Hi Quamera you are almost right , but my units are ok, when designing a BESS a key parameter is the rate of charge/discharge that you require the battery to operate. You can provide energy stored and the amount of time used to store/discharge it, or just the power output. Another key parameter is at which point in time you want the warranted energy at BOL (beginning of life) or EOL (end of life).
MWH are giving you the energy stored, the hours are telling you the rate of charge/discharge. A 400MWH battery , 2 hours, is a battery which is able to charge discharge at C/2 , in this case delivering an output of 200MW ( Is equivalent to what you said)
A BESS 200MWH 2 hours is physically bigger than a 200MWH 4 hours, the faster the charge discharge rate you need the more oversized battery you need. I.e. usually battery rating plates are given at C/5 ( 5 hours for either charge or discharge). I.e. for 200MWH , 4 hours (or 50 MW) you will need to install around 210MWH (nominal plate rating capacity); for a 2 hour, you will need 240MWH installed capacity(C/5). As you said the rating plate of the battery must be always higher than the warranted energy, the harder operating conditions, the higher is the oversizing ( high temperatures, high power outputs, short time between cycles, high EOL capacity....).
The 20 years is the design life, the battery failure in Lithium occurs due to the electronics failure embedded into each cell ( to control temperature/charge )...at the age of 20 years those electronics start to die, despite the battery capacity still can be 70% of the original capacity ( the battery chemistry aging is better than the electronics).
The depth of discharge is not constant during the battery life, commencing the life with a very shallow cycles that get deeper as the battery ages, but usually you charge up to 95% capacity and discharge down to 20-10%. Those are parameters that are controlled by the cell supplier, as the cycle is linked to degradation, and the supplier provides a warranty of the battery degradation and accumulated energy output up to year 15. ( If you operate out of the parameters window, the warranty is voided).
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