① Principle of photovoltaic power generation system
The basic principle of all power generation systems is to convert various types of secondary energy sources on the earth, such as coal, oil and gas, sunlight, and water energy, into electrical energy through specific technical means. The core principle of photovoltaic power generation is the photovoltaic effect, which refers to the phenomenon that semiconductor materials generate electromotive force when they are exposed to light and radiation. From the perspective of energy conversion, the core of photovoltaic power generation facilities is solar panels (also called photovoltaic modules). Photovoltaic materials are also called solar cell materials, and only semiconductor materials have this characteristic. At present, the semiconductor materials used for power generation mainly include: monocrystalline silicon, polycrystalline silicon, amorphous silicon and cadmium telluride. Regardless of the type of semiconductor material, a specific doping process is required to produce an N-type semiconductor with a higher concentration of electrons (equivalent to negative charges) and a P-type semiconductor with a higher concentration of holes (equivalent to positive charges). The P-type semiconductor and the N-type semiconductor are fabricated on the same semiconductor (usually silicon or germanium) substrate, and the space charge area formed at their interface is called a PN junction. PN junction has unidirectional conductivity, which is a characteristic used by many devices in electronic technology, and is the material basis of electronic components such as semiconductor diodes and bipolar transistors.
The essence of a solar cell is a large-area PN junction. When light irradiates a surface of a PN junction, such as a P-type surface, if the photon energy is greater than the forbidden band width of the semiconductor material, then each photon absorbed by the P-type region will produce a pair of free electrons and holes, electron-hole pairs It diffuses rapidly from the surface inward, and under the action of the junction electric field, an electromotive force that is roughly positively correlated with the light intensity is finally established (see Figure 1).
Generally speaking, a power generation system that uses the photovoltaic effect of solar cells to directly convert solar radiation into electrical energy is called a photovoltaic power generation system. Among them, the smallest indivisible solar cell assembly device with encapsulation and internal connection that can provide direct current output alone is the solar cell module, also known as the photovoltaic module. After a number of photovoltaic modules are connected in series, a circuit unit with a certain direct current input capability is formed, that is, a photovoltaic module string.
A certain number of photovoltaic module strings are collected through a DC combiner box, and then converted into a power source that meets the frequency and voltage requirements of the grid by an inverter and an isolation step-up transformer, which is a photovoltaic power generation unit. A DC power generation unit composed of thousands of photovoltaic modules assembled in a mechanical and electrical manner and with a fixed support structure is called a photovoltaic array or a photovoltaic array.
In the current mainstream power generation system types, in addition to photovoltaic power generation, other forms of power generation such as thermal power, hydropower, wind power, and even tidal and solar thermal power generation. Although the types of primary energy are different, the essence is based on the principle of electromagnetic induction. Cutting the magnetic field lines converts mechanical kinetic energy into electrical energy. From the perspective of later operation and maintenance management of the power generation system, currently only photovoltaic power generation systems do not involve mechanical and transmission components. This is the most significant difference between photovoltaic power generation systems and other power generation systems in terms of operation and maintenance.
② Overview of the development history of photovoltaic power generation
In the 1950s, scientists at Bell Labs in the United States invented a revolutionary practical silicon cell with a conversion efficiency of 6% for the first time, marking that photovoltaic power generation has officially become an industry. For more than half a century, photovoltaic power generation technology has gradually matured under the joint efforts of scientific research and industrial personnel from all over the world, and the application range has gradually expanded from the initial aerospace and other fields to the civil-scale power generation field. Based on the needs of responding to the fossil energy crisis, global climate change and environmental protection, achieving a clean energy transition and low-carbon development has gradually become a general consensus in most countries around the world. Since the beginning of the 21st century, governments of countries led by Europe and the United States have vigorously carried out policy support for the photovoltaic power generation industry. At the same time, China-based photovoltaic equipment manufacturing and supporting industrial chains have achieved rapid growth, and jointly promoted the rapid development of the global photovoltaic power generation industry. As of the end of 2019, the total global installed capacity of photovoltaic power generation reached 580 million kilowatts, and since 2015, China’s total installed photovoltaic power generation capacity and power generation have continuously ranked first in the world.
Photovoltaic power generation is one of the current mainstream technologies for clean energy. With the improvement of commercial economy and relative competitiveness, it will have broader development prospects in the energy field in the future. As large-scale photovoltaic power plants are built and put into operation, the importance of management in the later stage of grid connection (also called operation period) has become increasingly prominent. Doing a good job in the operation and maintenance of the power station during the operation period plays a vital role in ensuring the safe production and efficient power generation of the power station, and effectively improving the efficiency of asset operation and the rate of return on investment.
