Energy harvesting breakthroughs needed to power an IoT future

Scientists are hard at work

Among the many challenges facing the Internet of Things (IoT) is the problem of power. How to keep millions of tiny devices running without having to replace the batteries when the charge runs out? How to ensure a heart monitor continues to function without the trauma of regular surgery?

Many of the consumer wearables being unveiled at CES 2017 would certainly benefit from such a reliable and trouble free source of power. The barriers to experimentation with battery technology are low (you can generate electricity with a potato and some copper wire after all) and the potential rewards for designing an economic, practical power source high, meaning there is no shortage of experimentation going on, from obscure university labs to Elon Musk's giant battery factories.

Another less visible area of innovation is energy harvesting. The holy grail is a device that is powered only by ambient electromagnetic energy, such as the fields emanating from power cables, or the WiFi and other radio frequency (RF) signals that permeate the atmosphere. Such a device would be entirely self-supporting, with great practical, economic and environmental benefits. Indeed, some argue that the IoT will not be realisable without some major advances in this area.

The last two or there years have seen movement in energy harvesting on the macro scale, to the extent that solar is now the cheapest form of electricity generation in 60 countries and wind cheaper than fossil fuels in many others, but work is ongoing at the micro end too, although it is a little way behind. For example scientists at Stanford University in the US have produced a prototype called HitchHike, a device that combines a radio and a processor. HitchHike uses a process called backscattering to incoming radio waves and re-transmitting data on a different WiFi channel.

By hitching a free ride in this way, the researchers claim the battery life of the WiFi device can be dramatically improved to levels associated with protocols such as ZigBee (ZigBee devices can typically operate for 10 years or more on a single button battery). In the best case scenario the researchers claim that WiFi devices might be entirely self-sufficient, requiring batteries only for emergency backup. The prototype currently has dimensions of a few millimeters, but a much smaller version suitable for powering medical implants should be possible, the scientists claim.

Several groups are working on medical implants powered by subcutaneous solar cells. A number of these prototypes were tested last year by Swiss researchers who confirmed the validity of energy harvesting by having such cells implanted beneath the human skin. Devices to measure the power output of the cells were worn by volunteers in their daily routine in summer, autumn and winter, with influences such as season, weather and human activity analysed.

"The obtained mean power over the whole study period was 67 µW, which is sufficient to power a cardiac pacemaker," the scientists report.

Solar cells are also being woven into clothes. Designer Tommy Hilfiger showed off a jacket featuring flexible solar panels last year. Going a step further, scientists in China have developed a solar cell textile that can be woven into cloth. The textile retains a power-generation efficiency close to 1 per cent even after been bent more than 200 times, and can be illuminated from both sides. In another development US researchers have developed a concept for a hybrid solar pv and thremal energy system to harvest both solar light and heat that they said has the potential to reach 50 per cent energy efficiency. The system converts that energy into electricity and stores it for future use.

Meanwhile, scientists at the University of Cambridge have developed an ultra-low power transistor design that could run for months or even years at a time on ambient radiation. The design is being touted for use in wearable devices or implantable electronics. The transistor harnesses a tiny "leakage' of electrical current, known as a near-off-state current, for its operations.

"This leak, like water dripping from a faulty tap, is a characteristic of all transistors, but this is the first time that it has been effectively captured and used functionally. The transistors can be produced at low temperatures and can be printed on almost any material, from glass and plastic to polyester and paper," claims the university.

Static electricity is another possible source of energy. Triboelectric nanogenerators (TENGs) generate electricity by harvesting the charge that build up when two materials repeatedly touch each other and then separate, exchanging electrons. Researchers are experimenting with using this energy in devices that to power small medical implants. Scientists have also created hybrid energy harvesters that use the TENG to harvest the motion caused by the wind combined with a solar cell to exploit the suns rays. Another TENG application is harvesting the energy from key presses on a smartphone touchscreen.

With so many promising avenues of research we expect to see a lot more developments in energy harvesting during 2017.