If a company is designing anything electronic or electric in the future, it must first think about generating its own electricity, sometimes even without need for a battery. EnOcean has done that with wireless sensors and actuators for buildings, proclaiming, “No wires, hassle, no battery.” Indeed, the Internet of Things (IoT) nodes will only deploy in billions if they are fit-andforget and very low-cost, even disposable. A hand-held pollution sensor can be bought today that gets all its electricity from ambient 4G emissions, and energy-independent vehicles are already on sale that never plug in because daylight provides all their electric power. The game is changing completely.
Better harvesting needed
Energy harvesting is the order of the day, but the most efficient photovoltaics, thermoelectrics and piezoelectrics often use toxic elements. Electrodynamic harvesting often employs expensive rare earth magnets. Few forms of energy harvesting lend themselves to structural electronics such as smart load-bearing materials that are now increasingly replacing clunky components-in-a-box assemblies. Particularly significant is the ability to be layered with other forms of energy harvesting, energy storage, sensors, power conditioning circuits and sometimes displays.
Rugged flexible multimode energy harvesting is needed for wearable electronics, IoT and even low-cost longlife wind (smart sails, flags, airships) and wave power. That is why the newest invention in energy harvesting is attracting such attention: It employs nontoxic low-cost materials and, unlike most alternatives, can potentially generate microwatts in microelectromechanical systems up to megawatts at sea at high efficiency.
Rush to market
Triboelectric energy harvesting, invented only four years ago at Georgia Tech, will be sold in Chinese electronic devices in a few months. It has been demonstrated as electricity-generating flooring and as a self-powered heart sensor in a living sheep in China. Electricity-generating tires have been demonstrated on toy cars. A wider range of potential applications and efficiency improvements has been demonstrated than was ever achieved in the early years of alternatives. It has been combined with many other harvesting technologies in one plastic sheet and has even driven chemical synthesis. There has been a proof-of-principle of a 1-sqkm (.386-sq-mile) blanket generating 1 MW from wave energy.
What is it?
Triboelectricity is the creation of electrostatic charges on the surfaces of two dissimilar materials when they are brought into physical contact. Tribo (drag) is a misnomer. It has been known for millennia, but resulting sparks cannot be used. The propensity to become positively or negatively charged is only approximate: Morphology, formulation, impurity and surface oxides matter. Charge affinity in nano amp sec/watt sec of friction is measured by metal contact. Metals are triboelectrically active. Steel is neutral, but in tires steel reinforcement will double as an electrode.
Triboelectric energy harvesting is based on contactinduced charges generating a potential drop when the two surfaces are separated by a mechanical force. This can drive electrons to flow between the two electrodes built on the top and bottom surfaces of the two materials. Single impact or repeated reciprocating movement generates a form of AC electric power that can be converted to useful power for electrics and electronics by attaching power conditioning.
This is the triboelectric nanogenerator (TENG). Though “nano” means the TENG is based on smallscale physical change, these can be very large devices. There are four basic TENG operational modes and a huge number of physical variants of each. At least one active material must be a dielectric—fluoropolymers and silicones favored—to retain the created static charge. Following the first report of the TENG in January 2012 by Professor Zhong Lin Wang et al. at Georgia Tech, power density reached 500 watts/sq m and 15 MW/cu. m, instantaneous conversion efficiency reached 70%, and total energy conversion efficiency of up to 90% has now been demonstrated with relatively small devices. As with most other energy harvesting inventions, the industry is starting with small, low-power devices, in this case mainly laminar and flexible. The dielectric can double as a sensor.
New markets created
Triboelectric energy harvesting will expand the energy harvesting market rather than break into it. Forecasting is highly speculative, not least because the necessary finance and effort by the industry is not yet committed. Maybe a global market of $400 million will be served in 2027 for transducers and double that for the systems, rising to many billions over the following decade if takeoff is typical. The biggest value potential is above 1 watt. Most of the increasing number of proposals and demonstrations will fail commercially, but within 10 years the technology should overtake piezoelectric energy harvesting to become the fourth biggest seller after electrodynamic, photovoltaic and thermoelectric. This is because the TENG is not as limited by fragility, format and use of toxic metals as piezo is for affordable, desired devices.
Innovative experimental devices for new applications are frequently demonstrated, mostly in universities in the U.S., China and Korea. They promise considerable versatility of format, from wind-harvesting “grass” on rooftops to tribo balls in tubes harvesting multidirectional motion and rotational motion harvesting in transparent, disposable and stretchable versions. It lends itself well to physical integration with other filmbased harvesting for power increase, reduced intermittency (less battery or no battery) and improved security of supply.
When and why
First, it will be successful at low power. Self-powered sensors and disposables are good entry points because other options are problematic. Vibration harvesting at up to 1 MW may not be an opportunity of more than $100 million but is an entry point to develop random movement harvesting at much higher power. Success here could rapidly create billion-dollar businesses.
Reasons for success at low power are primarily due to being able to do new things such as self-powered thin haptic actuators, including transceivers, and secondly the low cost, being nontoxic and having other positive attributes. Wind, wave and random movement power (e.g., seats, flags, boats) might become the primary opportunity at 1 watt to 1 MW, with textile and plastic film formats compelling in these markets. However, feasibility and viability are far from proven.
What needs fixing
Success in any high-volume and/or high-value operation will only come when reliability, life and reproducibility are demonstrated and adequate; the marketplace is widely seeded with good working samples; and production is proven at acceptable yield and cost. There are additional challenges of standards and protection from moisture.
Only motion harvesting
The TENG is a form of motion harvesting, so appropriate motion must be tapped. Vibration harvesting has been demonstrated even with a disposable paper TENG, but vibration harvesters have an irritating habit of not efficiently harvesting the vibration frequencies that are actually encountered or being cost-effective since electronics and electrics take less power and batteries are lower cost up front. That market has never taken off. IDTechEx thinks that ultimately TENGs will do more business in harvesting random movement to create higher power electricity but also will be useful in single-hit applications like sensor/actuators.
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