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| Fig. 1: A wave breaking. (Source: Wikimedia Commons) |
Renewable energy has been explored in recent decades to supplement or replace fossil fuel usage, as a means of meeting humanity's increasing energy demand, while lessening environmental impacts associated with conventional fuel sources. Popular approaches have included wind, solar, geothermal, and hydroelectric energy generation. Ocean covers approximately 70% of the Earth and can provide renewable energy in the form of tidal energy, wave energy, current energy, temperature gradient energy, and salinity gradient energy. According to Falnes, global wave energy has the potential to produce up to 10 TW. [1] Little of this has yet been harnessed. Recent research into triboelectric power generation may yield an innovative approach to harnessing some of this large potential power source.
The triboelectric effect (coming from the Greek word 'tribo' meaning 'rubbing') is electrification which occurs at the interface between two surfaces in contact. This can occur when there is sliding contact at the interface, or as the two surfaces are brought together. Based on their molecular structure, materials have varying tendencies to either accept or donate electrons. For example, Nylon has a relatively high readiness to donate electrons, while Polyethylene readily attracts electrons, making the pair have a good interface for the triboelectric effect to occur (relatively high charge generation). [2] Since charge generated is also a function of surface area, research has gone into increasing the contact area through micropatterning the interface surfaces. [2]
This effect can be taken advantage of to generate power in a triboelectric nanogenerator (TENG). Multiple mechanisms for generation have been proposed including plane- contact separation, sliding contact, single-electrode, and liquid-solid contact. [2-4] The principles of power generation are similar in the various cases. Plane contact separation works as follows:
Two materials (one must be an insulator to prevent charge leak) with opposing triboelectric potential attached to metal electrodes start separated.
When a mechanical load is applied, the surfaces are brought together. The triboelectric effect causes equal and opposite charge to form on the two materials at the interface. These charges induce opposite charges to form at the attached electrodes.
As the two electrodes become separated (by mechanical work input), the charges create an electric potential (similar to parallel plate capacitor), so current flows between them through an attached load circuit, until the potential across them is zero again.
As they are brought back together again, completing the cycle, the triboelectric effect again creates charge at the interface. This induces opposite charge to form at the electrodes, which generates a current in the opposite direction to step 3.
TENGs can be a low-cost way to produce energy from the ambient environment. Depending on arrangement of TENG units, hundreds of volts can be produced with mechanical to electrical energy conversion efficiency of up to 70% (defined as the ratio of electrical energy out to mechanical energy in). In bench tests with high frequency contact cycles (kHz), power density up to 500W/m2 has been achieved, and roll-to-roll manufacture methods have brought costs of some triboelectric generators down to $3.00 /m2. [4,5] TENGs are being considered for applications such as self-powered wearable sensors and for reclaiming friction energy lost between automobile tires and pavement to boost fuel efficiency. [2,3] It is important to note however, that most real-world energy harvesting applications do not take advantage of high frequencies and ideal bench setups that produced such high power density and conversion rates. Ocean waves, for example, move at on the scale of hertz. [4]
The ocean generates a vast amount of kinetic energy that has the potential to be harvested. Systems for energy ocean energy capture include underwater turbines for tidal energy and bobbing buoys for wave energy. Ultimately, these approaches convert mechanical energy from waves into electricity through electromagnetic generators (EMG). These suffer from being heavy, are susceptible to corrosion, can have expensive infrastructure at sea, and typically inefficient at the low frequencies which ocean waves move at (< 2 Hz). [4] Furthermore, they only operate with peak efficiency when faced in the direction of motion. [4]
Spherical and duck-shaped designs of TENG generators have been tested for wave energy conversion, in which a nylon spheres roll inside of a sealed hollow Kapton body with metal electrodes, generating electricity as the interfaces roll with respect to each other. [4] The duck shaped devices were tested in realistic wave conditions (< 2 Hz) and measured to produce ~1 W/m2. [4] Large networks of these devices could produce higher power than EMG solutions at low oscillation frequency and respond better to the random ambient motion that waves produce. These TENGs are lightweight, cheap to produce, and can float on water. [4] Researchers envision nets of millions of such power generating devices floating in the ocean, generating megawatts of power per square kilometer, dependent on density of devices deployed. [6]
Triboelectric power generation is still in its infancy, but research is showing that the power density possible is rising rapidly, while costs to manufacture drop. [4,5] Harvesting ambient wave energy takes advantage of the large surface area of ocean available, and TENGs are lightweight and can float without requiring generator infrastructure built on a deep ocean floor. [4] However, many challenges lie in the way before TENGs can become a viable renewable energy technology. Among these are power transfer to the shore, cost to scale and manage large networks of devices, and device lifetime in ocean water. [4]
© Alex Gruebele. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. Falnes, "A Review of Wave-Energy Extraction," Mar. Struct. 20, 185 (2007).
[2] L. Dhakar, Triboelectric Devices for Power Generation and Self-Powered Sensing Applications (Springer Singapore, 2017).
[3] Y. Mao et al., "Single-Electrode Triboelectric Nanogenerator for Scavenging Friction Energy From Rolling Tires," Nano Energy 15, 227 (2015).
[4] Z. L. Wang, T. Jiang and L. Xu, "Toward the Blue Energy Dream by Triboelectric Nanogenerator Networks," Nano Energy 39, 9 (2017).
[5] C. Wu et al., "Ultrasoft and Cuttable Paper-Based Triboelectric Nanogenerators for Mechanical Energy Harvesting," Nano Energy 44, 279 (2018).
[6] J. Chen et al., "Networks of Triboelectric Nanogenerators for Harvesting Water Wave Energy: A Potential Approach toward Blue Energy," ACS Nano 9, 3324 (2015).
