Analysis of technical difficulties in concentrated photovoltaic power generation system

　　Solar photovoltaic technology refers to a forward-looking technology that can directly convert the sun's light energy into electrical energy and make full use of it. Its broad application prospects fascinate the world and continue to work hard for development, innovation and application. This issue will introduce you to the principles of solar power generation, solar cells, solar modules, photovoltaic controllers, photovoltaic inverters, etc.

　　Principles of solar power generation

　　Solar cells are devices that respond to light and convert light energy into electricity. There are many kinds of materials that can produce photovoltaic effects, such as: monocrystalline silicon, polycrystalline silicon, amorphous silicon, gallium arsenide, selenium indium copper, etc. Their power generation principles are basically the same. Now, a crystal is used as an example to describe the photovoltaic power generation process. P-type crystalline silicon is doped with phosphorus to obtain N-type silicon, forming a P-N junction.

　　When light irradiates the surface of the solar cell, part of the photons are absorbed by the silicon material; the energy of the photons is transferred to the silicon atoms, causing the electrons to migrate and become free electrons that accumulate on both sides of the P-N junction to form a potential difference. When the external circuit is turned on At this time, under the action of this voltage, a current will flow through the external circuit to produce a certain output power. The essence of this process is: the process of converting photon energy into electrical energy.

　　The manufacturing process of crystalline silicon solar cells

　　Silicon is one of the most abundant materials on our planet. Since scientists discovered the semiconductor properties of crystalline silicon in the 19th century, it has changed almost everything, even human thinking. At the end of the 20th century, silicon can be seen everywhere in our lives. Crystalline silicon solar cells have been industrialized the fastest in the past 15 years. The production process can be roughly divided into five steps: a. Purification process b. Rod drawing process c. Slicing process d. Battery making process e. Packaging process.

　　In the 1960s, scientists had already applied solar cells to space technology - power supply for communication satellites. At the end of the last century, as humans continued to reflect on themselves, they became more and more familiar with photovoltaic power generation, such a clean and direct form of energy. Not only in space applications, but also in many fields. Such as: solar garden lights, solar power household systems, independent systems for village power supply, photovoltaic water pumps (drinking water or irrigation), communication power supplies, cathodic protection of oil pipelines, optical cable communication pump station power supplies, seawater desalination systems, road signs in cities and towns, highways Highway signs, etc. Advanced countries such as Europe and the United States have incorporated photovoltaic power generation into urban power systems and natural village power supply systems in remote areas as development directions. The combination of solar cells and building systems has formed an industrialization trend. Solar photovoltaic glass curtain wall components are being used more and more. As several projects in Shanghai and Beijing enter substantial operation, this method will replace ordinary glass curtain walls. It has the characteristics of low reflected light intensity and good thermal insulation performance!

　　Solar cell module

　　Solar cell modules (photovoltaic modules) are composed of a certain number of solar cells connected in parallel through wire strings and packaged. The standard number of solar cells in a module is 36 pieces (10cm x 10cm), which means that a solar cell module can generate approximately 17V of voltage, which can effectively charge a battery with a rated voltage of 12V. The current output power of photovoltaic components ranges from hundreds of watts.

　　After the solar cells are packaged into components, they can provide sufficient mechanical strength, vibration resistance and impact resistance; have good sealing properties and can be anti-corrosion, windproof, hail-proof and moisture-proof; have good electrical insulation; can withstand ultraviolet radiation, etc. Potential quality problems may occur in the edge seal and the junction box on the back of the module.

　　According to the needs of photovoltaic engineering installation, when the application field requires higher voltage and current and a single component cannot meet the requirements, multiple components can be assembled in series or parallel into a solar cell array, also called a photovoltaic array, to obtain the required Voltage and current, its power can be determined according to the actual demand combination.

　　Solar photovoltaic controller

　　Photovoltaic charge controllers can basically be divided into five types: parallel photovoltaic controllers, series photovoltaic controllers, pulse width modulation photovoltaic controllers, smart photovoltaic controllers and maximum power tracking photovoltaic controllers.

　　1. Parallel photovoltaic controller. When the battery is fully charged, electronic components are used to shunt the output of the photovoltaic array to the internal parallel resistor or power module, and then dissipate it in the form of heat. Parallel photovoltaic controllers are generally used in small, low-power systems, such as systems with voltages within 12V and 20A. This type of controller is very reliable and has no mechanical parts such as relays.

　　2. Series photovoltaic controller. Mechanical relays are used to control the charging process and switch off the photovoltaic array at night. It is generally used in higher power systems, and the capacity of the relay determines the power level of the charge controller. It is relatively easy to manufacture a series photovoltaic controller with a continuous current of more than 45A.

　　3. Pulse width modulation photovoltaic controller. It switches the input of the photovoltaic array on and off with PWM pulses. As the battery becomes fuller, the frequency and duration of the pulses decrease. According to research by Sandia National Laboratory in the United States, this charging process forms a more complete state of charge, which can increase the total cycle life of the battery in the photovoltaic system.

　　4. Intelligent photovoltaic controller. Based on MCU (such as Intel's MCS51 series or Microchip's PIC series), the operating parameters of the photovoltaic power system are collected in real time at high speed, and single or multiple photovoltaic arrays are cut off and connected by software programs according to certain control rules. control. For medium and large photovoltaic power systems, distance control can also be carried out through the RS232 interface of the MCU in conjunction with the MODEM modem.

　　5. Maximum power tracking controller. Detect the solar cell voltage V and current I and multiply them to get the power P, and then determine whether the output power of the solar cell has reached the maximum at this time. If it is not running at the maximum power point, just adjust the pulse width, modulate the output duty cycle D, and change the charging The current is sampled in real time again, and a judgment is made whether to change the duty cycle. This optimization process can ensure that the solar cells always run at the maximum power point to fully utilize the output energy of the solar cell array. At the same time, the PWN modulation method is used to make the charging current into a pulse current to reduce the polarization of the battery and improve the charging efficiency.

　　Photovoltaic inverter

　　As an independent photovoltaic system, its DC power generation voltage is relatively low, so the power conditioning device, that is, the inverter, is absolutely indispensable.

　　There are two main types of inverters used in grid-connected systems to achieve AC power generation.

　　①Line rectification can use the signal in the power grid as the synchronization reference.

　　②Self-rectification determines the signal waveform through the internal circuit structure of the inverter, and then inputs it into the power grid.

　　Products can also be classified according to their application.

　　① The central inverter is used to rectify the output of large-scale photovoltaic systems with rated power in the range of 20 to 400kWp. The current mainstream products have self-rectifying designs, which are implemented through bipolar transistors and field-effect transistors.

　　② The series inverter is only allowed to receive signals transmitted through independent serial lines, so the rated power is between 1 and 3kWp.

　　③Duplex series inverters are equipped with various independent DC-DC inverters, which feed signals back to a central inverter device. Such a design can be applied to a variety of different component connection structures, so that the solar cells on each series line can output maximum power.

　　④The AC component inverter is installed on each photovoltaic component to convert the output of all components into AC.

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