Supercapacitor: what it is, where to apply and how to use it as a DC power source

Supercapacitors are a new class of sources close to powerful capacitors in function, and in fact - occupying a niche between capacitors and constant current sources. Not everyone knows what it is. Supercapacitors, ultracapacitors are meant by supercapacitors. The international designation EDLC is Electric double-layer capacitor, on the electrical circuits it is designated as R1.

Historical reference

In 1957, early versions of supercapacitors were developed by engineers at General Electric, but they did not have commercial applications due to their low efficiency. In 1966, Standard Oil accidentally discovered the double-layer condenser effect while working on fuel cells, which allowed the supercapacitor to function efficiently. The company did not commercialize the invention, but received a license to NEC. In 1978 she sold this technology as a "supercapacitor" for computers. In the USSR, EDLCs were first presented in 1978 in the publication of the magazine Radio No. 5 of the KI1-1 series with a capacity of 0.2 to 50.0 F.

The first supercapacitors for heavy-duty equipment were created in 1982 by PRI Ultracapacitor. It was only in the 1990s that progress was made in materials and production methods, which led to increased productivity and reduced cost of supercapacitors. They continue to develop and move into industrial battery technology using special electrodes and electrolyte.

Purpose of the electronic device

Supercapacitors (EDLCs) are electronic devices that are used to store extremely large amounts of electrical charge. They are also known as supercapacitors, double layer capacitors, or ultracapacitors. Instead of using a conventional dielectric, EDLC uses a mechanism for storing electrical energy - a two-layer capacitor. This means that they combine the work of conventional capacitors with the work of conventional batteries. The capacities achieved using this technology can be as high as 12,000 F. For comparison, the capacity of the entire Earth is only about 710 μF, which is more than 15 million times less than the capacity of the EDLC.

While a typical electrostatic capacitor can have a high maximum operating voltage, the typical maximum charge voltage of an EDLC is between 2.5 and 2.7 volts. EDLCs are polarized devices, meaning they must be connected to the circuit correctly, like electrolyte capacitors. The electrical properties of these devices, especially their fast charging and discharging times, are very promising for many industries where they can completely replace batteries.

Supercapacitor design and materials

Let's consider in more detail that is a supercapacitor. The design of EDLC is similar to that of electrolytic capacitors in that it consists of two foil electrodes, an electrolyte, a separator and a foil. The separator is sandwiched between electrodes, the foil is rolled or folded into a shape, usually cylindrical or rectangular. This folded mold is housed in a hermetically sealed enclosure impregnated with electrolyte. The electrolyte in the EDLC design, as well as the electrodes, is different from the electrolyte used in conventional electrolytic capacitors.

To conserve electrical charge, EDLC uses porous materials as spacers to store ions in pores at the atomic level. The most common material in modern EDLCs is activated carbon. The fact that carbon is not a good insulator limits the maximum operating voltage to 3 V.

Activated carbon is not an ideal material: charge carriers are comparable in size to the pores in material, and some of them cannot penetrate into smaller pores, which leads to leaks and a decrease in capacity storage.

One of the more interesting materials used in EDLC research, is graphene. It is a substance composed of pure carbon in a flat sheet only one atom thick. It is extremely porous and acts like an ionic sponge. The energy density achieved with graphene in EDLC is comparable to the energy density obtained in batteries.

However, while graphene EDLC prototypes have been made as proof of future concept, they are expensive and difficult to produce on an industrial scale, and this circumstance significantly slows down the use of this technologies. Despite this, graphene EDLC is the most promising candidate for future supercapacitor technology.

Advantages and disadvantages

Among the advantages of the device, the following should be highlighted:

1. Charge time. EDLCs have charging and discharging times comparable to those of conventional capacitors. Due to the low internal resistance, high charge and discharge currents can be achieved. It usually takes up to several hours to reach a fully charged battery. For example, like a cell phone battery, while an EDLC can be charged in less than two minutes.
2. Specific power. Battery Specific Capacity, or EDLC, is a measure used to compare different technologies in terms of power output divided by the total weight of the device. EDLCs have a power density 5-10 times that of batteries. For example, while lithium ion batteries have a specific power of 1-3 kW / kg, the specific power of a typical EDLC is about 10 kW / kg. This property is especially important in applications that require fast power drain from storage devices.
3. Cycle viability and safety. EDLC batteries are safer than conventional batteries when mishandled. While batteries can explode due to excessive heat when short-circuited, EDLCs do not heat up as much due to their low internal resistance.
4. EDLCs can be charged and discharged millions of times and have an almost unlimited lifespan, while batteries have a cycle life of 500 times or less. This makes EDLC very useful in applications where frequent energy storage and release are required.
5. EDLC has a lifespan of 10 to 20 years, with capacity decreasing from 100% to 80% over 10 years.
6. Thanks to their low equivalent impedance, EDLCs provide high power density and high load currents to achieve near instantaneous charging in seconds. Temperature performance is also strong, providing energy down to -40 C °.

EDLC have some disadvantages:

1. One of the disadvantages is the relatively low specific energy. The specific energy of the EDLC is a measure of the total amount of energystored in the device divided by its weight. While lithium-ion batteries commonly used in cell phones have a specific energy of 100-200 Wh / kg, EDLCs can only store 5 W / kg. This means that an EDLC with the same capacity as a regular battery will weigh 40 times more.
2. Linear discharge voltage. For example, a battery with a nominal voltage of 2.7 V, when at 50% charge, a voltage close to 2.7 V will still be output. An EDLC rated at 2.7V at 50% charge delivers exactly half its maximum charge of 1.35V. This means that the output voltage will drop below the minimum operating voltage of the EDLC device, and it must shut down before using up all the charge in the capacitor. The solution to this problem is to use DC converters. However, this approach introduces new challenges such as efficiency and noise.
3. They cannot be used as a permanent power supply. One cell is typically 2.7 V and if higher voltage is required, the cells must be connected in series.
4. The cost of conventional EDLCs is 20 times higher than that of Li-ion batteries. However, it can be reduced due to new technologies and mass production of supercapacitors.

Industrial application

Since EDLCs occupy the area between batteries and capacitors, they can be used in a wide variety of applications. Where the supercapacitor is used can be assumed based on its purpose. One interesting use is energy storage in dynamic braking systems in the automotive industry. Is to use electric generator, which converts kinetic energy into electrical energy and stores it in EDLC. This energy can then be reused to provide acceleration power.

Another example is low power applications where high throughput is not required but high life cycle or fast recharge is important. Applications include photographic flash, MP3 players, static storage devices that require a low power constant voltage source to maintain information, etc.

Possible future applications for EDLC are cell phones, laptops, electric vehicles, and all other devices that currently run on batteries. The most exciting advantage, from a practical point of view, is their very fast reload speed - this meant would be able to charge an electric car in a charger for a few minutes until it is fully charged battery.

EDLCs are used in many power management applications that require large numbers of fast charge / discharge cycles to short term needs in energy. Some of these applications are used in the following areas:

• voltage stabilization in start / stop systems;
• electronic door locks in case of power failures;
• regenerative braking systems;
• distribution chip;
• medical equipment;
• energy accumulators;
• consumer electronics;
• kitchen appliances;
• real-time clock data backup;
• standby power;
• wind energy:
• energy efficiency and frequency regulation;
• remote power supply for sensors, LEDs, switches;
• backup memory;
• power supply in burst mode.

Directions of development of supercapacitors

New promising developments of supercapacitors:

• Graphene Skeleton Technology supercapacitors will be key players in EDLC. In new trials in the UK transport fleet, they are being used to convert diesel cars into hybrids using the power from regenerative braking. Hybrid car system developed by Adgero and Skeleton Technologies called UltraBoost. During braking, the device becomes a generator, recovering kinetic energy that would otherwise be lost in the form of a body. At the heart of this technology is a bank of five powerful graphene-based supercapacitors known as SkelMod.
• Zap & Go, a UK startup, is launching a new type of charger specifically for business travelers. It uses graphene supercapacitors to charge phones for five minutes.
• Eaton offers solutions for coin-sized supercapacitors, large cells, small cylindrical cells, and modules. For example, its Supercapacitor XLR 48V module provides energy storage for high-power frequency charging / unloading systems in hybrid or electric vehicles, public transport, material handling equipment, heavy equipment and marine systems. XLR modules consist of 18 individual Eaton XL60 supercapacitors designed to provide 48, 6 V and 166 F with 5 mA for connection to systems requiring up to 750 V.

• Maxwell Technologies supercapacitors are used for regenerative braking energy storage in the Beijing subway system. China Railway Rolling Stock Corp. (CRRC - SRI) uses Maxwell 48 - V modules in two sets of energy saving regenerative devices braking for line no.8 of the system, an urban rail network that runs north-south through the capital China. Maxwell modules with 48V provide a long service life of up to 10 years and fast charging / discharging. Vishay offers 220 EDLC ENYCAP with a rated voltage of 2.7 V. It can be used in multiple applications, including power backup, surge support power, energy storage devices to collect energy, micro-UPS power supplies and recovery energy.
• Linear technology offers the LTC3350, a standby power controller that can charge and control a serial unit of up to four supercapacitors. Designed for automotive and other transportation applications, the LTC3350 offers the following features:
  • Power backup by charging the bank with up to four supercapacitors in the event of a power failure. It can operate with an input voltage of 4.5 to 35 V and more than 10 A of standby current charge.
  • Balancing and protection overvoltage protection for a series of supercapacitors.
  • Monitoring of voltage, current and temperature in the system.
  • Internal capacitor voltage balancers that eliminate the need for balancing resistors.

The developers of supercapacitors are constantly trying to modernize them and increase their specific capacity. It is obvious that in the future, batteries will completely replace supercapacitors. The results of research by Californian scientists have shown that the new type of ionistors is already several times superior in functionality to its counterparts.