Design scheme of distributed photovoltaic off grid system

The distributed photovoltaic off grid power generation system is mainly composed of photovoltaic modules, brackets, controllers, inverters, batteries and power distribution systems. It is mainly used in areas without or without power, mainly to meet the basic needs of life. There is no standard scheme for photovoltaic off grid system, but there are many types of off grid devices, which can be combined into different schemes to meet different user needs.

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01 Preliminary investigation

Prior to the design of the off grid system, the preliminary work needs to be done well. The first is to count the type and total power of users’ electrical loads, such as air conditioners, washing fans, water pumps, refrigerators and other electrical appliances. There are motors in them, which are inductive loads with large starting power. Electric lamps, televisions, induction cookers, computers and other electrical appliances are resistive loads; The second is to count the daily average power consumption, peak power consumption, day and night power consumption, etc; Third, first understand the climatic conditions of the user’s installation site, the average peak sunshine hour data, the number of consecutive rainy days in a year, and so on; Fourth, find out whether there is mains power or fuel oil generator; The fifth is to understand the user’s budget and economic situation, and the urgency of power use. After understanding these situations, you can suit the remedy to the case and start to design the scheme.

02 Project Design

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

The power of the off grid inverter shall be confirmed according to the user’s load type and power. The output power of the inverter shall be greater than the starting power of the actual load. The loads with motors for washing machines, air conditioners, refrigerators, water pumps, range hoods, etc. are inductive loads, and the starting power of motors is 5-7 times of the rated power. When calculating the power of inverters, the starting power of these loads should be taken into account. For users with high budget and high demand for electricity, the output power is calculated based on the sum of all load powers; For users with low budget, considering that all loads cannot be started at the same time, in order to save the initial cost, the output power can be multiplied by the sum of load power by a factor of 0.7-0.9.

The power of components shall be confirmed according to the daily power consumption of users. PV is a renewable energy, which is available every day. Therefore, the average daily power generation of modules should be 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. Generally, the weather conditions are lower than or higher than the average. In most areas, the light in winter is worse than that in summer, and in some areas, the difference is more than double. For users with high budget and high demand for electricity, the module power design basically meets the needs of the worst lighting season, that is, the battery can be fully charged every day in the worst lighting season. For users with low budget, if the power of the solar cell module is designed according to the worst case, the power generation will exceed the actual demand at other times of the year, causing waste. At this time, the module power can be based on the annual average light. In addition, the design capacity of the battery can be appropriately increased, and the electric energy storage can be increased to make the battery in a shallow discharge state, making up for the shortage of power generation in the worst light season.

The battery capacity is determined according to the user’s power consumption. When there is no PV, the battery provides electrical energy to the system load. For important loads, there should be enough budget, and continuous rainy days should be considered. For general loads such as solar street lamps, they can be selected within 2 to 3 days according to experience or needs. For important loads such as communication, navigation, hospital treatment, etc., they should be selected within 3 to 7 days, so that the normal operation of the system can be ensured within a few days. For ordinary poor families, the main consideration is price, so it is not necessary to consider rainy days. When the sun is good, they should be used more frequently; Use less when the sun is bad, and use less when there is no sun.

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03 Equipment Selection

The needs of off grid users are diverse, and the photovoltaic system is designed according to the requirements of users. At this time, it needs to be handled flexibly and not be confined to fixed formulas.

When selecting the inverter, it depends on the application situation. If it is only a simple lighting application, it is recommended to select PWM controller and correction wave inverter to save the initial cost; If there are air conditioners, washing machines, water pumps and other inductive loads containing motors, it is recommended to select MPPT controllers and power frequency inverters with strong load capacity. If it is a comprehensive load, it is recommended to select high-frequency inverter, taking into account the cost and load capacity.

soalr inverter

Small photovoltaic off grid power station, with limited investment, can consider selecting lead-acid colloid battery; Small and medium-sized photovoltaic off grid power stations have limited investment, but they hope to have a long service life. Lead carbon batteries can be considered; Medium and large photovoltaic energy storage power stations have sufficient budget and high requirements for return on investment, so lithium batteries can be considered.

Where there is mains supply, different priority modes can also be selected according to the actual situation. In places where the mains voltage is stable, the price is cheap, but the power supply time is short, it is recommended to choose the power grid priority mode; Where the electricity price of the city is high and the power supply is often cut off, it is recommended to select the battery priority power consumption mode for the users whose electricity demand is not particularly high; The price of municipal electricity is not very high. In places where power is often cut off, it is recommended that users with high demand for electricity should choose the photovoltaic power consumption mode first.

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