Mobile Energy Storage Systems
Mobile energy storage systems are being deployed in jurisdictions around the world, and—as demonstrated by a 2023 New Year’s Day mobile energy storage system fire—accidents can happen. We want to make sure communities are prepared for when these systems are deployed in their backyard. This blog will outline key considerations for mobile energy storage systems. To see the full requirements, check out the latest edition of NFPA 855, Standard for the Installation of Stationary Energy Storage Systems.
What is a mobile energy storage system?
An energy storage system (ESS) is a group of devices assembled together that is capable of storing energy in order to supply electrical energy at a later time.
A mobile energy storage system is one of these systems that is capable of being moved and typically utilized as a temporary source of electrical power. In practice, this is often a battery storage array about the size of a semi-trailer. Mobile energy storage systems can be deployed to provide backup power for emergencies or to supplement electric vehicle charging stations during high demand, or used for any other application where electrical power is needed.
While there are various types of ESS and many battery technologies, this blog will focus on the most prevalent type—lithium-ion battery energy storage systems. Many of these requirements apply to any type of mobile energy storage system; see NFPA 855 requirements for details on other technologies.
When does NFPA 855 apply to mobile energy storage systems?
The scope of NFPA 855 states that it applies to “mobile and portable energy storage systems installed in a stationary situation.” It also goes on to mention that the storage of lithium-ion batteries is included in the scope of the document.
The application section then limits the application of the standard to certain-sized mobile energy storage systems. For all types of lithium-ion batteries, the threshold is 20 kWh (72 MJ) before the requirements of NFPA 855 apply. For batteries in one- and two-family dwellings and townhouse units, that threshold is reduced to 1 kWh (3.6 MJ). For more information on residential ESS requirements, check out our previous blog on that topic.
When looking at how a mobile energy storage system works, we break its use down into three phases: the charging and storage phase, the in-transit phase, and the deployed stage. This is how I’ll break down the requirements as well.
Charging and storage
When charging and storing a mobile energy storage system, the requirements are relatively straightforward. The system should be treated as a stationary system as far as the requirements of NFPA 855 go. These requirements will vary based on whether the system is being stored indoors, outdoors, on a rooftop, or in a parking garage.
In-transit
While a mobile energy storage system is in transit from its normal charging and storage location to its deployment location, it typically travels on roads that are governed by the governmental transportation authority (in the US, that would the Department of Transportation). However, when the mobile energy storage system needs to be parked for more than an hour, it needs to be parked more than 100 ft (30.5 m) away from any occupied building, unless the authority having jurisdiction (AHJ) approves an alternative in advance.
Deployment documents
Before a mobile energy storage system is deployed, it needs to be approved by the AHJ, and a permit must be obtained for the specific use case. The permit application must include the following items:
Mobile Energy Storage System Permit Application Checklist
o Information for the mobile energy storage system equipment and protection measures in the construction documents
o Location and layout diagram of the area in which the mobile energy storage system is to be deployed, including a scale diagram of all nearby exposures
o Location and content of signage
o Description of fencing to be provided around the energy storage system and locking methods
o Details on fire protection systems
o The intended duration of operation, including connection and disconnection times and dates
o Description of the temporary wiring, including connection methods, conductor type and size, and circuit overcurrent protection to be provided
o Description how fire suppression system supply connections (water or another extinguishing agent)
o Maintenance, service, and emergency response contact information.
Deployed
There are restrictions on where mobile energy storage systems can be deployed. For example, they are not allowed to be deployed indoors, in covered parking garages, on rooftops, below grade, or under building overhangs. There is also a restriction on how long mobile energy storage systems can be deployed before they need to be treated as a permanent energy storage system installation, and that threshold is 30 days.
Additional limitations for where a mobile energy storage system can be deployed include a 10 ft (3 m) limitation on how close it can be to various exposures and a 50 ft (15.3 m) limitation on how close it can be to specific structures with an occupant load of 30 or greater. See NFPA 855 or the image above for more details on the exposures and occupancies.
An energy storage system contains a large amount of energy stored in a small space, which may make it the target for those who look to cause harm. For this reason, a deployed mobile energy storage system is required to be provided with a fence with a locked gate that keeps the public at least 5 ft (1.5 m) away from the ESS.
Conclusion
There are many applications where mobile energy storage systems can play a pivotal role in helping deliver electricity to where it is needed. While this technology has great practical applications and even more potential, it’s important to recognize that it also brings unique hazards. Adherence to the requirements of NFPA 855 can help keep our communities safe while embracing current technology.
Here are some additional NFPA® resources related to ESS safety:
- Energy storage system landing page
