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Fire protection challenges in data centres with Li-ion batteries: a case study
Above As demand grows for data storage systems, so too does the need to ensure fire safety in these facilities. The global pandemic brought with it a whole new set of challenges, forcing companies and schools to operate remotely and increasing demand for rapid access to information and data—estimated at 79 zettabytes in 2021.
KAVEH SOLEIMANI DEILAMANI
Aurecon
The growth of data centres and the use of new sustainable energy sources and battery storage has increased the fire protection challenge for these facilities.
Challenges of Li-ion batteries
Lithium-ion batteries (LIBs) are booming, particularly in both enterprise and hyper-scale data centre facilities in Australia, due to their high energy density, low maintenance, low selfdischarge, quick charging and longevity. However, the thermal stability of LIBs is relatively poor—their failure may cause fire and, under certain circumstances, explosion. This fire risk hinders the large-scale application of LIBs in electric vehicles and energy storage systems5 .
FM Global identifies thermal runaway (TR) as the major hazard for LIB energy storage systems (ESS) arising for different reasons, such as overcharging or over voltage. The heat generated from a single-cell fire has the potential to initiate TR in adjacent cells.
This process can be exacerbated by the LIB size, chemistry, condition, use and so on, and can be a challenge to control7. This is because the chemical reaction continuously creates oxygen and because of the cascading heat effect of one cell causing another to go into thermal runaway.
The organic electrolytes in many LIBs are highly flammable when heated, and fires can result in temperatures in excess of 1,200°C, releasing hot and corrosive solid compounds and toxic gases.
National Construction Code (NCC) and Australian standards do not cover the requirements of the fire prevention, detection and protection of LIBs, although Clause E1.10 and E2.3 of NCC 2019 Amendment 1 requires suitable additional provisions and smoke hazard management measures if special problems of fighting fire could arise because of the nature or quantity of materials stored or used in a building.
A multi-layer protection strategy that includes early detection and suppression may be the best solution for LIB fires.
The selected fire suppression methods should subdue any LIB fire and control any rise in battery temperature. Otherwise, TR reactions may continue and the battery could re-ignite, or it could spread to adjacent cells—a major challenge for LIB fire suppression systems. To control such fires, it is more important to cool the cells in a large battery pack, to prevent heat propagation, than to extinguish fires from a single cell.
At the moment there is no one solution for LIB-based fires, although water has been historically recommended because of its ability to cool the system.
Water spray can effectively extinguish LIB fires and reduce the maximum surface temperature of batteries, inhibiting TR propagation. However, the high conductivity of water may cause short circuit of cells and generate off-gases.
FIGURE 1: SCENARIO 1 FIRE GROWTH FIGURE 2: SCENARIO 2 FIRE GROWTH
SOURCE: KAVEH SOLEIMANI SOURCE: KAVEH SOLEIMANI
CASE STUDY: FIRE SPRINKLER SYSTEM DESIGN
While most clients and end users prefer not to have water spraying on electrical equipment in data halls and to deter the activation of sprinklers as much as practicable, fire sprinklers are still considered to be one of the most effective fire suppression methods.
In this case study, double interlock fire sprinkler pre-action systems were used to mitigate the risk of spraying water on electrical equipment.
AS 2118.1-2017 does not provide any specific requirements for the use of LIBs in a building. It classifies a data centre as Ordinary Hazard 3 (OH3), regardless of whether the data hall contains LIBs. By comparison, the National Fire Protection Association (NFPA) and FM Global (FMG) provide specific guidance on ESS with LIBs and specify discharge densities over area offering higher flow demands.
Even if NFPA or FMG are not specified for a data centre, more reliable fire control will be achieved if the discharge density of the fire sprinkler system follows NFPA or FMG requirements, with the remaining features of the fire sprinkler system following local codes and standards as much as practical for the ease of installation, constructability and maintainability.
The fire sprinkler system for this case was designed to NFPA 13 as the main dominate design code; however, requirements in the AS 2118.1-2017 also needed to be considered.
Data hall layout and fire scenarios
A centralised fire protection infrastructure, comprising on-site storage tanks and pumps, was considered for the data centre facility. The fire water storage tanks were combined with the fire hydrant system, but a dedicated fire sprinkler system with booster assembly, pumps and ring main were also included in the design.
For the fire sprinkler studies, two scenarios were identified, based on the possible fire growth in the data hall and the layout of the cold and hot aisles (in practice, it is not possible to evaluate all possible fire scenarios, so only two scenarios have been studied).
The fire scenarios in this study were not determined based on the Event Tree analysis (or equivalent methodology) and only two possible scenarios that have more impact on the activation of the sprinklers were considered, as the purpose of this study was not to establish the fire scenarios.
Due to the layout of the hot and cold aisles, the requirements of Clause 10.2.3 of AS 2118.1-2017 were not applicable to this data hall. The classification of the data hall with the LIBs within the racks was evaluated Extra Hazard Group 1 and Ordinary Hazard 3 in accordance with NFPA 13 and AS 2118.1, respectively.
The scenarios are as follows:
Scenario 1
A fire breaks out in the cold aisle and propagates to another cold aisle, as indicated in Figure 1. In this study, the activation of the sprinkler system in the hot aisle areas was excluded in the calculation, as it has been assumed fire developed to the adjacent cold aisle faster.
In accordance with NFPA 13, the required discharge density for the Extra Hazard Group 1 sprinkler system is 12.2 millimetres per minute at 230 square metres. However, an additional 30% needs to be considered for the sprinkler system area of operation, as per Clause 20.13.3 of NFPA 13. The minimum flow of 146.4 litres/min is required per activating head based on the coverage of the sprinkler heads as per NFPA.
In accordance with Clause 27.2.4.7.3.1 of NFPA 13, sprinklers under the obstruction shall not be required to be included in the hydraulic calculation of ceiling sprinklers. For this reason, only sprinklers in the cold aisles have been included in the hydraulic calculation.
PHOTO: LICHESS.ORG Lithium-ion batteries are booming, particularly in both enterprise and hyperscale data centre facilities, but they pose a significant fire hazard if thermal runaway occurs.
TABLE 1. OUTCOMES OF SCENARIOS 1 AND 2 IN THE FIRE SPRINKLER SYSTEM DESIGN CASE STUDY
Scenario 1
Scenario 2 Code
Hazard classification
NFPA 13 Extra Hazard Group 1
AS 2118.1 Ordinary Hazard 3
NFPA 13 Extra Hazard Group 1
AS 2118.1 Ordinary Hazard 3
Discharge density 12 mm/min over 300 m2
N/A*
12 mm/min over 300 m2
N/A
Minimum flow per head (L/min)
Number of activating heads
146.4 26
60
146.4 (Cold aisle) / 103.7 (Hot aisle)
60 18
38
18 Total calculated flow (L/min)
3980
1142
5635
2687
*AS 2118.1-1999 required discharge density of 5 mm/min @ 216 m2 for the OH3 sprinklers. This figure has only been mentioned for the comparison and it is no longer applicable based on the new code.
Scenario 2
Scenario 1 was based on the hypothesis that fire only propagates within the cold aisle.
Scenario 2, however, has assumed that fire penetrates to the hot aisle faster than it goes to the adjacent cold aisle contrary to AS 2118.1-2017, requiring the sprinkler heads in the void area not to be included in the hydraulic calculation.
Therefore, the sprinkler heads in the hot aisle would also be activated. This has been shown in Figure 2 (page 37). As the spacing of the sprinkler heads in the hot aisle is different (still compliant with the requirements of NFPA) for these heads, the minimum flow demand per sprinkler head is 103.7 L/min.
Results and discussion
A series of hydraulic calculations has been conducted to compare the results of the required demand for the sprinkler systems for the two scenarios, based on NFPA and the Australian standard.
The demands were calculated by simulating the sprinkler pipework reticulation with HYENA+. The sprinkler system demand using NFPA 13 methodology was also calculated using AutoSprink RVT 2021, which resulted in 3,963 L/min and 5,605 L/min for Scenario 1 and 2, respectively.
The results show the tremendous difference in the water supply demand for the sprinkler system between NFPA 13 and AS 2118.1-2017.
Despite this, there are no available prescriptive and regulatory requirements for the suppression of a LIB fire and no test results are yet available for the data centres, so authorities such as local fire brigades tend to require a fire sprinkler system with enhanced water supply where LIBs are being stored.
The enhanced water supply will not be achieved using Australian standards because AS 2118.1-2017 does not allow extra controlling measures for a building containing LIBs. To achieve the authorities’ requirements, sprinkler system demand may need to be worked out based on the higher water flows of international codes, such as NFPA 13, and storage of a minimum four hours may also need to be considered, similar to what is required for the fire hydrant systems in accordance with AS 2419.1.
As illustrated by the results, Scenario 2 demands a considerable amount of water compared to Scenario 1, which would have a major impact on the fire protection infrastructure, including tanks, pipes and pump sizes. Relevant studies may need to be considered by fire safety engineers to evaluate the risk of fire growth to the void areas (hot aisle) in data halls with similar arrangements.
The fire sprinkler pre-action system needs to be activated by the smoke detectors in the data halls, which should be determined based on the air exchange rate in the room. For the data hall of this study, a multipoint aspirating smoke detection system (MASD) should be specified.
If a client prefers a double-knock detection operation, two independent smoke detectors should be used in the room. However, in accordance with AS 1670.1-2018, only one detector in each area (the first knock) will be compliant to the local codes and the activation of the first knock (regardless of sensitivity) shall trip the Alarm Signalling Equipment (ASE) and call out the fire brigade.
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ACKNOWLEDGEMENT
The author would like to thank Trinh Lee, Hydraulic Engineer in Aurecon’s Vietnam office, for his help on the calculations of the sprinkler system using AutoSPRINK RVT 2021.