Supercapacitors and ultracapacitors once were somewhat costly components finding employment in equally costly applications. However, time and technology have transformed them into cost-effective alternatives for an array of power sources, especially batteries.
An Ultracap Takeover
With their ability to store ever greater levels of energy, ultracapacitors are slowly supplanting batteries in many designs and applications. Though they aren’t reapers of cell technology yet, these supercapacitors are poised for a major eventual takeover. So, how soon can we expect a larger deployment of supercapacitors as replacements for rechargeable and other batteries?
“Supercapacitors are already replacing batteries, such as in small-scale energy-harvesting applications, consumer, and commercial/industrial devices, as well as large-scale installations like wind turbines,” says Peter Buckle, vice president of sales and marketing at CAP-XX Limited.
“As environmental concerns increase, supercapacitors will continue to replace batteries. Supercapacitors allow designers to reduce the size and number of batteries required to power an application by handling the high-charge/discharge events in automotive apps and peak load leveling in portable electronics. The supercapacitor handles high power, allowing greater battery design and selection flexibility,” Buckle explains.
“In most instances, ultracapacitors are not inherently designed to replace batteries because they have lower energy densities than batteries. However, there are applications where a fast recharge and a higher current demand require an ultracapacitor over a battery,” says Chad Hall, vice president of sales and cofounder at Ioxus.
“Rather than replace batteries, designers are opting to pair them with an ultracapacitor for fast charges in higher numbers, which allows users to recharge quickly. Due to the high cycle life of an ultracap, users don’t need to replace the energy storage source for the life of the product. LED lights are a good representation of this,” Hall says.
“It must be noted that ultracapacitors are power devices (rapid charge and discharge) while batteries are energy devices (large energy storage capacity). Ultracapacitors store only a fraction of the energy of either primary or secondary batteries when compared on a cost and volume basis. Think power for seconds to minutes for ultracapacitors (a sprinter) and energy for minutes to hours (a marathon runner) with the batteries,” says Michael Everett, chief technical officer at Maxwell Technologies.
“Therefore, ultracapacitors aren’t a practical replacement for batteries in an application that requires extended energy output. They can replace batteries where the energy demand of the application is small and where a long operational lifetime, reliable operation in extreme temperatures, and rapid charge/discharge capabilities are preferred. Because of their complementary nature, solutions integrating ultracaps and batteries are becoming more common,” Everett says.
Innovation Drives Adoption
If necessity is the mother of invention, innovation must be its father. Although technological advances are evident, innovation these days seems to focus more on driving down prices, which in most cases makes any technology more viable and desirable. In the case of supercaps, we ask which significant technology breakthroughs within the past three to five years are making waves in the market now.
“Over that time, manufacturing cost has declined by a couple orders of magnitude. These cost reductions are mainly attributable to more efficient production processes for the activated carbon electrode material that stores and releases energy in an ultracapacitor, streamlined cell, and multi-cell module architectures to reduce part count and facilitate high-volume assembly, volume-related material cost reductions, and greatly improved power electronics,” says Everett.
“The next wave of technology improvements likely will be achievement of higher stable operating voltages, which will improve the energy density of the devices by a function of V2. As the stable operating voltage of the device increases, the energy available from that same device increases,” he says.
“This will not make them a direct competitor to batteries for high-energy applications, but it will enable ultracaps to deliver longer high-power run times proportional to the percentage of energy increase in the cell as a result of higher operating voltage,” says Everett.
“Low equivalent series resistance (ESR), which leads to high power, gives modern supercapacitors the ability to meet peak loads,” says Buckle. “Hybrid supercapacitors with one electrode from a battery and the other from a supercapacitor increase energy without sacrificing power or life.” Also, surface-mount technology will significantly increase the use of supercapacitors in portable electronics such as mobile phones, Buckle notes.
“The first automotive application to accept ultracapacitors for use in start/stop technology was a turning point in the market acceptance of ultracapacitors as a viable technology,” Hall says. “This has led to wider adoption of the technology in other markets, and engineers are beginning to see ultracapacitors as a solution to efficiency problems in designs. This will lead to further adoption in many more markets.”
Is It All In The Chemicals?
Ultracapacitors and/or supercapacitors are primarily electrolytic, chemical-based components, relying on some matter to store a charge. Electrolytic capacitors typically have relied on oil, paper, and other materials for this function. But what are the most viable chemicals or chemistries for today’s and tomorrow’s components?
“Currently, the best chemistry for an ultracapacitor is a symmetrical carbon/carbon electrode and acetonitrile (AN) electrolyte, as this delivers the highest performance and cycle life results at the lowest price point,” says Hall. “Other chemistries and constructions are being evaluated at many levels, but the lower costs of traditional ultracapacitors still rank higher for end users.”
“Competing chemistries each have their strengths. The name of the game in energy storage is value proposition: cost, volume, weight, operating environment range, and reliability. Those five parameters define the viability of any energy storage technology,” says Everett.
“In these areas, AN-based electrolyte chemistry coupled with high-surface-area conductive electrode materials are dominant factors. Power performance is superior in devices made with AN electrolytes versus any other electrolyte formulation. This applies across the entire temperature range,” Everett explains.
“Ultracaps with AN electrolyte outperform batteries from –10°C down to –40°C. Batteries are incapable of operating across this low temperature range. Further, caps made with other electrolyte formulations suffer serious performance limitations at –20°C and below and have higher ESR (equivalent series resistance) compared to AN-based devices,” he says.
“The two major supercapacitor chemistries today use aqueous and organic electrolytes. Organic electrolyte systems have the advantage in terms of power because of their higher cell voltage, up to 2.75 V, and dominate the large-cell market due to their high power and high energy when used in series configuration,” says Buckle.
“Aqueous, water-based systems have cheaper raw materials and often higher energy, but lower power. Ionic liquids, which do not use any solvents, offer possibilities in supercapacitor chemistry, but have yet to deliver in the mass market. And hybrid chemistries can be either water or organic,” Buckle says.
“Both organic and water-based systems based on metal oxides offer great promise, with aqueous lead-acid and nickel-hydroxide and organic lithium-ion systems being successfully commercialized,” he says.
CAP-XX has developed a supercapacitor module that delivers the necessary cranking current to start the engine in stop-start vehicles, also known as idle-stop and micro-hybrid vehicles (Fig. 1). The module promises to reduce wear on the battery in these vehicles, which turn the engine off when the vehicle is stopped in traffic and restart it when the driver releases the brake or engages the gears.
Using six of the company’s thin supercapacitors, the prototype stop-start supercapacitor module packs a peak current up to 300 A and is about the size of six DVD cases. Its capacitance is 150 F at 14 V, and its ESR is 4.5 mΩ.
For rugged, high-power transportation, alternative energy, medical, industrial, and consumer applications, the Ioxus iCAP series ultracapacitors sport a 3000-F capacity and specify the lowest ESR and highest power density available for energy-storage cells, according to the company (Fig. 2).
These ultracapcitors achieve what Ioxus calls the heaviest duty cycles possible and the longest life due to minimal temperature rise. They also offer weldable, double female threaded terminals, in addition to a 2.7-V working voltage, large-diameter terminals, and a single terminal design for bolted or welded connection.
Maxwell also offers 3000-F components, the 2.7-V Boostcap MC3000 power cells (Fig. 3). They feature a 2.7-V operating voltage, a low RC time constant, an ultra-low internal resistance, an operating temperature range from –40°C to 65°C, more than 1 million duty cycles, and threaded terminal or weldable post versions. The company also uses these power cells in its modular components.