Energy harvesting reaps energy that would otherwise go to waste—from sources ranging from solar rays to vibrations to ambient gradations in heat, ocean waves/tides, wind, and even the metabolisms of living organisms. These sources can be tuned to produce electricity that is safe, sustainable, and deployable in conditions not suited to the use of batteries.
Incorporating such energy harvesting into the Internet of Things (IoT) extends its reach to nearly any place on the globe—not just those that can access power lines or rechargeable batteries.
"Energy harvesting has the potential to expand the capabilities of the Internet of Things to heights previously unattainable without the technology," said ABI Research analyst Taylor Jensen, who is just putting the finishing touches on a study on energy harvesting for IoT applications.
Applications for IoT devices are expanding dramatically, targeting industries including automotive, aerospace, military, transportation, consumer electronics, industrial, buildings, home appliances, environmental monitoring, home automation, healthcare, and other increasingly "green" sectors. Ubiquitous communication standards such as Bluetooth, Wi-Fi, and cellular technologies have opened the door to IoT devices that monitor and manage widespread networks of sensors and actuators. With the addition of energy harvesting to power off-grid IoT devices, there are few corners of the Earth left that are out of reach.
Energy harvesting has matured through the use of a variety of environmental sources, including:
• Light (via photovoltaic cells).
• Wind (which drives turbines that generate electricity).
• Vibration (resonators drive transducers to generate electricity).
• Thermal (temperature differences converted to electrical energy by dipole oscillators)
• Radio Waves (electricity harvested from oscillating electromagnetic fields with antennas and diodes).
Today, IoT devices often depend on just one of the energy harvesting methods above; for example, solar cells work fine on sunny days, but not so well on cloudy days or at night. To compensate, the solar cells charge a battery during the day, then switch to battery back-up systems to provide power at night.
Yet batteries are not the long-term power solution for the IoT, according to Pat Pannuto, an assistant professor in the Department of Computer Science and Engineering at the University of California, San Diego.
"Every device with a battery is a device with a lifetime, and a short one at that," said Pannuto. "Today, we already average 10 or more connected devices per person in many parts of the world. Now imagine the transition from today's Internet of Things to tomorrow's Internet of Everything, a future with as many as a trillion connected devices; that is, hundreds of devices per person on the planet. We cannot become a world of battery-tenders; self-sufficient devices are fundamental to scale."
One answer available today has been demonstrated by Ji Li, a senior data and applied science manager at Microsoft. Li says 24/7/365 battery-free always-on IoT devices merely need to include multiple energy harvesters. Rather than using a single-source harvester that stores energy in a battery for times of darkness, no wind, etc., Li has demonstrated a method that combines several energy sources that together can provide the power requirements of an IoT device all day, every day, without the need for batteries.
According to Li, battery-free IoT devices require the use of machine learning (ML) to intelligently switch between multiple energy sources customized to provide the most efficient and most stable power supply for a particular application. For instance, managing the switch among thermal, kinetic, and photovoltaic energy sources with algorithms that compensate for the intermittent nature of each individual energy source allows ML to inform task-scheduler software that automatically switches among power sources.
"Batteries are the limiting factor in the lifetime of today's electronic devices. To extend service lifetime, devices must be able to obtain energy from their deployment context," said Pannuto. "That means scavenging energy from the nearby environment. As we push into more diverse environments, this also means scavenging from new and novel sources. In the future, intelligent instrumented systems will run on energy found wherever they are deployed. For instance, sensors embedded in concrete can harvest from the corrosion of the rebar. Sensors buried underground can harvest from electrogenic bacteria in the soil."
The use of energy from living organisms in the natural environment greatly boosts the energy available to power the IoT, according to Tian Li, an assistant professor of mechanical engineering at Purdue University. "Naturally occurring materials, such as cellulose—the main component in cotton and wood—are very promising for powering IoT devices," said Purdue's Li. "They are abundant, sustainable and cost-effective."
A recent proof-of-concept used living algae to harvest voltage differences produced by photosynthesis in plants, according to Christopher Howe, a professor in the Department of Biochemistry of the U.K.'s University of Cambridge. The amount of algae needed to power a typical in-the-field IoT device can fit in the space occupied by an AA battery, according to post-doctoral researcher Paolo Bombelli, also in the Department of Biochemistry of the University of Cambridge. An algae-based prototype there has been powering an IoT device continuously for nearly a year, said Bombelli, with longer-term trials underway.
Such proof-of-concept examples are just the beginning, according to Dong In Kim, director of the Energy Harvesting Communications Research Center at Sungkyunkwan University in South Korea. Kim specializes in harvesting energy from sources readily available "in the air," such as the RF waves emitted by television broadcast towers, Wi-Fi hubs, and even cellular telephone towers. Most of this energy is wasted today, since the number of receivers—TVs, portable computers, and smartphones, for instance—only occupy a tiny amount of space in the vast urban sprawls through which the RF energy is transmitted.
Even cosmic background radiation left over from the Big Bang is a potential source of energy, as it permeates the entire universe.
R. Colin Johnson is a Kyoto Prize Fellow who has worked as a technology journalist for two decades.
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