5 ms·

In NYC, I notice the underground subway stations are also really hot. The subways use electricity (no combustion engine), so there may be something going on oth

In NYC, I notice the underground subway stations are also really hot. The subways use electricity (no combustion engine), so there may be something going on other than the cars causing it.

jaggederest 13y ago
They still produce heat, quite a lot of it, both accelerating and braking. Also, don't underestimate the heat produced by people - even in the dead of winter large concert halls will be running air conditioning, not heating.
- bodyfour 13y ago
  A common rule-of-thumb is "100W per person" I think that's probably slightly high, but it's in the right ballpark. If you're standing in a crowd, you should expect that it'll be as hot as if every person was replaced with a 100W lightbulb.
  - ErsatzVerkehr 13y ago
    If you assume a 2000 (kilo)calorie per day diet, you get: 2000 Kcal/day * (4.18 Joules/calorie) * (1 day/86400 seconds) = 96.75 Watts
kalleboo 13y ago
My memory of the Stockholm Subway is that the air is cool. The Stockholm subway is built in more natural-style caves so maybe there's something to the concrete theory http://www.visitstockholm.com/ContentStoreFiles/Entity/1158/sl_konst_450x300.jpg http://www.visitstockholm.com/ContentStoreFiles/Entity/1158/...
- Someone 13y ago
  You would have to show that heat transfer between air and rock is higher than that between air and concrete. Without knowledge of what kind of concrete and rock are involved, it's hard to say something definitive. However, looking at http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html http://www.engineeringtoolbox.com/thermal-conductivity-d_429..., it seems "rock, solid" can have a much higher thermal conductivity than "concrete, dense" (and, as I expected, specific heat is comparable, as far as I can tell from http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html http://www.engineeringtoolbox.com/specific-heat-solids-d_154...) What also may help is higher tunnels. Temperatures near the ceilings of those tunnels will be quite a bit higher than at floor level. Stockholm might just have more high ceilings.
mmoche 13y ago
Most subways have air conditioners on the trains which expel heat into the tunnels, thus making them hot. Before air conditioning was introduced to the Boston subway system, the cars had little vents that pulled cool air into the trains when the cars were underground, but with the advent of air conditioning, the trains stay cold at the expense of the tunnels and stations.
gergles 13y ago
The subway itself is extremely poorly ventilated and all the cars are air-conditioned down to frigid temps (probably to make up for the ridiculously hot subway stations) -- the waste heat sucked out of the cars just gets dumped into the tunnels and the trains act as pistons to shove the hot air into the stations.
- ErsatzVerkehr 13y ago
  The air conditioners are, to a good approximation, just pumping energy out of the train cars that would be released anyway (i.e. the 100 Watts per passenger discussed below), plus whatever power they require themselves to run. Thus it seems very unlikely to me that train car air conditioning is a substantial contribution. One could argue that perhaps the passengers' metabolisms are increased slightly to make up for the extra heat loss due to a cooler environment. But this has got to be totally negligible with respect to the energy dumped by a stopping train.
pandaman 13y ago
One subway car weights about 30 tons empty, breaking from 15 m/s (~60 kmh) to a full stop it converts almost all its kinetic energy into heat so each car produces 15^2*30000/2 ~ 3MJ at every stop.
- matthiasl 13y ago
  An interesting thing is that many subway systems recover some of the kinetic energy by converting it to something more useful than heat. Some subways, for instance London's, are built with uphill slopes prior to stations, so that part of the kinetic energy is converted into potential energy. Many subways use regenerative electrical braking for part of coming to a stop, i.e. the kinetic energy is partially converted to electric energy which is used by other trains on the network. Some subways increase the efficiency of that by having storage, for instance a large battery, in stations. References: http://www.eenews.net/stories/1059982358 http://www.eenews.net/stories/1059982358 www.londonreconnections.com/2011/milestones-reached-on-the-jubilee-and-victoria-lines/ http://www.railway-energy.org/static/Regenerative_braking_in_DC_systems_103.php http://www.railway-energy.org/static/Regenerative_braking_in...
Dylan16807 13y ago
Why on earth would it matter if they use combustion or electricity? Internal combustion engines are pretty efficient. They're not as efficient as electric engines but the amount of energy burned is well within an order of magnitude.
- bodyfour 13y ago
  Electric engines can easily be above 90% efficiency. ICE's are around a third of that -- since they're Heat Engines, Carnot's Theorem even gives a theoretical maximum efficiency. So yes, an ICE will have an order of magnitude more waste heat. That's why a car engine requires liquid cooling to avoid destruction but a similarly powerful electric motor only has a small fan to keep air running over the windings.
  - ErsatzVerkehr 13y ago
    I'm not sure how good a model a heat engine provides for internal combustion. A heat engine assumes a fixed "working fluid" operating between heat reservoirs. In an internal combustion engine, there isn't strictly a "working fluid", since air and fuel are continually added, and exhaust vented away, and combustion adds energy without providing exactly a "heat reservoir".
  - Dylan16807 13y ago
    Why are you talking about waste heat only? Work energy degrades to heat too. The total amount of heat created is equal to the total energy used by the system. ICEs need three times as much energy to do the same work, so they output three times as much heat. This would be true even if electrical engines had 100% efficiency.
gaadd33 13y ago
There's a few reasons for that, one is due to steam pipes below ground, another is due to the AC units on the subway cars, I believe (its been a while since I read the math) that they put off as much heat as breaking from a full stop does, but they do it continuously. Finally breaking is a big factor but that only happens once per station and I believe the heat given off is comparable to only a fraction of what the AC units give off when stopped in the station. Finally, there's the poor ventilation that doesn't allow the air to circulate very well albit it is somewhat nice in the winter since then the station is warmer.

In NYC, I notice the underground subway stations are also really hot. The subways use electricity (no combustion engine), so there may be something going on other than the cars causing it.

jaggederest 13y ago

They still produce heat, quite a lot of it, both accelerating and braking. Also, don't underestimate the heat produced by people - even in the dead of winter large concert halls will be running air conditioning, not heating.

bodyfour 13y ago
A common rule-of-thumb is "100W per person" I think that's probably slightly high, but it's in the right ballpark. If you're standing in a crowd, you should expect that it'll be as hot as if every person was replaced with a 100W lightbulb.
- ErsatzVerkehr 13y ago
  If you assume a 2000 (kilo)calorie per day diet, you get: 2000 Kcal/day * (4.18 Joules/calorie) * (1 day/86400 seconds) = 96.75 Watts

kalleboo 13y ago

My memory of the Stockholm Subway is that the air is cool. The Stockholm subway is built in more natural-style caves so maybe there's something to the concrete theory http://www.visitstockholm.com/ContentStoreFiles/Entity/1158/sl_konst_450x300.jpg http://www.visitstockholm.com/ContentStoreFiles/Entity/1158/...

Someone 13y ago
You would have to show that heat transfer between air and rock is higher than that between air and concrete. Without knowledge of what kind of concrete and rock are involved, it's hard to say something definitive. However, looking at http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html http://www.engineeringtoolbox.com/thermal-conductivity-d_429..., it seems "rock, solid" can have a much higher thermal conductivity than "concrete, dense" (and, as I expected, specific heat is comparable, as far as I can tell from http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html http://www.engineeringtoolbox.com/specific-heat-solids-d_154...) What also may help is higher tunnels. Temperatures near the ceilings of those tunnels will be quite a bit higher than at floor level. Stockholm might just have more high ceilings.

mmoche 13y ago

Most subways have air conditioners on the trains which expel heat into the tunnels, thus making them hot. Before air conditioning was introduced to the Boston subway system, the cars had little vents that pulled cool air into the trains when the cars were underground, but with the advent of air conditioning, the trains stay cold at the expense of the tunnels and stations.

gergles 13y ago

The subway itself is extremely poorly ventilated and all the cars are air-conditioned down to frigid temps (probably to make up for the ridiculously hot subway stations) -- the waste heat sucked out of the cars just gets dumped into the tunnels and the trains act as pistons to shove the hot air into the stations.

ErsatzVerkehr 13y ago
The air conditioners are, to a good approximation, just pumping energy out of the train cars that would be released anyway (i.e. the 100 Watts per passenger discussed below), plus whatever power they require themselves to run. Thus it seems very unlikely to me that train car air conditioning is a substantial contribution. One could argue that perhaps the passengers' metabolisms are increased slightly to make up for the extra heat loss due to a cooler environment. But this has got to be totally negligible with respect to the energy dumped by a stopping train.

pandaman 13y ago

One subway car weights about 30 tons empty, breaking from 15 m/s (~60 kmh) to a full stop it converts almost all its kinetic energy into heat so each car produces 15^2*30000/2 ~ 3MJ at every stop.

matthiasl 13y ago
An interesting thing is that many subway systems recover some of the kinetic energy by converting it to something more useful than heat. Some subways, for instance London's, are built with uphill slopes prior to stations, so that part of the kinetic energy is converted into potential energy. Many subways use regenerative electrical braking for part of coming to a stop, i.e. the kinetic energy is partially converted to electric energy which is used by other trains on the network. Some subways increase the efficiency of that by having storage, for instance a large battery, in stations. References: http://www.eenews.net/stories/1059982358 http://www.eenews.net/stories/1059982358 www.londonreconnections.com/2011/milestones-reached-on-the-jubilee-and-victoria-lines/ http://www.railway-energy.org/static/Regenerative_braking_in_DC_systems_103.php http://www.railway-energy.org/static/Regenerative_braking_in...

Dylan16807 13y ago

Why on earth would it matter if they use combustion or electricity? Internal combustion engines are pretty efficient. They're not as efficient as electric engines but the amount of energy burned is well within an order of magnitude.

bodyfour 13y ago
Electric engines can easily be above 90% efficiency. ICE's are around a third of that -- since they're Heat Engines, Carnot's Theorem even gives a theoretical maximum efficiency. So yes, an ICE will have an order of magnitude more waste heat. That's why a car engine requires liquid cooling to avoid destruction but a similarly powerful electric motor only has a small fan to keep air running over the windings.
- ErsatzVerkehr 13y ago
  I'm not sure how good a model a heat engine provides for internal combustion. A heat engine assumes a fixed "working fluid" operating between heat reservoirs. In an internal combustion engine, there isn't strictly a "working fluid", since air and fuel are continually added, and exhaust vented away, and combustion adds energy without providing exactly a "heat reservoir".
- Dylan16807 13y ago
  Why are you talking about waste heat only? Work energy degrades to heat too. The total amount of heat created is equal to the total energy used by the system. ICEs need three times as much energy to do the same work, so they output three times as much heat. This would be true even if electrical engines had 100% efficiency.

gaadd33 13y ago

There's a few reasons for that, one is due to steam pipes below ground, another is due to the AC units on the subway cars, I believe (its been a while since I read the math) that they put off as much heat as breaking from a full stop does, but they do it continuously. Finally breaking is a big factor but that only happens once per station and I believe the heat given off is comparable to only a fraction of what the AC units give off when stopped in the station. Finally, there's the poor ventilation that doesn't allow the air to circulate very well albit it is somewhat nice in the winter since then the station is warmer.