With the rapid development of the economy, the problem of energy shortage has become increasingly severe. Faced with the dual pressures of energy demand and environmental protection, countries around the world have adopted strategies such as improving energy utilization efficiency, optimizing energy structure, and developing renewable energy. As a representative of new energy generation, solar photovoltaic (PV) power has been widely applied in practical production. However, unlike traditional power sources, the output power of PV systems fluctuates significantly with changes in environmental factors such as sunlight intensity and temperature. It is also characterized by uncontrollability. Therefore, if PV power is to replace traditional energy sources and achieve large-scale grid-connected power generation, the impact on the power grid cannot be ignored. Furthermore, as the proportion of PV systems in the power grid continues to increase, effective management of the impact on the power grid is essential to ensure safe and reliable power supply. The application of energy storage systems in PV systems can address the issue of power imbalance and meet the requirements for normal operation. Energy storage systems play a critical role in ensuring the stable operation of PV power plants. They not only guarantee the stability and reliability of the system but also provide an effective solution to dynamic power quality issues, such as voltage spikes, surges, voltage drops, and momentary power interruptions.
Here are four common energy storage technologies in PV systems:
1. Battery Energy Storage: Battery energy storage is one of the most promising storage methods. It is characterized by high reliability and modularization, making it suitable for distribution networks in areas with high requirements for power quality. Battery energy storage utilizes the oxidation-reduction reactions of battery electrodes to store and release energy. It can fulfill the energy demand during system peak loads and assist in reactive power compensation, helping to suppress voltage fluctuations and flicker. Common battery types include lead-acid batteries, lithium-ion batteries, sodium-sulfur batteries, and flow batteries. A study  analyzes the role of batteries in PV systems, highlighting their energy storage capabilities and ability to provide large instantaneous current to regulate the working voltage of solar cells. Another study  focuses on several common chemical energy storage technologies, selecting key technical indicators and collecting the latest application data up to 2011. Through data envelopment analysis (DEA) method, the study explores the advantages and application effects of various chemical energy storage technologies, providing suggestions and references for future research directions in chemical energy storage technology.
2. Supercapacitor Energy Storage: Supercapacitors are energy storage devices made of special materials with higher dielectric constant, greater energy density, and faster energy release compared to conventional capacitors. Supercapacitors can be divided into double-layer capacitors and electrochemical capacitors based on different energy storage principles. As an emerging energy storage component, supercapacitors have many advantages compared to other storage methods. They have high power density, long charge-discharge cycling life, high charge-discharge efficiency, fast charge-discharge rate, good performance at high and low temperatures, and long energy storage life. However, supercapacitors also have some drawbacks, such as low energy density and wide fluctuation range of terminal voltage, as well as the issue of voltage equalization in series capacitors. Considering the characteristics of batteries and supercapacitors, both technologies have strong complementarity in terms of technical performance. By combining them, the performance of energy storage devices can be greatly improved. A study  found that by paralleling supercapacitors with batteries, the hybrid energy storage system's power output capacity can be enhanced, internal losses can be reduced, and the discharge time can be increased. It can also reduce the number of charge-discharge cycles of batteries and extend their lifespan, as well as reduce the size of energy storage devices, improve the reliability and economy of power supply systems.
3. Flywheel Energy Storage: Flywheel energy storage is a mechanical storage method that offers advantages such as high efficiency, short construction period, long lifespan, high energy storage capacity, quick charging, fast discharging, unlimited charge-discharge cycles, and environmental friendliness. However, the maintenance cost of flywheel energy storage is relatively higher compared to other storage methods. A study  proposed a fuzzy control-based active power smoothing control for PV flywheel energy storage systems. The study analyzed the flywheel speed, power output, and smoothing coefficient of the PV flywheel energy storage system through simulation using the Matlab/Simulink platform, comparing it with two other devices: without flywheel energy storage and simple flywheel energy storage. Another study  introduced the flywheel energy storage system into PV systems based on maximum power point tracking and grid-connected inverter control to achieve stable power output to the grid. To ensure adjustable and stable electrical energy output from the PV system, the control strategy adopted current inner-loop control and voltage outer-loop control during charging and current inner-loop control and voltage outer-loop control during discharging. This approach enables the stable and controllable output of active power, effectively suppressing fluctuations caused by variations in sunlight and environmental temperature.
4. Superconducting Energy Storage: Superconducting magnetic energy storage (SMES) systems store the magnetic field energy generated by the power grid in coils made of superconducting wires and release the stored energy back to the grid when needed. The typical components of a SMES system include superconducting coils placed in a vacuum-insulated cooling container, cryogenic and vacuum systems, and power electronic devices for control purposes. The current in the closed-loop inductor formed by the superconducting coils circulates without any dissipation. Compared to other energy storage technologies, SMES has significant advantages. Due to its ability to store energy without losses for long periods, SMES achieves high energy return efficiency. It also has fast energy release, typically taking only a few seconds. The adoption of SMES makes it easy to regulate grid voltage, frequency, and active and reactive power. A study  proposed the use of SMES units to stabilize the voltage and frequency output from wind generators. The SMES unit was connected to the busbar of an asynchronous generator, and the active controller of SMES adopted the deviation of the asynchronous generator's speed as the control signal. Another study  suggested using voltage deviation as the control signal for the active controller of SMES to address frequent contact line short circuit faults and wind speed disturbances in wind farms. Various studies have demonstrated the excellent performance of SMES devices in improving wind farm stability.
These energy storage technologies play a crucial role in PV systems, ensuring their stable operation and providing secure and reliable power supply. With ongoing technological advancements and maturation, energy storage technologies will increasingly contribute to the development of renewable energy and the achievement of sustainable development goals.
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