Zhang Gong, a friend who is new to photovoltaics, installed a photovoltaic power station on his roof, using 120 340W modules, with a total module power of 40.8kW and an inverter of 40kW. After running for a few months, I checked the power generation with my mobile phone. The daily power generation was between 20 degrees and 240 degrees. When I checked the real-time power generation, Zhang Gong found that the output power of the photovoltaic power station in the past few months was only about 36kW. Up to 40kW, what's going on?
The output power of a photovoltaic system is generally difficult to reach the nominal power of the module. This is because the power of the module is tested under very good weather conditions and the module temperature is relatively low. This condition cannot be reached in normal weather. Photovoltaic Not all modules can receive solar energy, because they are not always at the best angle. And photovoltaic systems have losses: dust, shielding, shadows, etc. on the components, cable, switch, connector and other line losses; inverters, transformers, distribution cabinets and other equipment losses. In general large-scale power stations, the maximum output power of the system may only be about 85% -90% of the rated power of the components; in small distributed power stations, the maximum output power of the system may only be about 90% -95% of the rated power of the components.
1. Solar irradiance factor
We often say that 340W photovoltaic modules, the maximum power Pmax / W, means that its peak power is 340W in a standard test environment. only under standard test conditions (irradiance 1000W / m2, battery temperature 25 ° C), the output power of photovoltaic modules is "nominal power" (340W), and power will change when irradiance and temperature change.
2. System loss factor
The key factor affecting the power generation is the system efficiency. The main factors for system efficiency are: reduced efficiency caused by dust and shadow on the module, reduced power caused by the temperature of the module, impedance matching loss caused by the DC cable, series voltage of the module and inverter There are many factors such as the efficiency reduction caused by the voltage mismatch of the inverter, the MPPT tracking loss of the inverter, the power loss of the inverter itself, the AC cable power loss, and the transformer power loss.
System loss is the loss of comprehensive power generation, and the value varies depending on conditions. The factors that affect the maximum power output are components and DC cables. In order to increase the output power, various losses must be minimized. After the photovoltaic installation location is determined, it is difficult to change the installation angle of the module, the temperature of the module, the loss of the inverter itself, and the MPPT tracking efficiency. However, factors such as DC cable loss, DC cable impedance matching, and component and inverter voltage matching are controllable.
1) Minimize the length of the DC cable. The DC cable is used from the component to the inverter. The length of this cable has a great impact on the power generation of the system. On the one hand, it is the loss of the DC cable itself, and on the other hand, it is the impedance. Match, the inverter should be as close to the component as possible, the DC cable should be controlled within 20 meters, and the DC cable of each MPPT should be as consistent as possible.
2) The working voltage of the modules in series is as close to the rated voltage of the inverter as possible to reduce the losses of the inverter.
Then how to judge whether the photovoltaic system power generation is normal? First look at the output power of the inverter. When the weather is particularly good, if it can reach more than 90% of the module power, it means that there is no problem in the design of the system, and then calculate the power generation. Check the local average daily power generation hours, multiply the number of hours by 365, and then multiply the system efficiency by the average annual power generation. The general system efficiency is 0.8. For example, in Guangdong, the average daily power generation hours is 3.5, a 40kW Electricity, the average annual power generation is 40 * 3.5 * 365 * 0.8 = 40880 degrees. If the power generation is within this range, it means that there is no problem in the design and installation of the system.