Understanding Battery Energy Storage System (BESS) | Part 1 – BASICS

BESS is a stationary energy storage system (ESS) that stores energy from the electricity grid or energy generated by renewable sources such as solar and wind.

This energy is accumulated for later use in various scenarios, such as the following:

• Peak demand: BESS can help relieve the stress on the power distribution network during peak times by feeding the energy to cater to the excess demand. BESS can be conveniently charged again when the demand is lower than the supply.

• Peak shaving: It is the process of consumers reducing the amount of energy demanded from the grid during peak hours to avoid peak demand charges. BESS meets the remaining energy requirement.

• Frequency stabilization: An imbalance in power generation and demand impacts the stability of the frequency of grid power. BESS can help stabilize the frequency.

• Free energy from duck curve: During this scenario, energy from the source is still being generated despite oversupply. This scenario is experienced some days of the year in regions that depend heavily on Solar PV (photovoltaic). The energy generated during this time can be used to charge the BESS, which can discharge energy for later use for the scenarios mentioned above.

Constituents of BESS

The BESS as a system includes both hardware and software, which can be internal or external.  The following are the constituents of the BESS:

• Battery Cells, Modules and Racks: Various cells are connected in series and/or parallel connection to achieve the desired voltage and capacity of BESS. This arrangement together constitutes a module. Many modules are racked (connected) together in series and/or parallel to achieve the desired voltage and capacity of the overall BESS system (in the case of a single container BESS). More details about BESS design from cell to module to rack will be discussed in Part 2.

• Battery Management System (BMS): Integration of the cells into a module has a BMS (slave BMS). Connecting various modules requires the slave BMS from each module to be working with a Master BMS. More details about BMS connections (master-slave combination) will be discussed in Part 2.

• Power Conversion System (PCS): It is a bi-directional inverter that has the ability to convert alternating current (AC) from the grid or solar to direct current (DC) to charge the BESS. DC from solar can be sent to PCS via a DC-DC converter, and AC converted from solar Inverter can also be sent to PCS to charge the battery. PCS can convert DC from BESS to AC and supply to the grid. PCS can either be placed inside the BESS containerized solution when the container space is not utilized completely, or it can be a completely independent system to be placed outside the BESS.

• Energy Management System (EMS): It monitors and controls the energy flow of the BESS during charging and discharging. EMS collects and analyses the energy data of the system and runs the overall system. It can also support the remote monitoring of BESS usage.

• Battery Thermal Management System (BTMS): BESS can either have air-cooling or liquid-cooling based thermal management, which is used in the containerized BESS to ensure that the batteries do not operate in extreme temperatures. BESS operating without thermal management in high temperatures can have faster degradation of the battery capacity, resulting in reduced battery cycle life. The modern-day BESS are witnessing a shift towards the liquid-cooled system, which is claimed to be more efficient but slightly expensive. The energy used towards thermal management systems is referred to as auxiliary power losses.

• Fire Suppression System: BESS is generally a high-voltage DC system. A short circuit or other accidents can lead to fires. Hence, a fire suppression system is placed inside the BESS container to contain any fires arising due to unforeseen circumstances.

• Container: Either 20 feet or 40 feet containers are used for building a BESS. 20 feet containers are becoming popular these days with a capacity of more than 3.7MWh – this number is from one of my recently designed solutions.

• Transformer: It steps down the voltage from the grid to the necessary AC voltage to feed the PCS, which will then convert the AC to DC to charge the BESS. Similarly, when the battery discharges, its DC output goes to PCS, which converts DC to AC, and it is sent to the transformer to step up the voltage to send to the grid.

Sodium-ion battery is something to watch out for, which is theoretically cheaper and can last long. But time will tell when it will be commercially available for mass deployment.

Click here to read Part 2 of this article.

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This article was originally published in EVReporter Dec 2022 Magazine that can be accessed here.