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. Contact online >>
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.
Supercapacitors, bridging conventional capacitors and batteries, promise efficient energy storage. Yet, challenges hamper widespread adoption. This review assesses energy density limits, costs, materials, and scalability barriers.
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.
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.
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.
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.
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