28-07-2020 | By Liam Critchey
Energy harvesting devices are a big application area, especially for some of the more sophisticated technologies that are coming out―for example, electronic textiles (e-textiles). This is where energy harvesting devices harness the movement of the user, which in turn enables the different monitoring devices within the garment to be continuously powered. But this one example and there are many more like it, especially across the wearable and flexible electronics sectors.
Because of the rapid development of flexible and wearable electronic devices, alongside advances in wireless sensing nodes and implantable devices, there is currently a significant demand to create energy harvesting devices which have micro-milliWatts power output. While different energy harvesting devices have been devised over the years, there is still work to be done.
Solar cells are one of the biggest and most well-known energy harvesting devices. Still, for many of the applications mentioned above, they are not the most suitable (although some do use small cells to generate power). There are smaller, better-suited devices out there which are currently being developed, many of which utilise movement to create a power output. This class of devices includes electrostatic electrets, piezoelectric generators, electromagnetic generators, and triboelectrification generators. Recent advances have seen the emerging area of triboelectric nanogenerators (TENGs) becoming favoured due to its small size and great potential to turn kinetic movements into usable power output.
TENGs are tiny energy harvesting devices that are electrostatic kinetic energy harvesters, as well as a capacitive transducer because they possess an internal electric biasing effect which arises from a contract electrification effect. In short, they are a device that can be used to convert mechanical motions into electrical outputs using triboelectrification inductions, i.e. the generation of an electrical charge when two materials separate (after being in contact together).
TENGs first started without these internal biases and were seen as a type of variable capacitor that was pre-biased using an external source rather than internally. However, these first generation of TENGs were not self-sufficient (which is an issue when they are integrated somewhere without an external power source). Hence electrically biased TENGs are created, but this has also brought about its own set of issues that need to be corrected.
Even though the internal electrical bias has created TENGs that have a high power output relative to their size, they have a susceptibility to depolarising over time. A range of condition circuits have been under development to control loss of polarisation over time, but many of these have had to use components which consume much power, taking away the power output from devices. One of the novel ways that has been explored in recent years is the use of switches to control TENGs.
This has been regarded as a promising area. Still, switches to date have either had to be activated after each actuation of the device (and are not self-sustainable) and attempts to rectify this so far have also required the use of external and power-consuming components if the TENG is to function at both high and low voltage thresholds, making them unfeasible for several applications. So, there is a drive to create a self-actuation switch that can control more than one voltage threshold (high and low thresholds).
MEMS, which is shorthand for microelectromechanical systems, has been emerging technology for many years now and is often used to construct complex parts/components within your smaller-than-average devices―as well as in high-tech devices that require highly effective, but small components. The term is used to categorise both the micromechatronic devices that are fabricated, as well as the process used to create them, so it is a wide-ranging and expanding field of materials and components. In their most basic form, they are miniaturised mechanical and electro-mechanical elements that have been created using microfabrication methods.
Despite their name, some MEMS don’t have moving parts, but these devices still fall under the MEMS banner because they can be used as a replacement for parts that traditionally move (e.g. springs). In other cases, MEMS can be used to convert optical and electrical signals, so they showcase many characteristics that make them useful for use with small energy conversion devices such as TENGs.
One of the ways proposed to overcome the current issues with some TENGS is to employ a self-sustaining conditioning system using a MEMS switch. The aim of employing a MEMS switch to TENGs was to create a complete TENG system that could work at high voltages for high energy conversion applications without the need to use power consuming electronic components.
The switch device created by the researchers used both a MEMS plasma switch and an unstable Bennet doubler charge pump in a 2-stage circuit. The switch was kept separate from the TENG itself, meaning that no direct integration was required for it to have a positive effect on the performance of the TENG. The use of the Bennet doubler charge pump solved the issues surrounding voltage limitations in some TENG devices by creating an exponential charging process that had no saturation limit, enabling a large power output to be generated.
On the other hand, the MEMS plasma switch― a patterned silicon-based chip―was able to control the transfer of energy between the buffer and the final reservoir within the device. It was able to do so because it has a narrow hysteresis loop that could hold the voltage across a buffer capacitor continuously oscillating between different voltage levels. So, it offers a solution towards controlling the device at both low and high voltages, overcoming one of the main challenges with other switches. Other advantages of the switch are that no external power source is required, nor is any needed electric control, removing the need for external components to control and improve the performance of the TENG energy harvesting device.
The device created improves the energy harvesting efficiency of TENG devices compared to other switches (and non-switch-based approaches), and the use of such a switch offers a solution to several issues surrounding self-sustainability and external power sources. As it is a stand-alone device that does not require direct integration with the TENG, it is a solution that could also be used to solve some of the issues with other small energy harvesting devices.
TENGs are a device which is set to gain more traction in the coming years as flexible, wearable, and implantable devices become more commonplace. So, the groundwork being put in now to solve critical issues with these devices that could lay the platform for more efficient electronic networks and self-sustaining electronic systems in electronic textiles and devices of the future.