In recent years, many investments have been made in the Internet of Things (IoT), especially in machine-to-machine (M2M) interface technologies and big data processing. IoT comprises not only personal computers and smartphones connected through the Internet but connectivity with billions of “things” and devices. Powering those billions of things is a major drawback that design engineers are presently working on and trying to figure out efficient solutions. Though conventional energy sources like batteries can serve this purpose, they must be purchased, maintained, and disposed. In addition, when devices are installed in remote places, this adds to the hardship of maintaining power to these devices.
As an alternative, energy harvesting offers a profitable and straightforward means for powering remotely IoT devices and sensors. The market has already identified several possible technologies and has started to push investment towards their deployments. Power management ICs (PMICs) are being designed for harvesting energy, and a number of low-power MCUs that can operate at these levels to facilitate the growth of the Internet of Things.
Energy harvesting solutions allow electronic systems to operate in a standalone manner where there is no conventional power source. However convenient or how free this energy may be, harvesting energy has many facets and limitations. Energy harvesting is a workable solution, but not a simple or easy option for delivering power to IoT devices. Careful selection of power management ICs and energy storage devices with consideration of the power budget and harvesting efficiency are crucial in designing any energy harnessing devices.
Energy Harvesting: What Is It?
Harvesting energy is the process of capturing and converting small amounts of non-conventional power readily available within the environment, into electrical energy. This energy can be used directly or stored for future use. For remotely deployed devices that cannot access a local power grid, energy harvesting appears very useful in providing an alternative source of power to various electronic devices.
Energy can be harvested from radio energy (RF sources), captured from vibration, pressure by piezoelectric elements, or from light through photovoltaic cells. The energy generated is converted to electricity and stored in a durable storage cell like a capacitor. The energy harvesting system generally includes circuitry to produce or capture energy, and a storage device with some adjunct circuitry for power management and protection.
Energy harvesting technologies are not limited to only helping extend the battery life for IoT devices. They can also be used as an alternative power source in industrial, commercial, and medical applications like wearable electronics, implantable devices, remote corrosion monitoring, and structural monitoring, among others.
Why Is Harvesting Energy Important To IoT Devices?
The IoT has become one of the most promising and profitable market opportunities with a forecast of more than 30 billion connected devices by the year 2020. In the near future, almost every device—from sensors, instruments, vehicles, wearable electronics, and other embedded systems like thermostats and refrigerators—will be connected to the Internet.
Preferably those billions of things with tiny portable packages will be connected via wireless networks and have a long operating life. While batteries may seem an easy option, putting one within a tiny package in an inaccessible space is usually not a viable solution. Moreover, maintenance and battery replacement is not cost-effective. Considering the fact that we need adequate power, scavenging energy is an alternative approach that can address battery-related issues. In fact, energy harvesting can enable electronic systems to operate for years on ambient power sources.
Basic Building Blocks Of Energy Harvesting
The basic building blocks of an energy harvesting system typically consist of:
Transducer And Conversion Circuit: The transducer captures unconventional energy from different sources and transforms it into electrical energy. Typical transducer examples are photovoltaic cells to convert light, thermoelectric devices to convert heat, piezoelectric to convert vibration, and so on.
Energy Storage Device: Energy storage devices such as batteries and super capacitors are used to store the converted electrical energy.
Power Management Circuit: Power management circuits consist of a regulator which manages power as per the requirement of the system.
Present Trend And Technologies
As discussed, electrical energy can be harvested from different non-conventional energy sources like solar light, RF signals, and vibration. Each of these requires a form of power conversion circuit, energy storage devices, and a power management IC.
Harvesting Solar Energy: Small solar cells have photovoltaic cells that convert light into electrical energy. However, in the case of indoor applications, the ambient light is usually not very strong and the intensity is typically about 10 µW/cm². The power derived from indoor energy harvesting systems is limited by the size of the solar module, along with the intensity and spectral composition of the ambient light. Usually the electrical energy derived from the solar cells is used to charge a battery or supercapacitor to provide a stable power supply to the device. Today, these kinds of solar cells are extensively used in consumer and industrial applications including toys, watches, calculators, street lights, portable power supplies, and satellites.
Harvesting Kinetic Energy: Piezoelectric transducers produce electrical energy when subjected to vibration and movement. In this way, devices can convert the kinetic energy from vibrations into an AC voltage, which then can be rectified and regulated to provide power to the system. Energy can be harvesting from kinetic energy in a number of different ways. For example, a remote controller can collect the energy generated when a button is pushed by the user, and use this energy to send a low power radio signal. Similarly, piezoelectric transducers installed under the floor tiles can generate electrical energy from people walking on the floor that can be used to power small display systems or emergency lights.
Harvesting Thermal Energy: Thermoelectric energy harvesters work on the basis of the Seebeck effect. Here a voltage is produced depending on the temperature difference at the junction of two dissimilar conductors. This is done by using the electrical energy extracted from such thermal gradients inside the system, specially designed low-power circuits can operate for years. This technology is useful in reclaiming energy lost through heat. The latest technological developments may soon utilize human body heat to power wearable health sensors.
Tools For Development
Because of the small amounts of energy generated by these harvesting technologies, balancing power generation and power consumption in a system is of pivotal importance. Designers need to carefully estimate power requirements and select components accordingly. This operation involves trial and error, but sometimes detailed experimentation because an IoT device’s power requirements will vary based on its different modes of operation, such as active mode, sleep mode, etc. Having a development kit can help developers to perform early-stage experimentation and finalize the initial prototype of systems.
Industries investing in IoT technology have already introduced many IoT development kits. Estimates of power generation and power consumption can be accurately calculated using these industry standard tools.
Technical Challenges
The most crucial technical and practical challenge that developers face in constructing energy harvesting systems IoT devices is finding a viable energy storage solution. Initially products were designed to get power from non-rechargeable batteries considering their low cost, availability, and convenience. However, those energy sources were limited and needed to be replaced on a regular basis. To address this issue, OEMs started to use rechargeable batteries as an alternative primary energy storage.
Today, rechargeable batteries like Nickel Cadmium (NiCd) and Li-ion batteries are used in IoT devices. Despite being easily accessible, these batteries are known for their extremely high discharge rates and are limited by more or less 500 charge-discharge cycles per cell. This limits their long-term usefulness in IoT applications. Finding solutions for enhanced battery technologies is the pivotal challenge designers are facing today.
Besides this, designers are facing major drawbacks in increasing the efficiency of energy harvesting devices. The conversion efficiency of the transducers used for converting unconventional ambient power in electrical energy is typically limited to 10 percent. In addition, there are the energy losses in the circuits used for storing and converting the captured energy. Adding up all these losses, products would likely have access to about 1 percent of the energy source value. Therefore, designers need to do very careful analysis and modeling in order balance the available energy through harvesting energy and the circuit’s power needs.
Development of embedded electronic devices has seemingly become much more easy with the use of energy harvesting power management ICs. In terms of power management, there are a growing number of viable technologies available for powering IoT devices ranging from home appliances to wearable and flexible electronics. The most promising growth opportunities over the next several years are anticipated in wireless power transfer, thermoelectric, and solar harvesting technologies that can be used for powering IoT devices. In the coming years, we should see an increase in the availability of power management ICs designed for energy harvesting, as well as low-power MCUs, that will help advance the growth of the Internet of Things. This technological advancement will ripple throughout all the verticals of the market including consumer, industrial, and medical markets, creating new applications that we have yet to imagine.
