A power generation system that uses photovoltaic cells to convert solar radiant energy into electrical energy is called a solar photovoltaic power generation system.
Photovoltaic power generation systems generally consist of solar cell arrays, energy storage batteries, protection and control systems, inverters and other equipment.
1.Photovoltaic cell array
The smallest unit to realize photoelectric conversion is a single photovoltaic cell. The size is generally 4~100c㎡, which can only provide a voltage of 0.45~0.50V and a current of 20-25m/㎡, which is far lower than the actual power supply needs, so it cannot be directly When used as a power supply in practical applications, multiple photovoltaic cells are often organized in series and parallel according to power requirements, and packaged in a transparent shell (which can prevent damage to it from the outside world, prolong its life, and facilitate Installation and use) to form a smallest unit that can be used as a power source alone, namely photovoltaic cell modules. Photovoltaic modules, generally composed of 36 single cells, can generate a voltage of 12 to 16V, and the power ranges from a few tenths of a watt to a few hundred watts. It is also possible to connect multiple battery modules in series or parallel and install them on the bracket to form a photovoltaic cell array (mostly rectangular, so it is also called a photovoltaic square array).
The area of the photovoltaic cell array can be large or small. For example, to set up a square matrix of 10kw, an area of 70 to 80 square meters is required. The capacity of the photovoltaic system is expressed by the standard photovoltaic cell array power (the sum of the maximum power of the modules). The power of the photovoltaic system is closely related to the solar irradiance and the temperature of the photovoltaic cells in the photovoltaic module. The standard photovoltaic cell array power generally refers to the maximum power under the standard conditions of a solar radiation intensity of 1kW/㎡, an air quality of AM 1.5, and a cell temperature of 25°C.
The photovoltaic cell array can be placed in a flat panel type with simple structure and suitable for fixed installation occasions; it can also adopt a light-concentrating structure, usually using a flat reflector, a parabolic reflector or a non-Nell lens to condense the light, due to the increased incidence The irradiance of light can save the number of photovoltaic cells or increase the output power, but it is usually necessary to install sun tracking devices and rotating parts, which reduces the reliability.
2.Energy storage battery
Due to the instability and discontinuity of the output power of photovoltaic power generation, independent photovoltaic power generation systems often need to be equipped with energy storage devices to ensure reliable power supply to users. When the sun is full, the photovoltaic cell array is not only supplying power to the user, but also using the remaining energy to charge the battery. In the absence of sunlight at night or in rainy days, if the photovoltaic cell cannot generate electricity or has little output, the battery can supply power to the user.
Commonly used batteries include lead-acid batteries, silica gel batteries and alkaline cadmium-nickel batteries, among which lead-acid batteries have the best power-to-price ratio and are the most widely used.
3.Protection and control system
The protection and control system of photovoltaic power generation is mainly composed of electronic components, measuring instruments, relays, and control switches.
In a small-scale or independently operated photovoltaic power generation system, the protection and control functions are mainly battery protection to prevent overcharge and overdischarge. The controller that prevents the battery from overcharging and over-discharging is called the charge-discharge controller.
For large and medium-sized or grid-connected photovoltaic power generation systems, the protection and control system is responsible for balancing and managing system energy, protecting the battery and the entire system from working normally, and displaying the working status of the system. Sometimes it is necessary to be equipped with a data acquisition system and a microcomputer monitoring system.
The unidirectional conductivity of the diode can be used to prevent the battery from discharging through the photovoltaic cell when the sunlight is insufficient or even absent. These diodes are called anti-reverse charge diodes.
4.Inverter
An inverter is a power electronic device that converts direct current into alternating current. Both photovoltaic cells and storage batteries output direct current. The common electrical equipment for civil use all use alternating current, and the power grid is also an alternating current system. Inverters used in photovoltaic power generation systems generally convert low-voltage direct current into 220V alternating current, which is one of the key technologies for the popularization and application of photovoltaic cells.
According to the operation mode of photovoltaic power generation system, inverters can be divided into two types: one type is used for independent operation of solar photovoltaic power generation system, which supplies power to independent load; the other type is used for grid-connected operation of solar photovoltaic power generation system, which will emit Electric energy is fed into the grid, which is the core component and key technology of the interconnected photovoltaic power generation system. When the power output by the photovoltaic power generation system is due to the consumption of the load, the grid-connected inverter converts the remaining power into electrical energy whose voltage, frequency and other indicators are equivalent to the power grid, and then sends it to the power grid. On the contrary, if the electricity that the user obtains from the photovoltaic power generation system is insufficient, the grid-connected inverter will absorb the insufficient part from the grid.
The grid-connected inverter can also always output the maximum power of the solar battery when the output voltage and current change with the temperature of the solar battery and the solar irradiance.
According to the number of power conversion stages, photovoltaic grid-connected inverters can be divided into single-stage inverters and multi-stage inverters.
The single-stage grid-connected inverter uses fewer components and has a simple circuit structure, so the inverter has higher energy conversion efficiency and lower cost. However, it requires one step to implement various functions such as DC-AC inverter and Maximum Power Point Tracking (MPPT), and the control is relatively complicated: because there is no decoupling link, changes in grid operating conditions (such as low-frequency disturbances, etc.) will affect the power output of the photovoltaic power generation system. Impact: At the same time, the harmonics generated by the photovoltaic power generation system will also be directly injected into the grid, which reduces the efficiency and safety of grid operation to a certain extent.
The multi-pole inverter is mainly a two-stage inverter, which divides the entire control process into two steps: DC/DC and DC/AC. The DC/DC link is used to complete the adjustment of MPPT and DC voltage amplitude, DC The /AC link is used to complete grid-connected control, islanding detection and corresponding protection, etc. The controller design is relatively simple. However, the structure is more complex than single-stage inverters, more components are used, and the cost is higher.
According to whether there is a transformer in the system, photovoltaic grid-connected inverters can be divided into three types: Transformerless (Transformerless), Line-Frequency Transformer (LFT) and High-Frequency Transformer (HFT). kind. Grid-connected inverters that use high-frequency transformers and transformerless methods have advantages in cost, size, quality, and efficiency, and therefore have become a current research hotspot and development trend in low-power and distributed power generation systems.
Figures 2 to 6 show the structure of a single-stage transformerless grid-connected inverter, a bipolar transformerless grid-connected inverter structure, a grid-connected inverter structure isolated by a single-stage power frequency transformer, and a high-frequency transformer isolated structure. Grid-connected inverter structure.
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