Why is this bottom layer important?
This is a questionable setup though. You'd need massive amounts of sand dumped evenly, which requires more design and verification. Acrylic is also itself flammable.
A basic water based fire suppressor would not extinguish a battery fire but it will cool it and the room, limiting spread.
Let the experts design this kind of thing.
That's the thing: it will not, quite the contrary - unless it's many tons of water at once that quench the fire, the burning lithium will just go and create hydrogen gas that in turn recombines and leads to an even larger fire.
Modern EV fire suppression systems for parking garages use high pressure water mists to contain the fire to a single vechicle for example. This keeps neighboring cars cool and avoids them catching fire, and cooling down the burning vehicle may avoid spread to more battery banks or other flammable materials.
Water can and is used to cool batteries during battery fires, and more importantly, to cool the surroundings so that the fire cannot spread to, say, neighboring banks/cells or construction materials. Modern EV fire suppression systems for parking garages use high-pressure water mists to avoid the fire spreading to neighboring vehicles for example.
The recommendation about oil and water focuses on a larger container of liquid fuel that is on fire at the surface and heated far past the boiling point of water, such that dumping a large volume of water onto it all at once causes it to immediately boil and explode, spreading large amounts of oil as a mist in the air, which both spreads the fire and causes a much more violent combustion. A water-based fire suppression system (not a guy with a bucket of water), or a firefighter with a water hose, can absolutely extinguish such a fire.
Hacking together a sand container of acrylic may well do nothing to limit the fire while simultanously giving it more fuel (acrylic) and pathways to spread to (whatever the acrylic is near).
The point is: Don't hack together a fire suppression system, leave that to an expert please.
And dangerous for any moving parts?
