Located in the beautiful La Jolla in California, UC San Diego is known for its beautiful campus and world-renowned academic researches. However, less known to people, we also have a world-class advanced microgrid deployed on-campus that integrates renewable energy sources such as solar cells fuel cells, and large battery groups to supply power to the classrooms, HVAC, lightings, and electric vehicle charging stations. For our campus, it is crucial for the grid to maintain its stability to ensure constant power delivery to various medical and research facilities. To achieve this stability, the microgrid integrates emergency-backup diesel generators and distributed energy resources harvesting from various types of energy, making sure a constant supply of energy even when part of the renewable energy is not available. In addition, robust energy storage systems and advanced energy control algorithms are also crucial to predict and supply the fluctuating demand within the grid. As of 2018, the UCSD microgrid can supply 85% of its own power demand, and it is still growing in size as of today.
We see this similarity in wearable devices. Novel wearable electronics and sensing devices, such as flexible displays, touch panels, antennas, and sensors that detect ECG, blood pressure, temperature, biomarkers and electrolyte concentrations, or body movements, have all been widely reported. However, their untethered, sustainable and reliable operation on-body has been a challenge. Traditional Li-ion batteries are bulky and unsafe, whereas new form factor batteries features flexibility and stretchability are low in performance. At the end of the day, these wearable device needs to be frequently recharged, hence limiting their user-friendliness. Although various efforts have also been made to develop energy harvesters in wearable form factors (fiber, yarn, textile, patch), their integration into wearable systems is still in its infancy, suffering from reliability and efficiency. The ability to harvest energy via movements, sunlight, heat, or chemical reaction on-body has been presented, and the concept of an independent microgrid - a power grid that combines distributed energy sources and storage to reliably power loads managed by advanced predictive hierarchical control systems - can be of great inspiration.
In this work, we demonstrated the possibility of integrating two types of harvesters based on the completely different generation mechanism: biofuel cells (BFC) that harvest bioenergy using enzymatic oxidation of the abundant lactate in the sweat, and triboelectric generators (TEG) that harvester energy from frictions in body movements. Sweat-based BFC is advantageous for its high power density (up to several mW/cm2) and extended operation; yet is limited by the availability of sweat generated from lengthy, vigorous movements. On the contrary, TEG can instantly supply power upon movements but are rather transient and unstable. The integration of these two harvesters in the same textile-based system can thus reduce the limitations of both components and synergistically harvest energy from exercise: the instant reaction of TEG allows faster booting of electronics, while the BFC supplies constant energy after sweating, even after movements stopped.
Furthermore, we carefully evaluated the power consumption of two types of loads, namely, the low-power wristwatch and the high-power sensor-display system with a microcontroller (MCU). To power these devices, we incorporated printable, flexible supercapacitors with the optimal capacitance to power both devices while ensuring fast system booting. Similar to the battery group in the microgrid, the supercapacitor acts as the reservoir to regulate both the high-voltage input from the TEG and the low-voltage input from the BFC, powering the wristwatch continuously or the MCU in pulses. The integrated system, compared to the BFC-only system, can boost twice as fast, and compared to the TEG-only system, lasted at least 3 times longer.
In this work, we summarized the strategies to introduce microgrids to wearable systems using 3 key criteria: complementary component characteristics, commensurate energy ratings, and compatible form factors. In this work, the TEG and BFC's complementary characteristic allows the synergistic harvesting of bioenergy from exercise, which is stored by the supercapacitor with commensurate capacitance to power the applications. Moreover, everything is fabricated in printable, flexible, washable textile-based form factor, allowing them to integrate seamlessly on a shirt. This work, being an example of implementing these design concept, still are facing limitations and is in no way encompassing all the capabilities of a complete microgrid system. Yet we believe future work on the self-powered system will be greatly benefited from implementing the "microgrid" mindset towards building more practicality, efficiency, sustainability, and most importantly, reliability.
You can find more details about this work in the paper published in Nature Communications: Yin, L., Kim, K.N., Lv, J. et al. A self-sustainable wearable multi-modular E-textile bioenergy microgrid system. Nat Commun 12, 1542 (2021). DOI: https://doi.org/10.1038/s41467-021-21701-7
In addition, you can check out our other works for wearable and E-textile energy devices:
Lv, J. et al. Sweat-based wearable energy harvesting-storage hybrid textile devices. Energy Environ. Sci. 11, 3431–3442 (2018). DOI: https://doi.org/10.1039/C8EE02792G
Chen, X. et al. Stretchable and flexible buckypaper‐based lactate biofuel cell for wearable electronics. Adv. Funct. Mater. 29, 1905785 (2019). DOI: https://doi.org/10.1002/adfm.201905785
Yin, L. et al. High Performance Printed AgO-Zn Rechargeable Battery for Flexible Electronics. Joule. 5, 228-248 (2021). DOI: https://doi.org/10.1016/j.joule.2020.11.008