Knowledge and analysis of photovoltaic systems
- Photovoltaic systems equipment composition
- Photovoltaic module structure
- The main factors affecting the power generation of photovoltaic modules
The photovoltaic systems are mainly composed of photovoltaic modules, combiner boxes, power distribution cabinets, inverters, transformers and other equipment.
Photovoltaic systems equipment composition
In photovoltaic systems, there are many components listed in the following.
- Combiner box
Function: Connect several photovoltaic series and parallel to the confluence, which has the functions of monitoring and protection. The combiner box can reduce the wiring between photovoltaic modules and inverters, facilitate maintenance and improve photovoltaic systems reliability.
- Power distribution cabinet
Function: secondary confluence, which can protect the upper and lower levels in the application.
Function: The direct current generated by the solar photovoltaic array is converted into a sine wave alternating current by the inverter, and then input into the power grid. It is very important in photovoltaic systems.
Function: The transformer can play the role of step-up and step-down. When it increases the voltage, it can effectively reduce the loss of voltage.
- Photovoltaic modules
The basic unit of a photovoltaic module is a cell. A single solar cell cannot be directly used as a power source. Several single cells must be connected in series or parallel and tightly packaged into a module. Photovoltaic modules (or solar panels) are the most important part of photovoltaic systems, and their function is to convert solar energy into electrical energy to drive loads in photovoltaic systems.
At present, the battery components commonly used in photovoltaic power plants are: monocrystalline silicon photovoltaic modules, polycrystalline silicon photovoltaic modules and thin-film photovoltaic modules. From the appearance, the monocrystalline silicon photovoltaic module is dark blue, almost black, and the four corners of the monocrystalline solar cell are arc-shaped. The polysilicon is sky blue, and the polycrystalline cells are square with patterns on the surface in photovoltaic systems.
Thin-film photovoltaic modules mainly include perovskite, cadmium telluride (CdTe), copper indium gallium selenide (CIGS), and gallium arsenide (GaAs). Amorphous silicon cells are included in thin film cells. In addition, the recently popular heterojunction (HIT/HJT) is the deposition of amorphous silicon thin film on crystalline silicon, which is a combination product of single crystal silicon battery and amorphous silicon battery.
In terms of performance, the photoelectric conversion efficiency of monocrystalline silicon photovoltaic modules is about 21%, up to 24%, but the production cost is high. Because monocrystalline silicon is generally encapsulated with tempered glass and waterproof resin, it is durable and has a service life of up to 15 years, up to 25 years.
The photoelectric conversion efficiency of polysilicon photovoltaic modules is much lower, about 17%, but the production cost is lower, so it has been greatly developed. However, its lifespan is shorter than that of monocrystalline silicon panels in photovoltaic systems. In terms of cost performance, monocrystalline silicon is slightly better.
The heterojunction combines the advantages of crystalline silicon cells and thin-film cells. Compared with other photovoltaic cells, heterojunction cells have the advantages of high conversion rate and high stability. The main problem of heterojunction is the cost. First, the equipment investment is high, and second, the amount of silver paste is large, so the current cost performance is low.
The photovoltaic module is composed of solar cells connected in series and parallel, sealed with tempered glass, adhesive film and back plate, and aluminum alloy frame is installed around it, which has the advantages of strong wind resistance, hail resistance and convenient installation photovoltaic systems.
Photovoltaic module structure
A device that converts light energy into electrical energy through the photovoltaic effect. After solar cells made of semiconductors absorb sunlight, P-type semiconductors and N-type semiconductors generate electrons (negative) and holes (positive).
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Different wavelengths will also affect the conversion efficiency of solar panels. The light absorption band of solar cells: single polycrystalline silicon is generally 1100-1200nm, thin-film crystalline silicon thin films are generally 800nm, and some can reach 900nm. The wavelength of LED is 460-636nm, which cannot meet the power generation of photovoltaic cells.
- Interconnection belt
The interconnection belt is used to connect cells in series in photovoltaic systems. An interconnection strip is welded on the back of each cell, so that several cells are welded together in series to form a string of cells.
- Convergence belt
The carrier that connects the battery strings, connects the battery strings in series together, and finally leads out the positive and negative poles and connects them to the junction box. Both interconnect and bus strips are tin-coated copper strips in photovoltaic systems.
- Tempered glass
It is divided into coated glass and ordinary glass. Using low-iron ultra-white suede tempered glass, the light transmittance can reach more than 90%, and it can resist the sun’s ultraviolet radiation in photovoltaic systems.
Ultra-white refers to glass with low iron content, which is white when viewed from the side, and ordinary glass is green, so it is ultra-white and low-iron. The suede surface is to reduce the reflection of light and increase the anti-reflection treatment. Generally, there are sol-gel nanotechnology and coating technology. Tempering is the rapid air cooling of molten glass so that the surface is under pressure and the inside is under tension, so as to achieve the purpose of tempering in photovoltaic systems.
- EVA adhesive film
A thermosetting adhesive film, which has advantages in adhesion, durability, optical properties, etc., and is widely used in current components and optical products. At the same time, it is not sticky at room temperature and is easy to operate in photovoltaic systems.
The commonly used backplane material is TPT or polyvinyl fluoride composite film in photovoltaic systems, which has good environmental erosion resistance, insulation performance and good bonding performance with EVA.
- Aluminum alloy frame
It can protect the edge of the glass, cooperate with the silicone edge to strengthen the sealing performance of the module, improve the overall mechanical strength of the module, facilitate the installation and transportation of the module in photovoltaic systems.
- Silica gel
Function: bonding and sealing in photovoltaic systems.
- Junction box
The positive and negative lead wires of the component batteries, and the electrical box connected to the outside in photovoltaic systems.
The main factors affecting the power generation of photovoltaic modules
The inherent loss of the structure of the photovoltaic module itself
Power loss occurs during the process of cell packaging and assembly, partly due to the reduction of light incidence and absorption by materials such as glass and EVA, partly due to electrical connection loss, mainly due to the loss caused by connecting materials such as welding rods. Natural attenuation will also occur during the operation of photovoltaic modules in photovoltaic systems.
Hot spot effect
The shaded solar cell components in the series branch will be used as loads to consume the energy produced by other illuminated solar cell components. The shaded solar cell module will generate heat at this time, which is the hot spot effect in photovoltaic systems.
The generation of hot spots will not only affect the power generation efficiency, but also cause permanent damage to the photovoltaic modules and bring fire hazards to the power station in photovoltaic systems. According to statistics, severe hot spot effect will reduce the actual service life of solar cell modules by at least 30%. Over time, components may fail. There are many natural factors that cause the hot plate effect.
Junction boxes with bypass diodes are generally installed in the module to reduce the effect of hot spots. When a hot spot occurs, diodes in the junction box activate, shielding the string containing the offending cell.
The potential-induced attenuation effect is that the battery module is under high voltage for a long time, there is leakage current before the glass and packaging materials, and a large amount of charge accumulates on the surface of the battery sheet, which can cause the module power to attenuate by more than 50%, thus affecting the power output of the entire string. Coastal areas with high temperature, high humidity, and high salinity are most prone to PID phenomenon in photovoltaic systems.
Cracks are defects in cells. Due to the inherent characteristics of the crystal structure, crystalline silicon cells are very prone to cracking in photovoltaic systems. Different cracks have different effects on the function of the cell. The biggest impact on the function of the cell is the crack parallel to the busbar.
Based on the above reasons, the biggest impact on the function of the cell is the crack parallel to the busbar in photovoltaic systems. According to the research results, 50% of the failure chips come from cracks parallel to the busbar. The efficiency loss of the 45° inclined crack is 1/4 of the loss parallel to the busbar. Cracks perpendicular to the main grid lines hardly affect the thin grid lines, so the area of cell failure is almost zero.
Compared with the gate lines on the surface of crystalline silicon cells, the surface of thin-film cells is covered with a layer of transparent conductive film in photovoltaic systems, so this is one of the reasons why thin-film components have no cracks.
Sunshine intensity and temperature changes
The sunlight intensity is directly proportional to the photovoltaic photocurrent. When the sunlight intensity changes in the range of 1000-2100W/m2, the photocurrent always increases linearly with the increase of the sunlight intensity.
The sunshine intensity has little effect on the voltage in photovoltaic systems. Under the condition of fixed temperature, when the sunshine intensity changes in the range of 1000-2400W/m2, the open circuit voltage of the photovoltaic module remains basically unchanged. Therefore, the output power of photovoltaic cells is basically proportional to the intensity of sunlight.
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The higher the temperature of a photovoltaic module, the lower its operating efficiency. As the temperature of the module increases, its output voltage will drop: in the range of 20-100 degrees Celsius, the output voltage of each cell will decrease by about 5 millivolts for every 1 degree Celsius increase in the temperature of the module in photovoltaic systems.
As the temperature increases, the output current increases slightly. Overall, as the temperature of a component rises, its output power falls in photovoltaic systems. For every 1 degree Celsius increase in component temperature, the power is reduced by 0.35%.
Therefore, the output power varies with seasonal temperature changes. Under the same sunlight intensity, the output power in winter is higher than that in summer.
- The photovoltaic systems are mainly composed of photovoltaic modules, combiner boxes, power distribution cabinets, inverters, transformers and other equipment.
- The inverter converts the direct current generated by photovoltaics into alternating current for input into the grid.
- The phenomenon that the shaded solar cell module becomes a load, consumes energy and generates heat is the hot spot effect.
- The hot spot effect can be mitigated by bypass diodes.
- The cracks in the battery sheet are the cracks parallel to the busbar that are most likely to cause battery failure.
- The intensity of sunlight is directly proportional to the output power of photovoltaic cells.
- The output power of photovoltaic cells decreases as the surface temperature of the cells increases.