Understanding Battery Energy Storage System (BESS) | Part 2 – Advanced

The first part of this series covers the basics and constituents of BESS in the Dec 2022 issue of EVreporter magazine. In continuation, part 2 explores BESS in-depth and discusses BESS design and technical parameters in greater detail.

BESS Design

The market is shifting towards the 1500V DC system of BESS. Below is a possible design that can be used in such a high-voltage system.

Inputs for designing BESS

• 280Ah, 3.2V LFP Prismatic cell is very popularly used in BESS, and the same is considered for the above design.

• 44 number of cells connected in series in a module can also be increased to 48 and 52 series. The number of modules per rack can be 8 or 9, depending on the height of the module and the container selected.

• The number of racks in a 20 feet container can be 9 or 10.

The below image shows a line diagram of a popular type of BESS + Solar system:

Battery Thermal Management System (BTMS) – BESS operating without thermal management in high temperatures can lead to lower battery cycle life. On the other hand, batteries operating without thermal management in lower temperatures (sub-zero temperatures) can lead to lower output of energy from the BESS. Hence, keeping the BESS operation close to the ideal operating temperature of the battery, which is 25±2°C in the case of Lithium-ion batteries, is imperative. The temperatures sometimes vary from place to place depending on other environmental conditions such as atmospheric pressure, altitude, etc.

During the peak hours, typically sometime during the noon, the generation tends to be the highest, and if the demand is lower during the same period, a duck curve is expected.

Selection of battery type

BESS can be made up of any battery, such as Lithium-ion, lead acid, nickel-cadmium, etc. Battery selection depends on the following technical parameters:

• BESS Capacity: It is the amount of energy that the BESS can store. Using Lithium-ion battery technology, more than 3.7MWh energy can be stored in a 20 feet container. The storage capacity of the overall BESS can vary depending on the number of cells in a module connected in series, the number of modules in a rack connected in parallel and the number of racks connected in series.

• Power Rating (C rate of Charge and Discharge): It is the capability of the BESS to charge at a certain speed and discharge at a certain speed. It is directly proportional to the power input and power output, respectively.

• Cycle life: It is defined as the total number of charge and discharge cycles that the BESS can supply during its lifetime by the time it reaches its end-of-life (EOL). Depending on the life expected from the BESS, batteries such as Lead acid batteries (low cycle life) and Lithium Iron Phosphate (LFP) batteries (high cycle life) are used.

• Depth of Discharge (DoD): It is the percentage of energy discharged from the BESS out of the total energy storing capacity. Lower DoD can ensure higher cycle life of the BESS. Generally, the maximum DoD is set at 90% for BESS.

• Round-trip Efficiency: It is the percentage of energy delivered by the BESS during discharging when compared to the energy supplied to the BESS during charging. Flow battery technology has lower round-trip efficiency compared to Lithium-ion batteries. It means that higher energy is wasted (during charge-discharge) when flow batteries are preferred over Lithium-ion batteries.

• Usable Energy: For the above-mentioned BESS design of 3.19 MWh, energy output can be considered as 2.64 MWh at the point of common coupling (PCC). This is calculated at 90% DoD, 93% BESS efficiency, ideal auxiliary consumption, and realistically considering the conversion losses from BESS to PCS and PCS to Transformer.

• Operating Conditions: The operating temperature is key to selecting the type of battery. For example, an LFP battery would be considered in areas that experience high temperatures. In this scenario, using Lithium Nickel Manganese Cobalt Oxide (NMC) batteries would not be recommended.

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This article was originally published in EVreporter Magazine Jan 2023 issue that can be accessed here.

Read: Part 3 of this series