Supercapacitors have characteristics such as high power density and energy density, long service life, and compact size. When combined with other emerging battery technologies, they can meet the needs of high-performance power applications. This article analyzes these new energy storage solutions and their usage methods.
Nowadays, 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 batteries, mainly lead-acid batteries. Nowadays, engineers have more options to meet the demand for backup power, including advanced battery technologies such as lithium-ion and nickel hydrogen batteries, fuel cells, solar cells, and double-layer capacitors.
Lithium ion, nickel hydrogen batteries, and other battery technologies have made significant progress in providing reliable energy storage solutions. They have been applied in many designs and solved many cost issues in the past, but design engineers still face the same problem as when using lead-acid batteries, that is, all of these technologies are based on chemical reactions, their service life is limited and restricted by temperature, and the demand for high current will directly affect their service life. Therefore, these battery technologies still face some challenges in terms of durability and reliable applications.
Fuel cells are a newly emerging and very attractive battery technology that is gradually entering many applications, and there has been a lot of promotion for them recently. The ultimate application field of fuel cells is in automobiles, but during the transition period, they have already appeared in the backup power market. The key issues in using fuel cells as backup and main power sources are the start-up time and dynamic power response of these batteries. Although fuel cells have excellent energy density, their dynamic power is low, so they require an enhancement technology for power assistance and start-up.
At the same time, supercapacitors, also known as electrochemical double-layer capacitors (EDLC), also appeared. Compared with electrolytic capacitors, supercapacitors have very high power density and substantial energy density. In the past few years, these devices have been applied in many fields such as consumer electronics, industry, and automotive.
Nowadays, the best supercapacitors are ultra-high power devices with a power density of up to 20kW/kg, although their energy is still only a small fraction of that of batteries. The size of supercapacitors is very compact (small supercapacitors are usually only the size of stamps or smaller), but they can store much more energy than traditional capacitors, and the discharge speed can be fast or slow. They have a very long lifespan and can be designed for the entire lifecycle of end products. When combined with the latest technology of supercapacitors, high-energy batteries and/or fuel cells can achieve high power characteristics and long working life.
Although there are several supercapacitor manufacturers worldwide offering a variety of products, most double-layer capacitors are essentially constructed in a similar way. The structure of supercapacitors is very similar to that of electrolytic capacitors or batteries, with the main difference being the use of different electrode materials. In supercapacitors, the electrodes are based on carbon material technology, which can provide a very large surface area. The large surface area and small charge spacing give supercapacitors a high energy density. The capacity of most supercapacitors is calibrated in Farads (F), typically between 1F and 5000F.
According to application needs, supercapacitors can replace batteries or serve as smaller economical batteries. The equivalent series resistance (ESR) of supercapacitors is very small, which can provide and absorb very large currents; It adopts a "mechanical" rather than a chemical charge carrier mechanism, thus having a long and predictable service life, and its performance changes are also smaller over time. These features can benefit applications such as regenerative braking and other products that require fast charging, such as toys and tools.
Some applications are suitable for using battery/supercapacitor systems, and the design can be optimized to avoid excessive energy demand from the battery. These application examples include automotive applications (such as hybrid vehicles) and consumer electronics (such as digital cameras), where inexpensive alkaline batteries are used in conjunction with supercapacitors in digital cameras
Another fuel cell technology is proton exchange membrane (PEM), which is a high-efficiency energy conversion device with a continuous operating time comparable to hydrogen fuel cells. It meets the requirements of green environmental protection and can provide reliable backup power for many applications. Several characteristics of supercapacitors and PEM fuel cell systems make them highly suitable for use together as complementary devices. They are all devices with low voltage and high current. Supercapacitors have the characteristics of low ESR and large charge storage capacity, which can quickly provide high current with minimal voltage changes, and can produce a brief buffering response to peak power demand. This allows fuel cells to maintain their 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 provide power immediately. Because fuel cells typically require a start-up time of 10 to 60 seconds from start-up to full power operation, they require an energy buffer. Batteries or supercapacitors can serve as this energy buffer. Due to the low required buffering energy and the need for reliability assurance, supercapacitors are a better choice for this application. Nowadays, more and more fuel cell companies are considering using supercapacitors as a component of the entire backup power package.
To meet this demand, global supercapacitor manufacturers have provided 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. With the emergence of more supercapacitor products and the increasing availability of products, design engineers can use supercapacitors like other passive components.
Supercapacitors have made great progress in becoming standard components for backup power sources. About ten years ago, supercapacitors were just samples in the laboratory, sold in small quantities every year, with prices ranging from $1 to $2 per Farad capacity. Nowadays, these mass-produced devices are considered standard devices, priced at only 0.01 to 0.02 cents per Farad capacity. With the increasing production and decreasing prices of supercapacitors, many design engineers are using them as standard energy storage devices to meet the high power and reliability requirements of backup power sources.
