Understanding how a BESS works and the risks it poses is a critical step in safely and effectively incorporating a BESS into an electrical system.
Learning objectives
- Understand how a battery energy storage system (BESS) operates.ย
- Discover the benefits and drawbacks of using a BESS.
- Explore methods of mitigating the risks associated with a BESS, particularly the fire hazards.
BESS insights
- A BESS integrates battery storage, power conversion and advanced management systems to bridge the gap between inconsistent renewable energy production and fluctuating grid demand.
- While safety concerns like thermal runaway exist, modern BESS installations are becoming significantly safer through rigorous adherence to evolving industry standards like NFPA 855 and UL 9540.
Renewable energy sources continue to penetrate the power distribution industry, offering economically and ecologically sensible installations for new power generation across a wide variety of locations. While renewable energy sources are continually improving and increasingly recognized as foundational to the grid of the future, they present challenges at both individual installation and grid-wide scales. For widely adopted sources such as solar and wind, the nature of how they produce energy โ i.e., the sun shining and the wind blowing โ means that energy production may be inconsistent or misaligned with electrical system demand.
Battery energy storage systems (BESS), along with other forms of energy storage, enable electrical systems to ride through temporary fluctuations in renewable generation and allow energy produced during periods of excess generation to be used when demand is misaligned with production. Although their use is increasingly common in the electrical industry, BESSs remain a relatively new and continuously evolving technology, with ongoing questions surrounding designing practices, implementation strategies and safe operation.
Proper BESS implementation involves understanding how they operate, the associated benefits and drawbacks and the major risks โ along with appropriate mitigation strategies.
How a BESS works
A BESS consists of three major subsystems: storage, power conversion and protection. Each subsystem is necessary for a BESS to be complete.
The storage subsystem is composed of batteries. Each battery contains several cells that use electrochemical reactions to store electricity for later use. The process is then reversed when the stored energy is needed. These cells are grouped into modules to increase system capacity; however, increased capacity also poses additional risks, which will be discussed later.
Several battery technologies exist, each with a different chemical makeup that affects the capacity, physical size, longevity and ideal method of use. For large-scale storage, lithium-ion batteries are most prominently used. Although new battery technologies and chemistries are continuously being developed, tested and prototyped, lithium-ion technologies currently remain the most popular and cost-efficient choice.
Because batteries store energy as direct current (DC), BESSs require a way to convert stored energy into alternating current for use within electrical distribution systems. This is accomplished through inverters, which serve as the primary component of the power conversion subsystem. Bidirectional inverters allow BESSs to transfer energy into the battery for storage (see Figure 2).
To ensure efficient energy use and battery longevity, BESSs require proper maintenance. A battery management system (BMS) accomplishes this by monitoring the voltage, temperature, state of charge and current of the batteryโs cells and modules, as well as any imbalances between them. As such, the BMS is the first line of defense in battery protection.
An energy management system (EMS) is the central control platform for the entire BESS. It dictates when to charge or discharge the battery by looking at electricity prices, utility limitations, the application of the BESS and even the weather forecast. The EMS can be connected to a supervisory control and data acquisition system or other controls systems to have some degree of control over how the BESS operates. The EMS also serves as the link between the other components of the system (the BMS, the inverters and the source to provide/store energy from), making it a crucial aspect of a successful BESS.
Fire and environmental concerns
A common concern surrounding BESS installations is safety, especially concerning potential fire risks. A cursory review of online search results will return several incidents of large BESSs creating hazardous conditions, leading to understandable questions and pushback from concerned locals. However, as with any relatively nascent technology, it is important to understand how the type of BESS technology โ and the way it is installed and operated โ affect risks.
Lithium-ion battery technologies may experience thermal runaway if protections against overheating, overcharging or excessive charging rates are not in place. Thermal runaway occurs when an increase in an itemโs temperature generates further heat, creating a cascading effect that produces more heat and damages more components. Modern standards require BESSs to monitor battery cell heating, charging and charging/discharging rates to mitigate these risks. Additionally, modern BESSs โ especially modular BESSs โ are equipped with fire suppression capabilities.
Over the past two decades, the Electric Power Research Institute (EPRI) has conducted substantial research into BESS failures, including what technologies or components are most linked with fires. According to EPRIโs Battery Energy Storage Systems Failure Incident Database, most verifiable BESS failures originate from operational and construction-related issues, not design or manufacturing flaws. Additionally, the BESS elements that failed the most were in the BESS control systems or in the noncell component equipment, such as switching units, transformers or inverters โ not the battery cells themselves.
Of note in EPRIโs research database is the comparison of the number of BESS incidents against the amount of installed BESS capacity. While global BESS capacity has increased at almost exponential rates, the number of BESS failure incidents has remained relatively stagnant. Consequently, the failure rate has decreased from nine incidents per gigawatt of installed BESS in 2018 to less than one incident per gigawatt of installed BESS by 2023.
While this statistic does not mean that modern BESSs pose no failure risks, it strongly indicates that they are becoming safer. This is due to the electrical industryโs ongoing development of new standards and continual improvement of existing ones to better protect BESSs and prevent failures.
NEC and UL requirements for BESS
Article 706 of NFPA 70: National Electrical Code (NEC) was developed to ensure the safe installation and connection of a BESS to the overall system. One of the requirements is to ensure all BESS-related equipment is UL listed โ particularly to UL 9540: Energy Storage Systems and Equipment and UL 9540A โ and is compliant with all other fire codes and safety standards, including NFPA 855: Standard for the Installation of Stationary Energy Storage Systems, which will be discussed later.
Section 706.15 of the NEC requires a readily accessible disconnect for the entire BESS. The disconnect isolates the system in case of an emergency, like a fire or if maintenance is necessary. This disconnect must either be within the BESS equipment or within eyesight (i.e., within 10 feet) of the system. If the disconnect cannot be placed in either of those two conditions, the disconnect must be lockable, ensuring that it cannot be accidentally reactivated.
Section 706.20 of the NEC establishes installation requirements. A BESS must be properly ventilated in accordance with the manufacturerโs recommendations and the systemโs listing. The required ventilation rate to properly cool a BESS depends on the chemical makeup of the battery (e.g., lead-acid or lithium-ion). Additionally, equipment marking and working clearances must meet the requirement in Sections 110.21(B) and 110.26, respectively.
Section 706.30 addresses the requirements associated with the circuits connecting to the BESS. The maximum rated current of a circuit connected to the BESS is defined by the BESS nameplate rating or the output/input current ratings of the inverter or other power-converting device. Conductor ampacity must not exceed the rating of the overcurrent protective device (OCPD) or the BESSโs rated current. OCPDs must be sized for at least 125% of the maximum rated current. Any OCPD on the DC portion of the BESS must be listed for DC applications and must include the DC voltage, current and interrupting ratings. The OCPD location is determined by circuit routing. If it passes through a wall, floor or ceiling, the OCPD must be on the BESS side of the circuit.
Common BESS standards
Several industry standards, guides and certifications apply to BESSs. While a detailed investigation into each is beyond the scope of this article, engineers should familiarize themselves with each to better understand how to properly design, specify, commission, and/or operate a BESS.
IEEE 1547: Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces, establishes the requirements and recommendations for interconnecting distributed energy resources (DERs) โ including BESSs โ with electrical utilities. Many articles analyze IEEE 1547 and its requirements, especially related to its acceptable voltage and frequency tolerances, disconnection and reconnection strategies and system protection.
NFPA 855 applies to a broad range of storage systems, including BESSs, mechanical storage systems, fuel cell storage systems and flywheel energy storage systems. This standard provides general requirements for system interconnections, commissioning and operation and maintenance. It also includes specific requirements for BESSs used in stationary standby power applications (often called electrochemical energy storage systems). NFPA 855 includes requirements for equipment location and spacing, testing and documentation and fire control and suppression. NFPA 855 requires that BESSs be listed to UL 9540, with limited exceptions allowing inverters be listed to UL 1741: Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources and batteries be listed to UL 1973: Batteries for Use in Stationary and Motive Auxiliary Power Applications, establishes UL 1973, even when the entire BESS is not listed to UL 9540.
UL 1741 governs inverter testing and listing for DERs, including BESSs. Compliance with UL 1741 is often required to meet the latest IEEE 1547 requirements. Although the details within UL 1741 are less important to engineersโ day-to-day tasks, so long as the BESS unit is listed, the listing structure can cause confusion: inverters may be listed to UL 1741, UL 1741 Supplement A (SA) or UL 1741 Supplement B (SB). Each supplement improves upon the previous versionโs requirements, adding additional functionality related to safety and grid interoperability. UL 1741 SB is compliant with UL 1741 SA, which is in turn compliant with UL 1741.
In simplified terms, UL 1741 provides requirements for inverters, UL 1741 SA provides requirements for smart inverters and UL 1741 SB provides requirements for smarter inverters that must meet additional testing and interoperability functionality. Depending on jurisdictional requirements and the utility into which the BESS will interconnect, any of the three standards may be required. Currently, most inverters are manufactured to UL 1741 SA or SB requirements.
UL 1973 establishes construction and testing requirements for battery subsystems. Multiple versions of UL 1973 have been released in recent years, but most BESSs are built to the latest version of the standard.
UL 9540 provides requirements for the overall BESS, not just the battery and inverter subsystems. This standard also references UL 1973 and UL 1741.
