Transfer of power

Distributed sensors and portable devices could soon harness energy directly from the environment, rather than relying on batteries. Lou Reade reports.

Any mobile electronic device – from a phone to an environmental sensor – requires a power supply. And while modern batteries are sophisticated, they are not infallible. Who, for instance, can say that their phone has never died on them?

One solution could be to harvest energy from the surroundings, or to convert ‘waste’ energy into electrical power. Heat exchangers are well known for converting waste heat into usable energy, but now a range of ‘energy harvesting’ devices could gather energy from a far wider range of sources. It is the subject of numerous research projects. Recently, scientists have used everything from body heat to stray electromagnetic radiation as an energy source. Although the amount of energy involved is generally low, it can be enough to keep devices charged.

If commercialised, these techniques could have widespread implications: mobile phones could be charged by the simple act of walking; medical sensors could be powered by bodily functions such as the heartbeat; and environmental sensors might last almost indefinitely, as they would be constantly replenished with power. For now, however, research efforts are focused on ramping up the technologies that underpin these systems, including conductive polymers, liquid-metal alloys and organic semiconductors.

Liquid flexibility

Wearable technology is most often associated with clothing; electronic functionality added to fibres, for instance, or electronic devices embedded into clothes. However, attaching unobtrusive devices directly to the body could take the concept further.

Scientists at Purdue University in the US are using liquid-metal alloys to create devices that convert mechanical energy into electrical energy (Journal of Materials Chemistry A; doi: 10.1039/c9ta01249d). The liquid nature of the materials means they can be incorporated into highly flexible devices – and attached comfortably to the skin. The devices use the triboelectric effect to generate power. This relies on the fact the electrons can move naturally between dissimilar materials every time they are brought into contact – the same way that static electricity is generated.

The triboelectric effect harnesses static electricity on a microscopic scale. When the two dissimilar materials are carefully chosen – and the device is properly designed – the electron transfer can be harnessed as energy. Devices that use this principle are known as triboelectric generators (TEGs). ‘Using TEGs to harvest energy has been around for a few years, but the issue has been designing and optimising device performance,’ says Wenzhuo Wu, who leads the Purdue team.

One issue is to increase surface charge density, he says. This can be done by raising the dielectric constant of the material, which is often achieved by incorporating a polymer additive. However, this tends to reduce device flexibility. Wu’s team has used an alloy of indium and gallium – a liquid at room temperature – to modify the surface properties. It is highly conductive, which raises the dielectric constant, as well as being deformable and flexible.

‘We use a simple process to add small droplets of alloy to the polymer substrate, to make a composite of the two materials,’ he says.

The alloy forms a thin oxide layer on its surface, giving each droplet a protective outer ‘skin’. Wu likens the structure to a balloon filled with water – meaning it can flex its shape, but contains the liquid inside the outer ‘shell’.

This composite layer is sandwiched between two layers of another polymer. Vibration or movement from the body causes the different layers to touch one another and generate power through the triboelectric effect. Wu’s team has created a flexible polymer device that attaches to a finger. When the finger is moved, the mechanical energy is converted to electrical energy. This has been put to use as a self-powered user interface, to adjust music volume. The system has currently been tested indoors – but in future could be more mobile.

‘If you were out jogging, you could use this to control the volume of your iPod or phone without taking it out and adjusting it,’ he says.

Here, the effect is being used to run a user interface without the need for external power. However, generating enough to charge a mobile device, for instance, would require more research. ‘A lot of work will be needed to improve the energy harvesting performance, if we want to do that,’ he says.

Heat generator

Meanwhile, researchers at the University of Massachusetts Amherst, US, have used a thin layer of conductive polymer to create coated fabrics that efficiently convert body heat into electrical energy. Although they generate very low power, it would be enough to ‘trickle charge’ a mobile phone and keep it running for two days, according to chemistry professor Trisha Andrew, who leads the research. ‘You’re not charging from 0 to 100%, but instead you’re sustaining charge at around 70%,’ she says. ‘We’re getting around 10 micro Watts, which is also enough for a FitBit, or for glucose sensors.’

The underlying principle is called the thermoelectric effect. It relies on materials that have high electrical conductivity but low thermal conductivity. In this way, different sides of the material will be at different temperatures. Charges within the material migrate to the cold side. This generates a potential difference that can be harnessed when connected in a circuit. The effect is used to gather waste heat from power plants, where temperature gradients of up to 500°C can generate hundreds of watts of power. ‘We’re getting much less than that, but we don’t need as much,’ says Andrew.

Her team has used vapour deposition to coat fabrics like cotton with a conductive polymer called PEDOT-Cl (Advanced Materials Technologies; doi: 10.1002/admt.201800615). This imbues a fabric with enough conductivity to generate these small amounts of power. Andrew says the coated fabric is a practical way of incorporating thermoelectric generators into wearables. Traditional devices are made from alloys of tellurium, which is expensive, of limited availability and toxic. ‘You can’t put tellurium into a wearable device,’ she says. ‘The obvious answer is to use polymers.’

One happy accident of the research was finding that sweat – usually a killer for this type of device – actually made the effect stronger. This was because the moisture carried the heat of the body to the device more effectively. ‘It turned a disadvantage into a benefit,’ she says.

So far, the team has made a hat, an arm sleeve and a wristband using the material. Rather than charging devices directly, the output could also be used to charge a supercapacitor, such as those found in car key fobs or solar calculators, she says. As well as being used to trickle-charge mobile devices, she says the technology could also provide power for medical sensors.

Mobile power

While mobile devices are potential users of harvested energy, they could also act as the source. This is because they produce stray radio waves that could be collected and used to run low-power devices such as environmental sensors.

There are many wireless bands, operating at different frequencies. The Global System for Mobile Communications (GSM), for instance – used for mobile phones and other devices – operates at various frequencies between 380 and 1900MHz. There are also several WiFi frequencies in the GHz range. ‘We are surrounded with this wireless energy,’ says Atif Shamim, professor of electrical engineering at King Abdullah University of Science and Technology (Kaust) in Saudi Arabia. The advent of the ‘Internet of Things’ will see many more electronic devices imbued with wireless sensors that communicate over the internet, he says. ‘We expect billions of these sensors to be in operation. Our goal is to see if we can power them from the environment.’

Even the tiniest sensors need power. Incorporating a miniature ‘antenna’ into their structure will help them run on stray electromagnetic radiation, Shamim says, rather than requiring batteries. His team has developed a special structure of antenna that can harvest energy from different wireless bands at the same time (Nano Energy; doi: org/10.1016/j.nanoen.2018.09.022). Ordinarily, this type of device can only tap a single wavelength, he says.

First, the researchers used 3D printing to produce a series of square plastic substrates, then inkjet printed a ‘fractal’ antenna on the surface of each one, in silver. Fractals are ever-repeating patterns that are commonly found in nature. The shape of the antennae set up multiple resonances, allowing them to gather energy from wider parts of the spectrum. Joining five separate squares together into a cube shape enhanced this further.

‘We want to make the device low cost, which is why we’ve combined 3D printing and inkjet printing,’ Shamim says.

During a test, the team noticed that power levels jumped when somebody was actually using a mobile phone. Signals were also stronger near wireless routers. ‘Power levels will also increase when we move to 5G technology,’ he says, adding that future wireless applications such as unmanned aerial vehicles (UAVs) and autonomous cars will also provide more ‘free’ energy.
He says that ‘large area monitoring’ is a key potential use of this type of technology. It would allow cheap, disposable sensors – to detect forest fires, for instance – to be distributed widely across an area, secure in the knowledge that they would continue to be powered.
One potential argument he foresees in the future is whether this energy is actually ‘free’. Although it is ‘stray’ radiation, there is also a risk that harvesting it could reduce the quality of the mobile signal, though no studies have been done on this. ‘As long as we don’t affect the signal, there’s no problem – and there is lots of energy out there,’ he says.

Indoor harvest

Energy harvesting is not restricted to remotely located devices. Researchers in Japan have developed polymer-based photovoltaic cells that gather ambient indoor light – from lamps and lightbulbs – to power small electronic devices. The idea is not as crazy as it sounds, because many ‘home-based’ devices are battery-powered. And, while replacing these batteries is simple, the ongoing cost of doing this – and the extra weight – is something that could be avoided.

The photovoltaic cells, developed at Kyushu University, Japan, have been engineered to work more efficiently under artificial light than in natural sunlight. The researchers say they are around 16% efficient indoors – and around 6% outdoors. However, it is not really existing devices that will benefit, but those that are being developed for the future. ‘In the coming era of the Internet of Things, where information and communication systems are invisibly embedded in our living environments, wireless sensor network technologies will play a pivotal role,’ the researchers report.

They argue that it will be impractical to use batteries to power the growing number of sensors that will be incorporated into wireless network devices. In the same way, it is not feasible to power remote environmental sensors with batteries, they say.

The researchers tested a number of small-molecule organic semiconductors that had promising characteristics for harvesting ambient light (ACS Applied Materials and Interfaces; doi: 10.1021/acsami.9b00018). Those based on a molecule called BDT-2T-ID outperformed other similar devices, including one based on silicon. A set of six of these devices, connected in series, could produce ca 4V under dim light conditions, which would be enough to power microsensors. However, more work needs to be done to scale up the technology, say the researchers.

Mobile devices are always hungry for power, while an emerging generation of distributed sensors will have to operate without individual batteries. Even though solar energy is a useful ‘free’ resource, some of these emerging techniques could end up offering new sources of battery-free power.