Ultracapacitors - Review of Literature

The amount of electrical energy or charge a capacitor can store is determined by its capacitance. Capacitance is measured in Farads, named after Michael Faraday (Kosel, 2007, para.10). As a Farad (one coulomb per volt for each electrode) is a huge amount of electrical energy as far as electronic devices are concerned, many commercially available capacitors, use microfarads which is one-millionth of a Farad or Pico-farads which is one-millionth of a microfarad as their unit of capacitance (Kosel, 2007, para.10). Capacitors when charged gather electrons of opposite charges, hence the positive electrons gather on the negative plate and the negative electrons gather on the positive plate. Both plates receive the same amount of charge, but they are opposite. This causes the build up of electrostatic charge. To increase the amount of electrostatic charge, which determines capacitance, either the surface area of the electrodes needs to be increased or the separation between the plates needs to be reduced. The capacitance of a capacitor is dependent on four factors: area of the electrodes, distance between the electrodes, type of insulator, and temperature. The capacitance is directly proportional to the area of the electrodes and inversely proportional to the distance between them (Flower, 2003, p.231). Some insulators function better than others, for example, pure water is a better insulator than oil. In general the more resistance of the dielectric material, the better the efficiency of the capacitor. Temperature is the least important of all the four factors, although some conductors and insulators function better at very low temperatures.

One of the main advantages of capacitors when compared to batteries is their charging time. While batteries may take a few hours to charge, a capacitor will only take a few seconds. The charge and discharge cycle of capacitors are measured in time constants. A time constant is the time taken to charge 63.2% of the rated capacity of a capacitor. There are five time constants in one whole charge or discharge cycle. Each time constant charges 63.2% of the remaining charge on an incremental basis. During charging under each time constant the voltage increases proportionally with charge. The mathematical equation for time constant is (Time constant = Resistance * Capacitance). Another characteristic that indicates the performance of both batteries and capacitors is leakage current. Leakage current is the amount of power lost while a capacitor or battery when plugged into the load device is in the off state (Claudia, Ken, 2007, p.1). Characterizing leakage current, predicts how long a battery will last while the appliance is not in use. Power can also drain when a capacitor is running. To utilize power fully, the highest possible current and voltage must be used (How it Works Science and Technology, 2003, p.724). Low Power loss is one of the many advantages capacitors have over batteries. Capacitors can use more than 90% of their power even after repeated recharges. Where as, batteries lose a significant amount of their charge retention capability after each recharge.

With the population increasing dramatically, and with the country’s dependence on the depleting oil resources, it is essential that a new renewable power source be found before fossil fuels run out. Demand for oil has gone up drastically over the past few year. How long can the people live like this? New technologies have been making electric cars run mainly on batteries, but will this be enough? When capacitors replace batteries in electric cars, they will run and function better due to the fact that capacitors can be powered faster, providing a better startup and acceleration for the car, and future technologies are making them considerably lighter, and lasting longer. Capacitors were once unknown, as energy resources, but for the past five years scientists have been improving the quality and cost of capacitors exponentially. With their superior power output advantage over batteries, capacitors are sure to emerge as the main electric storage device replacing the omnipresent batteries. “In fuel cell vehicles, ultracapacitors have demonstrated a higher recovery of energy from braking than batteries, are considerably lighter, have a longer economic life, and are more environmentally friendly in their manufacture and disposal”(Miller, 2004, para.2). The earth is already filled with garbage full of toxins, batteries will only add to this problem by their poison liquid electrolyte. Now, what should the world go for, capacitors or batteries?