4 ms·

Could someone who took EE please remind me... how many drums of water could a fully-charged Tesla boil?

by mkmk 3y ago

gary_0 3y ago
That's not EE, that's basic math/physics. The largest Telsa Model S battery is 100kWh, which is 360MJ. Boiling 1L of water from room temperature takes 330kJ. 55 US gallons is about 200L water. 200 * 330kJ = 66MJ to boil the whole drum. 360MJ per battery / 66MJ per drum = 5.46 drums of water.
- mkmk 3y ago
  Nice, that's a pretty manageable amount of water to imagine a firefighter having access to. Are there better ways of discharging EV batteries?
- tzs 3y ago
  That's how many drums it could take from room temperature (20℃) to 100℃, but it would not be boiling yet. You need to put in more energy at 100℃ to make it boil. To go from 1 L @ 100℃ to steam at 100 ℃ would take in additional 2.26 MJ. A 200 L drum of room temperature water then could be used to tank 200 x (2.26 + 0.33) MJ = 518 MJ. They would only need one such drum, and one fully charged Tesla battery would be able to boil away 70% of the water. That actually works out nicely. Assuming a standard sized 55 gallon drum when the battery runs out juice there would still be about 25 cm of water in the drum. You want enough water to remain so that the heating element stay immersed because it is often bad for them to run in air.
  - prometheus76 3y ago
    But which method would take longer? Heating five 55-gallon drums to 100C each, or boiling one barrel of water until the battery is dead? Which is more efficient, time-wise?
    - tzs 3y ago
      My first guess is that heating multiple drums to 100℃ would be faster than boiling one drum. This is based on the rate of heat exchange by contact when two things of different temperature touch is proportional to the temperature difference. When the water is at 100℃ the difference between it and the heating element should be less than when the water is colder and so it should be transferring heat slower. By switching drums you are spending more time with the heating element in colder water so can transfer more energy. However there may be other factors. First, dH/dt = k ΔT, but k depends on the particular two things that are exchanging heat. I don't know if the k for the heating element and liquid water is the same as the k for the heating element and water that is undergoing a phase transition to steam. Someone who remembers their college thermodynamics a lot better than I do will be needed here. Second, the battery is going to have some limit on how fast it can transfer energy. That may be low enough that whether you use one drum or multiple may not matter.
- throwawaymobule 3y ago
  If I didn't know the context, this would sound like a contrived example of how great SI units are. Also, all teslas are fitted with the same batteries now, with a software limit. Don't know how charged the extra capacity is, or if it's spread out.
  - projektfu 3y ago
    How many cubic feet of water would a 5 BTU battery be able to bring to a boil?

Could someone who took EE please remind me... how many drums of water could a fully-charged Tesla boil?

by mkmk 3y ago

Could someone who took EE please remind me... how many drums of water could a fully-charged Tesla boil?

That's not EE, that's basic math/physics. The largest Telsa Model S battery is 100kWh, which is 360MJ. Boiling 1L of water from room temperature takes 330kJ. 55 US gallons is about 200L water. 200 * 330kJ = 66MJ to boil the whole drum. 360MJ per battery / 66MJ per drum = 5.46 drums of water.

mkmk 3y ago
Nice, that's a pretty manageable amount of water to imagine a firefighter having access to. Are there better ways of discharging EV batteries?
tzs 3y ago
That's how many drums it could take from room temperature (20℃) to 100℃, but it would not be boiling yet. You need to put in more energy at 100℃ to make it boil. To go from 1 L @ 100℃ to steam at 100 ℃ would take in additional 2.26 MJ. A 200 L drum of room temperature water then could be used to tank 200 x (2.26 + 0.33) MJ = 518 MJ. They would only need one such drum, and one fully charged Tesla battery would be able to boil away 70% of the water. That actually works out nicely. Assuming a standard sized 55 gallon drum when the battery runs out juice there would still be about 25 cm of water in the drum. You want enough water to remain so that the heating element stay immersed because it is often bad for them to run in air.
- prometheus76 3y ago
  But which method would take longer? Heating five 55-gallon drums to 100C each, or boiling one barrel of water until the battery is dead? Which is more efficient, time-wise?
  - tzs 3y ago
    My first guess is that heating multiple drums to 100℃ would be faster than boiling one drum. This is based on the rate of heat exchange by contact when two things of different temperature touch is proportional to the temperature difference. When the water is at 100℃ the difference between it and the heating element should be less than when the water is colder and so it should be transferring heat slower. By switching drums you are spending more time with the heating element in colder water so can transfer more energy. However there may be other factors. First, dH/dt = k ΔT, but k depends on the particular two things that are exchanging heat. I don't know if the k for the heating element and liquid water is the same as the k for the heating element and water that is undergoing a phase transition to steam. Someone who remembers their college thermodynamics a lot better than I do will be needed here. Second, the battery is going to have some limit on how fast it can transfer energy. That may be low enough that whether you use one drum or multiple may not matter.
throwawaymobule 3y ago
If I didn't know the context, this would sound like a contrived example of how great SI units are. Also, all teslas are fitted with the same batteries now, with a software limit. Don't know how charged the extra capacity is, or if it's spread out.
- projektfu 3y ago
  How many cubic feet of water would a 5 BTU battery be able to bring to a boil?