Understanding Battery Energy Storage System (BESS) | Part 3 – Project planning

The first part of this series covered the basics and constituents of BESS, and the second part covered the advanced aspects of BESS. In the third part of the series, Rahul Bollini explains project planning while competing for a BESS project.

Below are the points to be considered while planning to participate in a new BESS project:

1. Understanding the energy-to-power ratio of BESS

A lower energy-to-power ratio means faster charging, and a higher ratio means slower charging. Slower charging creates lower heat dissipation of the cells and ensures higher system efficiency. A higher ratio also indicates that the life of the battery will be longer.

2. Understanding the project life and making the necessary design

Project life not only means the years of the project but also the usage frequency, i.e., the number of charge-discharge cycles (per day or per year). A lower frequency of cycles ensures longer years of usability. One must also understand the operating temperature conditions of the project site and
accordingly set the cooling system (air cooling or liquid cooling) parameters of the BESS. This also creates a difference in the energy consumption by the cooling system to maintain the ideal temperature. The amount of energy consumed by the cooling system matters when the energy is supplied by the BESS (during the discharging and rest period). Accordingly, extra battery sizing must be added, or else the Depth of Discharge will increase and lead to a lower life of the BESS.

3. Understanding whole system efficiency to ensure enough energy at the point of measurement

Let us understand the diagram of on-grid connected BESS.

If energy is measured at the point of common coupling (PCC), the BESS capacity must be oversized to ensure that it discharges extra energy to cover the losses in DC cables from BESS to PCS, conversion losses of PCS, LV (low-voltage) cable losses from PCS to Transformer, conversion losses of Transformer while voltage step-up and MV (medium-voltage) cable losses from Transformer to PCC.

Similarly, energy sent from solar power generation must be calculated to incorporate losses at every stage and then incorporate the Wh efficiency of the batteries.

I have created a tool that I use for the projects where I design for BESS. It comes in very handy for energy calculation at each point and to accurately size the BESS. It also considers the annual degradation and calendar ageing parameters to ensure that correct BESS sizing can fulfil the required energy that can be discharged throughout the project lifetime (to avoid penalties).

4. PCS operating capacity

From this reference image, it is evident that the peak efficiency of an inverter (bi-directional inverter or PCS in this case) is when it is not operating at full capacity. Generally, in the PCS datasheet, only peak efficiency is informed, and thereby, in design, oversizing the PCS must be considered to ensure maximum round-trip efficiency of the overall system.

5. Cell Cycle Life is not equal to BESS Cycle Life

A common misconception of BESS project design is to assume that the cycle life value mentioned in the cell report provided by the cell manufacturer can be assumed to be the cycle life of the BESS. Cycle life changes when the cell becomes a module, when the module becomes a cluster and when the cluster becomes a container. The reason for this is external factors that add to the reduction of cycle life. For example, heat generated in a module is more than the same number of cells when they are not connected together. Another example is that the cycle life report has a minimum rest period, and the cycling goes on continuously. But in projects, a calendar ageing factor needs to be added where the cell retention capacity goes down because of the rest period of many hours. Additionally, cell testing in laboratory conditions does not have heat coming from its neighbouring cells, while this is the case in modules, and it can lower the cycle life.

6. Annual Degradation Chart from BESS supplier

It is very common for a cell manufacturer to claim 6000 cycle life, but what matters is the annual level degradation chart given by the BESS supplier. It is preferable to buy BESS from a supplier who understands the characteristics of the cells very well and provides an accurate annual degradation chart.

For example, below is the annual degradation chart provided for the BESS by the same company that manufactures the cells and claims the cell cycle life to be 6000 cycles at 0.5C charge and 0.5C discharge rate at 100% depth of discharge with 80% retention capacity.

7. Understanding the BESS voltage system

A 1000V battery system is preferred when using BESS for commercial and industrial (C&I) sectors to be able to give an output of 380V/400V AC 3 phase. A 1500V battery system is preferred when using BESS for grid connectivity because 1500V PCS’s output is 690V AC, which is then stepped up by the transformer to a higher AC voltage to send to the grid.

8. Deciding between air cooling and liquid cooling system for BESS

Both types of cooling mechanisms have their advantages and disadvantages. Air cooling is flexible to be used in most of the solution types, but liquid cooling is only used in 1500V systems. Air cooling solutions are cheaper but need regular maintenance, such as filter cleaning, and their power consumption becomes more inefficient in the long run.

9. Type of cell usage for BESS

Recently, the 280Ah cell has been preferred for BESS due to its large capacity, lower number of cells for a given system, and lower cost. The various parameters to check are the following:

• Original capacity of the cell (because first-year degradation is high).

• Types of cell design, such as z-stack type electrode-designed prismatic cells, are gaining popularity for their better performance.

280Ah cell-based BESS in 20 feet container is able to provide storage for slightly more than 3.7MWh in a 1500V system. The same cell-based BESS in 20 feet container in 1000V is slightly more than 2.5MWh.

Cells with a capacity higher than 300Ah will see a rise in the market in 2024 because of their ability to fit in the footprint of 280Ah and, therefore, will increase the overall capacity of the standard sizes of BESS containers.

10. Planning for projects more than 10 years

It is no surprise that there will be a few modules that will not perform as per expectation after 10 years. A regular module replacement strategy needs to be in place for projects that run for more than 10 years. Also, be prepared for a replacement of EMS any time after 5 years.

About the author:

Rahul Bollini is an R&D expert in Lithium-ion cells with 8 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: Evolving input trends for end-of-life lithium-ion cells and its relevance for battery recycling