Understanding battery energy storage system (BESS)| Part 6

FAQs about 5MWh BESS Architecture

In continuation to part 5 of the series (Understanding BESS), published in April 2024, part 6 focuses on deeper aspects of the architecture of a 5MWh liquid cooling container, which is gaining popularity across large-scale grid-connected projects.

What kind of single-unit BESS are used in large-scale BESS projects?

Large-scale projects use the most compact BESS containers with very high energy storage capacity. 3.727MWh in 20ft container with liquid cooling system was popular until last year which had 10P416S configuration of 280Ah, 3.2V LFP prismatic cells. This trend has shifted to 5.016MWh in 20ft container with liquid cooling system with 12P416S configuration of 314Ah, 3.2V LFP prismatic cells. For example, a 70MWh battery requirement would be fulfilled by 14 Nos. of 5MWh BESS systems. For a 2-hour storage project, a 35MW capacity PCS and transformer-integrated solution would be used. The actual energy discharged from the battery will be lower than 70MWh to maintain a healthy DoD (depth-of-discharge) for long cycle life, and the required PCS and transformer size would be slightly lower, but there are limited options for choosing the size of PCS and transformer integrated solution.

A simpler approach for this kind of project would be this SLD (single-line diagram).

What are the popular types of PCS sizes available?

Commercial and industrial applications use under 1000V battery systems, and the popularly available PCS ratings for such battery systems are 100kW, 150kW, 250kW, 500kW and 630kW. These PCS provide AC 3 phase output between 380V to 440V depending on the requirement of a given country. For higher PCS requirements, multiple PCS are added. These PCS come without isolation transformer to provide output with 3 phase 3 wire for industrial loads that require only 3 phase supply and there is also an option with isolation transformer to provide 3 phase 4 wire for single phase and 3 phase loads.

Utility-scale grid-connected applications use 1500V battery systems. They use popular PCS models such as 1250kW, 1500kW, 1725kW and 2500kW. For large projects, sometimes two PCS (with AC 3 phase 690V output) are integrated with a voltage boost transformer in a dedicated container that provides AC output between 10kV to 35kV depending on the requirement of the utility’s grid.

How easy is the system integration of BESS systems?

If shipped in fully assembled condition, BESS systems are easy to integrate and ready to use. However, each of these 5MWh systems weighs close to 42 tonnes, can be difficult to transport, and needs special attention during international or domestic shipping.

Additionally, there are many countries where local assembly is preferred to avail lower customs duty or for local value addition; hence, demand for SKD (semi-knocked down) BESS systems is rising. Very few BESS manufacturing companies want to provide liquid cooling solutions in SKD condition and take the responsibility of integrating these systems at the client’s location.

Is there 1 hour storage projects for peak energy demand management using BESS?

1-hour storage projects would need close to 1C discharge compatible cells. Such cells are not easily available beyond 230Ah cells that are used in e-buses and e-trucks and even their cycle life is not more than 4000 cycles at the cell level. 280Ah and above cells are designed not to cycle beyond 0.5C discharge to be able to function as per their optimal life cycle of a minimum of 6000 cycles. This graph shows a real-time cycle life comparison for cell cycling at 0.5C/0.5C and 1C/1C for a regular 280Ah energy storage cell.

The cycle life of 1C/1C can be as much as half the value of 0.5C/0.5C C rate, and the manufacturer strongly does not recommend 1C/1C. This has created a vacuum in the 1C discharge BESS supplier for peak demand management. This is where a company like XDLE Battery, manufacturing EV grade 2C continuous charge and discharge 280Ah cell (same dimensions as 280Ah ESS type cell) for mining trucks (1-hour charge and harsh operating conditions), is able to cater to the 1-hour backup storage market without much competition.

As shown, their cell cycle life graph at 1C/1C at 100% DoD shows 6500 cycles with 83% retention capacity. This translates to a system-level cycle life of 6000 cycles up to 15 years for 1C discharge function for peak energy demand management.

Cycle life expectations have been rising over the years, and there is a rise in the global market for 20-year BESS systems without replacement or augmentation. Many companies have launched energy storage variant 314Ah cells with 401Wh/L and 179Wh/Kg with up to 12000 cycles at 70% SoH. Some companies are claiming 15000 cycles, which should suffice for one cycle per day for 20 years at a system level with calendar ageing and higher temperature operating conditions. It could also cater to 2 cycles per day for 12 years at a system level with calendar ageing and higher temperature conditions.

“I will discuss this claim in detail in the next article, Part 7 of Understanding BESS, with a focus on cell cycle life expectations and BESS annual degradation pattern expectations under various scenarios.”

About the author:

Rahul Bollini is an R&D expert in Lithium-ion cells with 9 years of experience. He founded Bollini Energy to assist in deep understanding of the characteristics of Lithium-ion cells to EV, BESS, BMS and battery data analytics companies across the globe. Rahul can be reached at +91-7204957389 and bollinienergy@gmail.com.

Also read: Understanding battery energy storage system (BESS)| Part 5