Powertrain Design Using Ultracapacitors (Farad Capacitors)

Over the past decade, advances in materials and structural technology have transformed farad capacitors from an "immature" backup device to a very efficient way to store energy. Although the energy storage capacity of supercapacitors is very small compared to batteries, it can be charged and discharged very quickly, can transmit tens of thousands of high-power pulses during its service life, and can easily meet the design life requirements of the product. Supercapacitors can be charged and discharged very rapidly and can operate in any state of energy storage, even under fully discharged conditions, without any adverse effect on the life of the component. Supercapacitors have indeed become an indispensable component in product-oriented design in the field of power electronics. At present, many manufacturers have recognized the technical advantages and high practical performance of supercapacitors, and have begun to produce various systems based on supercapacitors.

　　Engineers basically need to meet peak power requirements in the design of major energy equipment, such as an engine or battery system, which needs to meet high loads, even if the demand lasts only a few seconds. Designing the entire system to meet high loads rather than average loads obviously leads to increased cost and decreased efficiency. Such systems could store the energy part of the primary energy device as electrical energy, thereby improving the design of such systems, such as using batteries as secondary energy storage devices, and releasing this part of the energy quickly when needed. Such a high-energy transmission method provides a dynamic output capability for the energy system to meet instantaneous peak power demands. But batteries are not very good at providing instantaneous peak power frequently; in this regard, supercapacitors are a better 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, low-power, but long-life backup component in video recorders (VCRs) and alarm clocks, and has little other use until its capabilities are recently discovered.

　　Over the past decade, advances in materials and structural technology have transformed supercapacitors from an immature back-up device to a very efficient form of energy storage. Supercapacitors have indeed become an indispensable component in product-oriented design in the field of power electronics. Now, many manufacturers have recognized the technical advantages and high practical performance of supercapacitors, and have begun to produce various systems based on supercapacitors.

　　1. What is a super capacitor

　　Dielectric capacitors are generally classified into three categories based on maintaining an electric field between a pair of electrodes, ordinary capacitors, electrolytic capacitors , and electrochemical double-layer capacitors. In terms of the capacity of these three types of capacitors, electrochemical double-layer capacitors (supercapacitors) top the list with certain advantages (up to several thousand Farads). This is because the electric field medium of the supercapacitor is composed of porous activated carbon and molecular-level electrolytic ions.

　　The structure of super capacitor

　　The specific surface area of the porous special surface of the activated carbon electrode in the Farad capacitor can reach 2000㎡/g, and the distance between charges is less than 10 angstroms. A supercapacitor with a good electrolyte has a voltage value of less than 3.0V.

　　Supercapacitors exhibit considerably low series impedance due to the infusion of high-conductivity electrolytes, high-conductivity electrodes and ion-fiber separators. Now, the energy density of commercial supercapacitors can exceed 5Wh/Kg, and the power density can reach 20kW/kg.

　　Supercapacitors are essentially based on electrostatic energy storage, which is a purely physical reaction and is fully reversible. The charging and discharging of supercapacitors is achieved by the movement of ions in the electrolyte, an energy storage process without any bonding or breaking of chemical bonds compared to the chemical reaction-based processes of battery technology. The cycle life of supercapacitors proved to be very good after millions of charge and discharge cycles.

　　A good way to compare the relative advantages of supercapacitor energy storage technologies and batteries and other energy storage technologies is to plot them on a Ragone diagram, which maps energy storage to power storage and primarily shows that energy density decreases as power density increases. little. It's a great way to quantify energy storage and categorize it for a variety of uses, from hauling to energy caching.

　　2. Characteristics of super capacitors

　　The characteristics of supercapacitors are quite different from those of batteries. The main differences are listed in the table below. Batteries store more electrical energy than supercapacitors of the same size, but in many applications where power determines the size of energy storage devices, supercapacitors may be a better solution.

　　1. Supercapacitors can deliver frequent pulses of energy without any detrimental effects, while many batteries experience reduced lifespan under frequent high-power pulse conditions.

　　2. Supercapacitors can be charged in a relatively short time, and fast charging often damages the battery.

　　3. The cycle cycle of supercapacitors is tens of thousands of times, while the life of batteries is usually hundreds to 1000, 2000 times.

　　4. Supercapacitors based on low internal resistance are more efficient than batteries; in practical applications, the conversion efficiency of supercapacitors of 84% to 95% is much higher than the average efficiency of most batteries below 70%.

　　5. The supercapacitor can be charged at any voltage value within its allowable voltage range, and can be fully discharged. This allows more freedom of design in the bus voltage control algorithm. The battery will also be damaged if it is over-discharged.

　　6. Calculating the energy storage value in the supercapacitor only needs to know the voltage and capacitance value. The capacitance value of the supercapacitor can be calculated in real time by measuring the changes in current and voltage. However, to obtain the energy storage value of the battery correctly requires multiple complex calculations, the capacity of the battery is usually unknown, and it is difficult to measure in real time.

　　7. Supercapacitors have a wider operating temperature range, and can even work normally at temperatures as low as -40°C. And most batteries cannot work at temperatures as low as -10°C.

　　8. Supercapacitors work by polarizing the electrolyte in a high specific surface area electrode, and the properties of the electrolyte, electrode, and separator materials determine the capacitance performance of the supercapacitor. Electrodes with high specific surface area and small charged ions determine high capacitance; while efficient electrolytes, separators and materials, and process design determine low impedance.

　　Because the energy storage of supercapacitors does not rely on chemical reactions, it is fundamentally different from batteries.

　　3. Market prospect and application

　　When designing a system it is natural to think of shape. The main energy reserve of the system should be able to meet the average endurance requirements and the relative instantaneous peak power requirements. However, it is not economical and impractical to meet such peak power. The system is significantly improved by being able to store electrical energy, take it from the main energy source when high power is required and then transmit high voltage pulses under control. At this point, supercapacitors provide a simple, reliable buffer for short-term power requirements and power ratings that do not match. This feature reduces the size and cost of the system and improves the performance and reliability of the system.

　　Example application

　　There are two main uses for supercapacitors. One is as a temporary supplementary energy and additional short-term functional energy when the main energy is insufficient. Here, when the supercapacitor is used as the main energy supply device, the supercapacitor has become another choice relative to the battery, and also has the function of backup energy when the main power fails.

　　The two functions of supercapacitors are peak energy supply. In this case, supercapacitors can not only be used alone in systems that require high power transfer, but can also be used as a follow-up energy source for batteries in systems that require not only continuous power discharge functions but also high-load pulse power. Here the supercapacitor acts as a relief to the battery during high power transmission, thereby increasing the battery life and reducing the size of the battery.

　　Although batteries are now commonly used as primary energy sources and energy storage/peak power transfer devices, supercapacitors are gradually being adopted as energy storage and high power transfer devices.

　　In fact, any application requires electrical energy storage and fast charging and discharging functions, which is the market potential of supercapacitors.