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

A Battery Energy Storage System (BESS) features more than just the battery cell that stores electricity - there are multiple other functions and components in a 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:

  • State of charge (SOC) management: Both overcharging and deep discharging can significantly reduce the service life of the cells. Hence, SOC management is key for cell protection and the economic efficiency of a BESS investment.
  • Balancing: Depending on the design, battery energy storage systems, for example with lithium-ion technology, consist of several cells. The battery balancer's task is equalising the charge as efficiently as possible. On the one hand, this reduces the risk of critical charge states of individual cells and, on the other, it takes into account their potentially different "cell health".
  • Temperature management: If batteries overheat, they pose a significant risk to their surroundings. However, even a moderate deviation from the appropriate temperature range can cause cell damage in the long term and reduce the lifespan of a BESS. Outside the optimum operating temperature, avoidable energy losses can also occur reducing efficiency.
  • Maintenance: The BMS analyses numerous parameters relating to the condition of the battery and notifies the system operator in due time when maintenance work is required to prevent a system failure.
  • IT-Security: To protect the battery and the BMS itself from unauthorised accessappropriate cybersecurity must be ensured.

Metering and communication

For efficient and effective energy management, the BMS must collect a range of operating data and communicate with other systems. Battery storage systems that provide system services, for example, transmit status messages to the grid operator and receive their commands simultaneously.

  • State of charge (SOC): Determining the state of charge (SOC) is central to grid-beneficial and economical operation. Connected systems need to "know" the SOC of the battery to utilise its charging capacity effectively.
  • Remote monitoring and control: Regardless of a BESS being used for arbitrage transactions, load shifting or ancillary services, successful participation in energy markets depends on the right timing of storing electricity and feeding it into the grid. The battery storage system must therefore constantly communicate with other systems - the grid operator, the power trader, etc. - transmitting its current status reports and receiving commands simultaneously. Remote monitoring and control of the system in real-time is also important so that the optimum location of the BESS can be selected independently of the location of those controlling it.
  • Performance measurement: The incoming and outgoing energy flows of the battery can be used to determine how efficiently and effectively the battery storage system is working. The ratio between stored and released electricity is also referred to as efficiency. The difference is mainly due to heat losses and the energy consumed by the system including the BMS.
  • Data logging and operational optimisation: The measurement and communication technology is used to log and evaluate the data collected in order to analyse and improve the performance, service life and efficiency of the BESS, as well as its communication with other systems over time. Last but not least, such data can help to model future revenues - including from new investments.

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