Energy Storage Technologies: A Comparison

BatteriesFuel CellsCapacitorsSupercapacitorsComparison Chart
Batteries, fuel cells, capacitors, and supercapacitors are all energy storage devices. Batteries and fuel cells rely on the conversion of chemical energy into electrical energy. Capacitors rely on the physical separation of electrical charge across a dielectric medium such as a polymer film or an oxide layer. Each type of device provides a different combination of power density and energy density.
Supercapacitors rely on the separation of chemically charged species at an electrified interface between a solid electrode and an electrolyte. Only supercapacitors can provide a combination of high power density and relatively high energy density.
A battery is a device that transforms chemical energy into electric energy. All batteries have three basic components in each cell ? an anode, a cathode, and an anode and their properties relate directly to their individual chemistries. Batteries are broadly classified into primary and secondary.
Primary batteries are the most common and are designed as single use batteries, to be discarded or recycled after they run out. They have very high impedance which translates into long life energy storage for low current loads. The most frequently used batteries are carbon-zinc, alkaline, silver oxide, zinc air, and some lithium metal batteries(like lithium-thionyl-chloride).
Secondary batteries are designed to be recharged and can be recharged up to 1,00 times depending on the usage and battery type. Very deep discharges result in a shorter cycle life, whereas shorter discharges result in long cycle life for most of these batteries. The charge time varies from 1 to 12 hours, depending upon battery condition, Depth of Discharge (DoD), and other factors. Commonly available secondary batteries are Nickel-Cadmium, lead-acid, Nickel-Metal Hydride, some lithium metal, and Li-ion batteries.
Some of the limitations posed by secondary batteries are limited life, limited power capability, low energy-efficiency, and disposal concerns.

Fuel Cells
Like a battery, a fuel cell uses stored chemical energy to generate power. Unlike batteries, its energy storage system is separate from the power generator. It produces electricity from an external fuel supply as opposed to the limited internal energy storage capacity of a battery.
A typical fuel cell requires a large amount of extraneous control equipment like fuel pumps, cooling systems, fuel tanks, and re-circulators that make them impractical for portable applications. New developments like the small direct methanol fuel cell (DMFC) can do away with a large amount of the extraneous systems. Fuel cells range in size from hand-held systems to megawatt power stations. Most large fuel cells operate at high temperatures (200 ?C to 1000 ?C); the proton-exchange membrane fuel cell (PEMFC) may be able to operate at room temperature.
Fuel cells operate most efficiently over a narrow range of performance parameters and at elevated temperature, rapidly becoming inefficient under high power demands. Fuel cells will be used in tandem with either batteries or supercapacitors to provide a high-energy, high-power combination. Use of catalyst metals, such as platinum, makes fuel cells an expensive proposition.

Capacitors use physical charge separation between two electrodes to store charge. They store energy on the surfaces of metallized plastic film or metal electrodes; thus, the capacitance is a function of the dielectric medium and the overlapping surface areas. The surface area is a critical feature as the opposing charges are in close proximity separated by a dielectric medium. Most configurations contain a layered arrangement with a separation distance on the micrometer scale which is volumetrically inefficient.
Electrolytic capacitors rely on a layer of oxide material deposited on a metal surface. Here again, the thickness is on the micrometer scale and is very inefficient. Most capacitors can handle large voltages because they contain healing mechanisms that overcome the dielectric breakdown of the charge separation medium.
When compared to batteries and supercapacitors, the energy density of capacitors is very low? less than 1% of a supercapacitor’s, but the power density is very high, often higher than that of a supercapacitor. This means that capacitors are able to deliver or accept high currents, but only for extremely short periods, due to their relatively low capacitance.

Supercapacitors are very high surface area activated carbon capacitors that use a molecule-thin layer of electrolyte, rather than a manufactured sheet of material, as the dielectric to separate charge. The supercapacitor resembles a regular capacitor except that it offers very high capacitance in a small package. Energy storage is by means of static charge rather than of an electro-chemical proces inherent to the battery. Supercapacitors rely on the separation of charge at an electrified interface that is measured in fractions of a nanometer, compared with micrometers for most polymer film capacitors.
In supercapacitors, the solution between the electrodes contains ions from a salt that is added to an appropriate solvent. The operating voltage is controlled by the breakdown voltages of the solvents with aqueous electrolytes (1.1 V) and organic electrolytes (2.5 to 3 V).
There are three types of electrode materials suitable for the supercapacitor. They are: high surface area activated carbons, metal oxide, and conducting polymers. The high surface electrode material, also called Double Layer Capacitor (DLC), is least costly to manufacture and is the most common. It stores the energy in the double layer formed near the carbon electrode surface.
The lifetime of supercapacitors is virtually indefinite and their energy efficiency rarely falls below 90% when they are kept within their design limits. Their power density is higher than that of batteries while their energy density is generally lower. However, unlike batteries, almost all of this energy is available in a reversible process.