Key characteristics of Energy Storage Systems?

Rated power capacity is the total possible instantaneous discharge capability in kilowatts (kW) or megawatts (MW) of the BESS, or the maximum rate of discharge that the BESS can achieve, starting from a fully charged state.

Energy capacity is the maximum amount of stored energy in kilowatt-hours (kWh) or Megawatt-hours (MWh). The energy capacity is often given as the so-called DC nominal capacity, which is the actual capacity of the Li-Ion battery at the battery terminals. Size DC nom. however, the capacity is not usable in the whole system, but the actual usable capacity is often described as the so-called AC nominal capacity which takes into account the battery operating window (which is generally 90-95% of the battery DC rated capacity) and the DC-AC-DC conversion efficiency (which is between 95-98%). As a rule of thumb, it can be said that the available AC system nom. the capacity is about 90% of the nominal capacity of the battery.

Storage duration is the amount of time storage can discharge at its power capacity before depleting its energy capacity. For example, a battery with 1 MW of power capacity and 2 MWh of usable energy capacity will have a storage duration of two hours.

Cycle life/lifetime is the amount of time or cycles a battery storage system can provide regular charging and discharging before failure or significant degradation.

Self-discharge occurs when the stored charge (or energy) of the battery is reduced through internal chemical reactions, or without being discharged to perform work for the grid or a customer. Self-discharge, expressed as a percentage of charge lost over a certain period, reduces the amount of energy available for discharge and is an important parameter to consider in batteries intended for longer-duration applications.

State of charge, expressed as a percentage, represents the battery’s present level of charge and ranges from completely discharged to fully charged. The state of charge influences a battery’s ability to provide energy or ancillary services to the grid at any given time.

Round-trip efficiency, measured as a percentage, is a ratio of the energy charged to the battery to the energy discharged from the battery. It can represent the total DC-DC or AC-AC efficiency of the battery system, including losses from self-discharge and other electrical losses. Although some battery manufacturers refer to the DC-DC efficiency, which does not describe the actual capacity of the system. AC-AC efficiency is typically more important to Energy Storage System operators, as they only see the battery’s charging and discharging from the point of interconnection to the power system, which uses AC.

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