Energy harvesting (also sometimes called scavenging) is a process of energy derivation from the ambient sources, e.g., sunlight, wind, heat, mechanical vibrations, etc. The main idea is that devices can collect the energy present in their surrounding environment in real time and use it to power themselves. This would mean that the operating time of the device would be limited only by the lifetime of its components. Clearly energy harvesting is an attractive alternative to battery powered systems, especially for devices operating on a long-term and with low power consumption.
The idea is not new. Prior to the development of internal combustion engines, power grids and batteries, many devices were developed to harvest energy. For example, wind mills used to collect the energy from the wind and use it to mill grain, thus harvesting energy and using it without storage in real time. Recently, energy harvesting got the attention again driven by the development of next generation portable electronic devices. The progress in the field of integrated circuits in the last decade has brought to life miniaturized electronics that are portable, wearable and stand-alone. The use of conventional power supply in such devices is limited and therefore different approach, such as energy harvesting, is needed.
Various devices exist that can harvest the surrounding energy: piezoelectric, electrostatic, thermoelectric and electromagnetic harvesting devices. Triboelectric nanogenerator (TENG) is another emerging mechanical energy harvesting technology that has been rapidly developing in the recent years. TENG uses triboelectric effect to convert mechanical energy into electricity. Due to triboelectric effect, after a contact between any two materials, charge transfer occurs and surfaces become electrically charged. When these charged surfaces start a relative motion, electric current is produced. The efficiency of such device is clearly depends on the materials in contact.
Recently, an international team of researchers from South Korea, US and UK has developed a high power density, flexible and long lasting TENG from Bombyx mori silkworm silk cocoons. As it was already mentioned, the performance of the TENG strongly depends on the materials that are in contact and generate the charge transfer. To get the desired efficiency, the team has used an alcohol treated silk and PTFE, which are classified as positive and negative materials in triboelectric series and generate opposite triboelectric charges on their surfaces after contact.
For the first time, a theoretical model was used to optimize the performance of the TENG. The model allowed to consider the effects of various parameters on the electrical output of the TENG. Surface roughness, materials, compression frequency and normal load applied are the parameters of the model. The model was verified with the experimental data and showed an excellent agreement.
Finally, the eco-friendly silk film based TENG was demonstrated to successfully power several types of portable electronic devices, such as LED, liquid crystal displays, wristwatch, etc. Besides using the TENG as a power supply, the team has demonstrated that it can be used as a stress/pressure or motion sensor – thanks to its flexibility and operating principle. Given all this, the newly developed device is a unique tool among the TENGs and is a step forward to the future of autonomous wireless biosensors.
Further details: Bhaskar Dudem et al. Exploring theoretical and experimental optimization towards high-performance triboelectric nanogenerators using microarchitecture silk cocoon films, Nano Energy (2020). DOI: 10.1016/j.nanoen.2020.104882