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Energy Harvesting for IoT

Things talking to each other—the IoT (Internet of Things)—was science fiction a few decades ago. Today, it’s common in many areas of many countries. The communications take place over wires, via 4G and 5G cellular networks (and some older 3G systems, too), and over hybrid systems that combine the cables and broadcast networks, with a wireless router at the endpoint.

The sensors and intelligent modules that are the basis of the IoT require power to function, to communicate, and to gather data. In the current state, that means batteries or direct access to electrical power. The combination of connection and power source present barriers to expansion of the IoT. Naturally, if there are barriers, there are people, organizations, academies, and governments seeking ways around, over, or through those barriers.

The IoT connects and facilitates data exchange between a multitude of smart objects of various shapes and sizes—such as remote-controlled home security systems, self-driving cars equipped with sensors that detect obstacles on the road, temperature-controlled factory equipment, and even heart rate monitors implanted in the human body. This “hypernetwork” is projected to reach trillions of devices in the next decade, boosting the number of sensor nodes deployed in its platforms.

Current approaches used to power sensor nodes rely on battery technology, but batteries need regular replacement, which is costly and environmentally harmful over time. Unfortunately, the current global production of lithium for battery materials may not keep up with the increasing energy demand from the swelling number of sensors. And there is the political aspect of where the raw materials are mined and refined.

Wirelessly powered sensor nodes could help achieve a sustainable IoT by drawing energy from the environment using so-called energy harvesters, such as photovoltaic cells and RF (radio frequency) energy harvesters, among other technologies. Large-area electronics could be key in enabling these power sources.

LAE for Thee

Large-area electronics have recently emerged as an appealing alternative to conventional silicon-based technologies thanks to significant progress in solution-based processing, which has made devices and circuits easier to print on flexible, large-area substrates. They can be produced at low temperatures and on biodegradable substrates such as paper, which makes them more ecofriendly than their silicon-based counterparts.

A team from the KAUST (King Abdullah University of Science and Technology) in Saudi Arabia assessed the viability of various large-area electronic technologies and their potential to deliver ecofriendly, wirelessly powered IoT sensors. Over the years, the team has developed a range of RF electronic components, including metal-oxide and organic polymer-based semiconductor devices known as Schottky diodes. These devices are crucial components in wireless energy harvesters and ultimately dictate the performance and cost of the sensor nodes.

With RF energy harvesting, ambient radio waves from cellular towers are rectified via the Schottky diode. The resulting charge is stored in a capacitive element. By cascading a series of these elements, an RF input signal captured with an antenna can be converted to a significantly higher DC voltage which a sensor can then use. This means that sensor batteries need not be replaced, saving time and money as well as increasing sensor network reliability.

Some of the benefits of RF energy harvesting:

• Works in dark locations unlike photovoltaic cells
• Provides power on demand, even in mobile conditions
• Provides tracking
• Can work as secondary battery

Key contributions from the KAUST team include scalable methods for manufacturing RF diodes to harvest energy reaching the 5G/6G frequency range. Such technologies provide the needed building blocks toward a more sustainable way to power the billions of sensor nodes in the future. The team is also investigating the monolithic integration of these low-power devices with antenna and sensors to showcase their true potential.

Other types of energy harvesting techniques and energy harvesting materials include thermoelectric, photovoltaic, piezoelectric, pyroelectric, electromagnetic, wind energy, and even vibration energy harvesting. In today’s global market, the major energy harvesting method uses light.

Light energy harvesting is a technology that includes solar cells, photodiodes, and photoelectric devices as well as optical fiber sensors for converting heat to electricity in order to generate electrical power from sunlight or other sources of natural light such as firelight (firefly), candlelight, etc. Solar panels are the best example of light energy harvesting devices.

Light energy harvesting is used in wireless sensor networks to provide power for the nodes, and batteries are not required so it helps in extending the lifetime of sensors, which also reduces maintenance cost because there is no need to replace or recharge batteries regularly.

Digging Deeper

While we may think of the IoT in terms of building HVAC (heating, ventilation, and air conditioning) sensors, autonomous vehicles, and smart home appliances, one of the growing arenas for the IoT is in a medical context: wearables and implants. This is a particularly sensitive area for energy management and design as the IoT sensor must be small, reliable, and sustainable—a life may depend on it functioning.

During the early days of the COVID-19 pandemic, construction was an essential industry and workers were allowed to continue projects. Many companies sought ways to protect their workers on jobsites by employing wearable sensors that could monitor heart rate and breathing to create an early warning system for illness.

RF-enabled wearable and implantable wireless sensors are fast becoming a promising interdisciplinary research area in information technology. These sensors offer an attractive set of e-health applications, including medical and physiological monitoring of body temperature, respiration, heart rate, and blood pressure. However, due to their small size, such sensors might have a very limited battery-enabled power supply.

As frequent recharge or even sensor replacement is not a practical solution for all applications, energy harvesting can be used as a technology to prolong the battery lifetime of these sensors. Exploring auxiliary sources of energy could directly impact the everyday use of IoT sensors and significantly help their commercial applications in telemedicine and other industries.

Keep Moving

As an alternative to RF energy harvesting, the NIST (National Institute for Standards and Technology) intends to study the statistical characteristics of the harvestable kinetic energy generated from human motion. This knowledge could help researchers to design efficient energy management protocols for low-power wearable medical sensors. It can also boost patient engagement and satisfaction by allowing them to spend more time in the comfort of their home and only interacting with their care centers when needed.

The BAN (body area networking) is a technology that allows communication between ultra-small and ultra-low-power intelligent sensors/devices that are located on the human body surface or implanted inside the body. In addition, the wearable/implantable nodes can also communicate to a controller device located in the vicinity of the body. These radio-enabled sensors can be used to continuously gather a variety of important health and/or physiological data (i.e. information critical to providing care) wirelessly.

Radio-enabled implantable medical devices offer a revolutionary set of applications such as smart pills for precision drug delivery, intelligent endoscope capsules, glucose monitors, and eye pressure sensing systems. By using the RF medium, communications and energy harvesting/charging are combined into a single framework, allowing miniaturization and reliability in the nano-sensors of the future. Although, the technology to create miniature-size devices for these applications is within reach, there are still several technical challenges, including interference issues, reliability, energy efficiency, and security issues that need to be addressed.

Market research firm Dataintelo reports the global energy harvesting system for the wireless sensor network market was estimated at $1.06 billion in 2021 and is expected to grow at a CAGR (compound annual growth rate) of 14.3% during the period 2021-2030. The increasing demand for cost-effective and reliable power solutions has contributed significantly to the growth of this market over the past few years. Energy harvesting systems are now being used widely in various applications such as building and home automation, consumer electronics, industrial, security systems, etc.

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