Photovoltaic power generation is characterized by high output levels during short periods at midday and low output levels at other times, as well as output during the day and no output at night. Energy storage technology has the characteristic of enabling the spatiotemporal transfer of electrical energy. By configuring energy storage rooms for photovoltaic power plants, the midday output of photovoltaics can be transferred to other periods, reducing the peak output of the power plant and reducing curtailment of solar power.
During the operation of a battery energy storage system, the principle is to minimize the number of charge-discharge cycles to extend its lifespan. During peak photovoltaic (PV) power generation periods, the battery energy storage system is charged to smooth out peak output from the PV power plant. After peak periods, the battery energy storage system is discharged. Discharge control helps smooth fluctuations in PV output and assists in peak shaving, maximizing the energy storage effect. Based on the different functions of energy storage discharge, three operating modes can be classified for energy storage systems: peak shaving, peak shaving + smoothing, and peak shaving + transfer.
Working Mode 1: Peak Shaving
During peak output periods of photovoltaic power plants, the charging of the battery energy storage system is controlled with peak shaving as the application objective. After the peak output period of photovoltaic power plants, and during the daytime output period of photovoltaic power plants, the power of the battery energy storage system is amplified and discharged to the lower limit of the SOE operating range of the battery energy storage system. Then the energy storage system stops working, ensuring that the working time of the energy storage system is within the power generation time of the photovoltaic power plant, without adding extra working time to the photovoltaic power plant, and reducing the impact of configuring the energy storage system on the working schedule of the photovoltaic power plant.
Working Mode 2: Peak Shaving + Smoothing
During peak photovoltaic (PV) power generation periods, the charging of the battery energy storage system is controlled with peak shaving as the application objective. Output fluctuations in large-scale PV power plants can be categorized into two types: slow changes in output, such as the periodic changes caused by day-night cycles; and sudden changes, such as a sudden drop in output due to cloud cover. The first type of change is large in amplitude but slow in nature; the second type is unpredictable and sudden, with output decreasing from full capacity to below 30% of the rated value within 1-2 seconds in severe cases. After the peak PV output period, the energy storage system is controlled to discharge, aiming to smooth out the decline in PV power generation during day-night cycles. Discharge is made until the SOE (State of Energy) of the battery energy storage system reaches the lower limit of its operating range. If it is already nighttime and the PV power generation drops to 0, but the SOE of the energy storage system is still greater than 0.2, the energy storage system is controlled to discharge at a constant rated power until the SOE drops to 0.2, and then the energy storage system is stopped.
Working mode 3: Peak shaving + transfer
During peak output periods of photovoltaic (PV) power plants, the charging of the battery energy storage system is controlled with peak shaving as the application objective. The output period of PV power plants is from 8:30 to 18:30, with the evening peak load occurring between 18:00 and 22:00. During this period, the PV power plant has basically no output, and the battery energy storage system can be controlled to discharge to assist in system peak shaving. In order to reduce the number of operations of the energy storage system and simplify its operation, the battery energy storage system is controlled to discharge at a constant power, and the discharge is kept below the lower limit of the SOE (State of Energy) operating range of the battery energy storage system before the energy storage system stops working.
As the proportion of photovoltaic (PV) power generation systems in the power grid continues to increase, their impact on the grid must be effectively managed to ensure the safe and reliable supply of electricity. The application of energy storage systems in PV power generation systems can solve the problem of power imbalance, thus meeting the needs of normal operation. Energy storage systems are crucial for the stable operation of PV power plants. They not only ensure system stability and reliability but are also an effective way to solve dynamic power quality problems such as voltage pulses, inrush currents, voltage dips, and transient power outages.
