Fun with numbers - August 28, 2019

Air conditioning FTW!

Part of what I do is training people, under the fancy name of "executive education," and to match the fancy name we tend to get nice AV equipment, color handouts, markers that work, fancy chairs, and climate-controlled rooms.

The least remarkable of those is also probably the most important. The air conditioning, not the markers.

For a large-ish event with around 60 people in a comfortably large room, with about 50% of heat losses to the exterior, the temperature would increase by almost 15 °C in a 90-minute session:

Okay, there's a lot of approximations in that calculation, but even 10 °C increase means that either it was too cold at the beginning or too warm at the end. So hurrah for air conditioning.

Seriously, how can people fall for this?

Today The Tesla Promotion Network Electrek, posted a news item about batteries. Apparently "[a] startup that spun out of Cambridge University claims a battery breakthrough that can charge an electric car in just six minutes."

That phrasing is unclear: how much charge? Even the slowest chargers in my neighborhood (6 kW) will give any electric car battery some charge in 6 minutes (0.6 kWh). Most people will read that to mean they could give a full charge to, say, a Tesla Model 3 in six minutes.

Say the Tesla has a 80 kWh battery; charging it in 6 minutes requires an average power of 800 kW; even using 480 V, that would mean the current would be almost 1700 A. That would be a really interesting current to see in a lithium-ion battery, or as the fire department calls it, "the initiating event of the fire."

For comparison, a typical gas pump can pump 3 l/s of gasoline, which at 34 MJ/l means that the gas pump can transfer energy at a rate of over 100 MW or around 125 times the rate of that fictional battery.



Not math: some infrastructure