THURSDAY, 22 APRIL 2021Imagine the climax of a superhero film. The villain plans to drain the planet of all of its energy to fuel their terrible scheme, and society as we know it collapses. At least, that is what it might sound like when someone hears the term ‘energy harvesting’ for the first time. Actually, energy harvesting is the extraction of small amounts of untapped energy from every day processes that would otherwise be lost, and is done by an array of new technologies. This energy is then stored and used in low-power applications. This can help to replace bulky, wasteful batteries while still powering our lives. As it turns out, not only is energy harvesting currently used over a much smaller scale than an evil plot, it may actually benefit the planet. Here, I will uncover the hidden world of energy harvesting through the unusual mechanisms of piezoelectricity, triboelectricity, and thermoelectricity.
Piezoelectricity: power beneath your feet | Piezoelectric materials generate a charge in response to mechanical stress, whether this is from a vibration in a building, stamping with your shoes or just bending small fibres. The reverse also works — applying a voltage can deform a piezoelectric material. Piezoelectricity was first demonstrated in 1880 by Jacques and Pierre Curie, who used crystals, such as quartz, tourmaline and topaz. It was later discovered that this piezoelectric effect arises from asymmetry within the crystal structure resulting from pressure. As a result, positive and negative ions in the material separate in response to the applied force, creating an overall charge.
Piezoelectric materials are used in a range of applications, such as in sensors, the ignition in electric cigarette lighters, as a time reference in quartz watches, and high-resolution microscopy. Currently, these are small-scale uses, but research is being done to adapt these materials for much larger scale applications.
Two graduate students from MIT’s Department of Architecture proposed the Crowd Farm, a project that incorporates piezoelectric elements into a network of floor tiles such that human steps can power lights in public places. One step can power two 60 W light bulbs for one second; around 30,000 steps can power a train for one second, and 84 million steps can theoretically power the launch of a space shuttle. With the average person in the UK taking 3,000 to 4,000 steps per day, this is a huge untapped resource. Another example is the Shibuya train station in Tokyo, Japan, where the floor tiles of the station have incorporated piezoelectric elements. Every time a person steps on one of these tiles, a message lights up on the station wall, and an LED board updates how much power has been generated that day. There are two main challenges these applications face: energy transfer must be efficient because steps are taken quickly, and the actual power generated by each step can be quite low, at just 0.1 W. However, with an estimated 2.4 million people passing through the station every day, a large amount of energy can be generated. Therefore, while piezoelectric materials are traditionally used for low-power energy harvesting, these devices can be scaled up to solve much larger problems.
Triboelectricity: giving your electronics a jump(er) start |
Most people have done the trick of rubbing a balloon on their jumpers to stick it to the ceiling, or to make their hair stand on end. To many, the static electricity that builds up from rubbing certain materials together is a fun, but unimportant physical phenomenon. This phenomenon is called the triboelectric effect, and much research has been done into taking advantage of it.
‘The triboelectric effect is a combination of contact electrification and electrostatic induction occurring when two surfaces touch and separate.’ explains Tommaso Busolo, PhD student at the Department of Materials Science at the University of Cambridge, who is working on developing novel triboelectric materials. While the mechanism has been exploited for many purposes, it is still not yet clear what causes it. ‘Contact electrification is poorly understood despite being known about since the ancient Greeks. The key unanswered questions are: what type of charge is being transferred (electrons, ions or molecules) and what is the timescale of this process (i.e. there is evidence that contact electrification happens at different timescales from seconds to hours)’.
Small devices that use the triboelectric effect to generate electricity are called triboelectric nanogenerators, and these are being incorporated into a variety of different applications. Fortunately, many of our clothes are made from triboelectric materials such as nylon, cotton, and silk, which means that we could use our own clothes to generate useful electricity. ‘My yarn is able to transform body movement into electricity and thus power small wearable biosensors.’ says Tommaso, who is making triboelectric textiles ‘durable, washable, and with scalable manufacturing methods’. Tommaso is excited to work on tailoring the fabrication process to achieve industry standard levels of energy output and durability. The electricity obtained from these devices could then be transmitted wirelessly into devices such as pacemakers or cochlear implants, therefore removing the need for surgery to replace batteries.
Thermoelectricity: a hot new energy source |
Heat is something that we let go to waste in our homes, our cars, or even in our bodies. Interestingly, a class of materials known as thermoelectric substances can make use of that ‘heat waste’ and convert it into electricity. Harvesting energy from a thermal gradient relies on the Seebeck effect: when two dissimilar materials are connected and one is heated up, the electrons flow towards the colder material, resulting in an electric current.
Thermoelectric generators have been used since the 1950s. Large thermoelectric generators are used in the Mars rover, Curiosity, and the Voyager 2 space probe. However, these generators only operate efficiently at high temperatures and use toxic components. Current research is focusing on developing this technology into much more portable and human-friendly applications. For example, small generators are being developed to harness human body heat to power electronics attached to the skin. In the future, the temperature of your hand or wrist may mean you never have to charge your phone or watch again.
Thermoelectric materials have the potential to make use out of a lot of wasted heat in transport, such as that produced by combustion engines in cars. The tyre company Goodyear has even invested in research looking into the possibility of utilising thermoelectric materials for their tyres. As the car moves, friction between the tyre and the road produces a lot of heat. In addition, piezoelectric materials can take advantage of the tyre deformation and convert this into electricity, which could be fed back into the vehicle to lessen its environmental impact.
For decades, it has become more and more important to recycle our waste materials, but modern technology has given us the ability to start recycling energy itself using unconventional energy sources. While the materials discussed in this article are not the solution to our energy crisis, they can help to alleviate the burden. In a few years, they may become more widespread and replace traditional, environmentally-harmful power sources for low-power applications. Importantly, electricity can be harvested from places you might not have previously considered.
Liam Ives is a 2nd year PhD student in Material Sciences at Selwyn College. Artwork by Marzia Munafo and Mariadaria Ianni-Ravn.