Grid-Connected Photovoltaic Energy Storage Systems

Photovoltaic power generation systems can be divided into grid-connected photovoltaic power generation systems and stand-alone photovoltaic power generation systems according to whether they are grid-connected or not. Grid-connected photovoltaic power generation systems mainly refer to photovoltaic systems that are connected to the grid and receive grid dispatch, such as various centralised or distributed photovoltaic power plants. Stand-alone photovoltaic power generation systems mainly refer to a variety of photovoltaic power generation systems that operate independently of the grid, such as solar street lights, rural household photovoltaic power supply, etc.

Grid-connected photovoltaic energy storage system

Grid-connected photovoltaic power generation systems are directly connected to the distribution network, and the electricity is directly input to the grid, so they are generally not equipped with energy storage systems. The configuration of energy storage in photovoltaic systems has become one of the research directions of large-scale energy storage systems.

The configuration of energy storage in grid-connected PV power systems is determined by the energy storage objectives, which can be specifically divided into: smooth output, economic dispatch, and microgrid composition.

1) Smooth output

Photovoltaic power generation is a process of converting solar energy into electrical energy, and its output power is affected by the intensity of solar radiation, temperature and other environmental factors and varies dramatically. The unstable power of PV power and the presence of harmonics make the connection of PV power to the grid a shock. An important objective of grid-connected PV systems with energy storage is to smooth the PV power output and improve the quality of PV power.

The capacity of the storage system is determined by the grid smoothing strategy, while the storage power is generally determined by the smoothing objective. Grid-connected PV smoothing strategies based on energy storage systems currently include fixed time constant low-pass filtering smoothing strategies, fuzzy control/SOC (storage battery state of charge) smoothing strategies, and PV power prediction smoothing strategies.

2）Peak regulation

After the PV power is connected to the grid, it needs to accept the grid dispatch, but its power output peak phase and grid load peak phase are not consistent, coupled with the power market peak and valley tariff factors, the use of energy storage systems to achieve PV power generation in the time coordinate shift, so that PV power to participate in grid peaking is also one of the current PV energy storage system research hotspots. Through power peaking, the access capacity of PV power in the grid and the economy of PV power can be improved.

The capacity of the storage system in this configuration is generally larger, and the cost of the storage system is higher and unreasonable charge and discharge control will seriously damage the life of the storage system, so the capacity and power configuration of the centralised energy storage system configured on the grid side is determined by the grid peaking requirements, energy storage charge and discharge control strategy, energy storage costs and other factors.

3) Micro-grid applications

Microgrid is a new type of grid structure proposed to promote the use of renewable energy, specifically a regional type grid form composed of renewable energy, energy storage system and load, as an independent whole, can either run in parallel or in isolated operation in off-grid state. The energy storage system as a component unit of the microgrid is the energy buffer link in the microgrid and plays an important role in improving the control stability, enhancing the power quality of the microgrid, maintaining the power balance of the microgrid and improving the anti-interference capability of the microgrid. In addition, the energy storage system in the microgrid can also be used as an emergency backup in case of power supply interruptions.

Energy storage systems in microgrids are generally configured in parallel with renewable energy generation systems and have an independent energy storage management system (e.g. battery control system, BESS), whose operation mode varies according to the microgrid operation mode (off-grid/on grid). The capacity and power configuration of the energy storage batteries depends on the different microgrid compositions and operating modes, and is also governed by the operating mode of the energy storage system.