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5. BESS SELECTION
Selection of battery energy storage for the grid application depends upon the specific application need and the way it competes the strength of different technology options. Hence, no single battery technology solution is the best in any cases.

The key factors which govern the selection of battery technology for a particular application are:
• Performance parameter of the battery
• Duration of the application and capability of the battery to meet the discharge duration.
• Cost economics

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Performance parameter

The key technical parameters which are important in selection analysis are.
• Energy density.
• Depth of discharge (DOD) or State of Charge
• Response time.
• Round Trip Efficiency
• Life cycle
Energy density is the amount of energy that can be stored/delivered from a storage system per unit mass (Wh)/kg. This parameter determines the battery weight and hence the space requirements for achieving a given performance target.
DOD or SOC (%) reflects the battery’s discharged or charged status as a percentage of its maximum capacity.
Response time is the time required for the battery to become operational online i.e., discharge energy. It can vary from milliseconds (ms) to few minutes (min).
Round trip efficiency is defined as the ratio of DC input to AC output. Full system Round trip efficiency includes the losses from the power conversion system, HVAC equipment loads, control system losses, and self-consumption.
Life cycle refers to the no. of charge-discharge cycle of a battery before falling to 80% of its original capacity. It depends upon the DOD and operating temperature.

Relative ranking derived and comparison of performance parameters for the four battery technologies is depicted below in figure-6

Discharge duration
Discharge duration measures how long a storage device maintains its output before reaching its cut off voltage. Discharge duration may range from few minutes (min) to few hours (hrs). Long duration applications demand large storage capacity in order to provide prolonged discharges (which may last for one or more hours). Contrary to long duration application, short duration applications require a fast and short charge and discharge capability (generally ranging between few minutes up to 1 hour).

Design considerations
Based on above discussed criteria, the design considerations for various renewable energy applications are:
Ramp Support: Renewable resource variability is fast and occurs frequently and thus battery storage with ramping capability is one of the key factor. Ramping support falls under frequent long application. So storage systems with response time in a minute, high power output and long life cycle are best suited for this application. The discharge duration for this application is few mins to few hours.

Energy Time Shift: Discharge duration depends mostly on the duration of the region’s off-peak and on-peak periods and the on- peak versus off-peak energy value. This application falls under frequent and long duration discharge duration application. Storage system with long discharge capability, high efficiency as economical operation, long life cycle will be the best suit. The discharge duration for this application is around 3 to 5 hrs.

Renewable Capacity Firming: As its primary use is to provide constant power, the storage used for capacity firming should be more dependable with long discharge capability as this falls under long duration discharge application. Discharge duration varies from 2 to 4 hrs.

Cost considerations
The capital cost of energy storage system comprises of following components
a) Energy Storage Equipment

b) Power Conversion Equipment

c) Power Control System

d) Balance of System
e) Installation

At present Lithium ion technology is positioned with high market share and least cost when compared with other technologies i.e Flow batteries and sodium sulphur.
6. BESS SIZING
Once the required technology is determined, next vital step is the optimal BESS sizing. The optimal solution refers to determination of optimal power and energy rating of the selected battery. Several factor which determine the sizing are forecast accuracy, application purpose, economical aspect, and battery parameters such as DOD, energy density, life cycle and efficiency.

The methodology for BESS sizing is based on the daily generation of wind/solar with further consideration of daily load dispatch. Based on the mathematical model generated, BESS is sized for different mean loading condition. The system is designed for the ‘worst’ generation and loading, which may fall during any day of the year. The BESS nominal power is then be determined considering both the above scenario and the load forcasting.

7. CONCLUSION
It is evident that there is a persistent need to integrate more renewable energy sources into future grid. Renewable energy sources, when coupled with energy storage, can immensely benefit the grid by offering various ancillary services and daily peak load reductions.
Some major challenges in the deployment of battery energy storage systems such as cost competitiveness of battery technology, limited engineering standards and evaluation tools, mitigation of safety risks associated with fire and explosion prevail which needs attention while BESS planning. However with increase in demand and usage, battery price is expected to continue to fall. All these aspects will make battery storage more viable for wide deployment of BESS across globe.