In some commercial areas, due to the limitation of transformer capacity, photovoltaic power generation is not allowed to sell electricity on the Internet. Some local power grids are not stable, and some places have very cheap power prices, but the price of electricity consumption is high and the peak valley price difference is large. If photovoltaic is installed in these places, it is suitable to use and separate the grid energy storage system. Its biggest feature is that it can be connected to the grid and can also store energy, It can also be run separately from the network.

There are four main ways to make profit in photovoltaic parallel off grid energy storage system: one is to use photovoltaic to supply power to load, which can be set at peak price to output, and reduce the cost of electricity; Second, it can charge the electricity price valley section, discharge at peak section, and make money by using the peak valley difference price; Third, if the power can not be sold online, the anti current system can be installed. When the photovoltaic power is greater than the load power, the excess energy can be stored to avoid waste; Fourth, when the grid is cut off, photovoltaic can continue to generate electricity without waste. The inverter can be switched to off grid working mode. The system can continue to work as a standby power supply, and photovoltaic and battery can supply power to the load through the inverter.

1、 Comparison of parallel off grid system technical route

And off grid optical storage system, including solar module, controller, inverter, battery, load and other equipment, there are many technical routes. According to the energy collection, there are two kinds of topology structures, namely DC coupling and AC coupling.

01 DC coupling

Working principle of DC coupling: when photovoltaic system is running, the battery is charged by MPPT controller; When the electrical load is required, the battery will release the electricity, and the current size is determined by the load. The energy storage system is connected to the grid. If the load is small and the battery is full, the photovoltaic system can supply power to the grid. When the load power is greater than the photovoltaic power, the grid and photovoltaic can supply power to the load at the same time. Because photovoltaic power generation and load power consumption are not stable, it is necessary to rely on battery balance system energy.

02 AC coupling

As shown in the figure below, the DC power generated by photovoltaic module is changed to AC through inverter, and directly feeds load or into power grid. The grid can also charge battery through bidirectional DC-AC bidirectional converter. The point of energy gathering is at the exchange end.

Working principle of AC coupling: including photovoltaic power supply system and battery power supply system. Photovoltaic system is composed of photovoltaic array and grid connected inverter; The battery system consists of battery bank and bidirectional inverter. The two systems can be operated independently, without interference, or separated from large power grid to form a micro network system.

▌▌▌▌ DC coupling and AC coupling are mature schemes at present. Each has its advantages and disadvantages. According to different application situations, the most suitable scheme is selected. The following is the comparison between the two schemes.

01 cost comparison

DC coupling includes controller, bidirectional inverter and switching switch; AC coupling includes grid connected inverter, bidirectional inverter and distribution cabinet. From the cost, the controller is cheaper than the grid connected inverter, and the switching switch is cheaper than the distribution cabinet. The DC coupling scheme can also be used as a control inverter, and the cost of equipment and installation can be saved. Therefore, the cost of DC coupling scheme is lower than that of AC coupling scheme.

02 Applicability Comparison

DC coupling system, controller, battery and inverter are serial, and the connection is close, but the flexibility is poor. The AC coupling system, grid connected inverter, battery and bidirectional converter are parallel, not closely connected and flexible. If in a installed photovoltaic system, it is necessary to install energy storage system, and the AC coupling is better. As long as the battery and two-way converter are installed, it will not affect the original photovoltaic system, and the design of the energy storage system has no direct relationship with the photovoltaic system in principle, so it can be determined according to the requirements. If it is a new off grid system, photovoltaic, battery and inverter should be designed according to the load power and power consumption of users. It is more suitable to use DC coupling system. But the power of DC coupling system is relatively small, generally under 500kW, and the larger system is better controlled by AC coupling.

03 efficiency comparison

From the perspective of photovoltaic utilization efficiency, the two schemes have their own characteristics. If the users have more loads in the daytime and less at night, the AC coupling is better. The photovoltaic module directly supplies power to the load through the grid connected inverter, and the efficiency can reach more than 96%. If the user has less load in the daytime and more at night, photovoltaic power generation needs to be stored in the daytime and reused at night. The DC coupling is better. The photovoltaic module stores the electricity to the battery through the controller, and the efficiency can reach more than 95%. If it is AC coupling, photovoltaic must be converted into AC through the inverter first, and then it will be converted into DC through bidirectional converter, Efficiency will be reduced to about 90%.

1、 System scheme design


Customer's electricity demand and lighting

Customer's electricity demand and lighting: the customer is a small factory in Beihai, Guangxi, which deals in processing, selling and refrigerating frozen seafood. The main power consumption equipment is freezer, processing equipment, packaging equipment, lighting, etc., among which, the important equipment is 5 freezers and packers, each power is 2kW, scattered in each room on the first-third floor, and no power interruption is allowed for more than 10 minutes.

The lighting conditions of Beihai are general in Guangxi, but the air quality is good. The annual effective utilization hours are 1080 hours, and the peak sunshine per day is 3.75 hours, which is more suitable for installation of photovoltaic.

02 system scheme design

The off grid energy storage has two schemes: DC coupling and AC coupling. According to the characteristics of users, the proportion of photovoltaic self-use is large and the power consumption in daytime is large. The method of AC coupling is better in terms of efficiency. However, considering that the reliability of customers for power consumption is not high and the budget is limited, the DC coupled parallel off grid control inverter is selected.

Considering the equipment is scattered and the power of single equipment is not large, five 10kW parallel off grid inverters are designed, the grid connection interface is connected to the grid, and one important load is connected to EPS interface.

The component power shall be confirmed according to the daily power consumption of the user. The average daily power consumption of users is 150 degrees, the local peak sunshine is 3.75 hours per day, and the efficiency of the off grid system is about 0.85. Therefore, 150 360W single crystal components with a capacity of 54kw are designed, which can generate 200 degrees of electricity per day, and the loss is removed to the user about 160 degrees, which can basically meet the needs of customers.

The battery capacity is determined by the power consumption of the user without light. Photovoltaic power generation can be used directly without electricity storage in daytime. It is estimated that the user uses 50 degrees of electricity every night. Five lithium batteries with 500v40ah are designed. The total power consumption is 10000vah. According to the discharge depth of 0.9, 90 degree power can be provided to customers.

Electrical scheme design: the components are 150, and 15 strings and 2 are used. Each inverter is connected to 30 components, and the battery bank, load and power grid are connected to corresponding breaker respectively.

03 electrical function commissioning

In order to adapt to different situations, and the off grid energy storage inverter has designed many functions, before application, it should be set according to the actual requirements of users. First, select grid connection mode or off grid mode. If it is grid connected mode, then select charging mode of battery, whether photovoltaic priority or municipal power priority, or municipal power is only bypass, and no charging; The mode of online access can choose the storage of the self-use allowance of photovoltaic power generation and the self-use allowance of photovoltaic power generation; The peak valley filling function can be selected for the places with large price difference between peak and valley.