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ENERGY STORAGE SYSTEMS Considerations for ensuring battery energy storage systems safety

From Nigel Charig

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The Battery Energy Storage Systems (BESS) market is growing rapidly worldwide and is expected to reach up to 1TWh by 2025. This growth is driven by the ever-expanding use and penetration of renewables and the drive for decarbonisation. With this growth comes a need to ensure the safety and reliability of such systems.

The Battery Energy Storage Systems (BESS) market is growing rapidly worldwide and is expected to reach up to 1TWh by 2025.
The Battery Energy Storage Systems (BESS) market is growing rapidly worldwide and is expected to reach up to 1TWh by 2025.
(Source: gemeinfrei / Unsplash)

This topic was discussed in a paper presented by iKenneth Rush and iiAhmed Elasser at the PCIM Europe Digital Days Conference, July 2020; we look at the paper’s key points below.

Variable and intermittent renewable energy sources such as PV Solar and Wind continue to grow in popularity – a trend that is expected to continue as battery costs decline and regulatory conditions ease. And the BESS is increasingly recognised as the ‘missing link’ in the electric power grid supply chain for such renewables, as it facilitates dispatchability and grid stability.

As with other markets, safety standards and best practices are vital for industry growth and maturity. Proper system design, component validation, and operating procedures are essential in reducing the risk of a safety incident and protecting life and property, while limiting propagation, should an incident occur. As with any complex system, emphasis must be on human safety first, followed by equipment. In the utility segment, where high voltage, high power, and high energy are ever-present, standards, regulations, safety operating procedures, and protection equipment are essential to ensuring industry growth.

BESS system components

The basic BESS system building block is the battery cell and module; for utility grid storage, Li-ion technology is typically deployed, due to its high power density, maturity, availability, and prevalence. Li-ion BESS systems need battery management systems (BMSs) at the cell, module, and string level. Cell BMSs are important in protection against issues like overvoltage, overcurrent, imbalance, and overheating.

Each battery module is also equipped with its own aggregate BMS system which monitors its overall safety and health as well as protecting it against overvoltages, overcurrents, and overheating. It also provides users and operators with important information such as State of Health(SOH), State of Charge (SOC), and charge/discharge voltages and currents.

Each module is also equipped with a fuse for short circuit protection.

BMSs are also increasingly provided at the battery string level to protect the string, and are commonly referred to as rack BMSs. Further protection levels are provided by other BESS elements. For example, power conversion equipment such as dc-dc converters and inverters provide additional short circuit protection and allow disconnection of the battery system to perform maintenance upstream.

Additional subsystems such as medium-voltage transformers, switchgear, purpose-built enclosures/containers, and unit controllers also contribute critically to ensuring system safety. Leading-edge BESSs integrate many of these subsystems into a single container to reduce cost, improve safety, and deliver a high-performance, reliable storage asset.

Although early BESS systems tended to be AC-coupled, the trend is now towards DC coupling as storage is co-located with solar and wind generators.

Common hazards and safety considerations

As for any industrial equipment, there are several common hazards and safety considerations associated with BESS systems. While those associated with Li-ion batteries are easily the most significant, many other hazards also require consideration. These can arise throughout the system’s operational life, from installation, to loading, commissioning, energisation, operation, and maintenance.

There is always the danger of personnel or equipment falling during battery module installation, so appropriate tools like forklifts, PPE and if appropriate restraining belts should be employed. PPE should also be used to protect technicians’ hands from pinches or cuts during installation and maintenance.

If Li-ion batteries are accidently exposed to high temperatures or fail due to electrical/mechanical problems or short circuits, excessive heat is generated within the battery module. If unprotected, this can create a thermal runaway condition and the release of flammable gases, battery deflagration, and, ultimately, fire and catastrophic failure. Any gases released are toxic as well as flammable, so can be harmful or even cause asphyxiation.

Additionally, BESS systems – which combine AC and DC voltages and currents – present a significant potential for electric shock or electrocution through accidental touching, or arc flash events.

To assure customer acceptance and a smooth maturation of this new technology and market, safety and reliability must be built in to all four phases of the BESS life cycle; from initial design and creation, through manufacturing and operational life, to end-of-life recycling.

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During the design phase, for example, considerations should include battery component design aspects such as cell electrodes, separators, current collection, and manufacturing processes, as well as containers and power conversion subsystems.

Designers, operators, and maintenance crews should also be given tools such as potential incident failure mode listings and mitigation steps to implement when an incident occurs. This Failure Mode and Effect Analysis (FMEA) is essential in developing strategies to deal with faults like short circuits and thermal runaways.

Standards and regulations

Impact of regulatory standards
Impact of regulatory standards
(Source: Kenneth Rush)

Standards and regulations are needed to allow for equipment interchangeability and to help customers, system integrators, and Authorities Having Jurisdiction (AHJ) to make informed decisions around proper risk mitigation. The Table shows the most relevant standards.

The standards listed above are centred around equipment safety. An Accidents Scenario Review (ASR) should be performed to address human safety during service and equipment interaction.

Assuring the future of battery energy storage

Energy Storage is still a small portion of the grid in Europe and North America with over 95 percent of the installed base being Pumped Hydro. Battery energy storage is a small part of overall grid storage, but it is growing by double digits annually. Of this growing base over 80 percent of currently installed and planned BESSs is Li-ion based.

While there are competing technologies, Li-ion is by far the market and technology leader due to its higher energy density, smaller weight and size, flexibility, and ease of installation. It does have certain disadvantages and issues, especially with respect to safety and reliability. In this article, we have looked at these issues within the context of BESS systems, together with ways to mitigate them.

Paper Authors for the PCIM Europe Conference 2020

iKenneth Rush was previously with GE Hybrid Renewables, Niskayuna, NY, USA
ii Ahmed Elasser is with GE Research, Niskayuna, NY, USA