Supercapacitors: a New Hero in the Spotlight

Until recently, supercapacitors (ultracapacitors) worked at fairly mundane tasks such as circuit protection and as short-term back-up supplies, but applications have expanded to electric/hybrid vehicles, renewable energy; and with energy harvesting, wearables and the Internet of Things come into play. The supercapacitor market is forecast to be at $11B by 2023, as supercaps soon will extend their range with chip-sized, MEMS-like micro-supercapacitors. Current trends include hybrid supercap/battery hybrid devices that behave more like an ideal battery; the goal is to replace rechargeable batteries with supercapacitors so charging fatigue is a thing of the past.

Supercapacitors, Ultracapacitors, or “Supercaps” exist between capacitors and batteries in terms of storage capability, and like batteries, they have the ability to release energy slowly. Unlike batteries, supercapacitors can charge in seconds, without capacity degradation like rechargeable batteries. They can endure virtually unlimited charge cycles. Supercapacitors have been in volume production as power energy storage cells since 1996 and are poised for high growth as the technology approaches the energy density of batteries. Supercaps also have a very long lifetime, irrespective of numerous charge cycles.
Supercapacitors have greater energy density than common capacitors, but less energy density than batteries. You can theoretically replace batteries with supercapacitors, but replacing an entire bank of batteries for the Tesla Model S with supercapacitors would take up too much volume.

For electric cars, supercapacitors have a place in start/stop systems, engine assist, and in charging stations. Supercaps also perform basic functions such as soaking up high current transients for batteries, which can increase battery life but come at the cost of higher design complexity and greater space requirements. Although supercaps are in use, batteries are the mainstay for electric vehicles in the near-term due to superior storage density. To encourage more electric vehicles on the market, Tesla Motors has opened its battery patents to everyone. Tesla CEO Elon Musk stated that this was done “in the spirit of the open-source movement, for the advancement of electric vehicle technology.”

He goes on to say, “Technology leadership is not defined by patents, which history has repeatedly shown to be small protection indeed against a determined competitor, but rather by the ability of a company to attract and motivate the world’s most talented engineers. We believe that applying the open-source philosophy to our patents will strengthen rather than diminish Tesla’s position in this regard.”

Supercapacitor fundamentals

Standard capacitors accumulate charge in the dielectric material between two opposing electrodes. Supercapacitors do not store energy in a dielectric. Supercaps store energy via a physical phenomenon first observed in 1957 by General Electric using carbon materials. Supercaps use the extremely high surface area of activated carbon, which relates to very high capacitance. These double-layer capacitors are also called Ultracapacitors or EDLCs (electrochemical double-layer capacitors). Composite materials, size, and geometry influence the farad values of supercapacitors.

Electrical charge builds up via ion migration inside a thin layer of activated carbon. When a voltage is applied across the capacitor’s electrodes, ions migrate to try to reverse the accumulating charge on the electrodes (the charging cycle). Negatively charged ions move towards the positive electrode and vice versa and create one positive and one negative layer. Absence of voltage makes the ions return, and they move in the opposite direction. This is the discharge part of the cycle. Exotic materials provide a boost: in development are graphene-based supercaps that have energy density comparable to NiMH batteries, but without the drawbacks of rechargeable batteries. Supercapacitors charge very quickly and are nearly indefatigable with respect to charging cycles.

Micro-Supercapacitors

Micro-supercapacitors are a micro-power source that would reside on-chip, and the planar graphene-based devices show promise of attaining a power density greater than that of electrolytic capacitors. In addition to being micro-sized MEMS-like devices, they are suitable for flexible micro-device applications where bending the micro-supercapacitors multiple times does not affect performance. This is ideal for wearables and IoT applications. Screen printing solid-state flexible micro-supercapacitors on glass, silicon substrates, and paper is under development.

This means a great deal to wearables and IoT markets. Imagine a rotating gear mounted with supercapacitors that, as it rotates, charges each supercap upon passing an energy source. Now imagine the gear as a MEMS device inside a chip. Present-day sensors that use energy harvesting are limited by a battery’s recharge-cycle-life and slow charging times. Supercapacitors enable quick charging times but as yet don’t have great energy storage capacity, thus batteries are necessary. One application that can benefit today is an instant-recharge tool where there is intermittent use and frequent holstering/docking is not an issue.

Applications

Present-day hybrid vehicles completely turn off the engine when they come to a stop, and then start with energy from supercapacitors. Supercapacitors have traditionally been used for applications that experience sudden bursts of energy or use energy in bursts.

Supercapacitors can be used as a backup power supply in the event of an outage to allow a proper shutdown sequence. Smartphones use supercaps for battery backup, real-time clocks, and camera flash. Smart meters use supercapacitors as back-up power supplies. USB products might use them for peak power assist. Supercapacitors are often used in solar systems, sensor networks, in energy harvesting applications and as zero watt standby electricity for computer monitors. With supercaps, monitors and other electronics require no electricity at all in idle or standby mode, providing the minimum power necessary to intercept the wake-up signal and initiate a wake-up sequence for the microcontroller. This set-up is referred to as zero-watt standby. Automatic water faucets use supercapacitors, with water flow for power generation in the next recharge cycle.

Supercapacitors are ideal for remote, wireless or battery-powered products, which fit in the Internet of Things (IoT). The small size of micro-supercapacitors doesn’t hurt, either. IoT devices are smart sensors that provide data about themselves or their environment over the internet as a contribution to an overall smarter process and is enabled by machine-to-machine communication between devices. IoT devices can take action without human intervention, although humans set up, monitor, and ultimately control IoT networks.

Source: Mouser article
by Lynnette Reese, Mouser Electronics

featured image: Maxwell K2 Series Ultracapacitors is a line of ultracapacitors for application in hybrids. image credit: Maxwell