There are two important factors that characterize power storage devices – energy density and power density. Energy density indicates the amount of energy the unit can store per unit of mass (or volume) of the device, while the power density is represented by the amount of energy the unit can deliver to the consumer in time (again, per unit mass or volume of the device). Batteries are most commonly used in portable devices due to their ability for storing large quantities of energy (which means they have a high energy density). However, they have a low power density, unlike the capacitors that can deliver a large amount of energy in short time, but cannot store it.
A supercapacitor, an ultracapacitor, or an electrochemical double layer capacitor (EDLC) differs from the conventional capacitor in that it has much higher capacity and energy density, while at the same time having a higher power density compared to a battery. Its properties additionally are the small equivalent serial resistance (ESR), long lifetime and small physical dimension and mass. These characteristics make it a convenient power source for devices that require high power and durability of the power unit.
What is Capacitor?
The capacitor is a passive electrical element, which accumulates energy in the electric field between the two conducting electrodes. The process of accumulating energy in the capacitor (charging the capacitor) involves accumulation of opposite polarity charges on the capacitor electrodes, thus generating a potential difference between the electrodes. The capacitor consists of two metal electrodes and a dielectric between them which ensures that there is no displacement of the charges directly from one electrode to the other. Electric charge can pass from one electrode to the other through the external circuit in which the capacitor is connected. When the external circuit of the capacitor is removed, the electrodes remain charged. The gathered charges on the electrodes attract each other and generate an electric field between them.
Electrostatic capacitors (capacitors with a dry separator) are considered a first development phase. Also known as classic capacitors, they are characterized with a very low capacitance and their main application is in the radio technology and filtering. The size ranges from a few pico-farads (pf) to low microfarads (μF). The second stage is represented by electrolytic capacitors (containing electrolyte (conductive liquid) between dielectrics and electrodes) which provide higher capacitance (rated in microfarads μF). These capacitors are used for filtering, buffering and signal coupling. They have a cell construction similar to that of a battery, but the anode and cathode are made of the same material. The third type is the supercapacitor, rated in farads (thousands times higher than the electrolytic capacitor).
A capacitor can have the following functions:
- Decoupling – prevents undesired coupling (current/energy transfers), at the same time maintaining voltage stability
- Filtering – removes/reduces unwanted frequencies
- Coupling – blocks DC component, thus separating different voltage levels
- Timing and wave shaping – can help in building timing (delay) circuits
What is Supercapacitor?
The electric / electrochemical dual layer capacitor is a unique energy storage device, since it has a much larger capacity than conventional capacitors and a much higher power density than batteries. This means that it can be used as a back-up power supply for electronic devices, load balancing, initiation or acceleration of hybrid vehicles motors, and storing electricity from solar or wind power plants. The supercapacitors consist of two metal electrodes that have been subjected to many layers of active nanoporous carbon, and between them there is an electrolyte membrane. The active charcoal is a powder made up of extremely small and round fragments that together form a sponge-like structure with nano-metered bundles, which results in a large effective electrode surface of a few hundred m2/g. Each layer of the activated carbon is quite conductive, resulting in a low internal resistance of the supercapacitor, while the contact between the adjacent layers represents some kind of dielectric, with a thickness of the order of nanometer and can withstand a voltage of 2 to 3 volts. The high effective electrode surface and the maximum amount of “dielectric” result in extremely high capacitance. Supercapacitors for higher voltages and currents are made by connecting several supercapacitors in series or parallel.
Active carbon is not the most suitable material for supercapacitors use. Namely, the dimensions of the free electrons are often larger than the pores in activated carbon, so the pores cannot accept them, thus limiting the storage capacity. Today’s research is focused on finding materials that will offer even more effective surface.
Supercapacitors have the following advantages:
- Very long cycle life – even after hundreds of thousands charging and discharging cycles the supercapacitor properties are slow degrading and are not subject to wear and aging as the electrochemical battery
- Low impedance (ESR) – parallel coupling with an electrochemical battery can improve the performance at impulse currents
- Quick charge and discharge – unlike rechargeable batteries, low impedance supercapacitors charge in a few seconds
- Easy charging – no need for detection until the end of the charge as there is no danger of overloading
- Viability of storage saving – low price per charge / discharge cycle, and the large number of cycles compensate for the lower energy density
- A supercapacitor can replace a large number of conventional capacitors
- Reduced voltage drop compared to devices running on batteries without supercapacitor in parallel
- Unlimited charging (unlike rechargeable batteries)
- Increased temperature range – allows the use of batteries at very low temperatures
- There is no chemical reaction on the electrodes – much slower aging and degradation of the properties compared to batteries and no significant heating
- Meets ecological standards
- Increased safety – supercapacitors do not explode due to overload
- Low energy density – approximately one-fifth (to tenth) of those of electrochemical battery
- Low voltage – need to be connected in series to get higher voltage values
- Linear discharge – the entire energy spectrum cannot be used and, depending on the application, not all energy is available
- Larger self-discharge
Difference Between Capacitor and Supercapacitor
1. Design of Capacitor and Supercapacitor
Conventional capacitors are designed by two electrodes and an insulator. Electrons move from one electrode to the other, and the charge is separated by a solid dielectric (ceramic, glass, plastic film, paper, aluminum oxide) between the electrodes. In supercapacitors, the electrodes are coated with activated carbon and instead of a solid dielectric, the two electrodes are separated by a liquid electrolyte.
2. Parameters of Capacitor and Supercapacitor
Super-capacitors have a higher energy density than normal capacitors (conventional <0.1; supercapacitor 1-10 Wh/kg). Other specifications are the faster charge and discharge time, lower specific power (W/kg), wider span of operating temperature, lower impulse current, limited operating voltage etc.
3. Application of Capacitor and Supercapacitor
Capacitors and supercapacitors find application in different segments – capacitors are often used as timing devices, filters, for smoothing the voltage in circuits, for tuning (in radios and TVs). Supercapacitors are used for load regulation of electric and hybrid vehicles, as conventional vehicle starters, in telecommunications, consumer electronics which require high power impulses (power tools, digital cameras, mobile devices), electricity storage in solar and wind power plants or as a backup power.
4. Cost of Capacitor and Supercapacitor
Generally supercapacitors are more expensive than conventional ones.
Capacitor vs. Supercapacitor : Table to show the difference between Capacitor and Supercapacitor
Summary of Capacitor and Supercapacitor
- Although based on the same principles as conventional capacitors, supercapacitors have design differences that result in a bigger area for storing much more charge (much higher capacity).
- Supercapacitors are filling the gap between dielectric capacitors and batteries. The main advantages of supercapacitors are the high power density, high efficiency, short charge and discharge times, long cycle life and the wide range of operating temperatures. In addition, supercapacitors are environmentally acceptable. Their main disadvantage is the very low maximum voltage which is not sufficient in some applications.
- Supercapacitors are more expensive than conventional capacitors, as they have high cost per watt.
- Capacitors are used for energy smoothing, as backup power, in signal processing, electronics and in many other industrial and commercial applications. Supercapacitors are now widely used as energy sources for integrated memory or microprocessors, load regulation of electric and hybrid vehicles, as motor starters, in telecommunication, in consumer electronics that require high power pulses, to save electricity in solar and wind power plants and so on.