- Power management technology must be matched to the application.
- Energy harvesting can replace battery or extend battery life.
- Power generation must be balanced with power consumption of the client device.
Energy-harvesting devices replace batteries in IoT sensors
In recent years, much attention has been placed on the Internet of Things (IoT) and machine to machine (M2M) markets and technologies. IoT and M2M refer not only to personal computers and mobile phones connected through the Internet but to the wireless interconnection of all of the billions of “things” and devices through the Internet or local area networks, to increase efficient utilization. With those billions of things come billions of batteries that must be purchased, maintained, and disposed of. Energy harvesting presents a straightforward solution for easily powering those remote devices using clean energy. Let’s take a closer look.
Wireless terminals equipped with sensors are included among the things and devices on the IoT. Wireless sensor terminals connected to a network will collect information about the environment surrounding the sensor terminal. Many types of sensors are used by wireless sensor terminals, including those for temperature, humidity, illumination, motion, pressure, stress, distortion, position, flow rate, and gas. The more numerous the sensor terminals placed, the greater the variety and accuracy the collected data will be. This information, often called big data, will help realize device control, monitoring, and prediction that were formerly unachievable, as well as new cloud-based services and businesses.
The development of IoT and M2M promises to have a great impact on our world, thanks to the evolution of semiconductor devices and the advancement of wireless technology. This is because device-level changes will enable equipment to become wireless, smaller, and more efficient. By mounting a battery, it will be possible to place equipment in various places without wiring.
The battery problem
As mentioned above, a key requirement for IoT and M2M is the ability to place wireless sensor terminals in all kinds of locations to collect data. But there is one big issue: the installation of power-distribution wires, or, in the case of battery use, the battery life or the time period for battery replacement. Nobody would find this a problem with 10 or 20 batteries, but when there are 10,000 or a million or a hundred million, there are concerns not only for battery costs but also the enormous scale of maintenance expenses. This is one reason the dissemination of wireless sensor terminals has become a concern.
Energy harvesting may provide a solution. Energy harvesting technologies use power generating elements such as solar cells, piezoelectric elements, and thermoelectric elements to convert light, vibration, and heat energy into electricity, then use that electricity efficiently.
These technologies can be produced now because semiconductors have achieved a balance between the improving performance of power generating elements and falling power consumption of active devices. This is the reason attention is being showered on this key technology that can solve the dissemination problem for wireless sensor terminals as part of the IoT.
Inside an energy harvesting terminal
A wireless sensor terminal consists of a sensor for sensing the surrounding environment, an MCU for processing collected data and for system control, and a wireless chip for performing wireless communications. A power IC matched to the power-generating element replaces the previous coin battery or dry cell battery (see figure 1).
The power-generating element must be selected after considering the type of energy to be collected from the surrounding environment, whether vibration, light, or heat. The most common elements used are solar, piezoelectric, or thermoelectric. It is also important that the power IC for use with the power-generating element efficiently collects the power from that element without loss, and that it supplies the stabilized power to a latter stage IC (see Figure 2).
Power generating elements
Let’s take a closer look at the generated power, output voltage, and generating environment for a solar cell, piezoelectric element and thermoelectric element (see Figure 3). The generated power for each element changes according to the size and generating environment. When integrating it into a device, it is necessary to comprehensively investigate the following:
- What kind of energy source will be obtained
- What size can actually fit on the device
- What balance between consumption power and generation power will be present in the device.
It is also necessary to select a power IC that is matched to the power generating element. In particular, the voltage/current/output characteristics (AC or DC) of the power generating element output will differ according to the element, and it is necessary to choose a power IC that will provide optimal results.
Wireless power requirements
Regarding the selection of wireless communications for a wireless sensor network terminal, the selection must be matched to the purpose (see figure 4), just as for the power generating element. Key aspects include the communication distance, the type of network to be built, the data transmission amount, the application, and the power consumption. When used in combination with energy harvesting, the key point is low power consumption, so wireless technologies in focus are EnOcean, ZigBee and Bluetooth LE.
Balancing generation with consumption
When using energy harvesting, there is a point to be considered—striking a balance between power generation and power consumption. This is because the device will not work if the power generation is smaller than the power to be consumed by the device. Although the generating characteristics of power generating elements are improving year by year, it is difficult to continuously deliver sufficient power to a device on an ongoing basis. A way to solve this is to collect the generated power in a capacitor and execute sensor operation at intervals, resulting in a method that balances the power generation with the power consumption.
To do this, the designer needs to have an accurate understanding of the generating environment for the power generating element, the power generated, and the time required, as well as the device power consumption and consumption time. Figure 5 illustrates the important points using power generating time, power collecting time, and power consumption time to solve the balance of power generation, power collection, and consumption.
Tools for energy harvesting development
In order to balance power generation and power consumption, designers need to calculate factors such as the power collection time for the power collecting element (capacitor) and the usable electric load, and thereby determine the optimal capacitor size. This operation requires trial and error, even when estimates of power generation and power consumption can be accurately calculated. In addition, when power generation and power consumption estimates are not accurate, it is necessary to calculate optimal values in each instance, or to confirm with an actual device. Spansion has prepared a web-based Easy DesignSim® tool that allows anyone to easily calculate and investigate energy harvesting. This can be used after a short registration process, so please try it.
Implementing the previously described development and investigation from step one would be considerably challenging. The Energy Harvesting Starter Kit simplifies and speeds the development of a wireless sensor terminal that uses energy harvesting (see Figure 6). The RF component operates at 2.4 GHz, and it includes an original protocol optimized for low power consumption. Those wishing to substitute the ZigBee or Bluetooth low energy wireless protocols can just change the RF component to the corresponding chip or module. The MCU is an FM3 MCU with a Spansion ARM Cortex M3 core base, so various customizations are possible if used in an ARM development environment.
Another starter kit allows the embedded designer to investigate using energy harvesting to drive a BLE Beacon. A solar cell or piezoelectric element can be connected as the power generating element, and the kit can also use a wireless power supply by AC input, USB power, or antenna connection.
Development of actual devices that use energy harvesting power ICs is moving forward in many locations and applications. In some cases, energy harvesting yields battery-free wireless sensor terminals. In other cases, using batteries and energy harvesting together extends battery life. In this way, there is an acceleration of the development of wireless sensor terminals that incorporate energy harvesting technology. In the coming years, we should see the placement of wireless sensor terminals having this technology in all manner of locations. Power management ICs designed for energy harvesting, as well as low-power MCUs, will help advance the growth of the Internet of Things.
Spansion®, the Spansion logo, Easy DesignSim®, and combinations thereof, are trademarks or registered trademarks of Spansion LLC in the United States and other countries. Other names used are for informational purposes only and may be trademarks of their respective owners.
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