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Design steps of distributed photovoltaic off-grid system

Mar 12, 2021

Design steps of distributed photovoltaic off-grid system

The distributed photovoltaic off-grid power generation system is mainly composed of photovoltaic components, brackets, controllers, inverters, batteries and power distribution systems. It is mainly used in areas without or lack of electricity to solve basic daily needs. There is no standard solution for photovoltaic off-grid systems, but there are many types of off-grid equipment, which can be combined into different solutions to meet different user needs.




1. Preliminary research


Before the off-grid system design, the preliminary work needs to be done. The first is to count the types and total power of users’ electrical loads, such as air conditioners, washing fans, water pumps, refrigerators and other electrical appliances, which contain electric motors, which are inductive loads and have high starting power, such as electric lights, TVs, induction cookers, computers and other electrical appliances. Resistive load; the second is to count the daily average power consumption, peak power consumption, day and night power consumption, etc.; the third is to understand the climatic conditions of the user's installation location, the average peak sunshine hour data, and continuous rain and rain throughout the year The number of days and so on; the fourth is to understand whether there is city power or fuel generators; the fifth is to understand the user’s budget and economic situation, and the urgency of electricity use. After understanding these conditions, you can prescribe the right medicine and start designing the plan.


2. Scheme design


The design of the off-grid system is mainly based on the data of the previous investigation to confirm the capacity of the system inverter, components and power storage. This design method is completely different from the grid-connected system and cannot be copied. The basis for the design and selection of these three equipments is different, but they also need to cooperate with each other.


The power of the off-grid inverter should be confirmed according to the user's load type and power. The output power of the inverter is greater than the starting power of the actual load. Loads with motors such as washing machines, air conditioners, refrigerators, water pumps, range hoods, etc. are inductive loads. The starting power of the motor is 5-7 times the rated power. When calculating the power of the inverter, the starting power of these loads should be considered Go in. For users with higher budgets and higher power requirements, the output power is calculated based on the sum of all load powers; users with low budgets, considering that all loads cannot be turned on at the same time, in order to save the initial cost, the output power can be Multiply the sum of load power by a factor of 0.7-0.9.


Component power should be confirmed according to the user's daily power consumption. Photovoltaic is a renewable energy source and is available every day. Therefore, the average daily power generation capacity of modules is greater than the daily useful power of users. As for the size, it depends on the local weather conditions, the user's budget and the demand for electricity. The general weather conditions are lower and higher than the average. In most areas, the sun is worse in winter than in summer, and the difference is more than double in some areas. For users with higher budgets and higher power requirements, the module power design basically meets the needs of the worst season of light, even in the worst season of light, the battery can basically be fully charged every day. For users with low budgets, if the power of solar cell modules is designed according to the worst-case scenario, the power generation will exceed the actual needs at other times of the year, resulting in waste. At this time, the power of the modules can be averaged annually. The design capacity of the battery can be appropriately increased, and the storage of electric energy can be increased, so that the battery is in a shallow discharge state to make up for the lack of power generation in the worst season.


The battery capacity is determined according to the user's power consumption. When there is no photovoltaic, the battery provides power to the system load. For important loads, there is sufficient budget and the number of consecutive cloudy and rainy days should be considered. For general loads such as solar street lights, it can be selected within 2 to 3 days based on experience or needs. Important loads such as communications, navigation, hospital treatment, etc. ~Select within 7 days to ensure the normal operation of the system within a few days. For ordinary poor families, the main consideration is the price, instead of rainy days. Use more when the sun is good; use less when the sun is bad, and no sun No need.



3. Equipment selection


The needs of off-grid users are diverse, and photovoltaic systems are designed according to user requirements. At this time, you need to deal with it flexibly and don't stick to a fixed formula.


When choosing an inverter, it depends on the application. If it is only a simple lighting application, it is recommended to use a PWM controller and a modified sine wave inverter to save the initial cost; if there are inductive loads such as air conditioners, washing machines, and water pumps that contain motors, it is recommended Choose MPPT controller and power frequency inverter, with strong load capacity. If it is a comprehensive load, it is recommended to use a high-frequency inverter, taking into account the cost and load capacity.



Small-scale photovoltaic off-grid power stations, with limited investment, can be considered for use of lead-acid gel batteries; small and medium-sized photovoltaic off-grid power stations, with limited investment, but hope to have a long life, can consider using lead-carbon batteries; medium and large photovoltaic energy storage power stations, there are enough Budget and high requirements for return on investment, you can consider using lithium batteries.


In places where there is commercial power supply, different priority modes can also be selected according to the actual situation. Where the mains voltage is stable and the price is cheap, but the power supply time is short, it is recommended to choose the power grid priority mode; where the mains power price is high and there are frequent power outages, users who have not particularly high power requirements are recommended to choose the battery priority. Electricity mode: The electricity price of the mains is not very high. In places where there are frequent power outages, for users with high demand for electricity, it is recommended to choose the photovoltaic power priority mode.