By Dorota Brzezinska
Abstract
Charging most industrial lead-acid batteries leads to hydrogen gas being emitted. In the absence of an adequate ventilation system, this causes hazards of explosions, especially if the batteries are located in a relatively small enclosure.
The article describes full-scale tests and CFD simulations of hazardous conditions that can occur in a non-ventilated battery room. It also demonstrates that different ventilation systems for battery rooms can provide different levels of effectiveness of hydrogen clearance.
Introduction
During the charging process for lead-acid batteries, gases are emitted from the cells. This is the result of water electrolysis, which produces hydrogen and oxygen. In accordance with the standard BS EN 62485-20141, the hydrogen concentration in battery rooms should be kept below 4% (by volume) of Lower Explosive Limit (LEL). This requirement can be reached only with the proper ventilation system. Basic parameters for such a system can be defined by the BS standard recommendations1.
Hydrogen is odorless, colorless and tasteless, so human senses cannot detect high concentrations of this gas. Hydrogen is also lighter than air and diffuses rapidly (diffusion coefficient equals 61.1 10-6 m2/s)2. The graph in Figure 1 shows densities of sample gases relative to the air2-5.
Figure 1: Densities of sample gases relative to the air.
The explosive limits of hydrogen vary between 4% and 74% and also have very significant parameters, in accordance with hydrogen explosive hazards. Figure 2 shows a comparison of explosive range for different flammable gases2-5.
Figure 2: Explosive range of sample gases.
Real-Scale Experiment Layout
This experiment involved measuring hydrogen dispersion in a small enclosure (20 m3). The real hydrogen gas was fed from a cylinder and supplied to the enclosure through a box imitating a battery. The upper surface of the box had 21 openings of 6 mm diameter each. Hydrogen concentration was measured by using the catalytic sensors at six heights over the emission source. The volume flow of the gas represented a flow similar to that during batteries’ charging process (Figure 3).
Figure 3: Measurement layout for hydrogen dispersion.
Experiment Results
The measurement results showed that hydrogen filled the entire room space very evenly, and the concentration under the ceiling and in the lower parts of the room expanded in almost the same time (Figure 4). The measurements show that hydrogen does not accumulate below the ceiling of the battery room, as was previously expected.
Figure 4: Hydrogen concentration above the emission source.
CFD Analysis of Ventilation System Effectiveness
Through the use of simulations, it has become possible to see the influence of ventilation on hydrogen dispersion in a battery room. The analysis was carried out using a real battery room as an example, as described above. As a model for analysis, the experiment assumed a battery room with a total volume 20 m3, in which 20 open lead batteries with a capacity of 2100 Ah each were powered. The calculations were based on the requirements outlined in the standard BS EN 62485-20141.
The first step in calculating hydrogen emission from the batteries was estimated as 0.35 m3/h simulation1. The simulations were realized with Fire Dynamic Simulator (FDS) version 6.3.2. The simulation results were compared with previously calculated theoretical emission time when hydrogen concentration reached the threshold points of 10%, 40% and 100% of LEL in the entire non-ventilated battery room (Table 1).
Table 1: Increase of hydrogen concentration in non-ventilated battery room.
The FDS simulation results presented in Figure 5 confirmed that the increase of hydrogen concentration in the battery room occurs uniformly over the entire enclosure, at a rate similar to the time calculated theoretically. However, because the lower part of the enclosure stays free of hydrogen, the simulated time of increasing hydrogen concentration was a little slower than calculated theoretically. The red color on the scale means the concentration of hydrogen is 100% LEL, which was equal to 3,4 x 10-3 kg/m3.
Figure 5: Hydrogen condensation increasing in the non-ventilated battery room.
After confirmation of proper FDS simulation of hydrogen dispersion phenomena, both mechanical and natural ventilation systems were verified. The parameters required for the analyzed battery room were calculated based on standard BS EN 62485-20141. The volume of mechanical air extraction was received as 42.3 m3/h and the opening area in natural ventilation was 0,12 m2 (Fig 6). In both cases, there was an exhaust ventilation opening in the ceiling of the battery room and a fresh air supply point in the wall, near the floor. The steady-state conditions of hydrogen concentration in a battery room ventilated both naturally and mechanically are shown in Figure 6.
Figure 6: Hydrogen condensation increasing in the ventilated battery room.
The simulation results in Figure 6 show that natural and mechanical ventilation systems can give different effectiveness of hydrogen removal. While they both provide a sufficient safety level, which keeps the hydrogen concentration below 20% of LEL, the natural ventilation appears to be more effective.
Conclusion
The full-scale experiments with hydrogen dispersion in a battery room confirmed that hydrogen does not accumulate below the ceiling, but that its concentration increases uniformly over the entire space of the room. This observation allowed formulation of the conclusion that if hydrogen concentration were to exceed the lower explosive limit, the volume of the explosive cloud would be equal almost to the total volume of the battery room and cause a high explosion hazard. The CFD simulations confirm those measurements, as well as the phenomenon that natural ventilation in the battery rooms could be more effective than a mechanical system.
Dorota Brzezinska is with Lodz University of Technology
References
1BS EN 62485:2014, Safety Requirements for Secondary Batteries and Battery Installations.
2Hydrogen Material Safety Data Sheet.
3Natural Gas Material Safety Data Sheet.
4Propane Material Safety Data Sheet.
5Gasoline Material Safety Data Sheet.