BESS Analysis - Methodology
As a user, it's essential to understand what Glint Solar calculates and how it performs its analyses — especially when it comes to battery systems. The platform assesses key battery performance parameters such as capacity, efficiency, and degradation over time. Below is a detailed clarification of the key terms and calculation methods used.
Battery Storage System
Capacity (DC) [kWh] – Installed usable battery container capacity at the beginning of the system’s lifetime.
Max C-rate – The maximum rate at which the battery can be charged or discharged, calculated as the DC rated power of a BESS container divided by its DC capacity.
Installed Capacity [MWh] – The total DC capacity of the project at the start.
Nominal Inverter Power [MVA] – The total, cumulative power rating of all MV stations in the system, without accounting for losses.
Nominal Supply Duration – The installed battery capacity of battery containers within single set divided by the MV station power.
Usable Capacity at POC – The AC capacity of the entire battery system during discharge, adjusted for single-way losses at the beginning of each year.
Energy Throughput – The cumulative energy (in GWh) imported from and exported to the grid each year, reflecting the system’s overall round-trip efficiency.
Capacity and Throughput Methodology
The overall AC capacity of the system is the sum of all battery set capacities on site. Each day, the DC capacity of the battery is reduced by a degradation increment. The AC capacity at the POC is derived from the DC capacity by accounting for the square root of the battery’s DC round-trip efficiency, inverter efficiency, transformer losses, and half of the cabling and auxiliary losses.
Daily system throughput is the total discharged energy each day. It depends on the system capacity, the number of cycles per day, and the maximum and minimum SOC limits. The C-rate does not affect the throughput. Those parameters, specified within the operations tab, apply to the DC side of the system, with round-trip losses impacting the energy imported and exported at the POC. A single cycle is defined as a full swing from minimum SOC to maximum SOC and back to minimum (or vice versa). As the degradation progresses, battery capacity decreases proportionally to the depth of each cycle.
Losses
Losses refer to the reduction in energy and power during the round trip — from importing energy at the POC to exporting it back. Various factors contribute to these losses, making it essential to understand them for accurate energy flow estimation. In Glint Solar, all BESS losses are defined by the user.
The energy dissipated by each loss, divided by the energy imported from the POC, is shown in the graph below. The ratio of energy exported back to the grid to the energy imported from the grid represents the system’s round-trip efficiency (RTE).
Auxiliary Losses
These are energy losses from supporting components such as cooling systems, control electronics, and battery management systems required for efficient operation. The user-defined percentage represents the total loss for the entire round trip, so the loss is split equally, with half applied to each direction.
Cabling Losses
Losses due to resistance and inefficiencies in the cables connecting the system components, causing some energy to dissipate as heat. Similarly to auxiliary the user-defined value is split for charging and discharging.
Transformer Losses
These are the energy losses that occur in medium voltage transformers during power conversion and transfer. A single direction transformer loss is calculated as 1-n(transformer), where n(transformer) stands for transformer efficiency [%].
Inverter Losses
Occur during the conversion between DC and AC power in the MV station. Unlike the transformer losses, the percentage of energy lost due to the inverter is dependent on the power supplied and the max power [kW] of the MV station.
The user-defined EURO efficiency stands for a weighted average operating efficiency corresponding to middle European climate. The inverter efficiency curve, modeled on a reference Sungrow inverter, is identical for both DC to AC and AC to DC conversions.
Battery Losses
For both charging and discharging, battery losses are modeled using the square root of the DC round-trip efficiency [%] defined in the battery profile. This approach evenly splits the overall efficiency impact between the two processes.
Inverter Power Limit
The DC discharge power from a single battery set cannot exceed the MV station (hence inverter) rated power [kW]. If it does, this will result in a following error:
"The power from the battery units with the given C-rate exceeds the inverter power."
This error will stop the analysis. To fix the issue, reduce the number of battery containers per set or lower the C-rate.
Degradation
Degradation is the gradual loss of DC capacity in a battery system over time. It is crucial since it directly impacts the system's performance and lifespan, indicating its state of health (SOH). In Glint Solar degradation is defined as a % of remaining capacity at the start of each year. Users can define their custom degradation curve or opt out for an automatic model.
Shelf Degradation: This refers to the capacity loss before the installation of the system. Essentially, the defined % will be discounted from the battery capacity at the start of the operation. If no value was specified then the shelf degradation will be equal to 0. For the automatic degradation model the shelf degradation is already removed from the calendar degradation, resulting in a slightly shallower degradation curve.
Automatic Degradation
The capacity loss is modeled using a reference Sony cell in LFP technology. The degradation process is divided into calendar degradation—dependent on elapsed time and SOC—and cyclic degradation—dependent on the number of cycles, cycle depth, and C-rate. The cell temperature is assumed constant.
It must be stressed that degradation varies significantly between different cell types, manufacturers, and even between individual specimens, making precise calculations impossible. However, the automatic model provides a reasonable estimation of the degradation level.
Warranty SoH
This value represents the battery’s state-of-health at which the warranty expires. Note, that even if the battery's SoH falls below the warranty threshold, the analysis will continue for the years of operation defined in the operations tab.
Additional resources
For more detail on BESS configuration, please see the articles below: