New energy storage options for high performance power applications

With its high power density and energy density, long life and compact size, supercapacitors meet the needs of high-performance power applications when combined with other emerging battery technologies. This article analyzes these new energy storage solutions and how they are used.

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Today's performance and reliability are essential for every design, and for engineers, energy storage has always been a fatal weakness in their designs. In the past, the solution for backup power was the battery, mainly lead-acid batteries. Now, engineers have more choices to meet the needs of backup power supplies, including advanced battery technologies such as lithium-ion and nickel-metal hydride batteries, fuel cells, solar cells, and double-layer capacitors.

Lithium-ion, nickel-hydrogen batteries and other battery technologies have made great strides in providing reliable energy storage solutions. They have been used in many designs and have solved many of the cost problems of the past, but design engineers are still faced with the same problems as when using lead-acid batteries, that is, all of these technologies are based on chemical reactions and their service life is limited. They are also limited by temperature, and the demand for large currents directly affects their service life. Therefore, these battery technologies face some challenges in terms of durability and reliability.

Fuel cells are a new and attractive battery technology that is gradually entering many applications, and there has been a lot of publicity recently. The ultimate application area for fuel cells is in cars, but during the transition they have emerged in the backup power market. A key issue with using fuel cells as backup power sources as well as primary power sources is the startup time and dynamic power response of these batteries. Fuel cells, despite their excellent energy density, have low dynamic power, so they require an enhancement technique for power assist and startup.

Also appearing in the same period are supercapacitors, or electrochemical double layer capacitors (EDLC). Supercapacitors have very high power density and substantial energy density compared to electrolytic capacitors. In the past few years, these devices have been used in many fields such as consumer electronics, industrial and automotive.

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Figure: Supercapacitors are very similar in structure to electrolytic capacitors or batteries. The main difference is that the electrode materials used are different.

Today, the best supercapacitors are ultra-high power devices with power densities up to 20 kW/kg, although the energy is still only a fraction of the battery. Supercapacitors are very compact (small supercapacitors are usually only stamp size or smaller), but they can store much more energy than conventional capacitors and can discharge very quickly or slowly. They have a very long service life and can be designed for the entire life cycle of an end product. When combined with the latest technology of supercapacitors, high energy batteries and/or fuel cells enable high power characteristics and long operating life.

Although there are several supercapacitor manufacturers offering a variety of products around the world, most double layer capacitors are basically constructed in a similar manner. As can be seen from the figure, the structure of the supercapacitor is very similar to that of the electrolytic capacitor or the battery. The main difference is that the electrode material used is different. In supercapacitors, the electrodes are based on carbon material technology and offer a very large surface area. The large surface area and small charge spacing make the supercapacitor have a high energy density. The capacity of most supercapacitors is measured by the pull (F), usually between 1F and 5,000F.

Depending on the application, supercapacitors can replace batteries or be smaller economical batteries. The supercapacitor has a small equivalent series resistance (ESR) that provides and absorbs very large currents; it uses a "mechanical" rather than a chemically charged carrier mechanism, resulting in a long and predictable lifetime, and As time goes on, its performance changes are also smaller. These features benefit regenerative braking and other applications that require fast charging, such as toys and tools.

Some applications are suitable for battery/supercapacitor systems, and the design can be optimized to avoid the battery being too large for energy requirements. Examples of these applications include automotive applications (such as hybrid vehicles) and consumer electronics (such as digital cameras). In digital cameras, inexpensive alkaline batteries are used in conjunction with supercapacitors (rather than using expensive lithium-ion batteries).

Another fuel cell technology is the Proton Exchange Membrane (PEM), a high efficiency energy conversion device with a continuous operating time comparable to that of a hydrogen fuel cell. It meets environmental requirements and provides reliable backup power for many applications. Several features of supercapacitors and PEM fuel cell systems make them ideal for use as complementary devices. They are all low voltage, high current devices. The supercapacitor has a small ESR and a large charge storage capacity, which can quickly supply a large current and a small voltage change, which can generate a short buffer response to the peak power demand. This allows the fuel cell to maintain its static operating point without reducing efficiency.

In all backup fuel cell applications, when the main power supply is disconnected, the backup power supply needs to be powered immediately. Because the fuel cell typically requires a 10 second to 60 second start-up time from startup to full power operation, it requires an energy buffer. A battery or super capacitor can act as this energy buffer. Since the required buffering energy is small and the reliability must be guaranteed, supercapacitors are a good choice for this application. Today, more and more fuel cell companies are considering using supercapacitors as an integral part of the entire backup power supply package.

To meet this demand, global supercapacitor manufacturers offer many batteries and modules for the backup power market. These batteries and modules can be placed in parallel/serial form to meet different capacity and voltage requirements. As more supercapacitor products come out and more and more products are available, design engineers can use supercapacitors like any other passive device.

Supercapacitors have made great strides in becoming standard devices for standby power supplies. About a decade ago, supercapacitors were just samples in the lab, with only a small amount of sales per year, and the price per Farah capacity was between $1 and $2. Today, these mass-produced devices are considered standard devices, and the price per Farad is only 0.01 to 0.02 cents. As the production of supercapacitors increases and prices drop, many design engineers are using supercapacitors as standard energy storage devices to meet the high power and reliability requirements of backup power supplies.

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