You ever be working and remember the British underground has been soaking in heat from the trains for a century and it's starting to cook them alive
and yet they haven't implemented any use of this carefully-hoarded resource to manage energy costs in chilly weather.
@munin @SwiftOnSecurity London doesn't get "chilly weather" this century.
I expect not, with all that untapped heat resourcing just leaking through the foundations like that!
@munin @SwiftOnSecurity A tube train draws up to 6MW of juice when starting from cold. There are about 180km of underground tube tunnels in the London network. Trains average 33km/h and run about 2-5 minutes apart, so there are probably about 100 trains running underground at any given time. That's a LOT of waste heat …
@cstross @munin @SwiftOnSecurity
Do you know what's the efficiency of pushing power into something (train grid? above-surface resistors?) when braking?
@munin @robryk @cstross @SwiftOnSecurity not with electric braking
@cliffordheath @robryk @cstross @SwiftOnSecurity
Even ignoring the eddy currents used within the braking motors that cause heat within those surfaces,
the train wheels on the rails will experience some degree of heating due to the friction required to slow the train on them.
There is always local heat produced.
@munin @robryk @cstross @SwiftOnSecurity it's minuscule compared to the regenerated electricity
I do not believe your assertion, because it conflicts with my prior knowledge around braking systems.
If you have a specific design to point to, that has actual numbers associated with it, we can discuss that.
@munin Trains only use mechanical brakes in emergency stops and below about 5km/hr where the electric braking doesn't work. The wheels are quite simply not a big enough heat sink. But you could have found that out yourself, you don't need me to spoon feed you
How large a fraction of power that's taken out of train's kinetic energy is transformed into heat by electrical braking? I would estimate that to be ~10%ish, because I vaguely remember 90%ish efficiency of electrical motors (incl. VFDs etc.) and I would expect that to be roughly same for the other direction.
How large a fraction of power that's taken out of train's kinetic energy is transformed into heat by electrical braking? I would estimate that to be ~10%ish, because I vaguely remember 90%ish efficiency of electrical motors (incl. VFDs etc.) and I would expect that to be roughly same for the other direction.
@robryk @munin Good thoughts but overall system efficiency is nowhere near that high unfortunately. ESS is the hot topic here, achievable savings can approach 40% (obviously not in long distance trains, where energy losses far outweigh kinetic requirements) https://www.sciencedirect.com/science/article/pii/S0378779623002572
@munin @cliffordheath @cstross @SwiftOnSecurity
Electric braking in trains is usually (always?) not eddy current braking, but running the motors as generators and connecting something to the motor. In somewhat older days that something was either the overhead wire (with engines set up so that they generate a higher voltage) or a bunch of onboard resistors (that is still sometimes the case in trams). I would expect that trains with a VFDful drive etc. do something more interesting there, where they are able to dump that into overhead wire at lower speeds. Obviously the question of ability of the overhead grid to sink that power (assuming insufficiently many other accelerating trains on the same overhead segment) is a separate issue.
@robryk @cstross @SwiftOnSecurity
Regardless of braking methodology, heat will be generated at the braking surface and there are no easy ways to dissipate that from the immediate environment.