Abstract:
The integration of renewable energy sources (RES) into the power grid has garnered significant attention in recent years due to their potential to reduce greenhouse gas emissions and fuel consumption. Microgrids, composed of distributed power sources, energy storage devices, energy conversion units, loads, monitoring, and protection systems, have emerged as a viable solution to harness this renewable energy. This paper presents the design and simulation of a standalone direct current (DC) microgrid, with a solar photovoltaic (PV) system as the primary power source and a battery-based energy storage system (ESS). The simulation model is developed in MATLAB/Simulink, and the system’s performance under various input conditions and load changes is analyzed. The results demonstrate the system’s effectiveness in maintaining stable power supply and energy management.
Keywords: Energy storage battery, microgrid, solar photovoltaic, DC-DC converter
1. Introduction
The global power system is transitioning towards decentralization and decarbonization to address environmental concerns and mitigate the impacts of climate change. The excessive reliance on fossil fuels for electricity generation has led to a rise in fuel costs, fossil fuel depletion, and increased greenhouse gas emissions. Consequently, there has been a surge in the exploration of alternative energy sources, particularly solar PV and wind energy. The rapid integration of these renewable energy sources into the utility grid has prompted the development of distributed energy systems (DES), which include distributed energy resources (DERs).
Microgrids, which originated from the deployment of renewable energy power plants, have emerged as a promising solution for enhancing the reliability and resilience of the power grid. Microgrids are small-scale power generation and distribution systems that can operate in both grid-connected and islanded modes. They integrate renewable energy sources, energy storage systems, energy conversion devices, loads, and monitoring and protection mechanisms to ensure a stable and reliable power supply.
In this paper, we focus on the design and simulation of a standalone DC microgrid, with a solar PV system as the main power source and a battery-based energy storage system. The system’s performance under different operating conditions is analyzed using MATLAB/Simulink. The remainder of this paper is organized as follows: Section 2 reviews related work in the field of microgrids and energy storage systems. Section 3 details the design of the standalone DC microgrid, including the PV system, energy storage system, and DC-DC converters. Section 4 presents the simulation results and discusses the system’s performance. Finally, Section 5 concludes the paper with a summary of the key findings.
2. Related Work
The development of microgrids has garnered significant attention from researchers and industry professionals due to their potential to address the challenges associated with integrating renewable energy sources into the power grid. The primary research directions in this field include energy management and control strategies for microgrids.
Several studies have focused on the development of centralized energy management systems for microgrids. Athira et al. [1] proposed a fuzzy control-based energy management system for an islanded DC microgrid to improve battery performance. In another study, Al-Dhaifallah et al. [2] developed an efficient short-term energy management system for a microgrid with renewable energy generation and electric vehicles.
Hybrid energy storage systems (HESS) have also been investigated extensively for microgrids. For instance, Athira and Pandi [3] presented an islanded DC microgrid prototype model with a hybrid energy storage system. The system employed a fuzzy logic controller to manage energy flow between the PV system, battery, and supercapacitor.
Power optimization in microgrids has also been a topic of interest. Han et al. [4] proposed a hierarchical energy management strategy for a PV/hydrogen/battery islanded DC microgrid. The strategy optimized energy distribution among different storage devices based on their characteristics and operating conditions.
While most existing research has focused on AC microgrids, DC microgrids offer several advantages, including reduced power conversion stages, simplified control mechanisms, and immunity to issues such as frequency synchronization and reactive power flow common in AC systems. Therefore, this paper focuses on the design and simulation of a standalone DC microgrid.
3. Design of the Standalone DC Microgrid
The standalone DC microgrid designed in this study comprises a solar PV system, an energy storage system (ESS) using batteries, DC-DC converters, and loads.
3.1 Solar PV System
The solar PV system serves as the primary power source for the microgrid. It consists of PV modules connected in series and parallel to achieve the desired voltage and current levels. A maximum power point tracking (MPPT) controller is employed to extract the maximum power from the PV array under varying irradiance and temperature conditions.
3.1.1 PV Cell Mathematical Model
The equivalent circuit of a PV cell is shown in Figure 2. It consists of a current source (Iph), a diode (D), and series (Rse) and parallel (Rp) resistances. The ideal output current (I) of the PV cell can be expressed as:
I=Iph−ID−Ish
where:
- Iph is the photogenerated current,
- ID is the diode current, given by the Shockley equation,
- Ish is the shunt current.
3.1.2 MPPT Controller
The MPPT controller adjusts the operating point of the PV system to maximize power output under varying solar irradiance and cell temperature. In this study, a lookup table (LUT) MPPT controller is used due to its fast response time and stability. The LUT contains precomputed duty cycle values for a range of irradiance (500–1300 W/m²) and temperature (25–55°C) conditions.
3.2 Energy Storage System (ESS)
The energy storage system consists of batteries that store excess energy generated by the PV system and supply power during periods of low or no solar irradiance. A bidirectional DC-DC converter connects the battery to the DC bus, allowing for both charging and discharging.
3.3 DC-DC Converters
DC-DC converters are essential components of the microgrid, facilitating power conversion and regulation. A unidirectional DC-DC boost converter is used to step up the voltage from the PV array to the DC bus voltage level. The bidirectional DC-DC converter manages the flow of energy between the battery and the DC bus.
3.3.1 Unidirectional DC-DC Boost Converter
The unidirectional DC-DC boost converter increases the output voltage of the PV array to match the DC bus voltage. Its circuit diagram.
3.3.2 Bidirectional DC-DC Converter
The bidirectional DC-DC converter allows energy to flow bi-directionally between the battery and the DC bus. It operates in buck mode during charging and boost mode during discharging. The converter circuit is designed for a 48V to 12V voltage conversion.
4. Simulation Results and Discussion
The standalone DC microgrid system was simulated in MATLAB/Simulink using the parameters listed in Table 1. The PV module’s rated output power is 100W, with a maximum power point voltage of 17.99V and current of 5.57A. The DC-DC converters are designed to convert the PV output voltage to a stable 48V DC bus voltage.
Table 1: Key Parameters of the Microgrid System
| Parameter | Value |
|---|---|
| Maximum Power | 100W |
| Open-Circuit Voltage | 21.84V |
| Short-Circuit Current | 6.11A |
| MPP Voltage | 17.99V |
| MPP Current | 5.57A |
| Output Voltage | 48V |
| Inductance | 0.54mH |
| Capacitance | 76.9μF |
| Switching Frequency | 10kHz |
4.1 MPPT Controller Performance
Shows the duty cycle generated by the MPPT controller under varying irradiance conditions. The controller extracts the duty cycle values from the LUT based on the measured irradiance and temperature, ensuring stable maximum power extraction.
4.2 System Voltage Regulation
The output voltage profiles of the PV module, DC-DC boost converter, and the 48V DC bus. The boost converter regulates the PV output voltage to maintain a stable 48V DC bus voltage, even under varying input conditions.
4.3 Energy Storage System Performance
The battery’s charging and discharging behavior is analyzed by connecting a 48V battery load to the DC bus. Figure 6 shows the battery’s state of charge and the DC bus voltage during charging and discharging cycles.
5. Conclusion
This paper presents the design and simulation of a standalone DC microgrid system based on a solar PV system and a battery-based energy storage system. The system’s performance under varying input conditions and load changes was analyzed using MATLAB/Simulink. The results demonstrate the effectiveness of the MPPT controller in maximizing power extraction from the PV array and the ability of the DC-DC converters to maintain a stable DC bus voltage. The energy storage system effectively manages energy during periods of low solar irradiance, ensuring a reliable power supply to the loads.
Future work can focus on enhancing the system’s resilience against grid faults and developing advanced control strategies for optimal energy management across different operating modes. Additionally, the integration of other renewable energy sources and energy storage technologies can be explored to further improve the microgrid’s sustainability and efficiency.