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Many nations worldwide have committed themselves to transitioning their energy sectors from being previously dependent upon fossil fuels to one that is entirely supported by renewable energy sources. To expand the portfolio of renewable energy sources, many scientists have developed energy harvesting technologies that can generate electricity from the ambient environment.
Can Energy be Harvested from the Environment?
The term energy harvesting, which is also known as energy scavenging, refers to the process where ambient energy present within the environment is converted into electrical energy that can be used to power a wide range of electronic devices and circuits.
For several years, scientists have investigated how to expand the range of this natural phenomena and exploit moisture or water vapor as a novel harvestable energy source. One of the key advantages of water vapor is its widespread availability, mainly due to its ubiquitous existence both on earth and within biological organisms.
Like renewable energy, the goal of advancing energy harvesting technologies is to improve the energy efficiency of homes, buildings and industries while simultaneously reducing or eliminating the burden that conventional power plants pose to the environment.
Despite having identical principles, energy harvesting technologies are limited in their ability to generate large amounts of energy that have been achieved by renewable energy sources such as wind turbines and solar panels.
Existing moisture-based energy harvesting technologies are also limited in their ability to provide continuous energy production, as bursts of power for these systems currently are less than 50 seconds in the ambient environment.
Although current energy harvesting technologies have been limited in their ability to produce comparably large amounts of energy, these systems can overcome some of the limitations associated with specific renewable energy sources. Solar cells, thermoelectric devices, and mechanical generators, for example, each have specific environmental requirements that can restrict their installation locations, as well as their potential to produce energy at a continuous rate.
Recent Advancements in Energy-Harvesting Technologies
The limitations of current energy-harvesting technologies are primarily due to the lack of a reliable and sustained conversion mechanism. To resolve these issues, several alternative energy harvesting strategies have been proposed and tested. In 2016, for example, researchers from the Beijing Institute of Technology created a high-performance chemical potential energy harvester (CPEh) comprised of a super hydrophilic three-dimensional assembly of graphene oxide. This system successfully converted chemical potential energy originally derived from moisture diffusion with a conversion efficiency of approximately 52%.
Another novel device concept was reported in 2018. A group of researchers from Tsinghua University in Beijing, China, in collaboration with the University of Waterloo, Ontario, created a self-powered wearable electronic device powered by moisture-enabled electricity. Moreover, this device was made up of strongly hydrophilic titanium dioxide (TiO2) nanowire networks (TDNNs) that contained three-dimensional (3D) nanochannels which, taken together, allow for the diffusion of water molecules from the environment. Once the molecules have diffused, the moisture-enabled electricity generator (MEEG), an open-circuit voltage of approximately 0.5 volts (V) could be generated. As compared to carbon-black generators, this MEEG device was found to perform at levels that were up to two orders better.
A New Approach using Protein Nanowires
In a 2020 Nature paper, a group of researchers from the University of Massachusetts in Amherst, Massachusetts, discussed its novel electric generator that was constructed from a thin film of protein nanowires. The purified protein nanowires, which originated from the microorganism Geobacter sulfurreducens, formed a mesh network in the 7 micrometers (µm) thin-film upon analysis by transmission electron microscopy (TEM).
Like previously studied protein-nanowire networks, the current-voltage, which arises between the protein nanowire film and the gold electrode placed above it, also displays ohmic behavior.
As water molecules made up of ionized species are adsorbed onto the nanowire network's surface, the ionized clusters of molecules donate charge to the nanowire. This exchange in electrons will provide the close loop with a current flow driven by the voltage that arises in response to a change in the moisture gradient.
Two critical components of the moisture gradients include a high density of both nanowires and surface function groups. Ambient moisture within the environment will provide an extensive reservoir of water molecules, which ensures that this exchange of electrons is continuous and can allow for a sustained electric output of approximately 0.5 V with a current density of approximately 17 microamperes square centimeter (cm).
Taken together, this device was able to produce currents at a consistent rate of 0.5 V for a total of 20 hours, following which the current reduced to 0.35 V. However, the current of the device was restored to 0.5 V within five hours, which demonstrates the improvement that this device has provided as compared to the transient current that was produced from previous ambient environment generators. This protein nanowire system's power output was found to be more than one hundredfold greater than that of other systems. The device maintained a stable direct-current voltage between 0.4 V and 0.6 V for more than two months in addition to excellent power output capabilities.
This system successfully and sustainably produced voltages from an environment with humidity levels as low as 20%, which is comparable to that which exists within a desert and when placed in an environment with 100% humidity.
References and Further Reading
Liu, X., Gao, H., Ward, J. E., et al. (2020) Power generation from ambient humidity using protein nanowires. Nature 578; 550-554. doi:10.1038/s41586-020-2010-9.
Beeby, S. P., Cao, Z., & Almussalam, A. (2013) 11 – Kinetic, thermoelectric and solar energy harvesting technologies for smart textiles. Multidisciplinary Know-How for Smart-Textiles Developers 306-328. doi:10.1533/9780857093530.2.306.
Zhao, F., Liang, Y., Cheng, H., et al. (2016) Highly efficient moisture-enabled electricity generation from graphene oxide frameworks. Energy Environmental Science 9; 912-916. doi:10.1039/C5EE03701H.
Shen, D., Xiao, M., Zou, G., et al. (2018) Self-Powered Wearable Electronics Based on Moisture Enabled Electricity Generation. Advanced Materials 30; 1705925. doi:10.1002/adma.201705925.
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