Understand the codes, standards for battery energy storage systems

BESS insights

• This will assist electrical engineers in designing a battery energy storage system (BESS), ensuring a seamless transition from traditional generators.

• This article discusses decarbonization and the transition from fossil-fuel-based backup generators to battery energy storage systems for building owners.

Decarbonization, electrification and elimination of fossil fuels are crucial for reducing our climate footprint. But how can building owners replace their fossil-fuel-burning backup generators? The solution lies in alternative energy sources like battery energy storage systems (BESS).

Battery energy storage is an evolving market, continually adapting and innovating in response to a changing energy landscape and technological advancements. The industry introduced codes and regulations only a few years ago and it is crucial to understand how these codes will influence next-generation energy storage systems (ESS).

A microgrid is a localized energy system comprising interconnected electrical resources that supply energy to users. For this article, consider a microgrid consisting of a single building equipped with a BESS and a solar generation system.

Addressing common BESS misconceptions

Figure 1: A simplified project single line showing both a battery energy storage system (BESS) and an uninterruptible power supply (UPS). The UPS only feeds critical loads, never losing power. The BESS is bidirectional, stores and supplies energy, but loses power when the utility is lost before it can restart in island mode after opening the utility breaker. Courtesy: Affiliated Engineers Inc.

Uninterruptible power supply (UPS) systems have been a familiar presence for years, known for their ability to enhance power quality and offer continuous power for critical loads. These systems typically supply power for a few minutes while the generator starts up. However, it is important to note that a BESS operates quite differently from a UPS (see Figure 1).

A BESS operates more similarly to a generator or utility plant connected to a microgrid. It can store and supply energy to an electrical system. While the BESS can start up quickly, it is not instant and there will be a brief voltage supply disruption during startup. As a precaution, the system will require a separate UPS to power sensitive or critical components, potentially including the controller for the BESS.

Another misconception worth addressing concerns solar panel operations during a power outage. Contrary to popular belief, solar panels do not generate power simply because the sun is shining. Without a live power source on the solar inverter’s alternating current (ac) side for reference, the inverter will automatically shut down, causing the solar panels to stop their direct current output. This reference source can be a utility grid, a BESS or, to a limited extent, a generator.

Defining energy storage system objectives

First, the building owner and consulting engineers must define project goals. The following questions can help determine the project’s objectives, informing the battery system design:

• What is the main issue the microgrid with battery energy storage would solve?

• Does the project prioritize resiliency? For instance, is there a need for a microgrid that can maintain service through inclement weather?

• Is there a long-term energy source involved, such as a set of fuel cells that may require time to reach capacity in response to an outage?

• Is the building’s own energy resource operating in parallel with the grid, with a battery assisting in maintaining power quality by smoothing transitions and fluctuations in demand?

• Is a portion of the battery capacity intended to be used as a financial solution to manage electricity rates — either by reducing peak kilowatt demand or shifting utility energy usage away from peak rates?

BESS codes and standards

Most are familiar with their local building, electrical and fire codes, but what do these regulations stipulate regarding a BESS?

International Building Code (IBC): Following IBC 2024 Chapter 27 Section 2702.1.3, emergency or standby power systems must be installed following the guidelines outlined in the International Fire Code IFC), NFPA 70: National Electrical Code (NEC) and NFPA 111: Standard on Stored Electrical Energy Emergency and Standby Power Systems. Below is an overview of what these referenced codes entail.

IFC Section 1207 addresses energy storage and the following highlights critical sections and elements:

• IFC 1207.1.3 features a table defining when battery systems must comply with this code section. It categorizes all lithium-ion technologies under “lithium-ion batteries.” Despite the six leading battery chemistry types having varying hazard performances, the code applies a uniform 20 kilowatt-hours (kWh) threshold for compliance. While it is essential to consider the specific lithium battery chemistry, note that it does not impact this code threshold.

• IFC 1207.3 requires third-party listings for ESS. The ESS must be listed in accordance with UL 9540, the Standard for Safety of Energy Storage Systems and Equipment. This can be indicated by a UL label or a label from another recognized testing authority if it meets the UL standard.

• IFC 1207.4.12 clarifies that a walk-in BESS enclosure is considered effectively unoccupied. An architect will appreciate this knowledge as a large BESS in a walk-in enclosure would otherwise classify as its own building. Additionally, according to IFC 1207.5.5, walk-in units must also have a fire suppression system installed.

• Table 1207.5 provides limits that pose a challenge until the accompanying text in IFC 1207.5.2 Exception 3, is considered. If the BESS is in a “dedicated use” building compliant with IFC 1207.7.1, then the kilowatt-hour limits outlined in the table do not apply. This exception is beneficial, especially considering that 600 kWh of energy capacity is approximately equal to a small portable diesel generator’s belly tank capacity.

• IFC 1207.6.1.2.1 mandates that battery enclosure ventilation must operate on standby power and comply with IFC 1203.2.5. Manufacturers typically design the enclosures with this requirement in mind. If accessory power is needed for heating, ventilation and air conditioning (HVAC), ensure it comes from a source recognized as “legally required standby” under NEC 701.

• IFC 1207.7.4 stipulates a two-hour fire barrier separation requirement if the BESS is installed inside a building that is not dedicated for its use.

• IFC 1207.8.3 requires a minimum 10-foot separation between the BESS and any building. However, note this is a minimum requirement; a greater separation may be necessary per the BESS manufacturer’s specifications or the owner’s insurance provider. Certain exceptions also exist that can reduce this code-required separation to 3 feet under specific conditions.

The NEC presents significant requirements. Several sections with the NEC are relevant, including Sections 695, 700/701/702, 705 and 706.

• NEC 706.15 specifies signage requirements.

• NEC 705 Section 705.12 regulates overcurrent device and bus sizing for microgrids.

• If the microgrid system feeds any emergency or legally mandated loads, the design must adhere to NEC 700/701. Otherwise, it operates as a NEC 702 system.

• NEC 695 provides electrical code requirements for fire pump installations, referencing NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection. If the building has a fire pump, it is crucial to collaborate with the local fire marshal. This ensures a comprehensive understanding of how an inverter-based microgrid electrical system can meet the specified technical requirements and guarantee reliable fire pump operation when needed.

NFPA 111 outlines the requirements for BESS in emergency or standby power systems under IBC, NEC 700, or 701. Due to its reference in IBC, this standard is mandatory for supporting emergency or legally required systems in jurisdictions where IBC codes are applicable. According to Section 5.2.1, a bridging system is the UPS that maintains BESS control functionality during the transition from a utility outage to microgrid operation in island mode. Because NFPA 111 is directly required by the IBC for emergency and code-required standby systems, the entire standard must be adhered to as if it were code when the BESS supports NEC 700 or 701 loads.

NFPA 855: Standard for the Installation of Stationary Energy Storage Systems provides essential guidelines for BESS installation and every BESS must comply with this standard. While many requirements in the IFC and NEC reference NFPA 855, not all its provisions are explicitly stated within the fire code.

For instance, Section 4.3.5 addresses signage requirements not mentioned in International Code Council codes and supplies guidelines for commissioning, startup and testing. Consulting this collection of codes is crucial to ensure all signage and related construction requirements are thoroughly detailed in specifications and drawings.

Various BESS concerns

It is unwise to assume a battery system can be installed outside in a Minnesota winter or an Arizona summer without considering additional factors. Precaution measures such as shading, heating, cooling, or other steps may be necessary to ensure the battery system’s operability and longevity. Ideally, if considering an enclosed system from a single vendor, these considerations will have already been addressed.

Many are familiar with insurance options like FM, which imposes additional requirements beyond IFC regarding separation distance. It is crucial to engage building owners and their insurance representatives before proceeding with system installation. Effective communication with the insurance representative is essential, as they need to understand the battery technology, fire hazard, fire suppression technology and other relevant details to provide meaningful feedback to the project team.

Battery systems use solid-state inverters to provide ac power, which comes with certain limitations. For instance, consider the differences between the solid-state inverter and traditional generators. When sizing a generator, it’s crucial to input loads into the sizing calculator to confirm the generator has sufficient starting kVA to start and support the loads without exceeding voltage drop when the automatic transfer switch transfers the loads onto the generator.

This consideration becomes even more important with battery inverters, although specific sizing calculators for battery systems are not yet available. To address this, one method is to check the inverter specifications and plot the shutdown curve for the inverter against the motor-starting curve for the fire pump or largest air handling unit. Additionally, plot the inverter output curve against the breaker protecting the wire feeding the BESS. This comparison helps ensure the compatibility and adequacy of the inverter power connection.

Consult with the inverter manufacturer or system manufacturer for specific considerations. They can provide guidance on arc flash hazards, short circuit currents, system impedances, protection relay settings and input impedances, among others. It’s noteworthy that most large-scale inverters are designed based on balanced three-phase power input and output. When these systems supply power to a building with a four-wire utility feed, it is advisable to include a transformer between the BESS and the building microgrid. This transformer can generate the neutral fourth wire and introduce impedance on the connection.

BESS will likely encompass a network-connected controller and the BESS manufacturer may require access to this data connection. Coordinate with the client’s information technology team to address this aspect, as well as internet protocol addresses, firewalls and access permissions, before initiating wiring to facilitate a smooth integration.

Electrical considerations for BESS

It is important to consider inverter limitations when discussing breaker tripping and arc flash calculations. In a microgrid powered by batteries, the inverter output sets the limit for short-circuit current and energy that can be delivered during a fault. Assessing whether coordinated breaker tripping is necessary involves comparing the inverter curve to the breakers to determine the trip points.

Figure 2: This shows the breaker versus inverter output limit. Notice the breaker curve far to the right of the inverter limit. The inverter will shut down when it reaches its output limit. Courtesy: Affiliated Engineers Inc.

In many cases, the inverter may need to be significantly larger than the calculated building load to ensure it can generate the required current for coordinated tripping. This underscores the significant mechanical advantage of rotating engines and alternators, as their rotating mass provides stored energy capable of sustaining a fault long enough to trip breakers (see Figure 2).

Battery systems experience a decrease in charge capacity (energy capacity) over time. This degradation rate is influenced by various factors and may differ based on the technology used. While batteries in most lithium iron phosphate systems may endure for 20 years, they are unlikely to retain 100% charge capacity throughout this period. Refer to the battery vendor’s warranty details to determine the additional battery capacity required for the BESS to maintain performance targets over the system’s life span.

Determine whether the battery is supplying power to a building with systems capable of load shedding or returning to service based on battery state of charge. Collaborate with the energy model engineer to identify pickup and drop-off points for each load level, which can be adjusted to extend the battery life.

Sustainability with renewable energy and BESS

Battery energy storage represents a critical step forward in building sustainability and resilience, offering a versatile solution that, when applied within the boundaries of stringent codes and standards, ensures safety and reliability. Embracing these advancements enables building owners to reduce carbon footprints and enhance operational efficiencies, preparing for a future powered by renewable energy sources.

Building owners and engineers are encouraged to integrate a BESS in their upcoming projects to drive sustainability, resilience and operational efficiency.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.