Supercapacitors
One of the most critical aspects of an uninterruptible power supply (UPS) is its backup or battery backup system, where supercapacitors are now beginning to play a role.
A supercapacitor looks like a normal capacitor, except that it offers high capacitance in a small package. Energy storage is done by static charging rather than an electrochemical process, inherent in lead-acid uninterruptible power supply batteries. Applying a voltage differential across the positive and negative plates charges the supercapacitor (this concept is similar to an electrical charge that builds up when walking on a carpet).
Their design makes them ideal for small uninterruptible power supply installations where they are used in favor of a battery pack or to reduce the potential for battery discharge during momentary power grid failures.
The amount of energy that can be stored depends on the active material used in the design of a supercapacitor. It can potentially reach up to 30 kW of stored energy.
A supercapacitor (also known as an electric double-layer capacitor, electrochemical double-layer capacitor, or ultracapacitor) consists of two electrodes constructed of a highly activated carbon material, which can be woven. Whereas a regular capacitor consists of conductive foils and a dry separator, the supercapacitor goes into battery technology by using special electrodes and some electrolyte. There are three types of electrode materials suitable for the supercapacitor: large-area activated carbons, metal oxide, and conductive polymers. The high-surface electrode material, also called a double layer capacitor (DLC), is less expensive to manufacture and is the most common. It stores the energy in the double layer formed near the surface of the carbon electrode.
Activated carbon electrodes provide a large cross-linked area on which an active material such as ruthenium oxide is deposited. The material provides a huge area, for example 1000 square meters per gram of material used. Cellulose paper with polymeric fibers to provide reinforcement is typically used as a spacer between the electrodes. The electrolyte is usually dilute sulfuric acid. Ruthenium oxide is converted to ruthenium hydroxide through a chemical reaction and this allows energy to be stored.
To operate at higher voltages, the supercapacitors are connected in series. In a chain of more than three capacitors, a voltage balance is required to prevent any cell from reaching an overvoltage.
The energy within a supercapacitor is readily available, and this is one of its greatest advantages. When attached to an existing battery pack, they can inhibit battery cycling through momentary interruptions, helping to extend equipment life. The lifespan of a supercapacitor is typically ten years (twice that of an average UPS battery). They can also operate in a wide temperature range (minus 30 to 45 degrees Celsius).
Other advantages
o Virtually unlimited life cycle: it can be cycled millions of times.
o Low impedance: improves load handling when placed in parallel with a battery.
o Fast charging: supercapacitors charge in seconds.
o Simple charging methods: no need for full charge detection; no danger of overload.
Limitations
o Linear discharge voltage avoids the use of the entire energy spectrum.
o Low energy density: it generally contains between one fifth and one tenth of the energy of an electrochemical battery.
o Cells have low voltages: series connections are needed to obtain higher voltages. It is necessary to balance the voltage if more than three capacitors are connected in series.
o High self-discharge: the rate is considerably higher than that of an electrochemical battery.
While the electrochemical battery delivers a constant voltage across the spectrum of usable energy, the supercapacitor voltage is linear and drops evenly from full voltage to zero volts. Because of this, it cannot deliver the full load. If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment shuts down, the supercapacitor reaches that threshold within the first quarter of the discharge cycle. The remaining power slides into an unusable voltage range. A DC-to-DC converter could correct this problem, but such a regulator would add costs and introduce a 10-15 percent loss of efficiency.
The charging time of a supercapacitor is about 10 seconds. The ability to absorb energy is largely limited by the size of the charger. The charging characteristics are similar to those of an electrochemical battery. The initial charge is very fast; the coverage charge requires more time. Steps must be taken to limit the current when charging an empty supercapacitor.
In terms of charging method, the supercapacitor looks like a lead-acid battery. Full charge occurs when a set voltage limit is reached. But unlike the electrochemical battery, the supercapacitor does not require a full charge sensing circuit. Supercapacitors consume as much energy as necessary. When they are full, they stop accepting charges. There is no danger of overload or “memory”.
Supercapacitors are relatively expensive in terms of cost per watt. Some design engineers argue that it would be better to spend the money to provide a larger battery by adding additional cells. But the supercapacitor and the chemical battery are not necessarily in competition. They improve each other.