Composition and Working Principle of Photovoltaic Power Generation Systems

Composition and Working Principle of Photovoltaic (PV) Power Generation Systems

A photovoltaic (PV) power generation system is primarily composed of PV modules, a controller, an inverter, batteries, and other accessories (batteries are not required for grid-connected systems). Based on whether it relies on the public power grid, PV systems are divided into off-grid and grid-connected types. Off-grid systems operate independently without relying on the utility grid. They are equipped with energy-storage batteries to ensure stable system power supply, capable of providing electricity to loads during nighttime or prolonged cloudy/rainy days when solar generation is insufficient.

Regardless of system type, the working principle remains the same: PV modules convert sunlight into direct current (DC) electricity, which is then converted into alternating current (AC) by an inverter, enabling power consumption or grid connection.

1. Photovoltaic (PV) Modules

PV modules are the core component of the entire power generation system. They are made by combining individual photovoltaic cells, which are cut into different sizes using laser or wire-cutting machines. Since the voltage and current output of a single solar cell are very low, multiple cells are first connected in series to achieve higher voltage, then in parallel to increase current. The assembly includes a blocking diode (to prevent reverse current flow), and is encapsulated within a frame made of stainless steel, aluminum, or non-metallic materials. It is sealed with tempered glass on the front, a backsheet on the rear, filled with nitrogen gas, and hermetically sealed. Multiple PV modules connected in series and parallel form a PV array (also known as a solar array).

Working Principle: When sunlight strikes the semiconductor p-n junction of a solar cell, electron-hole pairs are generated. Under the influence of the electric field at the p-n junction, holes move toward the p-region and electrons toward the n-region. When the circuit is closed, current flows. The primary function of PV modules is to convert solar energy into electrical energy, either storing it in batteries or directly powering electrical loads.

Types of PV Modules:

• Monocrystalline Silicon:Efficiency ≈ 18%, up to 24% — the highest among all PV types. Typically encapsulated with tempered glass and waterproof resin, making them durable and long-lasting (lifespan up to 25 years).

• Polycrystalline Silicon:Efficiency ≈ 14%. Similar manufacturing process to monocrystalline, but with lower efficiency, lower cost, and shorter lifespan. However, it is simpler to produce, consumes less energy, and has lower production costs, leading to widespread adoption.

• Amorphous Silicon (Thin-Film):Efficiency ≈ 10%. Made using a completely different thin-film process, requiring minimal silicon and energy. Its main advantage is better performance under low-light conditions.

2. Controller (Used in Off-Grid Systems)

The solar charge controller is an automatic device that prevents battery overcharging and over-discharging. Equipped with a high-speed CPU microprocessor and high-precision A/D converter, it functions as a microcomputer-based data acquisition and monitoring control system. It can quickly collect real-time operational data, monitor system status, and store historical data, providing accurate and sufficient information to evaluate system design and component reliability. It also supports serial communication for centralized management and remote control of multiple PV sub-stations.

3. Inverter

An inverter converts the DC electricity generated by solar panels into AC electricity, making it compatible with standard AC-powered appliances. The PV inverter is a key balance-of-system (BOS) component and includes special features such as Maximum Power Point Tracking (MPPT) and islanding protection.

Types of Solar Inverters:

• Standalone Inverter:Used in off-grid systems. The PV array charges the battery, and the inverter draws DC power from the battery to supply AC loads. Many standalone inverters include built-in battery chargers that can recharge the battery using AC power. These inverters are not connected to the grid and do not require islanding protection.

• Grid-Tied Inverter:Feeds AC power back into the utility grid. Its output waveform must match the grid’s phase, frequency, and voltage. It automatically shuts down if the grid is disconnected for safety. It does not provide backup power during a grid outage.

• Battery Backup Inverter:A special inverter that uses batteries as its primary power source and includes a charger to recharge them. Excess power can be fed back into the grid. During a grid outage, it can supply AC power to designated circuits, and thus includes islanding protection.

4. Battery (Not Required in Grid-Connected Systems)

The battery is the energy storage unit in a PV system. Common types include sealed lead-acid, flooded lead-acid, gel, and nickel-cadmium alkaline batteries. Sealed lead-acid and gel batteries are the most widely used.

Working Principle: During the day, sunlight hits the PV modules, generating DC voltage and converting light into electricity. This power is sent to the controller, which prevents overcharging, and then stored in the battery for later use when needed.