Is a battery energy storage system right for your project?

Learning objectives

• Understand what a battery energy storage system (BESS) is.
• Learn how a BESS is applied to an electrical system.
• Comprehend why a BESS is uniquely suited to renewable energy applications.

BESS insights

• Battery energy storage systems (BESS) are rechargeable batteries that can store energy from various sources and distribute it on demand for energy management purposes.
• BESS can be useful without renewable sources, but they are uniquely suited for the incorporation of renewable sources into electrical systems.

Battery energy storage systems (BESS) are current candidates for cleaner energy in providing power for electrical distribution systems. During design for projects, electrical engineers need to have a basic understanding of the components, applicable applications and benefits that BESS may have on new and existing electrical systems.

BESS can be incorporated into several different types of applications and in conjunction with renewable and distributed generation technologies and the utility grid. Solar photovoltaic (PV) (see Figure 1) and wind energy sources are inherently intermittent when applied in nonutility scales as the energy they harness from the world around them waxes and wanes. BESS can smooth these intermittent sources into more reliable, steady sources of power, helping to maximize the benefits of solar PV and other renewable energy technologies by reducing power fluctuations on the system.

BESS can be used strategically for cost-saving and backup power applications. BESS will also be an important part of future grid plans for distributed energy resources as localized generation increases.

BESS codes and standards

The following codes and standards should be considered when designing a BESS.

• International Building Code.

• International Electrical Testing Association.

• International Fire Code (IFC) Section 1207.

• NFPA 70: National Electrical Code Articles 705 and 706.

• NFPA 72: National Fire Alarm and Signaling Code.

• NFPA 855: Standard for the Installation of Stationary Energy Storage Systems.

• National Electrical Safety Code.

• Institute of Electrical and Electronics Engineers (IEEE).

• IEEE 1547: Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces.

• IEEE 2800: Standard for Interconnection and Interoperability of Inverter-Based Resources (IBRs) Interconnecting with Associated Transmission Electric Power Systems.

• UL: 1741 Inverters, Converters, Controllers and Interconnection System Equipment for Use With Distributed Energy Resources.

• UL 9540: Energy Storage System Requirements.

It should be noted that the above codes are still catching up to the state of the market. The primary concern of the codes is fire safety. IFC (2021) Section 1207 and NFPA 855 are some of the more recent codes written specifically for energy storage systems.

Components of a BESS

A BESS comprises several main components. Each component within the BESS could be its own discussion, but for this article, they will be briefly discussed with a general overview. There are two main configurations of BESS, container and cabinet, both of which incorporate the major components of a BESS as discussed within this article.

Figure 1: A floating solar photovoltaic array used in conjunction with a battery energy storage system. Courtesy: CDM Smith

Container configurations are preconfigured with all components integrated into a single container (see Figure 2). Shipping containers, similar to those on cargo ships, are typically used as enclosures for container configurations with the same standard sizes. In general, the nominal output power for these types of units can range from 250 kilowatts (kW) up to 2 megawatts (MW), depending on the manufacturer and are more common for large-scale facilities or for electric utilities.

The components within these containers are all provided and tested by a single manufacturer to ensure conformance with equipment and project requirements. These containers can also be paralleled together to increase the overall capacity required by the system being served. These types of containers are better used for large-scale applications.

Cabinet configurations are commonly built at the project site and components can be provided from several manufacturers. Because of this, these types of configurations require more coordination of system components to ensure the entire system is properly functioning. As with the container type configuration, all the components are housed in these individual cabinets. The enclosure of this type would be like an outdoor enclosure for a motor control center or other comparable types of equipment.

Battery system: The battery is the most critical component of the BESS because this is the piece of equipment that will store the energy to provide power. When thinking about batteries, the immediate thought may be to jump to household AA batteries that many small electronics use.

However, while the batteries for the BESS are fundamentally similar (storing chemical energy that is converted to electrical energy for power consumption), they are constructed and packaged differently. The main components that comprise the overall battery are battery cells, which are connected in series to form modules. The modules are then combined in parallel to form racks (see Figure 3), with several racks being connected in series or parallel to create the energy storage system with the required capacity, voltage and current of the system.

Figure 2: Battery enclosure, transformers, switchgear and inverters: all part of a battery energy storage system. Courtesy: CDM Smith.

Lithium-ion batteries are most prevalent for these types of systems. Other battery technologies are being developed for better use in BESS, so this discussion of batteries may differ in the coming years. Following are the two main types of battery chemistries.

Lithium nickel manganese cobalt oxide (NMC) is the battery chemistry typically used in the automotive industry. It is used in stationary applications as well because of its high energy density. This high energy density allows the battery to have a smaller footprint without losing the ability to store high amounts of energy. Compared to other lithium-ion chemistry discussed below, NMC has a lower thermal runway temperature and is considered less thermally stable. The cost for NMC can also be more expensive because it requires rare earth materials.

Lithium iron phosphate (LFP) is the battery chemistry more commonly used in stationary applications because it has a lower energy density than NMC. LFP has a higher thermal runaway temperature and is more thermally stable, which makes it a better option for applications where fire safety may be a concern. LFP also has a lower cost as compared to NMC because LFP does not require rare earth materials for construction and is able to have higher maintained charge.

BESS management and conversion

Figure 3: Individual battery cells make up the larger battery energy storage system. Courtesy: CDM Smith

The battery management system (BMS) provides controls and monitoring for the battery packs, typically within each module. This system serves several important functions, including:

• Monitoring individual battery cells in each module in real time.

• Maintaining battery operation in normal and safe states.

• Provides battery protection and optimizes battery performance.

Within electrical systems, there are two different types of power: alternating current (ac) and direct current (dc). ac power is typically used in power distribution systems to provide power for a wide range of equipment, from homes to industrial-type applications.

However, dc power is typically used in power electronics (e.g., laptops, batteries, renewable energy systems) and only used for highly specific distribution/transmission applications. The batteries for the BESS operate and store energy as dc power. To allow facilities such as homes, office buildings, industrial applications to use the BESS, an inverter or power conversion system is required to convert the dc to ac power. These inverters are bidirectional and allow the ac power to be converted to dc power as well to allow charging of the batteries.

The energy management system (EMS) acts as the overall controller for the BESS, not to be confused with the controller for the BMS that is solely for batteries. This is typically used in microgrid-type applications, but it is not required for all types of BESS, such as a BESS operating independently from other power sources on an electrical system.

Operation of the EMS is achieved by having the EMS communicate with the entire system — including the battery system, inverter and power system — to determine when to charge and discharge power. This is primarily determined by the type of application as discussed in the following paragraphs.

Although components such as heating, ventilation and air conditioning , fire suppression, transformers and cabling are essential to the success of a fully functional BESS and for providing additional protection of the system, they are not discussed in detail within this article.

Applications and benefits of BESS

BESS may offer a level of independence from the utility grid and can be used with renewable generation systems and traditional fossil fuel-based generation systems in residential/commercial/industrial distribution systems, microgrids or electric utility systems. The following applications are written with a renewable integration focus in mind.

• Renewable smoothing: If a large cloud passes over a solar field, a PV array can lose significant power output. If the wind ceases to blow, a wind turbine will lose significant power output. The electrical system (e.g., building or industrial plant) being powered from these sources will immediately need power from the utility. This type of on-and-off surge can negatively affect an electrical grid and, depending on the utility, may not be allowed for interconnection. A BESS can smooth these fluctuations by acting as an in-between source, automatically injecting electrical energy when a renewable source temporarily loses power.

• Peak shaving. Peak demand can significantly affect many electrical utility bill costs. It is typical to see this peak used as the billing rate for an entire month of usage, even if the peak was only attained for a few minutes. BESS can be designed to target these peaks and reduce them by taking on chunks of the demand load that would otherwise be billed by the utility. Similar techniques include scheduled power or commanded power, where the energy market is monitored to determine when it would be most cost-effective to use on-site-distributed generation to power facility loads in lieu of the utility source. This means that BESS can be used to reduce energy costs even without an on-site generation source (e.g., solar, wind or fossil fuel-based generator). The batteries charge when energy costs are low and supplement facility power (discharge) when utility demand costs are high.

• Emergency/standby backup. BESS can also be used as a backup power source upon loss of utility power. This practice is referred to as “islanding.” To accomplish this, the system complexity is enhanced. Using some additional components, the system would need to take measurements, perform automatic switching and trip/reclose circuit breakers. Islanding using BESS is more practical for smaller energy consumption facilities and some BESS require another source (such as a diesel generator) to provide a voltage synchronization source. A properly sized backup battery system can provide complete independence from utility grid interruptions. However, unexpected islanding can cause major issues to an electrical system that is not prepared for it. Utility workers and maintenance staff should always be made aware of a system that can be islanded during utility power outages. Utilities may require extra interconnection stipulations to enable safe isolation.

BESS are commonly sorted into three categories based on size. All three sizes are applicable to renewable energy systems. Front-of-the-meter or utility-side BESS can range upward from 10 megawatt-hours (MWh) into the hundreds of MWh. Behind-the-meter or customer-side commercial and industrial BESS can range from 50 kilowatt-hours (kWh) to 10 MWh. Behind-the-meter residential are generally less than 50 kWh. Power system study requirements from utilities are often a significant barrier to interconnection. The complexity and cost of these interconnection requirements increases with the size of the BESS.

A graphic example of how renewable energy is captured in excess against facility demand is shown in Figure 4.

Along with the applications discussed above, BESS provide some not-so-obvious benefits. In addition to cost savings and the potential for energy independence, BESS helps reduce greenhouse gas emissions when used in conjunction with renewable energy power sources. This has a real impact on the social cost of carbon, a concept of the real dollar cost of damage done to society by fossil fuels. Unused energy from a renewable source will otherwise be wasted if a battery system is not present and the demand is not high. Overall energy efficiency is improved with this practice and, thus, a reduction in power provided by nonrenewable sources may be achieved.

One trend coined by the uniquely name “duck curve” could be corrected with the widespread use of BESS. The curve resembles a sitting duck and portrays the timing imbalance between peak demand and drop-off of solar production per day. Peak demand generally happens around sunset each day, which is when solar power starts to wane.

Batteries are also sources of immediate backup power. Unlike a gas-engine generator that needs time to get up to speed, a charged battery can provide rated power as soon as it is switched to, allowing for more seamless transfers.

Maintenance and lifetime expectancy

Figure 4: A graphic example of how renewable energy is captured in excess against facility demand. Courtesy: CDM Smith

The BMS is the main hub for operations and maintenance information. However, some maintenance items are unique to battery systems. The BMS can automate these items if configured properly.

• Occasional power cycling: Both main types of lithium-ion batteries will degrade faster if kept at 100% charge all the time. If a BESS system at a facility is only used for backup and rarely gets discharged, it could lose maximum capacity faster than the average expected life of 5 to10 years.

• Temperature performance: A cold battery does not perform as well as a warm battery. Most manufacturers will specify their batteries at an ideal operating temperature of 60° to 80°F. At this temperature, the battery can fully charge and discharge at rated specifications. However, most BESS will live at temperatures higher or lower than this. Both extremes can shorten the life of the battery, but cold temperatures specifically will also hurt performance. BESS perform best in temperature-controlled environments that can be set by an operator or a BMS.

A battery’s function and operating life can significantly impact its expected life. A 10-year life is possible in ideal conditions while a five-year life is more likely in adverse environments (i.e., extreme ambient temperatures, frequent full discharges).

The end of life does not necessarily mean that the battery completely fails; it simply does not achieve its rated amp-hour capacity. A cause of complete failure in lithium-ion batteries would be thermal runaway. Thermal runaway is a condition that begins when heat dissipating from a battery is less than the heat being generated by the battery. The increase in heat can increase the reaction rate and vice versa causing the runaway condition. High ambient temperatures and age of the battery are likely culprits in these scenarios.

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