Solar photovoltaic power generation system is a system that converts solar energy into electric energy by using solar cell modules and other auxiliary equipment. Generally, solar photovoltaic power generation system is divided into independent system, grid connected system and hybrid system. According to the application form, application scale and load type of solar photovoltaic power generation system, the solar photovoltaic power generation system can be divided into the following types: small solar photovoltaic power generation system, solar photovoltaic power generation simple DC system, large solar photovoltaic power generation system, solar photovoltaic power generation AC and DC power supply system, Grid connected solar photovoltaic power generation system, hybrid power supply solar photovoltaic power generation system, grid connected hybrid solar photovoltaic power generation system.
The independent solar photovoltaic power generation system forms a circuit in its own closed-circuit system, which directly converts the received solar radiation energy into electrical energy to supply the load through the solar cell pack, and stores the excess energy in the form of chemical energy in the battery after passing through the charging controller. The grid connected power generation system converts the received solar radiation energy into electric energy through the solar cell pack, and then becomes high-voltage DC after high-frequency DC conversion. After inverter inversion, it outputs sinusoidal AC current with the same frequency and phase as the grid voltage to the grid.
- Composition of independent solar photovoltaic system
The scale and application forms of solar photovoltaic power generation system vary, such as the system scale span is large, ranging from 0.3 ~ 2W solar courtyard lamp to MW solar photovoltaic power station; Its application forms are also diverse, and it can be widely used in many fields such as home, transportation, communication, space and so on. Although the scale of photovoltaic system is different, its composition, structure and working principle are basically the same. The independent solar photovoltaic system is composed of solar cell array, battery pack, controller, DC / AC converter and electric load.
1) Photovoltaic module array
In the solar photovoltaic power generation system, the most important is the solar cell, which is the core component to collect sunlight. A large number of solar cells are combined to form a square array of photovoltaic modules. Solar cells are mainly divided into crystalline silicon cells (including monocrystalline silicon Monoc Si, polycrystalline multi Si, ribbon silicon Ribbon / sheeetc SI), amorphous silicon cells (a-Si) and non silicon cells (including copper indium selenide CIS and cadmium telluride CdTe).
For technical and material reasons, the power generation capacity of a single solar cell is very limited. The practical solar cell is a battery system composed of several single cells in series and parallel, which is called battery module. In recent years, as the mainstream technology of solar cells, the raw material price of crystalline silicon cells has been rising, resulting in a sharp rise in the cost of crystalline silicon cells, which makes the cost advantage of amorphous silicon cells more obvious. In addition, thin-film battery (which greatly saves the use of raw materials and greatly reduces the cost) has become the development direction of solar cells, but its technical requirements are very high. As the most mature thin-film battery at present, amorphous silicon thin-film battery is the variety with the most growth potential in thin-film battery.
2) Battery
Battery pack is an energy storage device in solar photovoltaic power generation system. It converts the DC power converted from solar radiation energy into chemical energy and stores it for load application. As the input energy of solar photovoltaic power generation system is extremely unstable, it is generally necessary to configure batteries to make the load work normally. The electric energy generated by the solar cell is stored in the battery in the form of chemical energy. When the load needs power supply, the battery converts the chemical energy into electric energy and supplies it to the load. The characteristics of battery directly affect the working efficiency, reliability and price of solar photovoltaic power generation system. The selection of battery capacity should generally follow the following principles: first, on the premise of meeting the power consumption of the load, store the electric energy generated by the solar cell module during the day as much as possible, and at the same time, store the electric energy required by the load in the scheduled continuous rainy days. For more information about batteries, visit tycorun.com。
The battery capacity is affected by the power demand of end load and sunshine time (power generation time). Therefore, the ampere hour capacity of the battery is determined by the predetermined load power demand and continuous sunshine free time. At present, valve regulated sealed lead-acid batteries and deep discharge liquid absorption lead-acid batteries are commonly used in solar photovoltaic power generation systems.
3) Controller
The function of the controller is to make the solar cell and battery work efficiently, safely and reliably, so as to obtain the highest efficiency and prolong the service life of the battery. The controller controls the charging and discharging of the battery, and controls the solar cell module and battery to output electric energy to the load according to the power demand of the load. The controller is the core part of the whole solar power generation system. The charging and discharging conditions of the battery are limited by the controller to prevent the battery from reverse charging, overcharging and over discharging. In addition, the controller shall also have the functions of circuit short-circuit protection, reverse connection protection, lightning protection and temperature compensation. Because the output energy of solar cells is extremely unstable, the quality of controller charge and discharge control circuit is very important for the design of solar power generation system.
The main function of the controller is to make the solar power generation system always near the maximum power point of power generation, so as to obtain the highest efficiency. Charging control usually adopts pulse width modulation technology (PWM control mode), so that the whole system always operates in the area near the maximum power point PM. Discharge control mainly refers to cutting off the switch when the battery is out of power and the system fails (such as open circuit or reverse connection of the battery). At present, a “sunflower” controller which can track both the control point PM and the solar movement parameters has been developed, which improves the efficiency of fixing solar cell modules by about 50%. With the development of solar photovoltaic industry, the function of the controller is becoming more and more powerful. There is a trend to integrate the traditional control part, converter and monitoring system. For example, the spp and SMD series controllers of AES company integrate the above three functions.
4) DC / AC converter
In the solar photovoltaic power generation system, if it contains AC load, DC / AC converter should be used to convert the DC generated by solar cell module or the DC released by battery into AC required by the load. After the DC generated by the solar cell module or the DC released by the battery is modulated, filtered and boosted by the inverter main circuit, the sinusoidal AC with the same rated frequency and voltage as the AC load is obtained and provided to the system load. According to the excitation mode, the inverter can be divided into self-excited oscillation inverter and other excited oscillation inverter. The inverter has the functions of circuit short-circuit protection, undervoltage protection, overcurrent protection, reverse connection protection and lightning protection.
5) Electric load
Solar photovoltaic power generation system is divided into DC load system and AC load system according to the load nature.
The independent photovoltaic power generation system currently faces the following two problems:
(1) The energy density is small, the overall utilization efficiency is low, and the investment in the early stage is large.
(2) It is easy to charge and discharge the storage battery in advance, which increases the cost of the initial photovoltaic power generation system by about 25%. The battery accounts for 43% of the investment cost in the 20-year operation cycle. Most batteries can not reach the designed service life. In addition to the defects of the battery itself and inadequate maintenance, the unreasonable operation and management of the battery is an important reason for the early failure of the battery. Therefore, for independent solar photovoltaic power generation system, improving energy utilization and studying scientific system energy control strategy can reduce the investment cost of independent photovoltaic system.
- Grid connected solar photovoltaic power generation system
The grid connected solar photovoltaic power generation system is composed of photovoltaic cell array, controller and grid connected inverter. Without energy storage through the battery, the electric energy is directly input into the public grid through the grid connected inverter. Because the electric energy is directly input into the public power grid, the configuration of battery is exempted, the process of energy storage and release of battery is saved, the energy loss is reduced, the occupied space and system investment and maintenance are saved, and the cost is reduced; The power generation capacity can be large and can ensure the reliability of power supply of electric equipment. However, since the inverter output is connected in parallel with the power grid, the consistency of the electrical characteristics such as voltage, phase and frequency of the two groups of power supplies must be maintained, otherwise it will cause the charge and discharge between the two groups of power supplies, resulting in the internal friction and instability of the whole power supply system.
The main components of grid connected solar power generation system are inverter or power regulator (PCU). PCU converts the DC generated by the solar photovoltaic power generation system into standard AC that meets the requirements of the power department. When the power department stops power supply (such as the failure of the public power grid), PCU will automatically cut off the power supply. When the power output of the solar photovoltaic power generation system exceeds the actual power required by the system load, the excess power will be transmitted to the public power grid. In rainy days or at night, when the power output of the solar photovoltaic power generation system is less than the actual power required by the system load, the power required by the system load can be supplemented through the public power grid. At the same time, it is also necessary to ensure that the solar photovoltaic power generation system will not feed electric energy to the public power grid in case of public power grid failure or maintenance, so as to make the system operate stably and reliably. Grid connected solar power generation is the development direction of solar photovoltaic power generation and a potential energy utilization technology in the 21st century.
The grid connected solar photovoltaic power generation system requires the inverter to be connected to the public power grid. Due to the diversity of solar panel installation, in order to maximize the conversion efficiency of solar energy, the grid connected inverter is required to have a variety of combined operation modes to realize the best solar energy conversion. At present, the common grid connected inverters in the world include centralized inverter, series inverter, multi series inverter and component inverter.
1) Centralized inverter
Centralized inverters are generally used in large-scale solar photovoltaic power stations (more than 10kW). Many parallel photovoltaic modules are connected to the DC input end of the same centralized inverter. Generally, high-power inverters use three-phase IGBT power modules, while low-power inverters use field effect transistors. At the same time, controllers with DSP are used to control the quality of inverter output power, making it very close to sine wave current. The most important characteristic of the centralized inverter is the high power and low cost of the system. The centralized inverter photovoltaic power generation system is affected by the matching of photovoltaic modules and partial shading, which reduces the efficiency of the whole photovoltaic power generation system. At the same time, the reliability of the whole photovoltaic power generation system is also affected by the poor working state of a photovoltaic unit group. The latest research direction is to use space vector modulation control technology and develop new inverter topology connection to obtain the high efficiency of centralized inverter photovoltaic power generation system.
Solar max (solar max) centralized inverter can be attached with an interface box of photovoltaic array to monitor each photovoltaic module. If one photovoltaic module in the photovoltaic array does not work normally, the system will transmit this information to the remote controller, and the photovoltaic module can be stopped through the remote controller, Thus, the power output of the whole photovoltaic system will not be reduced and affected due to the failure of a photovoltaic module.
2) Series inverter
Series inverter has become the most popular inverter in the international market. The series inverter is based on modularization. Each photovoltaic series (1 ~ 5kW) passes through an inverter, which has maximum power peak tracking at the DC end and is connected to the public grid at the AC end. Many large solar photovoltaic power plants use series inverters. The advantage of series inverter is that it is not affected by the module difference and shading between series, and reduces the mismatch between the best working point of photovoltaic module and inverter, so as to increase the power generation. These technical advantages not only reduce the system cost, but also increase the reliability of the system. At the same time, the concept of “master-slave” is introduced between the strings, so that when a single group of photovoltaic modules can not meet the operation of a single inverter, the system connects several groups of photovoltaic modules together to make one or several modules work, so as to output more electric energy. The latest concept is that several inverters form a “team” to replace the “master-slave” concept, which further improves the reliability of the system. At present, transformerless series inverter has played a leading role in solar photovoltaic power generation system.
3) Multi series inverter
Multi series inverter uses the advantages of centralized inverter and series inverter to avoid its disadvantages. It can be applied to thousands of watt photovoltaic power stations. In the multi series inverter, different individual power peak tracking and DC / DC converters are included. These DC currents are converted into AC through an ordinary inverter and connected to the public grid. Different rated values of photovoltaic strings (such as different rated power, different number of modules in each string, different manufacturers of modules, etc.), photovoltaic modules of different sizes or technologies, strings in different directions (such as East, South and West), different inclination angles or shadows can be connected to a common inverter, and each string works on their respective maximum power peak. At the same time, the length of DC cable can be reduced, and the shading effect between strings and the loss caused by the difference between strings can be minimized.
4) Modular inverter
Module inverter is to connect each photovoltaic module with an inverter, and each module has a separate maximum power peak tracking, so as to better cooperate between the module and the inverter. It is usually used in 50 ~ 400 W photovoltaic power stations, and the total efficiency is lower than that of series inverter. Because it is connected in parallel at the AC side, it increases the complexity of the AC side wiring of the inverter and makes maintenance difficult. In addition, what needs to be solved is how to connect the power grid more effectively. The simple way is to connect the power grid directly through the ordinary AC socket, which can reduce the cost and equipment installation, but the safety standards of local power grids do not allow this. Power companies are prohibited from connecting the power generation device directly to the ordinary socket of ordinary household users.
The biggest feature of the grid connected solar photovoltaic power generation system is that the DC generated by the solar cell module is converted into AC that meets the requirements of the municipal power grid through the grid connected inverter, and then directly connected to the public power grid without configuring the battery. It can make full use of the electric energy generated by the photovoltaic array, so as to reduce the energy loss and reduce the cost of the system. However, a special grid connected inverter is needed in the system to ensure that the output power meets the requirements of the power grid for voltage, frequency and other electrical performance indicators. Due to the efficiency of the inverter, there will still be some energy loss. This system can usually use the municipal power and solar photovoltaic power generation system as the power supply of the local AC load in parallel, reduce the load power shortage rate of the whole system, and the grid connected photovoltaic system can play a role in peak regulation of the public power grid. However, as a distributed power generation system, grid connected solar photovoltaic power generation system will have some adverse effects on the power grid of the traditional centralized power supply system, such as harmonic pollution, island effect and so on.
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