-power silicon designs and networks optimized for low-power applications will lay the foundation for a new era of power management, reducing significant costs and waste for manufacturers and consumers.
High-performance, low-power power management solutions
Silicon Labs has introduced the EFR32MG22 (MG22) series SoCs, optimized Zigbee solutions that bring industry-leading energy efficiency to IoT applications such as smart home sensors, lighting controls, and building and industrial automation.
The EFR32MG22 and EFR32MG22E Zigbee SoC solutions are part of the Wireless Gecko Series 2 platform. The MG22 family offers an optimized Zigbee SoC solution, integrating a high-performance, low-power 76.8 MHz ARM® Cortex®-M33 core with TrustZone. MG22 enables you to create energy-efficient applications, while the MG22E (“E” for Energy Conservation) further enhances energy-saving advantages by extending battery life and supporting completely battery-free designs. The MG22 SoC combines ultra-low transmit and receive power (+6 dBm at 8.2 mA TX, 3.9 mA RX), 1.4 µA deep sleep mode power, and low-power peripherals, providing an industry-leading energy-saving solution for Zigbee protocol applications, including Green Power.
Silicon Labs’ EFP0111GM20 Energy-Friendly Power Management IC (PMIC) is a flexible, highly efficient, multi-output power management IC that provides complete system power for EFR32 and EFM32 devices, with three output voltage rails and primary cell battery Coulomb counting capabilities. The EFP0111 boost Bootstrap PMIC can operates within a voltage range of 1.7 to 5.2, with a quiescent current as low as 150 nA. The EFP0111GM20 supports a wide range of batteries from 1.5 to 5.5 volts, offering flexibility for different battery technologies while enhancing the power efficiency of EFR32 and EFM32.
Silicon Labs’ Si10xx Sub-GHz Wireless MCUs combine high-performance wireless connectivity technology with ultra-low-power microcontroller processing in a compact 5 x 6 mm form factor. The devices support frequency bands ranging from 142 to 1050 MHz, including an integrated advanced packet handling engine and a link budget capability of up to 146 dB. The devices are optimized by reducing TX, RX, active, and sleep mode currents, and supporting fast wake-up times, lowering energy consumption for battery-powered applications. The Si106x MCU is pin-compatible with Si108x devices, with flash capacity scale from 8 to 64 kB, and robust analog and digital peripherals, including ADC, dual comparators, timers, and GPIO. All devices are designed to comply with the 802.15.4g smart metering standard and support worldwide regulatory standards, including FCC, ETSI, and ARIB specifications.
Conclusion
Energy harvesting technology has become quite popular and is expected to become even more widespread in the coming years due to the many benefits it offers for embedded system design. A properly designed energy harvesting system, once it overcomes the initial power-on reset, can operate indefinitely. With careful system design, the lifespan of an energy harvesting system can be extended to over 20 years. Thin-film batteries are commonly used in energy harvesting systems due to their ultra-thin profile and low leakage characteristics. The ability to design self-sustaining embedded systems without the need for a main power supply or conventional replaceable batteries opens up new possibilities for applications and paves the way for new areas in embedded system development. The MG22 series Zigbee SoC solutions, EFP0111GM20 energy-efficient power management IC, and Si10xx Sub-GHz wireless MCU introduced by Silicon Labs can provide excellent power consumption control for energy harvesting systems, ensuring long-term operation of embedded systems without the hassle of battery replacement.