Components And Working Principle Of Photovoltaic Power Generation Systems
Photovoltaic systems consist of PV modules, controllers, inverters, batteries and other accessories (grid-connected systems do not require batteries). There are two types of PV systems, off-grid and grid-connected, depending on whether they are dependent on the public grid or not. Off-grid PV systems are equipped with storage batteries to ensure that the system power is stable and can be used to supply loads when the PV system is not generating electricity at night or when it is not generating enough electricity on rainy days.
Regardless of the form, the working principle is that the PV module converts light energy into direct current, which is then transformed into alternating current by the inverter, ultimately achieving the function of electricity consumption and access to the grid.
1.Photovoltaic modules
The PV module is the central part of the entire power generation system and consists of a PV module sheet or a combination of different sizes of PV modules cut by a laser cutting machine or steel wire cutter. As the current and voltage of a single PV cell is very small, it is connected in series to obtain a high voltage and then in parallel to obtain a high current, which is outputted through a diode (to prevent current return) and then encapsulated in a stainless steel, aluminium or other non-metallic frame, fitted with the glass above and the backsheet on the back, filled with nitrogen and sealed. The photovoltaic modules are combined in series and parallel to form a photovoltaic module square, also called a photovoltaic array.
How it works: Sunlight shines on a semiconductor p-n junction, forming a new hole-electron pair, and under the action of the electric field of the p-n junction, holes flow from the p-zone to the n-zone, and electrons flow from the n-zone to the p-zone, forming a current when the circuit is connected. The function is to convert solar energy into electrical energy and send it to the battery for storage or to drive the load.
Module types：
①Monocrystalline silicon: photoelectric conversion rate ≈ 18%, up to 24%, is the highest conversion rate of all photovoltaic modules, generally using tempered glass and waterproof resin encapsulation, robust and durable, service life generally up to 25 years.
②Polycrystalline silicon: photoelectric conversion rate ≈ 14%, similar to the production process of monocrystalline silicon, the difference of polycrystalline silicon is lower photoelectric conversion rate, lower price and shorter life, but polycrystalline silicon material is easy to manufacture, save electricity consumption, low production cost, so it is vigorously developed.
3. Amorphous silicon:with a photovoltaic conversion rate of ≈10%, it is a thin-film solar cell made in a completely different way to monocrystalline and polycrystalline silicon, with a greatly simplified process, little silicon material consumption and much lower electricity consumption; its main advantage is that it can generate electricity even in low light conditions.
2. Controller (for off-grid systems)
The PV controller is an automatic control device that can automatically prevent the battery from being overcharged and overdischarged. Adopting a high-speed CPU microprocessor and a high-precision A/D analogue-to-digital converter, it is a microcomputer data acquisition and monitoring control system, which can not only quickly collect the current working status of the PV system in real time and obtain the working information of the PV station at any time, but also accumulate the historical data of the PV station in detail, which provides an accurate and sufficient basis for assessing the rationality of the PV system design and testing the reliability of the quality of the system components, and also has The serial communication data transmission function allows for centralised management and remote control of multiple PV system sub-stations.
3.Inverter
An inverter is a device that converts the direct current generated by photovoltaic power into alternating current. A photovoltaic inverter is one of the important balance of systems in a photovoltaic array system and can be used in conjunction with general AC powered equipment. Solar inverters have special functions to match the PV array, such as maximum power point tracking and islanding effect protection.
Solar inverters can be divided into the following three categories：
1.Stand-alone inverters: used in stand-alone systems, where the PV array charges the batteries and the inverter uses the DC voltage from the batteries as the energy source. Many stand-alone inverters also integrate a battery charger and can charge the batteries with AC power. Generally these inverters do not come into contact with the grid and therefore do not require islanding protection.
2.Grid-connected inverters: The output voltage of the inverter can be fed back into the commercial AC supply, so the output sine wave needs to be the same phase, frequency and voltage as the supply. The grid-connected inverter will have a safety design that will automatically switch off the output if it is not connected to the mains. If the grid power supply trips, the grid-connected inverter does not have the ability to back up the power supply.
3.Battery Backup Inverter: A special type of inverter where the battery is used as its power source, together with the battery charger in it to charge the battery, and if there is excess power, it will be fed back into the AC power supply. This type of inverter can provide AC power to a specified load when the grid power supply is tripped, and therefore needs to have an islanding effect protection function.
4. Battery (not required for grid-connected systems)
The battery is the equipment for storing electricity in the photovoltaic power generation system. At present, four types of batteries are used: lead-acid maintenance-free batteries, ordinary lead-acid batteries, colloidal batteries and alkaline nickel-cadmium batteries, and widely used are lead-acid maintenance-free batteries and colloidal batteries.
Working principle: the sunlight shines on the photovoltaic module during the day, generating DC voltage, converting light energy into electricity and then transmitting it to the controller, after the controller's overcharge protection, the electricity from the photovoltaic module is transferred to the battery for storage, for use when needed.
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