Detailed explanation of solar photovoltaic technology knowledge

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 have attracted people all over the world and have made continuous efforts to develop, innovate and apply it. This issue will introduce the principles of solar power generation, solar cells, solar cell modules, photovoltaic controllers, photovoltaic inverters, etc.

Principle of solar power generation

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

When light shines on the surface of a solar cell, some 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 gather on both sides of the P-N junction to form a potential difference. When the external circuit is connected, under the action of this voltage, current will flow through the external circuit to generate 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 and its role. Crystalline silicon solar cells have been the fastest to be industrialized 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 manufacturing process e. packaging process.

Application of Solar Cells

In the 1960s, scientists had already applied solar cells to space technology - power supply for communication satellites. At the end of the last century, in the process of continuous self-reflection, people have become more and more familiar with photovoltaic power generation, a clean and direct form of energy. It is not only used in space, but also in many fields. For example: solar garden lights, solar power generation household systems, independent systems for village power supply, photovoltaic water pumps (drinking water or irrigation), communication power supply, cathodic protection of oil pipelines, optical cable communication pump station power supply, seawater desalination system, road signs in towns, 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 into the development direction. The combination of solar cells and building systems has formed an industrialization trend. Solar photovoltaic glass curtain wall components are increasingly used. 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 modules

Solar cell modules (photovoltaic modules) are made up of a certain number of solar cells connected in series and parallel by wires and packaged. The standard number of solar cells in a module is 36 (10cm x 10cm), which means that a solar cell module can generate a voltage of about 17V, which is just enough to effectively charge a battery with a rated voltage of 12V. The output power of current photovoltaic components ranges from hundreds of watts.

After the solar cells are packaged into modules, they can provide sufficient mechanical strength, vibration resistance and impact resistance; have good sealing, can be anti-corrosion, windproof, hailproof, moisture-proof; have good electrical insulation; can resist ultraviolet radiation, etc. Its potential quality problems may occur in the sealing of the edges and the junction box on the back of the module.

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

Solar Photovoltaic Controller

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

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

2. Series photovoltaic controller. Use mechanical relays to control the charging process and cut off the photovoltaic array at night. It is generally used in higher power systems. 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 in PWM pulse mode. When the battery is nearly full, the frequency and time of the pulse are shortened. According to the research of Sandia National Laboratory in the United States, this charging process forms a more complete charging state, 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 the single-channel or multi-channel photovoltaic arrays are disconnected and connected by software programs according to certain control rules. For medium and large photovoltaic power systems, distance control can also be performed through the RS232 interface of the MCU and the MODEM modem.

5. Maximum power tracking controller. After detecting the voltage V and current I of the solar cell, multiply them to get the power P, and then determine whether the output power of the solar cell has reached the maximum. If it is not running at the maximum power point, just adjust the pulse width, modulate the output duty cycle D, change the charging current, perform real-time sampling again, and make a judgment on whether to change the duty cycle. Through this optimization process, it can ensure that the solar cell always runs 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 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.

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

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

Products can also be classified according to their application.

① The central inverter is used to rectify the output of large photovoltaic systems with a rated power range of 20 to 400 kWp. The current mainstream products have a self-rectification design, which is achieved through bipolar transistors and field-effect transistors.

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

③The compound series inverter is equipped with various independent DC-DC inverters, which feed back signals to a central inverter. Such a design can be applied to various 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 element to convert the output of all components into AC.

In the first issue, we gave you a general introduction to the principles of solar power generation, solar cells, solar cell modules, photovoltaic controllers, photovoltaic inverters, etc. Next, we will start to explain in depth. In this issue, we will focus on the detailed process of solar cell manufacturing, the difference between monocrystalline silicon and polycrystalline silicon, the difference between monocrystalline silicon and polycrystalline silicon cells, and the concept of inverters, connecting the knowledge together for readers to learn and understand.

Manufacturing process of crystalline silicon solar cells

The manufacturing process of crystalline silicon solar cells is described as follows:

(1) Slicing: Use multi-wire cutting to cut the silicon rod into square silicon wafers.

(2) Cleaning: Use conventional silicon wafer cleaning methods, and then use an acid (or alkaline) solution to remove 30-50um of the damaged layer on the surface of the silicon wafer.

(3) Preparing a velvet surface: Use an alkaline solution to anisotropically etch the silicon wafer to prepare a velvet surface on the silicon wafer surface.

(4) Phosphorus diffusion: Use a coating source (or liquid source, or solid phosphorus nitride sheet source) for diffusion to form a PN+ junction with a junction depth of generally 0.3-0.5um.

(5) Peripheral etching: The diffusion layer formed on the peripheral surface of the silicon wafer during diffusion will cause a short circuit between the upper and lower electrodes of the battery. The peripheral diffusion layer is removed by masked wet etching or plasma dry etching.

(6) Remove the back PN+ junction. Wet etching or grinding is usually used to remove the back PN+ junction.

(7) Making upper and lower electrodes: Use vacuum evaporation, chemical nickel plating or aluminum paste printing and sintering. First make the lower electrode, then make the upper electrode. Aluminum paste printing is a widely used process.

(8) Making anti-reflection film: In order to reduce the reflection loss, a layer of anti-reflection film should be covered on the surface of the silicon wafer. The materials for making anti-reflection film include MgF2, SiO2, Al2O3, SiO, Si3N4, TiO2, Ta2O5, etc. The process methods can be vacuum coating, ion coating, sputtering, printing, PECVD or spraying.

(9) Sintering: Sinter the battery chip onto a nickel or copper base.

(10) Test classification: Test classification according to specified parameter specifications.

It can be seen that the manufacturing process of solar cell chips is basically the same as that of semiconductor devices, and the production process equipment is also basically the same, but the process processing accuracy is far lower than the manufacturing requirements of integrated circuit chips, which provides favorable conditions for the large-scale production of solar cells.

The difference between monocrystalline silicon and polycrystalline silicon

The difference between single crystal silicon and polycrystalline silicon is that when molten silicon solidifies, silicon atoms are arranged in a diamond lattice to form many crystal nuclei. If these crystal nuclei grow into chips with the same crystal plane orientation, single crystal silicon is formed. If these crystal nuclei grow into chips with different crystal plane orientations, polycrystalline silicon is formed. The difference between polycrystalline silicon and single crystal silicon is mainly reflected in physical properties. For example, in terms of mechanical properties and electrical properties, polycrystalline silicon is not as good as single crystal silicon. Polycrystalline silicon can be used as a raw material for pulling single crystal silicon. Single crystal silicon can be regarded as the purest substance in the world. General semiconductor devices require silicon purity of more than six nines. Large integrated circuits have higher requirements, and the purity of silicon must reach nine nines. At present, people can already produce single crystal silicon with a purity of twelve nines. Single crystal silicon is an indispensable basic material in modern science and technology such as electronic computers and automatic control systems.

High-purity silicon is extracted from quartz. Taking single crystal silicon as an example, the refining process goes through the following steps: quartz sand - metallurgical grade silicon - purification and refining - deposition of polycrystalline silicon ingots - single crystal silicon - silicon wafer cutting.

It is not difficult to refine metallurgical grade silicon. It is mainly prepared by reducing quartz sand with carbon in an electric arc furnace. The purity of the reduced silicon is about 98-99%, but the silicon used in the semiconductor industry must be highly purified (the purity of electronic grade polysilicon requires 11 nines, while the purity of solar cell grade silicon requires only 6 nines). In the purification process, there is a key technology of "trichlorosilane reduction method (Siemens method)" that my country has not mastered. Due to the lack of this technology, more than 70% of polysilicon in my country is discharged through chlorine during the refining process, which not only has high refining costs, but also causes serious environmental pollution. Every year, my country extracts a large amount of industrial silicon from quartz stone and exports it to Germany, the United States, Japan and other countries at a price of US$1 per kilogram. These countries process industrial silicon into high-purity crystalline silicon materials and sell them to my country's solar energy companies at a price of US$46-80 per kilogram.

After obtaining high-purity polysilicon, it must be melted into single crystal silicon in a single crystal furnace and then sliced ​​for use in integrated circuit manufacturing, etc.

The difference between monocrystalline silicon and polycrystalline silicon cells

Due to the difference in the early production process of monocrystalline silicon cells and polycrystalline silicon cells, there are some differences in their appearance and electrical performance. From the appearance: the four corners of monocrystalline silicon cells are arc-shaped, and there are no patterns on the surface; the four corners of polycrystalline silicon cells are square corners, and there are patterns similar to ice flowers on the surface.

For users, there is not much difference between monocrystalline silicon cells and polycrystalline silicon cells. Both monocrystalline silicon cells and polycrystalline silicon cells have good lifespan and stability. Although the average conversion efficiency of monocrystalline silicon cells is about 1% higher than that of polycrystalline silicon cells, since monocrystalline silicon solar cells can only be made into quasi-squares (their four corners are arcs), when forming solar cell components, part of the area cannot be filled, while polycrystalline silicon solar cells are squares and do not have this problem, so the efficiency of solar cell components is almost the same. In addition, due to the different manufacturing processes of the two solar cell materials, the energy consumed in the manufacturing process of polycrystalline silicon solar cells is about 30% less than that of monocrystalline silicon solar cells, so polycrystalline silicon solar cells account for an increasing share of the total global solar cell production, and the manufacturing cost will be much lower than that of monocrystalline silicon cells, so the use of polycrystalline silicon solar cells will be more energy-saving and more environmentally friendly.

The concept of inverter

Inversion refers to rectification. The process of converting AC power into DC power by a rectifier is called rectification. Then the process of converting DC power into AC power is called inversion. The circuit that completes the inversion function is called an inverter circuit, and the device that realizes the inversion process is called an inverter.

Why must photovoltaic inverters be used in solar photovoltaic power generation systems? At present, my country's power generation system is mainly a DC system, that is, the electricity generated by solar cells is used to charge batteries, and the batteries directly supply power to the load. For example, the solar lighting system used more in Northwest my country and the microwave station power supply system far away from the power grid are all DC systems. This type of system has a simple structure and low cost, but due to the different DC voltages of the load (such as 12v, 24v, 48V, etc.), it is difficult to achieve system standardization and compatibility. In particular, household appliances, such as fluorescent lamps, televisions, refrigerators, electric fans and most power machinery work with AC power, that is, most of them are AC loads, so photovoltaic power supplies powered by DC power are difficult to enter the market as commodities. The purpose of setting up an inverter in a solar photovoltaic system is to convert DC power into AC power, so as to meet the needs of most user loads.

In addition, if the power line is damaged or forced to shut down, the inverter will stop supplying power to the electrical equipment or the grid. If the power line voltage is low or undervoltage, or there is a large disturbance, a sensor for a "non-island" inverter is used to sense this situation. When this happens, the inverter will automatically shut down the power supply to the grid or transmit the power to other places to prevent it from becoming an "island" of power generation. The so-called island effect means that after a grid failure, the photovoltaic grid-connected power generation system connected in parallel to the grid can still work and is in an independent operation state .