The benefits that supercapacitors bring to the energy storage market are considerable. Yet, few designers seem aware of these small but powerful components.
Perhaps that is why some analysts feel that supercapacitors are still a small niche market that is being held back from wide adoption by a high price-level and lack of visibility in the industry. But an increasing number of analysts see these devices as a mature technology with a growing range of applications. For example, Market Tech Research predicts that supercapacitors will drive $3.5 billion in energy storage revenues in 2020, accounting for 5 percent of the battery energy storage market in 2020.
Asia-Pacific has trumped all other regional markets to become a major manufacturer in the global supercapacitors market in 2012, noted a recent Transparency Market Research (TMR) report. Advances in applications such as regenerative breaking, HEVs, micro-hybrids/stop-start systems, and aerospace/military applications have spurred the growth in supercapacitor technologies. These new and innovative applications are expected to drive the global ultracapacitors market to a considerable degree in the coming years.
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Supercapacitors, also known as ultracapacitors or electronic double-layer capacitors (EDLC), have a growing role to play in energy storage systems. Elon Musk, CEO of Tesla Motors, is credited with saying that these capacitors will even “supersede” battery technology. What makes supercapacitors and ultracapacitors different from their more common brethren? How do they compare with batteries and fuel cells? What is the growth market and application space for these capacitors on steroids? This article will answer these questions.
Superconductor Differences
Supercapacitors are electrochemical capacitors with energy densities that are order of magnitudes greater that densities of conventional capacitors. The amount of energy that can be stored in a capacitor per volume of that capacitor is called its energy density. Energy density is measured volumetrically (per unit of volume) in watt-hours per liter (Wh/l).
Compared to batteries, supercapacitors have a low internal resistance, which means they can achieve high power densities (Figure 1). Power density refers to the speed at which energy can be delivered to/absorbed from the load.
Figure 1: The maximum power for supercapacitors. (Source: Author Rendered)
Another advantage of supercapacitors is their fast charge and discharge rates. These devices achieve charging and discharging through the absorption and release of ions. Thanks to the supercapacitors’ low internal series resistance, high current charging and discharging is achievable without any damage to the parts.
The two main disadvantages of supercapacitors are low per-cell voltage and unsuitability in AC and high-frequency circuits. Depending upon the material composition of the supercapacitor, the cell voltage load can range between 2.1 V to 4 V. But the typical value is 2.7 V. If an application requires a higher voltage, then several cells must be connected together in series.
Supercapacitive devices are not well-suited for AC and high-frequency circuits due to time-constant issues. For supercapacitors, the RC time constant has a different connotation then the usual reference to filters for common capacitors.
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One way to utilize RC time constants is as a measure of the time it takes for the capacitor to be charged and discharged. These time periods are significant for power and energy applications using capacitors for energy storage. Conventional capacitors will have RC time constants on the order of microseconds to tens of microseconds. Conversely, supercapacitors have RC time constant of roughly a second. This slower response time is due to the larger capacitance (Farads).
However, this larger capacitance makes supercapacitors ideal for energy storage. When compared with other energy storage devices—such as batteries—the superconductors are considered extremely fast. This makes them ideal for an application needing to store and return quick surges of energy, such as is the case of regenerative braking.
“Historically the ability of capacitors to act as a ‘store of energy’ was so minuscule compared to batteries that they were never seriously considered as an alternative,” noted Jamil Kawa, Synopsys Scientist. . “Supercapacitors and ultracapacitors have dramatically increased the volumetric capacity for energy storage. It is not yet on par with batteries or fuel cells, but is close enough to be competitive with them given that it has some qualities that are superior to batteries.”
Batteries have long been the standard for energy storage. How can they be bested by a capacitor, even a ‘super” one? For a start, batteries lose the capability to be recharged after several hundreds or thousands of cycles. However, for all practical purposes, capacitors have an infinite lifetime of charging and discharging cycles.
Secondly, capacitors have very low internal resistance compared to batteries. They are capable of delivering higher instantaneous power (energy-per-unit time) than batteries. “This is very important,” explains Kawa. “While a battery can have a larger energy content than a competing supercapacitor, the latter can deliver higher instantaneous power.”
Lastly, for the Internet-of-Things (IoT) applications with an energy-harvesting mechanism, the capability to incorporate such powerful energy storage devices into a chip is a critical requirement. Supercapacitors and micro-batteries are good candidates for these needs.
Supercapacitors Versus Other Energy Storage Technologies
How do supercapacitors compare to other storage devices like batteries and even fuel cells? Figure 2 provides a good comparison of where each category of batteries and supercapacitors fit in the power density / energy density matrix. Fuel cells are at the head of the pack in energy density but have a rather low power density. Batteries, in general, still have higher energy density than supercapacitors, but, supercapacitors are not far behind, and some excel over batteries in power densities.
Figure 2: Rechargeable batteries verses supercapacitors for on-chip storage solutions. (Source: Courtesy of Synopsys)
Applications
Today, supercapacitors and ultracapacitors are used to stabilize fluctuating loads and for providing a quick charge for mobile electronics. Further, these capacitive devices serve as a power buffer to mitigate voltage swings and much more.
Large supercapacitors and ultracapacitors, like KEMET's, are increasingly making it to industrial applications where an initial high torque is needed. Smaller supercapacitors, especially the ones that can be integrated in small-form modules, are going to be powering applications where a battery replacement is not an option, e.g., energy harvesting.
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In most applications, supercapacitors will not replace batteries but will likely co-exist with them. According to Kawa, supercapacitor devices will be used where high levels of instantaneous power are needed and for recharging quickly as with energy harvesting technologies. Batteries will be needed for the “dark periods” when renewable energy sources are intermittent.