The usefulness of the “Internet of things” will depend on small sensors put just about anywhere to capture data about the environment or track products and people. To have the greatest impact, such sensors will need to work even without being plugged in or depending on batteries that have to be recharged or replaced. That’s why engineers are developing sensors that can get by with energy harvested from ambient sources, such as light, changes in temperature, and radio waves. Here are some examples.
The University of Washington’s Shyam Gollakota and his team have created a way of connecting devices to the Internet that consumes very little power and does not require batteries. The devices capture radio-frequency energy that is all around us from radio, TV, and cell phone networks. Gollakota and colleagues are not the first to harvest radio-frequency energy, but their technology goes a step further because it also can facilitate communications over Wi-Fi—which normally would require much more power than can be captured from ambient radio waves. They do this with technology they call Wi-Fi backscatter. They have developed tags with antennas that switch between reflecting and absorbing the ambient radio waves. In absorbing mode they gather power; in reflecting mode they can scatter the radio-frequency energy in ways that a nearby Wi-Fi router can detect. The tag uses fewer than 10 microwatts of power to communicate via these reflections, compared to the watt of power it could take to actually transmit Wi-Fi signals. Today, the tag can communicate over a range of two meters, but researchers hope to extend that to 20 meters. The researchers demonstrate the technology in this video and explain it in this paper.
Another group of University of Washington researchers has developed a sensing device that takes advantage of the gradual temperature changes in the environment. The sensors could monitor water leaks or structural issues in buildings and send wireless alerts. The key to making this technology work is an accordion-like bellows inside the device filled with a gas that expands or contracts based on the temperature; this movement is converted into electricity. The researchers demonstrated that a temperature change of just 0.25 °C is enough to power a sensor node, transmit data to a receiver more than five meters away, and update an e-ink display showing the temperature. The sensor is described in the paper “Powering Wireless Sensor Nodes with Ambient Temperature Changes” and shown in this video.
Antenna on Chip
Researchers from Stanford and the University of California, Berkeley, have made a self-sufficient radio the size of an ant. Its antenna harvests enough power from an incoming signal to execute the command embedded in it. At a cost of only a few pennies to make, these radios could give engineers an affordable way to connect almost any type of device to the Internet. The chip was described in a paper presented at the 2014 Symposium on VLSI Circuits.
Researchers at Georgia Tech are creating nanogenerators that can harvest energy from the electric charge produced when two different materials rub together. This means it could be possible to take advantage of the otherwise wasted mechanical energy that comes from blowing wind, waves, or walking, says Zhong Lin Wang, the professor leading the research. The researchers have demonstrated how generators can illuminate up to 1,000 LED lights, but the hope is to eventually power smartphones or enable sensors to power themselves to detect motion, vibrations, leaks, or explosions. Research on the generators has been described in several papers, including an article in Nature Communications in March that explains how the generator can charge electronics. Lin Wang explains the “triboelectric effect” produced by two different materials in this video.
Eventually, we might be able to run phones and tablets on harvested ambient energy, after such devices get reworked to require dramatically less power. But for now, energy harvesting is promising for only small sensors and radios. Today’s versions of the cell phones and laptops we use every day require orders of magnitude more power.
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