What are the components and their functions in a Battery Energy Storage System (BESS)?

Definition

(Electric) battery is the common term for galvanic cells or groups (batteries) of galvanic cells. There are various storage technologies, with very different specifications. What they all have in common is that they store the incoming electrical energy electrochemically.

This distinguishes battery cells from heat storage systems or mechanical (potential) energy storage systems such as pumped storage power stations. While the waste heat from battery storage systems is occasionally used to generate space heating, for example, this is not a function.

The distinction between disposable batteries (primary batteries) and rechargeable batteries (secondary batteries) is not made for battery energy storage systems (BESS); they are rechargeable by definition. 

While everyday batteries usually consist of not much other than the galvanic cell itself, the charging and application devices are equipped with components necessary to use them. In larger storage systems, however, they are integrated into the systems. These components can be split up into two groups, power electronics and control units.

Power electronic functions

Power electronics is the electrotechnical link between the actual battery cells and the connected systems, i.e. power supply, or loads such as machines or households and, of course, the power grid. Its main task is to convert the incoming and outgoing electricity into the required form.

Electric power converters

Power converters are the central components of power electronics. There are several types of converters, rectifiers, inverters, DC-to-DC converters and AC-to-AC convertera. The latter generally play no role in battery storage systems. This makes the other three types of converters all the more important. This is because battery cells - regardless of whether they are large storage units, mobile phone batteries or other standard household batteries - all must be charged with direct current (DC). However, the grid, which usually supplies them with electricity, carries alternating current (AC) (with only a few exceptions).

For this reason, the charger of any battery with a mains connection must contain a rectifier that converts the incoming alternating current into direct current. If the battery storage system is then to feed the stored energy back into the grid, the direct current from the cell must be converted into alternating current via an inverter.

DC-to-DC converters are power converters that convert direct current from one voltage level to another. They are required, for example, if a BESS is to be charged directly via a photovoltaic (PV) solar system, which also produces direct current, but at a different voltage than the battery requires for charging.

Exactly which power converters a BESS requires depends on the specific use case - for example, as pure grid storage, as an emergency power generator for a data centre or to shift the load of an electric car charging station with a PV connection.

Harmonic filter (power line conditioner) 

Harmonics are frequencies above the specified mains frequency that can destabilise the power grid. They occur when non-linear loads such as motors, LED lamps, or batteries are connected to the grid. With harmonic filters in the power electronics, however, battery storage energy systems can help to reduce harmonics and thus increase their usefulness to the grid.

Safety 

There are several components that protect the battery system from external influences and disturbances in the grid and, conversely, protect their surroundings and the grid from undesirable effects coming from the power storage system. Overvoltage protection is a safety component that is part of the power electronics.

The Battery Management System (BMS)

The battery management system (BMS) is basically a computer that controls all charging and discharging processes. The BMS can perform the following functions.

Energy or load management

The battery management system controls the charging and discharging of the battery according to predefined criteria such as the availability of electricity (for example, if connected to a solar system) or the demand of the grid or other loads. This function is crucial for a sound and beneficial usage for both, the owner and the grid.

Safety, cell protection and maintenance

The BMS monitors the battery storage system for any risks and takes appropriate measures to prevent damage to the system and hazards to the environment. It performs the following tasks: