Introduction to Bidirectional Energy Storage and Microinverters

  1. Bidirectional energy storage inverter

Bidirectional energy storage inverter is also called bidirectional grid-connected inverter or dual-mode converter. It can not only realize off-grid and grid-connected power generation functions, but also realize bidirectional flow control of electric energy: it can convert alternating current into direct current, and can also convert direct current into alternating current. During the day, the power generated by the photovoltaic modules can supply power to the local load or be integrated into the grid through the bidirectional energy storage inverter, or charge the energy storage system; at night, the electric energy in the energy storage system can be released for the load to use as needed. In addition, the grid can also charge the energy storage device through the inverter. The bidirectional energy storage inverter can be applied to the grid-connected power generation system with electric energy storage requirements, and can also be combined with the string inverter to form an independent photovoltaic power generation system. The principle is shown in Figure 1.

figure 1

The bidirectional energy storage inverter is powered by the battery pack, converts the direct current into alternating current, and establishes a power grid on the alternating current bus. The string inverter automatically detects whether the photovoltaic array has enough energy, and detects whether the AC grid meets the grid-connected power generation conditions. When the conditions are met, it enters the grid-connected power generation mode and feeds the AC bus, and the system starts. After the system works normally, the bidirectional energy storage inverter detects the power consumption of the load, and the power fed into the grid by the string inverter is first used by the load. If there is remaining electric energy, the bidirectional energy storage inverter converts it into direct current to charge the battery pack; if it can, the bidirectional energy storage inverter converts it into direct current to charge the battery pack; if the string inverter feeds the When the electric energy is not enough for the load, the bidirectional energy storage inverter converts the DC power supplied by the battery pack into AC power and feeds it into the AC bus for the load to use. The modular decentralized independent power supply system composed of this basic unit can also be connected to other power grids.

In areas without photovoltaic power generation subsidies and peak-to-valley electricity prices, the use of bidirectional energy storage inverters can store excess photovoltaic energy in the power storage system for use at night, maximizing the self-generated and self-consumption of photovoltaic power generation systems. The power storage system can also be charged with cheap night-time valley electricity. Photovoltaic power generation is used to meet the electricity consumption during the day, and the stored electricity is used from the evening to the peak electricity consumption at night, thereby reducing the user’s electricity bill.

As the key equipment of energy storage system and micro grid system, bidirectional energy storage inverter will be widely used in distributed photovoltaic power generation system.

  1. Micro inverter

Micro inverters are also called component inverters. The micro-inverter can be directly fixed behind the module, and each photovoltaic module corresponds to a micro-inverter with independent DC-AC inverter function and MPPT function. At present, a micro-inverter can connect two or four photovoltaic modules to form two or four independent MPPT inputs, and the maximum output power can reach 1200W, which can be widely used in various distributed photovoltaic power generation systems. The photovoltaic power generation system composed of micro-inverters is more efficient, reliable and intelligent. During the life cycle, the power generation of the micro-inverter system can be increased by up to 25% compared with photovoltaic power generation systems using other inverters.

Micro-inverters effectively overcome the shortcomings of centralized inverters and string inverters, and have the following characteristics.

① Maximize power generation. The micro-inverter does MPPT for each individual component, which can obtain the highest power from each component, and the total power generation can be increased by up to 25%

② Strong adaptability to the application environment. Micro-inverters have low requirements for the consistency of photovoltaic modules. In practical applications, such as shadow occlusion, cloud and fog changes, dirt accumulation, inconsistent module temperature, inconsistent module installation inclination angles, inconsistent module installation orientations, small module cracks and module efficiency degradation When the internal and external non-ideal conditions are unequal, the problem component will not affect the other components, thereby not significantly reducing the overall power generation efficiency of the system.

③ Can quickly diagnose and solve problems. Using power carrier technology, the micro-inverter photovoltaic system can monitor the working status and power generation performance of each component in the system in real time.

④I don’t need DC cables, but the AC side needs more wiring costs and expenses.

⑤ Avoid single point of failure. The traditional centralized inverter is the weak link and fault-prone unit of the entire photovoltaic power generation system. The use of micro-inverters not only eliminates this weak link, but its distributed architecture ensures that the entire system will not stop due to a single point of failure. Work.

⑥Construction and installation are quick, simple and safe. The application of micro-inverter makes the photovoltaic power generation system get rid of the dangerous high-voltage DC circuit, and the component performance does not have to be completely consistent during installation, so there is no need to select and match the photovoltaic components, which reduces the installation time and cost by 15%~25%, and can be installed at any time. Make flexible changes and expansions to the system.

⑦ The internal main circuit of the micro-inverter adopts the spectrum vibration soft switching technology, the switching frequency is up to several hundred dry hertz, the switching loss is small, and the conversion efficiency is high. At the same time, a high-frequency transformer with small size and light weight is used to realize electrical isolation and power conversion, and the power density is high. High efficiency, high power density and high reliability applications are realized.