Basics of BESS

(Battery Energy Storage System) & Project Planning

Basic Terms in Energy Storage

• Cycles: Each number of charge and discharge operation

​

• C Rate: Speed or time taken for charge or discharge, faster means more power.

​

• SoC: State of Charge, the present battery charge percentage

​

• DoD: Depth of discharge the battery, the decrease in the SoC during one discharge.

​

• RTE: Round trip efficiency, efficiency of energy for energy that went in and came out.

​

• SoH: State of health is existing energy storing capability compared to when it was new.
• Calendar Aging: The battery’s SoH goes down by age regardless of you use it or not.

Battery Pack

Cell

Liquid Cooling Unit

High Voltage Box

MV Skid containing PCS and Transformer

PCS (Power Conversion System)

• Unlike Solar Inverters which are unidirectional, PCS has bi- directional capability, meaning it can allow movement of power in both directions.
• PCS converts LV AC power coming from the grid to DC power to charge the BESS.
• PCS converts DC power discharged from the BESS to LV AC power to feed to the grid.
• LV AC voltage is typically 690V for grid connected BESS projects.
• LV AC voltage is typically 380V/400V/415V for commercial and industrial energy storage projects, without the need for a Transformer.

Grid Following PCS

Grid following PCS (along with energy source) synchronizes its energy output with the grid’s voltage and frequency.

​

Grid following PCS track the grid angle and magnitude to inject or absorb active and reactive power.

​

Grid following PCS are dependent on the grid to provide a stable voltage and frequency and cannot operate in islanded or off-grid mode and does not support black start function.

​

Grid following PCS are popularly used in on-grid BESS projects.

Grid Forming PCS

Grid forming PCS (along with energy source) has the ability to provide voltage and frequency support to the grid during power outages.

​

Grid forming PCS can operate independently and help restore the grid after a blackout.

​

Grid forming PCS is a growing trend these days as the grid is working with more renewable energy.

MV/LV Transformer

• Most Projects require the BESS to deliver energy to a measuring point after MV/LV Transformer.

• The losses for this type of arrangement is lower than delivering power to a

measuring point after HV/MV Transformer.

• MV/LV Transformer is not arranged in MV skid arrangement in Indian projects. Globally MV skid arrangement is followed.

​

• Typical MV in India is 11kV or 33kV. This transformer converts it to LV AC 690V.

​

• There is a switchgear after the MV/LV transformer.

Switchgear

• Switchgear consists of circuit breakers, fuses and circuit protection devices to protect, control and isolate the system.
• New regulations globally are adopting to switchgears with SF6 (Sulphur Hexaflouride) gas insulation.
• Switchgear using SF6 gas is designed to not release the gas into the atmosphere because SF6 is a very strong greenhouse gas.
• SF6 is a non-toxic gas with high stability and has an inert nature. It is non-flammable and 5 times heavier than air.

View of a 100MW/200MWh BESS Project

View of a 50MW/100MWh using Sodium- ion

• Some Projects require the BESS to deliver energy to a measuring point after HV/MV Transformer. Typical HV in India is 66kV, 110kV and 220kV.
• The losses for this type of arrangement is higher than delivering power to a measuring point after MV/LV Transformer because there are additional losses of MV cable and HV/MV Transformer conversion for two-way.
• This kind of arrangement still achieves 85% RTE in the beginning of the project. The cost for doing this type of project is higher.
• Eventually the RTE goes <85% because of the reducing RTE of the battery system. Going <85% RTE attracts penalties in most projects.

ngle Line Diagram

of a BESS Container

��Single Line Diagram of a BESS Project

• A common misconception of BESS project design is that the cycle life of the cell provided by the cell manufacturer can be directly assumed to the cycle life of the BESS.
• Cycle life changes when the cell becomes module, when module becomes cluster and when cluster becomes container.
• 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 cells when they are not connected together. Also, laser welding on the cell adds to the resistance of current flow and increasing the heat generation.
• Another example is that cycle life report has minimum rest period and the cycling goes on continuously.
• But in projects, calendar aging factor needs to be added where the cell retention capacity goes down because of rest period for many hours. Additionally, cell testing in laboratory conditions do not have heat coming from its neighbouring cells while it is the case in modules and it can lower the cycle life.

Why BESS?

• Peak demand: An increase in demand can lead to significant stress on the power distribution network. BESS can help relieve the situation by feeding the energy to cater to the excess demand. BESS can be conveniently charged again when the demand is lower than the supply.
• Peak shaving: It is a process of reducing the amount of energy demanded during the peak hours when the energy rates are on the higher side. It helps the consumer avoid peak demand charges. The remaining energy is utilized from BESS.
• Frequency stabilization: The stability of the frequency of the grid power is affected when there is an imbalance in the power generation and the energy demanded. BESS can help stabilize the frequency during such a scenario.
• Free energy from duck curve: During this scenario the energy generation from source is still being generating despite oversupply. This scenario is sometimes experienced on some days of the year in some regions of the world that depend heavily on Solar PV (photovoltaic). The energy during this time can be considered free and this energy can be used to charge the BESS, which can discharge energy for later use for the above mentioned scenarios.

Business Opportunities

• Domestic Manufacturing: With growing projects in India, there’s a significant opportunity for manufacturers to establish production units for BESS in India.
• Rural Electrification: Expanding access to electricity in remote areas using energy storage as part of decentralized solar microgrids.
• EV Charging Infrastructure: BESS provides an opportunity for businesses to set up integrated EV charging and storage stations to cater to peak demands.
• Renewable Integration: BESS solutions are increasingly required to stabilize grid and manage the variable nature of renewable energy sources.
• Energy as a Service (EaaS): New business models offering storage solutions for enterprises, utilities, and even residential consumers, providing scalability and flexibility.

CAPEX, OPEX - Detailed Cost Structure

Capital Expenditure (CAPEX) Initial Investment Costs:

• Battery Storage (DC side): 70-80% of total CAPEX (e.g., Lithium-ion batteries cost per kWh).

​

• Inverters and Transformers: 12-20% of CAPEX (depends on storage hours, if it requires HV/MV transformer).

​

• Energy Management System (EMS): Up to 3% (manages energy storage and usage pattern).

​

• Civil Infrastructure, Transportation and Installation: Up to 6% (land, leveling, construction, transportation and installation).

OPEX for Maintenance and Replacement Replacement/Maintenance Costs:

• Battery Maintenance: Battery capacity augmentation is required for

projects with more than cycles specified by manufacturer, specially for operation in high temperature areas.

• Inverters and Transformers Replacement: Design life of Inverters is 8-12 years and design life of transformer is higher.
• EMS Replacement: Design life of EMS is 6-8 years.
• Labor and Administration: For staff, technical support, and management.
• Auxiliary power costs to power the HVAC and monitoring systems.

​

• Storage Capacity Losses: Battery degradation of 1.5-3% annually due to

battery cycling and calendar aging, depending on number of cycles per day.

• Reduction in RTE: About 2% additional loss in RTE during the project’s

lifetime due to increasing internal resistance of the cells with time. It takes the overall project RTE below 85% with time and attract penalties.

Financial Viability:

• Product Quality Assessment
• ROI and Profitability of the Project
• System Reliability, Cost of Replacement and Augmentation, if required
• Operation and Maintenance Costs.

Regulatory Compliance:

​

• Adherence to Policies
• Grid Standards

Project Sustainability

• Environmental Impact
• Recyclability

• Long-term cost of storing unit energy, for example to store 1kWh
• Round Trip Efficiency (RTE)
• Footprint of the energy storage device
• Supports the required charging time and discharging time
• Maintenance requirement and associated costs
• Availability of components during project lifetime
• Technology existence time to understand its true lifetime
• Number of companies producing the technology and its popularity

• Capacity Augmentation in BESS projects is defined as when additional BESS capacity is added to an existing project to increase the overall BESS capacity and reduce the depth-of- discharge of the BESS in a project.
• It is an alternative to having a large capacity in the beginning.
• It is a useful way to reduce CAPEX and use the upcoming revenue generation to fund additional capacity in future. It is a very useful strategy for OPEX projects.
• BESS typically have a very high degradation in the initial two years and it can be higher than the allowed degradation and hence capacity augmentation makes up for it.

Project Overview and Sizing Planning

Real Data Provided by a BESS

Real Data Provided by a BESS

Round Trip Energy (RTE) Calculation at Measuring Point�Round trip calculation is done keeping the losses below in mind:

• HV/MV Transformer step-down losses (if measuring point is after HV/MV Transformer)
• MV Cable losses (if measuring point is after HV/MV Transformer)
• MV/LV Transformer step-down losses
• LV Cable losses
• PCS Conversion from LV AC Power to DC Power
• DC Cable losses
• BESS Round Trip Efficiency (differs based on C rating use)
• DC Cable losses
• PCS Conversion from DC Power to LV AC Power
• LV Cable losses
• MV/LV Transformer step-up losses
• MV Cable losses (if measuring point is after HV/MV Transformer)
• HV/MV Transformer step-up losses (if measuring point is after HV/MV Transformer)

• BESS can be combined with renewables for off-grid solutions.
• It must be noted that on-grid BESS projects take auxiliary power from the grid and this component would be missing in off-grid projects.
• Auxiliary power is consumed during the battery charging, discharging and during its idle state.
• For 24 hours solution using BESS and renewables, BESS capacity must be sized well to cover the reducing BESS capacity with time or capacity augmentation should be planned.
• Some off-grid projects even consider having diesel generators to synchronize with the solution as a redundancy and to be used for during battery downtime.

BESS Auxiliary Power Consumption

Real Data Provided by a BESS