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supercapThe supercapacitor, also known as ultracapacitor or double-layer capacitor, differs from a regular capacitor in that it has a 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.

We group capacitors into three family types and the most basic is the electrostatic capacitor, with a dry separator. This capacitor has a very low capacitance and is used to filter signals and tune radio frequencies. The size ranges from a few pico-farad (pf) to low microfarad (uF). The next member is the electrolytic capacitor, which is used for power filtering, buffering and coupling. Rated in microfarads (uF), this capacitor has several thousand times the storage capacity of the electrostatic capacitor and uses a moist separator. The third type is the supercapacitor, rated in farads, which is again thousands of times higher than the electrolytic capacitor. The supercapacitor is ideal for energy storage that undergoes frequent charge and discharge cycles at high current and short duration.

Faradis a unit of capacitance named after the English physicist Michael Faraday. One farad stores one coulomb of electrical charge when applying one volt. One microfaradis 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 the electric double-layer capacitor, which led to the development of an early type of supercapacitor in 1957. There were no known commercial applications then. In 1966, Standard Oil rediscovered the effect of the double-layer capacitor by accident while working on experimental fuel cell designs. The company did not commercialize the invention but licensed it to NEC, which 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 modern supercapacitor is not a battery per se but crosses the boundary into battery technology by using special electrodes and electrolyte. Several types of electrodes have been tried and we focuse on the double-layer capacitor (DLC) concept. It is carbon-based, has an organic electrolyte that is easy to manufacture and is the most common system in use today.

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 they would reduce the service life. To achieve higher voltages, several supercapacitors are connected in series. This has disadvantages. Serial connection reduces the total capacitance, and strings of more than three capacitors require voltage balancing to prevent any cell from going into over-voltage. This is similar to the protection circuit in lithium-ion batteries.

The specific energy of the supercapacitor is low and ranges from 1 to 30Wh/kg. Although high compared to a regular capacitor, 30Wh/kg is one-fifth that of a consumer Li-ion battery. 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 from full to zero voltage. This reduces the usable power spectrum and much of the stored energy is left behind. Consider the following example.

Take a 6V power source that is allowed to discharge to 4.5V before the equipment cuts off. With the linear discharge, the supercapacitor reaches this voltage threshold within the first quarter of the cycle and the remaining three-quarters of the energy reserve become unusable. A DC-to-DC converter could utilize some of the residual energy, but this would add to the cost and introduce a 10 to 15 percent energy loss. A battery with a flat discharge curve, on the other hand, would deliver 90 to 95 percent of its energy reserve before reaching the voltage threshold. Table 1 compares the supercapacitor with a typical Li-ion.
 

Function

Supercapacitor

Lithium-ion (general)

Charge time

Cycle life

Cell voltage

Specific energy (Wh/kg)

Specific power (W/kg)

Cost per Wh

Service life (in vehicle)

Charge temperature

Discharge temperature

1–10 seconds

1 million or 30,000h

2.3 to 2.75V

5 (typical)

Up to 10,000

$20(typical)

10 to 15 years

–40 to 65°C (–40 to 149°F)

–40 to 65°C (–40 to 149°F)

10–60 minutes

500 and higher

3.6 to 3.7V

100–200

1,000 to 3,000

$2 (typical)

5 to 10 years

0 to 45°C (32°to 113°F)

–20 to 60°C (–4 to 140°F)

Table 1: Performance comparison between supercapacitor and Li-ion

Courtesy of Maxwell Technologies, Inc.

Rather than operating as a stand-alone energy storage device, supercapacitors work well as low-maintenance memory backup to bridge short power interruptions. Supercapacitors have also made critical inroads into electric powertrains. The virtue of ultra-rapid charging and delivery of high current on demand makes the supercapacitor an ideal candidate as a peak-load enhancer for hybrid vehicles, as well as fuel cell applications.

The charge time of a supercapacitor is about 10 seconds. The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited by the charger. The initial charge can be made very fast, and the topping charge will take extra time. Provision must be made to limit the initial current inrush when charging an empty supercapacitor. The supercapacitor cannot go into overcharge and does not require full-charge detection; the current simply stops flowing when the capacitor is full.

The supercapacitor can be charged and discharged virtually an unlimited number of times. Unlike the electrochemical battery, which has a defined cycle life, there is little wear and tear by cycling a supercapacitor. Nor does age affect the device, as it would 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 functions well at hot and cold temperatures.

The self-discharge of a supercapacitor is substantially higher than that of an electrostatic capacitor and somewhat higher than the electrochemical battery. The organic electrolyte contributes to this. The stored energy of a supercapacitor decreases from 100 to 50 percent in 30 to 40 days. A nickel-based battery self-discharges 10 to 15 percent per month. Li-ion discharges only five percent per month.

Supercapacitors are expensive in terms of cost per watt. Some design engineers argue that the money for the supercapacitor would better be spent on a larger battery. We need to realize that the supercapacitor and chemical battery are not in competition; rather they are different products serving unique applications.Table 2 summarizes the advantages and limitations of the supercapacitor.
 

Advantages

Virtually unlimited cycle life; can be cycled millions of time

High specific power; low resistance enables high load currents

Charges in seconds; no end-of-charge termination required

Simple charging; draws only what it needs; not subject to overcharge

Safe; forgiving if abused

Excellent low-temperature charge and discharge performance

Limitations

Low specific energy; holds a fraction of a regular battery

Linear discharge voltage prevents using the full energy spectrum

High self-discharge; higher than most batteries

Low cell voltage; requires serial connections with voltage balancing

High cost per watt

Table 2: Advantages and limitations of supercapacitors

 

Text taken from  batteryuniversity.com