Energy Harvesting: The Place of Triboelectrics

BY DR. PETER HARROP, CHAIRMAN, IDTECHEX

Triboelectrics will only be a small part of the energy harvesting EH market projected by IDTechEx to be over $7 billion in global market value in 2027. This value is struck at the transducer level. It is a multiple of that at system level. EH produces electricity where it is needed and without emissions. It is an increasingly vital enabling technology in everything from medical devices to electric vehicles. If we include toys and novelties where photovoltaics PV is widely adopted, the most popular EH technologies in numbers deployed and market value are electrodynamic and photovoltaic and will stay that way. The new IDTechEx Research report, Energy Harvesting: Off- Grid Microwatt to Megawatt 2017-2027 gives the detail, explaining why thermoelectrics is overtaking piezoelectrics to be number three in market value but still a long way behind.

Versatility and reinvention

Photovoltaic and electrodynamic options are and will remain the most successful not because they are exceptionally versatile. They keep being reinvented in new forms, recent examples being solar roads and electrodynamic airborne wind energy (AWE) using autonomous tethered multicopters.

What else?

In 5-10 years there may be significant business in magnetostrictive, capacitive (electrostatic) and triboelectric energy harvesting because they promise to be versatile too from stretchable forms to wave power and car tires but not to rival the established technologies in that timeframe. They were merely academic only a few years ago, dismissed as a maybe another option at MEMS level but unlikely candidates at high power where most of the EH value market lies. From that perception to today we have a burgeoning number of improvements and applications emerging but only in research laboratories in the main.

Triboelectric EH

Triboelectric energy harvesting development programs now vary from triboelectric nanogenerators TENGs at microwatts to milliwatts created on organic materials up to the possibility of generating hundreds of watts from car tires to charge traction batteries – a form of regeneration. When IDTechEx was guest presenter at the inaugural meeting of the Japan Energy Harvesting Association in late 2015, we learnt of work there on using triboelectrics for car tire pressure monitoring in real time. One problem is the generation of power in spikes of high impedance/ high voltage rather like piezoelectrics, this being ill suited to the needs of electronics and electrics but there are many potential benefits emerging.

The basics

Triboelectrics is based on contact electrification with electrostatic induction. It employs moving dielectrics. A potential is created by the triboelectric effect due to the charge transfer between two thin organic/inorganic films that exhibit opposite tribo-polarity. This potential can cause current to flow in an external circuit. Triboelectrics is a promising because of high output power density, ultrahigh energy conversion efficiency, low weight, cost-effective, environmentally friendly materials, reliability and high adaptability of format to different applications. After the first TENG report in January 2012, output power density improved five orders of magnitude in 12 months. Area power density reached 313 W/m2, volume density reaches 490 kW/ m3, and a conversion efficiency of about 60% has been demonstrated like the theoretical potential of previous leader, piezoelectrics. The triboelectric generator has three basic operation modes: vertical contact-separation mode, in-plane sliding mode, and single-electrode mode with different characteristics suitable for different applications.

Uncontrollable and unstable

Inherently uncontrollable and unstable characteristics mean converted electrical energy is unstable: it cannot be utilized to directly power electronic or electrical devices. Storage elements such as capacitors or batteries and power conditioning are needed to stabilize and regulate the power output for direct application at a level of complexity similar to piezoelectrics. Understanding integration performance and basic charging characteristics is paramount and the influence of the load capacitance and TENG parameters on the charging characteristics is poorly understood. Similar to a resistive load, there should be an optimum load capacitance for maximized energy storage, while a mismatch between the TENG and the load capacitance can greatly decrease the energy storage efficiency.

New understanding

In 2015, GeorgiaTech and Chinese researchers established that charging behaviour strongly depends on the load capacitance. For multicycle charging with a bridge rectifier, the saturation charging SC behaviour is analogous to utilizing a dc voltage source with internal resistance to charge a load capacitor. The TENG with larger SC transferred charges and smaller inherent TENG capacitance can charge a load capacitor to a higher saturation voltage under unlimited charging cycles and the TENG with larger inherent capacitance can provide a smaller charging time constant. Optimum load capacitance matching TENG impedance is observed for maximum stored energy. It is linearly proportional to the charging cycle numbers and the inherent capacitance of the TENG.

Triboelectrics for wearable technology

Also in the surge of new discoveries in 2015, researchers from Hanyang University Korea reported the electrical responses of a textile substrate-based triboelectric nanogenerator T-TENG, including nanostructured surface configurations provided by Al nanoparticles and PDMS, in which no intricate fabrication process was performed or required. Along with analysis of the working principle and finite element simulation, the textile-based output power density of 33.6 mW/cm2 was obtained from the periodic mechanical stress of an adjustable bending machine, and this was used to activate commercially available LED bulbs. Additionally, they demonstrated the generation of sufficient energy from clothes attached to a commercial arm sleeve to power the LED devices from the activity of a human arm. Their practical cloth approach may contribute to a general framework for developing functional and self-powered wearable electronic devices, they suggested.

Vibration harvesting and power from tires

New variants are now regularly being announced such as vibration harvesting with 36Hx acoustic bandwidth (relatively wide), combined electrostatic/ triboelectric energy harvesting, integral energy storage and use of various inorganic triboelectric materials. A floating oscillator “FO-TEG” has been designed by KAIST Korea. A group of University of Wisconsin-Madison engineers and a collaborator from China are developing a nanogenerator that harvests energy from a car’s rolling tire friction. Professor Xudong Wang says the nanogenerator provides an excellent way to take advantage of energy that is usually lost due to friction. “The friction between the tire and the ground consumes about 10 percent of a vehicle’s fuel,” he says. “That energy is wasted. So if we can convert that energy, it could give us very good improvement in fuel efficiency.” The nanogenerator relies on an electrode integrated into a segment of the tire. When this part of the tire surface comes into contact with the ground, the friction between those two surfaces ultimately produces an electrical charge. They used a toy car with LED lights to demonstrate the concept. They attached an electrode to the wheels of the car, and as it rolled across the ground, the LED lights flashed on and off. The movement of electrons caused by friction was able to generate enough energy to power the lights, supporting the idea that energy lost to friction can actually be collected and reused. “Regardless of the energy being wasted, we can reclaim it, and this makes things more efficient,” Wang says. “I think that’s the most exciting part of this, and is something I’m always looking for: how to save the energy from consumption”.

The researchers also determined that the amount of energy harnessed is directly related to the weight of a car, as well as its speed. Therefore the amount of energy saved can vary depending on the vehicle-but Wang estimates about a 10-percent increase in the average vehicle’s gas mileage given 50-percent friction energy conversion efficiency. In 2015, The State University of New York Binghamton envisioned triboelectric energy harvester inside a capsule endoscope that can generate power from natural contractions of gastrointestinal tract. The periodic contacts and separations of two triboelectric materials inside the capsule endoscope create an alternating current that can be used to charge the capsule endoscope battery, which is used for imaging.

Forecasts

In the IDTechEx Research report, Energy Harvesting: Off-Grid Microwatt to Megawatt 2017-2027 it is predicted that triboelectrics will reach the marketplace and have an important part to play alone and in combinations with other forms of harvesting to balance out intermittency and reduce and sometimes eliminate the need for energy storage. However, we are only in the early stages of these developments. Its eventual success is likely to be with devices generating under 10 watts when active.

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