In recent years, the development of the Internet of Things has attracted a large amount of investment, especially in the field of machine-to-machine (M2M) interface technology and big data processing. The Internet of Things refers not only to the connection of personal computers and smartphones via the Internet, but also to the connection between billions of "objects" and devices. However, how to power these billions of devices is a challenge that design engineers are currently thinking about and needs to find effective solutions. Although traditional battery power supplies can solve power problems, they must go through procurement, maintenance, and post-processing procedures. Moreover, when the device is installed in a remote location, it is more difficult to maintain the power supply.
As an alternative, energy harvesting technology can directly power remote IoT devices and sensors with great performance. There are several viable technologies on the market, and most of them have already started to deploy applications. These include energy harvesting PMIC products and a large number of low-power microcontrollers that can meet the power needs of remote IoT devices, driving the growth of the Internet of Things.
With an energy harvesting solution, electronic systems can operate independently without traditional power supplies. However, no matter how convenient and flexible this solution is, energy harvesting technology has certain versatility and limitations. This is a viable solution, but it is by no means a simple solution. PMIC products and energy storage devices must be carefully selected from the perspective of power distribution. In addition, energy harvesting efficiency is also an important factor in the design of energy harvesting equipment.
This article describes the benefits, builds, and trends of energy harvesting, energy storage, and power management solutions.
What is energy harvesting?
Energy harvesting refers to the process of collecting a small amount of non-conventional energy that is readily available in the environment and converting it into electrical energy. The amount of electricity obtained can be used directly or stored for future use. For remote deployment devices that cannot use the local grid, energy harvesting solutions excel in providing alternative power to a wide range of electronic devices.
The energy collected can be derived from radio energy (RF source), vibrational kinetic energy of the piezoelectric element, pressure energy, or light energy of the photovoltaic cell. The collected energy is then converted to electrical energy and stored in a durable storage battery, such as a capacitor. Energy harvesting systems typically include circuitry for generating or harvesting energy, as well as storage devices with additional circuitry for power management and protection.
Energy harvesting technology applications are not limited to extending the battery life of IoT devices, but can also be used as an alternative power source for industrial, commercial, and medical applications such as wearable electronics, implantable devices, remote corrosion monitoring, and structural inspection.
Why is energy harvesting so important for IoT devices?
The development of the Internet of Things has become the most promising and profitable market opportunity. It is predicted that by 2020, there will be more than 30 billion IoT devices. In the near future, almost every device, from sensors, instruments, cars, wearable electronics, and embedded systems such as thermostats and refrigerators, will be connected to the Internet.
Ideally, these billions of small, portable devices will connect to the wireless network and have a long life. The battery seems to be a good choice, but installing a battery in a small device is usually not feasible. In addition, the cost of battery maintenance and replacement is not low. Considering that we need ample power, energy harvesting will be a viable solution to battery problems. In fact, energy harvesting can support electronic systems that operate on environmental power for several years.
Basic Building Blocks for Energy Harvesting The basic building blocks of an energy harvesting system typically include:
Transducer and conversion circuit: The transducer takes unconventional energy from different sources and converts it into electrical energy. Examples of typical transducers include: photovoltaic cell conversion light energy, thermoelectric device conversion thermal energy, piezoelectric element conversion vibration kinetic energy, and the like.
Energy storage devices: such as batteries and supercapacitors, can be used to store the electrical energy generated by the conversion.
Power Management Circuit: The power management circuit consists of a voltage regulator that performs power management according to the requirements of the system.
Figure 1: Basic building blocks of an energy harvesting system
Today's trends and technologies <br> As mentioned earlier, we can collect electrical energy from a variety of different non-traditional energy sources such as sunlight, RF signals and vibrational kinetic energy. Power conversion circuits, energy storage devices, and PMIC products are required for each type of energy harvesting.
Collecting solar energy: Small solar cells contain photovoltaic cells that convert light energy into electricity. However, for indoor applications, ambient light is usually not very intense and typically has a strength of about 10 μW/cm2. The energy collected by the indoor energy harvesting system is limited by the size of the solar module, ambient light intensity, and spectral composition. In general, the energy collected by a solar cell can be used to charge a battery or supercapacitor to provide a stable power source for the device. Today, such solar cells are widely used in consumer and industrial applications such as toys, watches, calculators, street lights, mobile power and satellites.
Collecting kinetic energy: Piezoelectric transducers generate electricity when they are vibrated and moved. Therefore, the device can convert the kinetic energy generated by the vibration into an AC voltage, and the AC voltage is adjusted to supply power to the system. There are many different ways to collect the energy of automatic energy. For example, the energy generated by a user pressing a remote control button can be collected for transmitting a low energy radio signal. Similarly, piezoelectric transducers mounted underneath the floor tiles can also generate electrical energy when someone passes by, and can power small displays or emergency lights.
Table 1: Comparison of performance parameters of different energy harvesting techniques
Collecting thermal energy: The working principle of the thermoelectric collector is based on the Seebeck effect, which generates a voltage based on the temperature difference between the junctions of two different conductors. Utilizing the electrical energy generated by temperature changes in the system, it is possible to operate systems that have been powered for several years, especially for low-power circuits. This technique is useful in recovering heat loss. The latest technological developments will use the body's heat to power the health sensors of the wearable device.
Development Tools <br> Since energy harvesting technology produces very little energy, it is important to maintain a balance between energy production and energy consumption in the system. Design engineers need to carefully evaluate energy requirements and select the appropriate components. Depending on the mode of operation, such as activation mode, sleep mode, etc., the energy requirements of IoT devices will vary. Tests are required during the design process or errors can occur, and sometimes detailed experiments are required. Therefore, using a development kit can help developers with early experiments and complete the initial prototype of the system.
Today, the industry that invests in IoT technology has cited a variety of IoT development kits. After using industry standard tools, energy production and consumption can be accurately calculated and evaluated. For example, Cypress/Spansion has launched a network-based Easy DesignSim design tool that makes it easy for all users to calculate and investigate energy harvesting.
Figure 2: Energy Harvesting Starter Kit, which includes the original wireless protocol optimized for minimum energy consumption and an FM3 microcontroller with an ARM Coretex M3 core.
Cypress's Energy Harvesting Starter Kit simplifies and accelerates the development of wireless sensor modules with energy harvesting technology. This low-power wireless protocol can replace low-power wireless protocols such as ZigBee or Bluetooth. The microcontroller in the kit is Cypress's FM3 microcontroller based on the Spansion ARM Cortex M3 core, which can be customized for different ARM developments.
Figure 3: The starter kit that uses energy harvesting technology to drive low-power Bluetooth beacons can be used with solar cells, piezoelectric components or other traditional accessories
Another energy harvesting starter kit helps developers build energy-efficient Bluetooth beacons using energy harvesting technology. Energy harvesting can be performed using solar cells or piezoelectric elements. In addition, the kit can be powered by USB power and comes with a wireless module with BLE low-power Bluetooth conversion.
Figure 4 IoT Development Starter Kit for Energy Harvesting
Technical Challenges <br> The biggest technical challenge and operational challenge for developers in designing an IoT device energy harvesting system is to find a viable energy storage solution. The original product was designed for power from non-rechargeable batteries due to its low cost and high availability and convenience. However, the energy resources of non-rechargeable batteries are limited and need to be replaced periodically. To solve this problem, manufacturers began to use rechargeable batteries as the main energy storage.
Today, rechargeable batteries such as nickel-cadmium batteries and lithium batteries have been used in IoT devices. Although these batteries are very convenient to use, they have a very high discharge rate, and each battery can only be charged and discharged about 500 times, which limits the long-term application of the battery in IoT applications. Therefore, finding solutions to improve battery technology is a key challenge for design engineers today.
In addition, designers need to overcome the major drawbacks of providing energy harvesting equipment. The conversion efficiency of a transducer used to convert non-conventional environmental energy into electrical energy is typically limited to 10%. In addition, circuits used to store and convert energy have energy losses. Together with all losses, the product can only get about 1% of the energy source. Therefore, designers need to perform very careful analysis and modeling to balance the available energy through energy harvesting and the power requirements of the circuit.
This article provides an overview of the latest technologies and trends in energy harvesting, energy storage, and power management solutions. With the energy harvesting PMIC product, the development of embedded electronics seems to be much simpler. In terms of power management, there are more and more viable power supply technologies that can supply power to various IoT devices such as home appliances, wearable devices, and electronic products. In the next few years, the technology that predicts the most development potential is wireless power transmission, thermoelectric technology and solar energy collection technology that powers IoT devices. In the future, we will see more PMIC products designed for energy harvesting, as well as low-power microcontrollers to drive the advancement of the Internet of Things. Technological advances will run through vertical markets such as the consumer, industrial and medical markets, creating new technology applications beyond imagination.
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