Classification and introduction of photovoltaic systems

Photovoltaic system definition: A photovoltaic system is a system that converts solar energy into electrical energy using solar cell modules and other ancillary equipment.

Classification and introduction of solar photovoltaic systems

Generally we divide photovoltaic systems into independent systems, grid-connected systems and hybrid systems. According to the application form of the solar photovoltaic system, the application scale and the type of load, the photovoltaic power supply system is divided into more detailed. The photovoltaic system can also be subdivided into the following six types: small solar power system (Small DC); simple DC system (Simple DC); large solar power system (Large DC); AC, DC power supply system (AC/DC); Utility Grid Connect; hybrid power supply system (Hybrid); grid-connected hybrid system. The following describes the working principle and characteristics of each system.

1. Small solar power supply system (Small DC )

The system is characterized by only DC load in the system and relatively small load power. The whole system is simple in structure and easy to operate. Its main use is general household household systems, various civilian DC products and related entertainment equipment. For example, this type of photovoltaic system has been widely used in western China, and the load is a DC lamp, which is used to solve the problem of home lighting in areas without electricity.

2. Simple DC system (Simple DC)

The characteristic of this system is that the load in the system is DC load and there is no special requirement for the use time of the load. The load is mainly used during the daytime, so the battery is not used in the system, and the controller is not needed. The system structure is simple and direct use. The photovoltaic module supplies power to the load, eliminating the need for energy storage and release in the battery, as well as energy loss in the controller, improving energy efficiency. It is commonly used in PV pump systems, some daytime temporary equipment, and some tourist facilities. The figure below shows a simple DC PV pump system. This system has been widely used in developing areas where there is no pure tap water for drinking, and has produced good social benefits.

3 Large Solar Power System (Large DC)

Compared with the above two kinds of photovoltaic systems, the photovoltaic system is still suitable for the DC power supply system, but the solar photovoltaic system usually has a large load power, and the corresponding system is ensured to reliably supply the load with a stable power supply. The scale is also large, and it needs to be equipped with a large array of photovoltaic modules and a large battery pack. The common application forms include communication, telemetry, monitoring equipment power supply, rural centralized power supply, beacon lighthouse, street light and so on. Some rural PV power plants built in some non-electric areas in the west are adopted in this form. The communication base stations built by China Mobile and China Unicom in remote and non-grid areas are also powered by this kind of photovoltaic system. Such as the communication base station project of Wanjiazhai, Shanxi.

4 AC, DC power supply system (AC/DC)

Different from the above three solar photovoltaic systems, the photovoltaic system can provide power for both DC and AC loads, and has more inverters in the system structure than the above three systems for converting DC power to AC power to meet The need for AC load. Usually, the load of such a system is also relatively large, and the system is also large in scale. It is used in some communication base stations with AC and DC loads and other PV power plants with AC and DC loads.

5 Grid Connect System (Utility Grid Connect)

The biggest characteristic of a solar photovoltaic system is that the direct current generated by the photovoltaic array is directly connected to the mains network after being converted into an alternating current that meets the requirements of the mains grid, and the power generated by the PV square array in the grid-connected system is not only supplied with the AC load. In addition, excess power is fed back to the grid. On a rainy day or night, the PV array is powered by the grid when it does not generate electrical energy or the generated electrical energy does not meet the load demand. Because the electric energy is directly input into the power grid, the configuration of the storage battery is eliminated, the process of storing and releasing the battery is omitted, and the power generated by the PV square array can be fully utilized to reduce the energy loss and reduce the cost of the system. However, a dedicated grid-connected inverter is needed in the system to ensure that the output power meets the requirements of the grid power for voltage, frequency and other indicators. Because of the efficiency of the inverter, there will still be some energy loss. Such systems are typically capable of using a combination of utility and solar photovoltaic modules in parallel as a source of local AC load. Reduces the load shortage rate of the entire system. Moreover, the grid-connected PV system can peak the utility grid. However, as a kind of distributed power generation system, the grid-connected photovoltaic power supply system will have some adverse effects on the grid of the traditional centralized power supply system, such as harmonic pollution and islanding effect.

6 Hybrid Power System (Hybrid)

In addition to the solar photovoltaic module array, this solar photovoltaic system uses an oil machine as a backup power source. The purpose of using a hybrid power supply system is to take advantage of various power generation technologies and avoid their respective shortcomings. For example, the advantages of several of the above-mentioned independent photovoltaic systems are less maintenance, and the disadvantage is that the output of energy depends on the weather and is unstable. The hybrid power supply system using diesel generators and photovoltaic arrays can provide weather-independent energy compared to a single energy independent system. Its advantages are:

1. The use of hybrid power systems can also achieve better utilization of renewable energy. Because independent systems that use renewable energy are usually designed for the worst-case scenario, because renewable energy is variable and unstable, the system must be designed with minimal energy generation. Since the system is designed according to the worst case, at other times, the capacity of the system is too large. The excess energy generated during the peak period of solar radiation is not wasted and wasted. The performance of the entire stand-alone system is thus reduced. If the worst month situation is very different from other months, it may result in wasted energy equaling or even exceeding the design load.

2. Has a high system utility. In a stand-alone system, because the change and instability of renewable energy will cause the system to fail to meet the load demand, that is, there is a load shortage, the use of the hybrid system will greatly reduce the load shortage rate.

3. Less maintenance and use less fuel than systems with diesel generators alone.

4. Higher fuel efficiency. In the case of low load, the diesel engine's fuel utilization rate is very low, which will cause fuel waste. Comprehensive control is possible in the hybrid system to allow the diesel engine to operate near rated power, thereby increasing fuel efficiency.

5. Load matching for better flexibility. After using the hybrid system, because the diesel generator can provide more power on the fly,

Therefore, the hybrid system can be applied to a wider range of load systems, such as the use of larger AC loads, shock loads, and the like. It also better matches the load and system power generation. Simply turn on the backup energy at the peak of the load and you can do it easily. Sometimes, the size of the load determines the need to use a hybrid system. Large loads require large currents and high voltages. If you just use solar energy, the cost will be high.

Hybrid systems have their own drawbacks:

1. Control is more complicated. Because a variety of energy sources are used, the system needs to monitor the working conditions of each energy source, deal with the interaction between each sub-energy system, and coordinate the operation of the entire system, which makes its control system more complex than independent systems. Processing chips for system management.

2. The initial project is large. The design, installation and construction of the hybrid system are larger than the independent project.

3. More maintenance is required than a standalone system. The use of the oil machine requires a lot of maintenance work, such as changing the oil filter, fuel filter, spark plug, etc., and also need to add fuel to the fuel tank.

4. Pollution and noise. Photovoltaic systems are noise-free, non-emission, clean energy, but because of the use of diesel engines in hybrid systems, noise and pollution are inevitable.

Many communication power sources and civil aviation navigation equipment power supplies in remote and unpowered areas are powered by hybrid systems because of the high requirements on power supplies, in order to achieve the best price/performance ratio. Many rural photovoltaic power plants built in Xinjiang and Yunnan in China use a light/wood hybrid system.

7 grid hybrid power supply system (Hybrid)

With the development of the solar photovoltaic industry, there has been a grid-connected hybrid power supply system that can comprehensively utilize solar photovoltaic module arrays, utility power and backup oil engines. This kind of system is usually integrated with the controller and the inverter. It uses the computer chip to fully control the operation of the whole system, comprehensively utilizes various energy sources to achieve the best working condition, and can also use the battery to further improve the load supply guarantee rate of the system. For example, AES's SMD inverter system. The system provides qualified power for local loads and can operate as an online UPS (uninterruptible power supply). It is also possible to supply power to the grid or to obtain power from the grid. The way the system works is usually to operate the mains and solar power in parallel. For local loads, if the PV modules generate enough electrical energy to use the load, it will directly use the energy generated by the PV modules to supply the load. If the energy generated by the PV module exceeds the demand of the immediate load, the excess energy can be returned to the grid; if the PV module generates insufficient power, the utility will be automatically activated, the utility will be used to supply the local load, and when local When the power consumption of the load is less than 60% of the rated commercial capacity of the SMD inverter, the utility will automatically charge the battery to ensure that the battery is in a floating state for a long time; if the utility power fails, the utility power is cut off or the mains is powered. If the quality is unqualified, the system will automatically disconnect the mains and turn it into an independent working mode. The battery and inverter will provide the AC power required for the load. Once the utility power returns to normal, that is, the voltage and frequency are restored to the above normal state, the system will disconnect the battery and switch to the grid-connected mode to be powered by the mains. In some grid-connected hybrid power supply systems, system monitoring, control, and data acquisition functions can also be integrated into the control chip. The core components of such systems are controllers and inverters.

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