Energy Storage System Safety:
Comparing Vanadium Redox Flow and Lithium-Ion–Based Systems

By Matthew Paiss

The field of large-format stationary energy storage systems (ESS) is expected to experience significant growth in all sectors of the power grid, from residential to utility installations. The specific technology and chemistry selected for a particular project takes into account many factors, with safety as a higher priority for many of these design decisions.

This article compares the safety considerations for lithium-ion batteries and vanadium redox flow batteries, and how the systems function and behave.

As of January 2018, there were approximately 732 MW of operational stationary electro-chemical ESS installations behind the meter.1 Drivers for the wide deployment of ESS include both cost reduction and operational resiliency, as well as additional grid services, including, but not limited to:

  • Local and statewide energy storage incentives and mandates
  • Reduced demand charges
  • Load shifting for time-of-use savings or arbitrage
  • Grid support services, such as frequency regulation and ramping needs
  • Smoothing or buffering of intermittent renewable resources (PV or wind)
  • Back-up of electrical loads in the event of outages


Concerns expressed by several groups of stakeholders — property owners, insurance underwriters, fire services, and building code officials — include the risk of overheating through flammable and toxic gas production, thermal runaway, leakage of hazardous materials, and stranded energy in damaged batteries.

Electrical Shock/Arc Flash

Electrical shock presents a risk to workers and emergency responders, since most ESS cannot be “turned off,” with the exception of some flow batteries. Damaged batteries represent the potential for a significant hazard because it is not possible to discharge the stored energy in the damaged cells safely. This is referred to as “stranded energy” and presents unique mitigation hazards.

Arc flash or blast is possible for systems operating above 100 V. Li-ion systems operate from 48 Vdc–1000 Vdc, depending on the battery design. Flow batteries, on the other hand, do not present the same potential for short circuit fault current, and therefore do not create as great a shock or arc-flash hazard when the system is off.


Toxicity or corrosion risks may be present in aqueous electrolytes or from off-gassing produced by over-heating aqueous or vaporized electrolytes. In addition, lithium-ion batteries and flow batteries in fire scenarios may generate toxic gas from the combustion of hydrocarbons, plastics, or acidic electrolytes.


Either aqueous or vaporized electrolytes may create fire hazards. Charging aqueous batteries (including flooded lead acid and AGM) can electrolyze water into hydrogen and oxygen. Battery systems with this hazard should be equipped with exhaust and H2 detection systems.

When li-ion cells are exposed to temperatures over 80°C (176°F), they can generate heat at a faster rate than they are able to dissipate it, presenting a thermal runaway risk. This can occur from a variety of modes, including thermal or mechanical abuse, or manufacturing defects. Thermal runaway fires can produce temperatures above 2000°F while forcefully venting vaporized flammable and toxic electrolyte gases. Gas or aerosol-based fire suppression systems in li-ion battery systems may not be effective at stopping either the thermal runaway process or complete combustion, since cooling — not oxygen reduction — is required to stop either process.

Confined or enclosed spaces may present deflagration hazards when great quantities of flammable gases reach both the explosive range and auto-ignition temperatures, especially since ignition sources also arise due to the electrical nature of the components. Because of the dense configuration of many li-ion cells within modules, prevention of thermal runaway is critical and is one of the primary functions of a battery management system.

Ventilation, Exhaust and Deflagration Venting and Protection

One of the primary concerns with installing li-ion ESS inside structures is the generation of flammable gasses created during thermal runaway and cell venting. Depending on the quantity of cells that enter runaway and the cause and conditions, the volume and type of gasses created can vary widely. Flammable gases produced during overheating can include carbon monoxide, hydrogen fluoride, hydrogen chloride, methane, ethane, ethylene, and propylene.2 Depending on the rate of heating, gas production can be quite rapid and may vent from the cell with significant pressure. In fact, the rate of gas release could exceed the design capacity of the exhaust system.

Fire Suppression

The need for engineered fire suppression systems is challenging for fire protection engineers, since current fire codes provide little in the way of recommendations. As a result, some members of the profession are taking a proactive approach in the early stages, based on limited available test data. Early large-scale fire tests with li-ion ESS have shown that cooling the cells during suppression is critical to terminating the production of flammable combustion gases. However, both cabinet design and rack configuration make active cooling with water more complex.

An installation in an existing building equipped with a fire sprinkler system may still not allow the water to contact modules containing cells on fire. A ConEdison report2 provides recommendations that include a cascading response, where suppression systems may include a gas phase agent for initial discharge and deploying water if heat buildup continues.

Flow Batteries

Flow batteries are typically based on two aqueous electrolytes serving as either the

 anolyte or catholyte, with different charges that are pumped from separate storage tanks across a membrane in a fuel cell. Power is only produced when the pumps and control systems are operating, so there is no risk from “stranded energy” as there is with other electrochemical batteries.

One such technology, the Vanadium Redox battery (VRB), does not represent the same fire or deflagration risk as li-ion based ESS. While not flammable, the electrolyte in VRB systems is corrosive. It is composed of a sulfuric acid-based solution similar to common automotive lead-acid batteries. While very similar to such batteries, VRBs are notably different and deemed safer than lead-acid.

  1. Unlike traditional lead-acid batteries, VRBs do not include lead. Therefore, VRBs do not have the toxicity issues of lead that conventional car batteries have. The only potential source of toxicity in a VRB is when Vanadium is in powder form, but the concentration levels of Vanadium are so low that when it is mixed into liquid form in the final product and put into operation, the VRB is deemed non-toxic. Some VRB batteries may also include hydrochloric acid, but will still be at a similar pH.
  2. VRBs have a lower concentration of sulfuric acid than traditional lead-acid batteries. 

Leaks must be expected in any hazardous-fluid handling equipment. Secondary containment is typically designed into the system and standard corrosive PPE is required for handling liquid. Reliable leak detection, annunciation, and containment is paramount.

In the area of shock hazard, a flow battery produces voltage only when electrolytes are in a cell stack. If the motors are turned off and fluids drain from the cell stack, then the cell stacks have no measurable voltage at the terminals for most designs. Currently, flow batteries are found only in commercial, industrial, and utility-scale applications, but manufacturers are expected to introduce residential flow battery systems in the future. Lifespan operations and maintenance must be factored into ownership models. While flow batteries have lower efficiency and energy density than lithium-ion, they offer longer life and increased safety benefits. 

Matthew Paiss is with Energy Response Solutions.

1U.S. Department of Energy (DOE) Global Energy Storage Database, http://www.energystorageexchange.org.

2Consolidated Edison and NYSERDA. (2017). Considerations for ESS Fire Safety, DNV-GL.