With the issuance of new policies for distributed photovoltaic (PV) systems, these systems have garnered significant attention from industry professionals. In practical applications, PV systems can be divided into grid-connected and off-grid types. Grid-connected systems convert the direct current (DC) generated by the system into alternating current (AC) and feed it into the power grid. Off-grid PV systems, on the other hand, are self-sufficient and flexible in application, commonly seen in solar street lights, traffic signals, and agricultural pest control lights. In areas not covered by the power grid, the importance of off-grid PV systems becomes particularly apparent. Here, we mainly introduce the components of an off-grid PV system. What components make up a PV system, and what are their functions? Let’s unravel these questions for you.
Components of an Off-Grid PV System:
A typical PV system mainly consists of a photovoltaic array, a charge-discharge controller, energy storage equipment or an inverter, and a load. The configuration is shown in the diagram below.
When sunlight irradiates the PV array, solar energy is converted into electrical energy. Due to environmental influences, the output current from the PV array is unstable and needs to be converted into a stable current through a DC-DC converter before it can charge the battery. The battery then supplies power to the load. In the case of grid-tied power sales, a battery is not required. Instead, a grid-tied inverter converts the DC current into AC current, which is then fed into the power grid. Therefore, off-grid PV power generation systems must use battery storage, while grid-tied systems do not necessarily need it.
The control system continuously samples the output voltage and current from the PV array to determine whether the PV power generation system is operating at its maximum power point. Based on tracking algorithms, it adjusts the PWM signal's duty cycle, controlling the output voltage of the PV array to approach the maximum power point. In the overcharge and over-discharge control module of the battery, the system will automatically shut down or activate when the battery voltage reaches certain preset levels.
Below, we will introduce each component of an off-grid PV power generation system.
1. Photovoltaic Array:
● PV systems utilize photovoltaic array components, which convert solar energy directly into electrical energy based on the principle of the photovoltaic effect. The smallest unit used for photoelectric conversion is a photovoltaic cell, which produces a voltage of about 0.45V and a working current of about 20~25mA/cm². These cells are encapsulated in series and parallel to form a photovoltaic array component.
l When solar cells are exposed to light and connected to a load, the photogenerated current flows through the load, establishing a terminal voltage. The working condition of the solar cell can be represented by the solar cell load characteristic curve. This curve shows the relationship between output voltage, output current, and output power of the photovoltaic cell under specific sunlight intensity and temperature, referred to as I-V and P-V characteristics. The P-V characteristic curve indicates that the output power is approximately a downward parabola. The peak of this parabola represents the maximum power point under a certain sunlight intensity, and the corresponding voltage is called the maximum power point voltage. To enhance the efficiency of a PV system, it must operate near the maximum power point on the P-V curve.
● Key Technical Parameters of the Photovoltaic Array:
1. Short-Circuit Current (Isc): The maximum output current under specific sunlight intensity and temperature.
2. Open-Circuit Voltage (Voc): The maximum output voltage under specific sunlight intensity and temperature.
3. Maximum Power Point Current (Im): The current corresponding to the maximum power point under specific sunlight intensity and temperature.
4. Maximum Power Point Voltage (Um): The voltage corresponding to the maximum power point under specific sunlight intensity and temperature.
5. Maximum Power Point Power (Pm): The maximum power that the solar cell array can output under specific sunlight intensity and temperature.
2. DC-DC Converter:
l The electrical energy produced by photovoltaic panels varies with weather, temperature, and load, making it unstable and difficult to use directly. A DC-DC converter, which is an electronic power converter, is used to control and transform this energy into a form suitable for the load or grid. The basic function of the power electronic converter is to convert one form of electrical energy into another to meet the requirements of different loads. In off-grid PV systems, DC-DC converters are used to perform tasks such as maximum power point tracking, battery charging, PID automatic control, and voltage step-up or step-down.
● The relationship between the output voltage and input voltage of a DC-DC converter is achieved by controlling the on-off time of the switch, usually done with a PWM signal. The main principle is to keep the period (T) constant and adjust the on-time of the switch to control the voltage.
● Types of Power Electronic Converters:
1. AC-DC Converter
2. DC-AC Converter
3. DC-DC Converter
4. AC-AC Converter
● In off-grid PV systems, a DC-DC converter is used, and it can be divided into various types based on its operation: step-down (Buck), step-up (Boost), step-up/step-down (Buck-Boost), and others.
3. Battery:
l In standalone PV systems, energy storage devices are essential. Although there are various energy storage methods such as capacitors, flywheels, and superconductors, most medium and large PV systems use maintenance-free lead-acid batteries due to their convenience, reliability, and cost-effectiveness.
● However, lead-acid batteries have their drawbacks, including high cost and being the weak link in the system. Improper management can lead to premature failure, increasing overall system costs.
4. Photovoltaic Control Module:
l The PV control module, with a microcontroller as the center, provides the optimal charging current and voltage for the battery, charging it quickly, smoothly, and efficiently. It also minimizes battery loss during charging, extending the battery's lifespan while protecting it from overcharging and over-discharging. If a DC load is used, the solar controller can provide stable DC power to the load and implement automatic switching functions through control sensors (light control, sound control, etc.).
5. Off-Grid Inverter:
l For residential off-grid PV systems, since some loads are AC, an off-grid inverter is needed to convert the DC generated by the PV modules into AC for the AC load. Off-grid inverters and grid-tied inverters have similar main circuit structures, but grid-tied inverters need to consider safety when connected to the grid, such as frequency, phase, and anti-islanding control. Off-grid inverters do not need to consider these factors.
● The performance indicators of inverters are crucial for ensuring the overall performance and long-term stable operation of off-grid PV systems.
Applications of Off-Grid PV Systems:
Off-grid PV systems are widely used in remote mountainous areas, areas without electricity, islands, communication base stations, and street lighting, among other applications.
To learn more about PV systems or to inquire about customized PV power systems, please contact us.
Post time: Jun-26-2024
