A supercapacitor or a double layer capacitor has much higher capacitance than a normal capacitor. Its capacitance, usually expressed in Farads, is thousands of times higher than that of electrolytic capacitors.
Supercapacitors can be charged and discharged frequently, making them suitable for situations where high current is required for a short time. The voltage limit of a supercapacitor is 2.7 V. High voltage is also possible but it will reduce the life of the supercapacitor. It is possible to combine several supercapacitors in series to achieve higher voltage, but this will increase the internal resistance and reduce the overall capacitance.
It is theoretically possible to charge and discharge a supercapacitor an infinite number of times. An electrochemical battery is different, as it has a fixed life cycle. So, the life of a superpower is much more than a battery. A supercapacitor weakens to 80% of its full capacity in 10 years under normal conditions. However, applying more voltage than specific voltage can cause severe damage to supercapacitor and shorten its lifetime. There is no effect of hot and cold temperature on supercapacitors, like in electrochemical batteries. Table 1 gives a comparison between supercapacitors and lithium-ion batteries.
Applications in solar power
The solar power industry is a well-known case of using batteries for power storage. Battery life in the industry is 3-5 years, depending on the load demand curve. The inconsistent supply of the solar PV cells often negatively affects battery life. PV cell production depends on the climatic conditions, making them very unpredictable and unstable. Battery life is severely damaged by these output fluctuations, which interrupts the battery charging and discharging cycle. Batteries are not high in power density, but are only high in energy density. Sometimes, the load demands high surge current from battery for a short period of time which can have a serious impact on the lifespan of the battery. This irregular discharging can severely damage the battery life.
In a solar PV system, the hybrid energy storage system (HESS) is designed by combining a supercapacitor with a battery to increase the energy density of the system. This system has more advantages than the individual use of a supercapacitor or battery.
The stress on batteries can be reduced by using a hybrid system of supercapacitors and batteries. The operating and maintenance cost of new system will be less because it decreases the size and rate of discharge of the battery and, therefore, increases the battery life. This hybrid storage system will also improve the power quality. The energy in the supercapacitor is stored in physically separated negative and positive charges. The supercapacitor acts as a buffer when used with a battery. In this way, it protects the battery from high power drain. Supercapacitors have unlimited life cycles, high power density, fast charging time and less equivalent series resistance. Due to these advantages, supercapacitors have already replaced batteries in many applications.
The battery has a high energy density and the supercapacitor has a high power density so the combination of both will make a perfect hybrid system. At peak power requirements, the supercapacitor's high power density allows a sufficient energy supply within a short period of time. The supercapacitor can quickly be charged after discharge. On the other hand, the battery will supply continuous power to load for a long period of time due to its high energy density.
Supercapacitors can also reduce battery size because during peak hours the energy will be provided by the supercapacitor, so there is no need to design a large battery to meet peak load requirements. Battery life will also increase because the battery will not undergo frequent discharge. Therefore, the addition of a supercapacitor will reduce the cost of operating and maintaining the system.
Applications in wind power
Wind power is one of the fastest growing renewable power generation technologies. However, wind energy is one of most unpredictable energy sources, because it depends on variable wind speed. A change in wind speed affects the power quality of the grid because it produces fluctuations in the turbines output power.
Modern wind turbines feature directly adjustable rotors to eliminate the active power fluctuation. This smooths the power output, but it offers limited capabilities for power adjustment. The reactive power fluctuation of the system is removed by using power compensation devices. But the active power fluctuations cannot stabilize by using power compensation devices.
The voltage bus of wind farms can be stabilized by using energy storage equipment. It is also possible to adjust the active and reactive power by adding a storage device. Studies show that the power quality of the grid is greatly affected by the fluctuating power at 0.01 to 1 Hz. The power quality of the grid is greatly affected by the power fluctuation in this frequency band. A short term storage device can be used to suppress the fluctuation of wind power in this frequency band. Therefore, a storage device which is capable of realizing its energy in a short interval of time has many applications in wind power system.
Supercapacitors can be used in wind power systems to solve high current fluctuations. This will be most suitable due to their high current charge and discharge properties. The long life of supercapacitors also makes them an ideal option for use in wind power. Energy will be stored in the supercapacitor when the wind is strong. When the wind speed fluctuates, the supercapacitor will begin to discharge to smooth out the system’s output power, enabling a more efficient grid system.