Abstract:
In recent years, the rapid development of solar cells has been driven by the urgent need to address environmental pollution and energy crises through natural energy sources such as solar power. However, the intermittent and stochastic nature of solar power generation poses challenges to the frequency stability of power systems when integrated. To mitigate these issues, energy storage devices are often deployed in distributed power sources and solar-integrated distributed generation to suppression the fluctuations in solar output and ensure smooth grid integration of solar power. Given the limitations of single-type energy storage in achieving both high speed and large capacity, it is necessary to leverage composite storage systems combining energy and power storage for complementary advantages. This paper proposes an automatic control method for smoothing photovoltaic (PV) power generation in microgrids using a hybrid energy storage system (HESS) consisting of a battery bank and supercapacitors. The method is validated through simulation examples.
1. Introduction
Solar energy, as a renewable and clean energy source, has significant potential for addressing energy crises and environmental issues. However, the variability and unpredictability of solar radiation lead to fluctuations in photovoltaic power output, affecting the stability of power systems. To address this, energy storage systems (ESSs) are employed to smooth photovoltaic power generation. A single type of energy storage cannot fully meet the requirements of high speed and large capacity, necessitating the use of hybrid energy storage systems.
2. Literature Review
Extensive research has been conducted on capacity allocation for hybrid energy systems internationally. Yan et al. used a low-pass filter for hybrid energy storage but encountered time lags affecting matching accuracy under different operating conditions. Gou et al. adopted Fourier transform to directly decompose electrical energy in power systems for efficient power allocation. However, Fourier transform is a frequency-domain method that only analyzes signal frequency characteristics. Liang used variational mode decomposition (VMD) to decompose photovoltaic converter output and allocate capacities for composite storage devices. Nevertheless, the subjective setting of VMD penalty factor α and modal number K, along with the neglect of important information in VMD residuals, impacts the accuracy of hybrid energy storage capacity configuration.
3. System Overview and Modeling
3.1 System Architecture
The proposed system architecture for hybrid energy storage in photovoltaic-integrated microgrids. This architecture combines supercapacitors and batteries to form a composite energy storage system.
3.2 Photovoltaic Array Model
The equivalent circuit of photovoltaic module, accurately depicting the current-voltage characteristics of the module based on its operating mechanism.
The corresponding electro-hydraulic properties are given by the equation:
I=Iph−I0[exp(nkTq(U+IRs))−1]−RshU+IRs
Where I and U are the output current and voltage of the photovoltaic cell, respectively; Iph is the short-circuit current; Rsh is the shunt resistance; I0 is the diode saturation current; q is the Coulomb constant; Rs is the equivalent series resistance of the photovoltaic cell; k is Boltzmann’s constant; n is the ideality factor; and T is the cell temperature.
4. Maximum Power Point Tracking (MPPT)
MPPT adjusts the Boost boost converter to maximize the output of the photovoltaic panel, which is then converted to AC power using an inverter and fed into the photovoltaic system. The composite energy storage system consisting of DC-DC converters, batteries, and supercapacitors smooths and controls the power of the photovoltaic-integrated microgrid.
5. Hybrid Energy Storage System
5.1 Model Analysis
This paper proposes a composite energy storage system based on parallel connections of energy converters, inductors, and direct connections, offering flexibility in configuration and control, enhancing overall energy storage efficiency. The hybrid storage mode adopts a parallel connection. Typically, the system discharges at minimum charge levels and stores energy at maximum loads, using a Buck-Boost power converter for bidirectional energy flow.
5.2 Capacity Ratio Design
Rational capacity ratio design for the hybrid energy storage device improves overall economy and stability. The maximum current in the battery branch in steady state is given by:
Ibpeak=D(1−γ)Ihybrid(1+RRb)
Where Ibpeak is the peak battery current; Ihybrid is the hybrid energy storage current; Rb is the equivalent internal resistance of the battery; D is the pulse duty cycle; T is the power pulse period; and γ is the system power enhancement factor.
5.3 Control Strategy
The hybrid energy storage system controls discharge depth and charging power source selection, requiring consideration of multiple factors. Research on load status and capacity allocation in the hybrid energy storage system analyzes power variations caused by temperature changes and irradiance. Based on this, a bidirectional DC-DC converter main circuit with two control modes is proposed.
6. Simulation and Results
Simulations were conducted using the MATLAB/Simulink platform to validate the effectiveness of the proposed hybrid energy storage for smoothing photovoltaic power generation in microgrids. The PV-integrated microgrid power system wiring diagram.
Simulations were designed with six photovoltaic cells in parallel, an irradiance of 1000 W/m², a short-circuit current of 28.5 A, an open-circuit voltage of 21.75 V, and a maximum output power of 506.25 W. The system included a 40 F supercapacitor, a 50 V DC busbar voltage, a 200 V rated voltage battery with a 200 Ah capacity, and an ideal operating voltage range of 110-190 V. Temperature ranged from 19-25 ℃, and irradiance varied sinusoidally between 1000-3000 W/m², simulating sudden changes in weather-induced irradiance.
MPPT tracked the maximum photovoltaic array output power, set at 950 W. The hybrid energy storage system charged when photovoltaic output exceeded 950 W and discharged when it fell below 950 W. Supercapacitors provided instantaneous power to mitigate power fluctuations during significant PV output changes. As irradiance varied, photovoltaic array output current and voltage changed, continuously optimized under MPPT control to maintain maximum power point position. During low photovoltaic generation, energy storage relied primarily on the battery bank. During photovoltaic generation, supercapacitors supplied power to ensure system stability.
7. Conclusion
This project studied the dynamic variations of solar power sources in microgrids, established mathematical models for hybrid energy storage (supercapacitors and batteries), and researched a novel MPPT method based on bidirectional DC-DC power conversion technology for efficient and stable grid power control. Experiments demonstrated that the composite energy storage system rapidly and effectively achieved efficient energy utilization and release under the action of a power comparator. Regardless of temperature and lighting conditions, the photovoltaic-integrated microgrid’s output power could be efficiently regulated by the system to ensure stable power output. MPPT technology stabilized the operating state of the photovoltaic array, ensuring optimal performance under illumination. The proposed hybrid storage approach maximizes the advantages of batteries and supercapacitors, achieving efficient and stable distributed photovoltaic grid integration, which is crucial for deep exploration and utilization of solar energy resources.