Pengfei Wang is a Senior BESS Applied Technology Manager at RWE Clean Energy, where he provides technical support for BESS project development. His previous experience includes roles at Fluence Energy, General Electric and TNEI. Pengfei holds a PhD in Electrical Engineering from Newcastle University in the United Kingdom.

An Important but Often Overlooked Aspect of BESS Project Design and Development

The installation of battery energy storage systems (BESS) has been growing rapidly in the United States and worldwide since 2021, driven by the continuously falling cost of lithium-ion batteries and favorable government policies and incentives. While the industry is maturing quickly, there are still gaps because BESS remains a relatively new technology. One critical but often overlooked aspect of BESS project development is the technical requirements and financial implications of BESS auxiliary power.

What Is BESS Auxiliary Load?

In addition to the power required to charge its batteries, a BESS also requires power for its auxiliary loads. BESS auxiliary loads typically fall into the following three categories:

● Control and communication equipment, such as the battery management system and network switches;

● Thermal management systems, such as HVAC or chillers;

● Fire safety systems, such as fire alarms, control panels and gas ventilation systems (if present).

These auxiliary loads are essential for ensuring the safe and efficient operation of BESS projects. Therefore, providing a reliable power supply for these auxiliary loads is crucial.

BESS Auxiliary Power Supply Circuit Design

Most BESS products on the market require an external power supply circuit for their auxiliary loads, although some have built-in circuits and do not need an external supply. When an external auxiliary power supply is required, project owners or their EPC (engineering, procurement and construction) contractors are typically responsible for designing, furnishing and installing the auxiliary power supply circuit. This includes auxiliary power transformers, switchboards and cables.

"While it is an important aspect, a comprehensive approach, such as the total cost of ownership method, should be used for BESS product evaluation and selection"

The auxiliary power supply circuit must be designed to meet the BESS product's technical requirements, which vary by product. For example, the rated voltage of the auxiliary power supply might be 400V, 480V, or 208V.

The circuit must also be sized based on the peak auxiliary load of the selected BESS product and the specific project configuration. Each BESS product has a unique auxiliary load design and peak auxiliary load. Even for a specific product, the peak auxiliary load may vary depending on the use case (e.g., C-rate, charging/discharging profile) and ambient temperature. Additionally, the peak auxiliary load for a project may increase over time due to augmentation—adding new BESS equipment to offset battery capacity losses caused by degradation. Accurately determining the peak auxiliary load is essential to properly size the auxiliary power supply circuit.

Backup Auxiliary Power Supply

For certain projects, backup power must be provided for the BESS auxiliary load as required by the BESS supplier or fire codes. Some BESS suppliers mandate uninterrupted power to maintain the operation of thermal management systems, ensuring battery temperatures remain within desired limits to minimize degradation.

BESS fire safety standards, such as NFPA 855, outline minimum requirements for backup power for fire safety systems. For instance, NFPA 855 specifies a minimum of two hours of standby power for gas detection systems and combustible gas concentration reduction systems. If a BESS product cannot meet these backup power requirements as mandated by the code or the Authority Having Jurisdiction (AHJ), an external backup power source needs to be provided. Options for backup power include local distribution network feeders (if available with sufficient kVA rating) or backup generators.

BESS Auxiliary Power Cost

The cost of the auxiliary power supply circuit and any required backup power sources must be accounted for in the project's capital expenditures. Project owners are also responsible for the electricity costs associated with the BESS auxiliary load during operation.

The electricity cost for auxiliary loads depends on the energy consumption (kWh) and the pricing structure set by independent system operators or utilities. For example:

  • In ERCOT, the BESS auxiliary load must be metered separately from energy used for battery charging and is charged at the retail rate.
  • In CAISO, only the BESS auxiliary energy consumption in idle mode is charged at the retail rate.

Accurately estimating auxiliary power energy costs is a critical aspect of evaluating the financial viability of a BESS project.

Conclusion

As discussed above, auxiliary power is a vital consideration in BESS project design and development. While it is an important aspect, a comprehensive approach, such as the total cost of ownership method, should be used for BESS product evaluation and selection.