In the past decade, the development of materials and structural technology has transformed Farad capacitors from an "immature" backup device into a highly effective energy storage method. Although the energy storage capacity of supercapacitors is very small compared to batteries, they can charge and discharge very quickly, transmit tens of thousands of high-power pulses during their lifespan, and easily meet the design life requirements of products. Supercapacitors can charge and discharge very quickly, and can operate in any energy storage state, even under fully discharged conditions, without any adverse effects on the lifespan of the components. Supercapacitors have indeed become indispensable components in product design in the field of power electronics. At present, many manufacturers have recognized the advantages of supercapacitor technology and high practical performance, and have begun to produce various systems based on supercapacitors.
Engineers need to meet peak power requirements in the design of major energy equipment, such as for an engine or battery system, which needs to operate at maximum load, even if this requirement only lasts for a few seconds. Designing the entire system to meet the maximum load rather than the average load will obviously lead to an increase in cost and a decrease in efficiency. This system can store a portion of the energy in the main energy equipment as electrical energy, thereby improving the design of such systems, such as using batteries as secondary energy storage devices and quickly releasing this energy when needed. This high-energy transmission method provides a dynamic output capability for the energy system to meet the demand for instantaneous peak power. However, batteries cannot be effectively used to frequently provide instantaneous peak power; In this regard, supercapacitors are the best choice.
Farad capacitors, also known as electrochemical double-layer capacitors (EDLC) or supercapacitors. It has been around for about 10 years and was first used as a low energy storage, low-power, but long-life backup component in video recorders (VCRs) and alarm clocks, with almost no other uses until its functionality was recently discovered.
In the past decade, the development of materials and structural technologies has transformed supercapacitors from an immature backup device into a highly effective energy storage method. Supercapacitors have indeed become indispensable components in product design in the field of power electronics. Nowadays, many manufacturers have recognized the advantages of supercapacitor technology and high practical performance, and have begun to produce various systems based on supercapacitors.
1、 What is a supercapacitor
According to the dielectric capacitance that maintains an electric field between a pair of electrodes, it is usually divided into three categories: ordinary capacitance, electrolytic capacitance, and electrochemical double-layer capacitance. In terms of the capacity of these three types of capacitors, electrochemical double-layer capacitors (supercapacitors) rank first with an absolute advantage (up to several thousand Farads). This is because the electric field medium of supercapacitors is composed of porous activated carbon and molecular level electrolytic ions.
Composition of supercapacitor structure
The specific surface area of the porous special surface of the activated carbon electrode in Farah capacitors can reach 2000 ㎡/g, and the distance between charges is below 10 angstroms. A supercapacitor with a good electrolyte has a voltage value of less than 3.0V.
Due to the injection of high conductivity electrolyte and the presence of high conductivity electrodes and ion fiber isolation layers, the supercapacitor exhibits a relatively low series impedance. Nowadays, the energy density of commercial supercapacitors can exceed 5Wh/kg, and the power density can reach 20kW/kg.
Supercapacitors essentially operate based on electrostatic energy storage methods, are purely physical reactions, and are completely reversible. The charging and discharging of supercapacitors are achieved through the movement of ions in the electrolyte, and this energy storage process, compared to battery technology based on chemical reactions, does not involve any chemical bonding or disconnection. After millions of charge and discharge cycles, it has been proven that the cycle life of supercapacitors is very good.
A good way to compare the relative advantages of supercapacitor energy storage technology with batteries and other energy storage technologies is to draw them on a Ragone diagram, which corresponds energy storage and power storage and mainly shows that energy density decreases with increasing power density. This is a good method of positioning energy storage methods by quantity and categorizing them into various uses such as traction and dragging to energy caching.
Ragone image
2、 Characteristics of Supercapacitors
There are many differences in the characteristics of supercapacitors compared to batteries. The main differences are listed in the table below. Batteries store more electricity than supercapacitors of the same size, but in many applications where power determines size, supercapacitors may be the best solution.
1. Supercapacitors can transmit energy from frequent pulses without any harmful effects, while many batteries experience reduced lifespan under frequent high-power pulse conditions.
2. Supercapacitors can complete charging in a relatively short amount of time, while fast charging often damages the battery.
3. The cycle period of supercapacitors is tens of thousands of times, while the lifespan of batteries is usually several hundred to 1000 or 2000 times.
4. Supercapacitors based on low internal resistance have higher efficiency than batteries; In practical applications, the conversion efficiency of supercapacitors ranging from 84% to 95% is much higher than the average efficiency of most batteries, which is below 70%.
5. Supercapacitors can be charged at any voltage value within their allowable voltage range and can be fully discharged. This allows for more flexible design in the bus voltage control algorithm. And overdischarging the battery can also cause damage.
6. To calculate the energy storage value in a supercapacitor, only the voltage and capacitance values need to be known. The capacitance value of supercapacitors can be calculated in real-time by measuring the changes in current and voltage. However, obtaining the correct energy storage value of a battery requires multiple complex calculations, and the capacity of the battery is usually unknown, making real-time measurement difficult.
7. Supercapacitors have a wider operating temperature range and can even function normally at temperatures as low as -40 ℃. Most batteries cannot function at temperatures as low as -10 ℃.
8. Supercapacitors work through the electrolyte in polarized high specific surface area electrodes, and the characteristics of the electrolyte, electrode, and isolation layer materials determine the capacitance performance of supercapacitors. The high specific surface area of the electrode and the small charged ions determine the high capacitance; Efficient electrolytes, isolation layers and materials, as well as process design, determine low impedance.
Because the energy storage of supercapacitors does not rely on chemical reactions, they have fundamental differences from batteries.
3、 Market prospects and applications
When designing a system, shape is a natural thought, and the main energy reserve of the system should be able to meet the average durability requirements and relative instantaneous peak power requirements. However, meeting such peak power is uneconomical and impractical. The system is significantly improved by being able to store electrical energy, which is obtained from the main energy source when high power is needed and then transmitted under control by emitting high-voltage pulses. At this point, supercapacitors provide a simple and reliable buffer when the short-term required power does not match the rated power. This feature reduces the shape and cost of the system and improves its performance and reliability.
Example application
Supercapacitors have two main uses. The first one is used as a temporary supplementary energy and additional short-term functional energy when the main energy is insufficient. When supercapacitors are used as the main energy supply device here, they have become another option compared to batteries, and also serve as a backup energy source when the main power fails.
The second function of supercapacitors is peak power supply. In this case, supercapacitors can not only be used independently in systems that require high-power transmission, but also as a follow-up energy source for batteries in systems that require not only continuous power discharge function but also high load pulse power. The supercapacitor here serves as a relief for the battery during high-power transmission, thereby increasing the battery's lifespan and reducing its size.
Although batteries are widely used as the main energy source and energy storage/peak power transmission devices, supercapacitors are gradually being adopted as energy storage and high-power transmission devices.
In fact, any application requires energy storage and fast charging and discharging functions, which is the market potential of supercapacitors.
