Uninterruptible power supply (UPS) systems provide two functions: continuous power conditioning to the IT load, and short-term power to bridge the gap while the emergency generators are coming online following the loss of the utility supply.
In the context of data centres operating as bidirectional microgrids, two key questions arise. Which battery technologies are most suitable as Battery Energy Storage Systems (BESS) and what are their sustainability credentials?
About Battery Energy Storage Systems
For grid interaction, the BESS consists of three main components:
- Battery Management System
- Energy Management System
- Power Conversion System
The data centre sector has traditionally used lead acid batteries with static UPS system, but that situation is gradually changing. According to a Frost and Sullivan 2021 report, lithium ion batteries will, by 2025, account for 38.5% of data centre energy storage. Its growing popularity is reportedly due to its durability and smaller footprint; Li-ion achieves ten times the number of recycles compared with traditional lead acid batteries, which although are cheaper to acquire need more frequent replacement and are both bulkier and heavier.
However, in sustainability terms for the data centre sector, it is not a straight fight between Li-ion and lead acid. There are challenges with the use of lithium throughout its lifecycle, from its extraction which utilises high volumes of water to recycling constraints. Lead acid batteries, by comparison, benefit from a long-established recycling supply chain which can recover more than 98% of components.
In the white paper, Sustainability Considerations for Battery Option Selection in Data Centre Energy Storage System Clayton Lim, Associate Director at i3 Solutions Group and a contributing member of the GHG Abatement Group, explores the main factors that influence decision-making associated with current battery storage technologies together with important sustainability indices that ought to be considered. It examines the following battery types:
- Sodium-sulphur (NaS)
- Sodium-nickel-chloride (NaNiCl)
- Nickel-cadmium (NiCd)
- Vanadium Redox Flow Battery (VRFB)
- Zinc Bromine Flow Battery (ZBFB)
- Lithium-ion (Li-ion)
Emerging battery types which logically could be considered viable alternatives to Lithium-ion include technologies such as the Vanadium Redox Flow Battery (VRFB), metal-air battery and sodium-sulphur battery.
The white paper states, “Liquid-metal batteries appear to be a potential gamechanger in the various UPS types given its superiority in upfront system cost, operating cost, cycle life, response time, footprint and geographical dependency.”
New pressure and regulatory concerns make battery choice increasingly important
Mandates such as SECR (Streamlined Energy and Carbon Reporting), CSRD (Corporate Sustainability Reporting Directive) and EED (Energy Efficiency Directive) will oblige data centre companies to report their sustainability efforts; this will also cover battery choices.
Significant factors include energy density which refers to the amount of energy that is available for storage in certain areas, volume, or mass. This, in conjunction with power density and the right battery type, determines the most suitable battery technology for optimal system selection.
The focus of the new white paper is on the sustainability characteristics of various chemical batteries for BESS requirements. However, it should be noted that other energy storage options are available which may be or become applicable for data centre power requirements. These include; kinetic flywheels, compressed gas storage, and potentially pumped hydro, tidal current and gravity storage.
Criteria for sustainable battery choices for BESS
The new white paper concludes that many interacting factors should be considered when selecting the appropriate UPS BESS. These include the type of application, sustainability performance indicators, investment and revenue return opportunities, technical performance, and location factors. Critically, the environmental impacts of a battery technology must be taken into consideration from a whole life point of view.