BU-209: How does a Supercapacitor Work?

The supercapacitor, also known as ultracapacitor or double-layer capacitor, differs from a regular capacitor in that it has very high capacitance. A capacitor stores energy by means of a static charge as opposed to an electrochemical reaction. Applying a voltage differential on the positive and negative plates charges the capacitor. This is similar to the buildup of electrical charge when walking on a carpet. Touching an object releases the energy through the finger.

There are three types of capacitors and the most basic is the electrostatic capacitor with a dry separator. This classic capacitor has very low capacitance and is mainly used to tune radio frequencies and filtering. The size ranges from a few pico-farads (pf) to low microfarad (μF).

The electrolytic capacitor provides higher capacitance than the electrostatic capacitor and is rated in microfarads (μF), which is a million times larger than a pico-farad. These capacitors deploy a moist separator and are used for filtering, buffering and signal coupling. Similar to a battery, the electrostatic capacity has a positive and negative that must be observed.

The third type is the supercapacitor, rated in farads, which is thousands of times higher than the electrolytic capacitor. The supercapacitor is used for energy storage undergoing frequent charge and discharge cycles at high current and short duration.

Farad is a unit of capacitance named after the English physicist Michael Faraday (1791–1867). One farad stores one coulomb of electrical charge when applying one volt. One microfarad is one million times smaller than a farad, and one pico-farad is again one million times smaller than the microfarad.

Engineers at General Electric first experimented with an early version of supercapacitor in 1957, but there were no known commercial applications. In 1966, Standard Oil rediscovered the effect of the double-layer capacitor by accident while working on experimental fuel cell designs. The double-layer greatly improved the ability to store energy. The company did not commercialize the invention and licensed it to NEC, who in 1978 marketed the technology as “supercapacitor” for computer memory backup. It was not until the 1990s that advances in materials and manufacturing methods led to improved performance and lower cost.

The supercapacitor has evolved and crosses into battery technology by using special electrodes and electrolyte. While the basic Electrochemical Double Layer Capacitor (EDLC) depends on electrostatic action, the Asymmetric Electrochemical Double Layer Capacitor (AEDLC) uses battery-like electrodes to gain higher energy density, but this has a shorter cycle life and other burdens that are shared with the battery. Graphene electrodes promise improvements to supercapacitors and batteries but such developments are 15 years away.

Several types of electrodes have been tried and the most common systems today are built on the electrochemical double-layer capacitor that is carbon-based, has an organic electrolyte and is easy to manufacture.

All capacitors have voltage limits. While the electrostatic capacitor can be made to withstand high volts, the supercapacitor is confined to 2.5–2.7V. Voltages of 2.8V and higher are possible, but at a reduce service life. To get higher voltages, several supercapacitors are connected in series. Serial connection reduces the total capacitance and increases the internal resistance. Strings of more than three capacitors require voltage balancing to prevent any cell from going into over-voltage. Lithium-ion batteries share a similar protection circuit.

The specific energy of the supercapacitor ranges from 1Wh/kg to 30Wh/kg, 10–50 times less than Li-ion. The discharge curve is another disadvantage. Whereas the electrochemical battery delivers a steady voltage in the usable power band, the voltage of the supercapacitor decreases on a linear scale, reducing the usable power spectrum. (See BU-501: Basics About Discharging)

Take a 6V power source that is allowed to discharge to 4.5V before the equipment cuts off. By the time the supercapacitor reaches this voltage threshold, a linear discharge only delivers 44% of the energy; the remaining 56% is reserved. An optional DC-DC converter helps to recover the energy dwelling in the low voltage band, but this adds costs and introduces loss. A battery with a flat discharge curve, in comparison, delivers 90 to 95 percent of its energy reserve before reaching the voltage threshold.

Figures 1 and 2 demonstrate voltage and current characteristics on charge and discharge of a supercapacitor. On charge, the voltage increases linearly and the current drops by default when the capacitor is full without the need of a full-charge detection circuit. This is true with constant current supply and voltage limit that is suitable for the capacitor rated voltage; exceeding the voltage could damage the capacitor.

The charge time of a supercapacitor is 1–10 seconds. The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited by the charger’s current handling capability. The initial charge can be made very fast, and the topping charge will take extra time. Provision must be made to limit the inrush current when charging an empty supercapacitor as it will suck up all it can. The supercapacitor is not subject to overcharge and does not require full-charge detection; the current simply stops flowing when full.

Table 3 compares the supercapacitor with a typical Li-ion.

The supercapacitor can be charged and discharged a virtually unlimited number of times. Unlike the electrochemical battery, which has a defined cycle life, there is little wear and tear by cycling a supercapacitor. Age is also kinder to the supercapacitor than a battery. Under normal conditions, a supercapacitor fades from the original 100 percent capacity to 80 percent in 10 years. Applying higher voltages than specified shortens the life. The supercapacitor is forgiving in hot and cold temperatures, an advantage that batteries cannot meet equally well.

The self-discharge of a supercapacitor is substantially higher than that of an electrostatic capacitor and somewhat higher than an electrochemical battery; the organic electrolyte contributes to this. The supercapacitor discharges from 100 to 50 percent in 30 to 40 days. Lead and lithium-based batteries, in comparison, self-discharge about 5 percent per month.

Supercapacitor vs. Battery

Comparing the supercapacitor with a battery has merits, but relying on similarities prevents a deeper understanding of this distinctive device. Here are unique differences between the battery and the supercap.

The chemistry of a battery determines the operating voltage; charge and discharge are electrochemical reactions. In comparison, the capacitor is non-electrochemical and the maximum allowable voltage is determined by the type of dielectric material used as separator between the plates. The presence of electrolyte in some capacitors boosts the capacitance and this may cause confusion.

Since the supercap is non-chemical, the voltage is free to rise until the dielectric fails. This is often in the form of a short circuit. Avoid going higher than the specified voltage.

Applications

The supercapacitor is often misunderstood; it is not a battery replacement to store long-term energy. If, for example, the charge and discharge times are more than 60 seconds, use a battery; if shorter, then the supercapacitor becomes economical.

Supercapacitors are ideal when a quick charge is needed to fill a short-term power need; whereas batteries are chosen to provide long-term energy. Combining the two into a hybrid battery satisfies both needs and reduces battery stress, which reflects in a longer service life. Such batteries are being made available today in the lead acid family.

Supercapacitors are most effective to bridge power gaps lasting from a few seconds to a few minutes and can be recharged quickly. A flywheel offers similar qualities, and an application where the supercapacitor competes against the flywheel is the Long Island Rail Road (LIRR) trial in New York. LIRR is one of the busiest railroads in North America.

To prevent voltage sag during acceleration of a train and to reduce peak power usage, a 2MW supercapacitor bank is being tested in New York against flywheels that deliver 2.5MW of power. Both systems must provide continuous power for 30 seconds at their respective megawatt capacity and fully recharge in the same time. The goal is to achieve a regulation that is within 10 percent of the nominal voltage; both systems must have low maintenance and last for 20 years. (Authorities believe that flywheels are more rugged and energy efficient for this application than batteries. Time will tell.)

Japan also employs large supercapacitors. The 4MW systems are installed in commercial buildings to reduce grid consumption at peak demand times and ease loading. Other applications are to start backup generators during power outages and provide power until the switch-over is stabilized.

Supercapacitors have also made critical inroads into electric powertrains. The virtue of ultra-rapid charging during regenerative braking and delivery of high current on acceleration makes the supercapacitor ideal as a peak-load enhancer for hybrid vehicles as well as for fuel cell applications. Its broad temperature range and long life offers an advantage over the battery.

Supercapacitors have low specific energy and are expensive in terms of cost per watt. Some design engineers argue that the money for the supercapacitor would be spent better on a larger battery. Table 4 summarizes the advantages and limitations of the supercapacitor.

References

[1] Source: PPM Power
[2] Source: Maxwell Technologies, Inc.