The Electrifying World of Energy Harvesting Liam Ives explores how we can harness the wasted energy from everyday processes Imagine 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 12
The Electrifying World of Energy Harvesting
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. Lent 2021
