As the global push for carbon neutrality accelerates, the installed capacity of renewable energy sources such as wind and photovoltaic power has been increasing year by year. However, due to local climate conditions, the output power of renewable generation often experiences abrupt ramping or steep drops, posing challenges to the frequency regulation margin of power systems. The inherent variability of renewable output, combined with complex grid impedance characteristics, can lead to frequency oscillations in large-scale centralized or distributed grid integration scenarios, thereby raising stability concerns and affecting load security. By integrating battery energy storage systems, we can achieve peak shaving and valley filling for power system loads, alleviating the peak regulation pressure on thermal power units. Furthermore, using battery energy storage systems as primary frequency regulation resources can balance load fluctuations and keep system frequency within allowable limits.
Extensive research has been conducted worldwide on battery energy storage systems. For instance, studies have highlighted the prospects and advantages of storage systems. Others have performed cost–benefit analyses for generation-side and load-side storage. Some have focused on control and management strategies to enhance system stability. Additionally, projects integrating photovoltaic + storage and offshore wind + storage have been designed to relieve peak regulation pressure and improve energy utilization. Research on hybrid storage optimization and configuration models has explored the complementary advantages of multiple flexible resources across different segments of new power systems.
To better mitigate the impact of renewable energy grid integration on power system stability, this paper analyzes the application and role of battery energy storage systems in grid-connected renewable power generation. We take a specific photovoltaic‑storage integrated microgrid project as a case study to evaluate the economic viability of the battery energy storage system.
1. Role of Battery Energy Storage Systems in Grid-Connected Renewable Power Systems
Taking a power system comprising wind and photovoltaic generation as an example, we analyze the functions of a battery energy storage system in such a grid-connected renewable power system.
1.1 Peak Shaving and Valley Filling
Renewable generation exhibits long‑term daily fluctuations and a mismatch with load profiles—i.e., an anti‑peak regulation characteristic. This increases the requirement for both upward and downward reserve capacity in the power system after grid connection. During the evening peak load period (typically 19:00–22:00), photovoltaic systems produce no power, while wind farms may operate at full capacity during the lowest load point of the day (e.g., 24:00). Consequently, a certain proportion of “curtailed solar” and “curtailed wind” occurs each year due to insufficient transmission capacity. Under such circumstances, a battery energy storage system can store the electricity generated by wind farms during the lowest load period and release it during the evening peak, effectively shifting energy across time. This maximizes the utilization of transmission lines to match load trends, reduces the need for thermal unit up‑ and down‑regulation, and achieves peak shaving and valley filling.
By configuring a battery energy storage system, the equivalent load—defined as the sum of the daily load and the renewable output—can be constrained within the maximum and minimum effective power limits for renewable power injection. This avoids curtailment and load shedding, enhances the grid’s ability to absorb renewable energy, lowers the demand for reserve capacity, and improves overall system efficiency.
1.2 Stabilizing the Power System
The short‑term ramp rate of renewable generation output must satisfy power system stability requirements. Table 1 lists the typical active power change limits for grid‑connected renewable systems.
| Installed capacity (MW) | Max change in 10 min (MW) | Max change in 1 min (MW) |
|---|---|---|
| < 30 | 10 | 3 |
| 30–150 | 10–50 | 3–15 |
| > 150 | 50 | 15 |
Smoothing the power fluctuations of renewable generation involves controlling the storage and release of energy through the battery energy storage system to suppress minute‑level active power fluctuations at the point of common coupling. The total power \( P = P_{\text{BES}} + P_{\text{NE}} \) (where \( P_{\text{BES}} \) is the output of the battery energy storage system and \( P_{\text{NE}} \) is the renewable output) must satisfy the limits in Table 1.
Two main active power control algorithms are used for battery energy storage systems: the point‑wise limit method and the low‑pass filter method.
With the point‑wise limit method, the output power of the battery energy storage system at time \( j \) is constrained by:
\[
\max(\Delta P_{10}(j)-P_{y,10},\; \Delta P_{1}(j)-P_{y,1}) < P_{\text{BES}}(j) < \min(\Delta P_{10}(j)-P_{y,10},\; \Delta P_{1}(j)-P_{y,1})
\]
where \(\Delta P_{10}(j)\) is the change in BES output over the past 10 minutes, \(P_{y,10}\) is the maximum allowable fluctuation in 10 minutes, \(\Delta P_{1}(j)\) is the change over the past 1 minute, and \(P_{y,1}\) is the maximum allowable fluctuation in 1 minute.
The low‑pass filter method smooths the output by filtering the input signal. The BES output at time \( j \) is:
\[
P_{\text{BES}}(j) = \frac{\tau}{t} \left[ \sum P(j) – \sum P(j-1) \right]
\]
where \(\tau\) is the time constant, \(t\) is the control period, and \(P(j), P(j-1)\) are the total powers at times \(j\) and \(j-1\). The time constant is related to the cutoff frequency \(f_c\) of the low‑pass filter:
\[
\tau = \frac{1}{2\pi f_c}
\]
1.3 Primary Frequency Regulation
Primary frequency regulation addresses short‑term rapid load variations. When grid frequency exceeds the dead‑band, the system autonomously provides or absorbs active power. Different energy sources have different primary frequency regulation dead‑bands: e.g., thermal units: \(50 \pm 0.033\) Hz; hydro: \(50 \pm 0.05\) Hz; photovoltaic: \(50 \pm 0.06\) Hz; wind: \(50 \pm 0.10\) Hz.
For a battery energy storage system, primary frequency regulation adjusts its output or absorption in real time based on frequency deviations, enabling rapid response to load changes and maintaining frequency stability. This approach offers fast response and high precision. Compared to thermal units, a battery energy storage cell can respond much faster and can independently or jointly perform primary frequency regulation. According to relevant power system dual‑assessment rules, equipping a renewable plant with a battery energy storage cell can fulfill or improve functions such as peak regulation and primary frequency regulation.
For a 300 MW thermal unit, the primary frequency regulation amplitude is 8% of rated capacity (24 MW). The load adjustment per hertz is 160 MW/Hz. For a frequency deviation between 0.033 Hz and 0.183 Hz, the corresponding regulation power is 0–24 MW. Each time the frequency exceeds the dead‑band, the thermal unit provides \(\pm\)0.2% of rated power, i.e., \(\pm\)600 kW. If we assume that a battery energy storage system independently handles primary frequency regulation, the single charge/discharge duration is only about 10 seconds. Assuming equal probability of upper and lower frequency excursions, a battery energy storage cell rated 600 kW/0.5 h is appropriate. With a rational state‑of‑charge (SOC) management strategy, the battery energy storage cell can cycle many times per day while remaining near 50% SOC with shallow depth of discharge, ensuring a long service life.
To further reduce the required capacity of the battery energy storage cell and keep the SOC within a suitable range, a dual‑boundary improved smoothing control algorithm can be adopted. By frequent but shallow actions, the SOC of the battery energy storage cell is optimized, reducing the overall capacity demand from the power system.
2. Case Study: A Photovoltaic‑Storage Integrated Microgrid Project
2.1 Project Overview and Main Equipment
The microgrid project consists of an 800 kW photovoltaic system, a 250 kW/500 kWh lithium‑iron‑phosphate battery energy storage system, and the user’s electrical loads. The nominal voltage of the battery energy storage system is 10 kV. When surplus photovoltaic power is available after meeting self‑consumption, it is stored in the battery energy storage cell and later discharged during peak demand hours to supply the grid. The main equipment list is given in Table 2.
| Equipment | Specification | Quantity |
|---|---|---|
| PV modules | 550 W monocrystalline silicon | 1455 |
| Inverters | 33 kW rated power | 22 |
| LiFePO₄ battery cells | 3.2 V / 130 Ah each | 1224 |
| Power conversion system | 250 kW | 1 |
| 10 kV step‑up transformer | 10 kV/0.4 kV, 800 kVA | 1 |
The lithium‑iron‑phosphate battery energy storage cell shown above is typical of modern storage systems used in such projects.
2.2 Main Operational Functions of the Microgrid
2.2.1 Black Start Capability
The microgrid stores surplus photovoltaic power in the battery energy storage cell. When the utility grid fails, the point of common coupling disconnects, and the battery energy storage cell can perform a black start—powering the loads from the storage cell and photovoltaic system without relying on the grid.
2.2.2 Voltage‑Current Dual‑Closed‑Loop Operation
The output of the lithium‑iron‑phosphate battery energy storage system adopts two bus sections. When multiple storage cells under one bus operate in voltage‑current dual‑closed‑loop mode, the other bus runs under the microgrid grid‑connected control strategy. This dual‑loop mode ensures stable charging/discharging control, maintains DC bus voltage balance, and improves the stability and efficiency of the battery energy storage cell.
2.2.3 Energy Management System (EMS)
The EMS ensures efficient, stable, safe, and reliable operation of the microgrid and optimal utilization of the photovoltaic system. Its functions include data and status monitoring, device management, system fault protection, information storage and recording, distribution automation, smart metering, smart consumption, video and environmental monitoring, and comprehensive energy management.
2.3 Economic Evaluation of the Battery Energy Storage System
The lithium‑iron‑phosphate battery energy storage cell in this microgrid primarily operates in an economic peak‑shaving and valley‑filling mode. During low‑demand periods, it charges from the grid; during high‑demand periods, it discharges energy back to the grid.
With a local peak‑valley electricity price difference of 0.7 RMB/kWh and a depth of discharge of 90%, the annual revenue from the battery energy storage cell is:
\[
0.7 \times 500 \times 90\% \times 365 \approx 115{,}000\ \text{RMB}
\]
In addition to saving electricity costs, the integration of the battery energy storage cell reduces the required transformer capacity, thereby saving the cost of purchasing a box‑type transformer.
3. Discussion
Supporting grid‑connected renewable power generation is a vital application area for battery energy storage systems. The functions of peak shaving/valley filling and primary frequency regulation enable existing power systems to better accommodate renewable energy, reduce the need for thermal standby capacity, and mitigate curtailment issues. With improved minute‑level forecasting of renewable output and the deployment of battery energy storage cells, the schedulability and dispatchability of renewable generation are enhanced. This increases friendliness toward the grid and reduces the consumption of fast frequency regulation resources.
However, under current business models, relying solely on these functions may not be economically viable. Moreover, the economic benefit of the battery energy storage cell can vary significantly with seasonal changes. Therefore, further exploration of additional roles for battery energy storage cells in grid‑connected renewable systems—or improving the performance of the battery energy storage cell across all seasons—represents a promising research direction.
4. Conclusion
This paper has analyzed the application and role of battery energy storage systems in grid‑connected renewable power generation. Taking a photovoltaic‑storage integrated microgrid project as an example, we evaluated the economic performance of the battery energy storage system. The analysis shows that in grid‑connected renewable power systems, a battery energy storage cell primarily serves three functions: peak shaving and valley filling, stabilizing the power system, and primary frequency regulation. Based on the local peak‑valley electricity price difference, the battery energy storage cell operating in the economic peak‑shaving mode can achieve an annual revenue of approximately 115 thousand RMB. Additionally, it reduces electricity costs and saves the expense of a box‑type transformer. Thus, the application of a battery energy storage cell not only improves the reliability and stability of the power system but also delivers measurable economic benefits.
The results confirm that incorporating a battery energy storage cell into a grid‑connected renewable project is a technically effective and economically attractive strategy. Future work should focus on developing more advanced control algorithms to further optimize the duty cycle of the battery energy storage cell, enhance its seasonal adaptability, and explore new revenue streams such as ancillary service markets. Ultimately, the widespread adoption of battery energy storage cells will be a cornerstone of the transition to a low‑carbon, resilient power grid.