July 5, 2013 @ 10:10 pm
It’s very definitely waste heat from the cars.
Typical car engine runs just under boiling – 80 C differential from ‘room temperature’.
The hot sections of most cars will weigh about 250kg, specific heat of steel is 0.49 j/g per degree K
Meaning each car brings about 6000kj of waste heat into the garage, plus another 100kj+ for each second they run in the confines of the garage.
Air has a specific heat of roughly 1 j/g per degree K, and a kilogram of air takes up nearly a cubic meter. So a car can heat 200 cubic meters (a cube roughly 6m) of air quite uncomfortably warm, which is much larger than the area the car occupies.
This is just back-of-the-envelope math, but air has extremely low specific heat by volume, so it’s very easy for a hot engine to heat up a nonsensically large space."
Air is humid, the Enthalpy is significantly higher than the specific heat of dry air. Latent heat is the major factor in the real world. And car exhaust is loaded with water vapor (and CO2).
See examples here: http://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.h...
Latent heat is why auditoriums get hot, when filled with people.
The heat source in this case is cars, which are literally radiators. Cars are actively driving through the space and even at rest they are radiating heat. Wine doesn't generate heat, plus concrete and other construction materials retain heat moreso than a thin-walled or bare earth cave.
Not that much. The "warmth" is also true in garages or levels where there seldom are cars coming and going. One every while. Even when the garage is closed.
The other reasons stated (the thermal behavior of the structure) are more promising.
How does the outside air temperature affect the temperature of underground parking lots? Are they as hot or less hot during the winter?
CO is deadly; air must be recycled very quickly. All else being equal, it seems like ventilation would bring the facility to outside ambient temperature quickly.
"For enclosed parking facilities, ANSI/
ASHRAE Standard 62-1989, Ventilation
for Acceptable Indoor Air Quality specifies
a fixed ventilation rate of below 7.62
L/s·m2 (1.5 cfm/ft2) of gross floor area.2
Therefore, a ventilation flow of about
11.25 air changes per hour is required for
garages with 2.5 m (8 ft) ceiling height.
However, some of the model code authorities
specify an air change rate of four
to six air changes per hour. ..."
As a mechanical engineer I know that this could be simplified to a lumped capacitance heat transfer problem. If you wanted to get a little fancier you could setup a thermal circuit, but that wouldn't take more than a few minutes. Looking up a few constants and knowing the number of cars in the garage would enable you to know not just why the garage is hot, but roughly how hot it should be on a given day.
has an underground parking lot. The lowest row of openings facing the camera view are air vents that lead to the garage; is has (supposedly) been geogaphically positioned so that thermal currents coming up off of the ocean blow up through the parking lot and ventilate it with no consumption of energy from the grid. I'll be able to get back to you in october with an answer as to if it actually works.
>* The cars generate a lot of waste heat as they drive around within the garage.
Cars remain exceptionally hot long after they've been parked. That has little to do with their carrying heat from the surface, and much to do with the fact that they run on a constant stream of controlled gasoline explosions. Once parked, they will radiate that heat into the garage. I think this is the most likely explanation for why garages tend to be hot.
Also, for weak evidence: I have never paid that close attention to it, but I'm pretty sure that I've been cold in a parking garage when I came back to my car very late at night.
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.
2000 Kcal/day * (4.18 Joules/calorie) * (1 day/86400 seconds) = 96.75 Watts
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.
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.
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.
I've also noticed that underground subway station platforms can get very hot in the summer, and it's worse when trains come by and their AC systems are spewing hot air into the station.
I also think most of the heat the cars bring into the garage comes from heat radiating off the engine block and not heat that was absorbed from a hot road.
As for the cars, they heat up outside because the sunlight is able to pass through the windows and heat the seats faster than they the air can dissipate heat through the windows.
You see, I live in a country that gets very hot during summers. The ambient goes to 45 easily. And the only sensible solution is to park some place where the sun rays cannot (a) directly hit your car because that would cause the car to become uncomfortably hot (b) cannot be reflected through to your car via multiple other surfaces (e.g. other cars, walls, etc).
The underground section of the parking lots perfectly suit this criteria, even though it's technically the same building with under-and-over ground parking facilities. If you're wondering where I'm talking about, it's the parking lot at the Bahrain City Center. (One can hope that very few people residing in Bahrain read hackernews, and my supposedly secret strategy doesn't leak out to everyone).
What I could do would be to take actual thermometric readings over the course of this summer for underground vs. above-ground parking spots.
The next factor is the heat load of the cars. It is not just that they produce heat, it is that a substantial portion of that heat is latent - combustion produces hot water vapor. The elevated temperature within the garage means more water vapor and hence latent heat within the microenvironment.
Finally, the quality of the good air is relative. It is going to be drawn from an urban environment at street level. And drawn at high volumes and low velocity (to reduce noise). Human comfort in an underground parking garage is not a meaningful design consideration.
There's a positive feedback loop until the point that street-level urban air begins to provide some equilibrium. Or at least that would be my guess.
in contrast, vapid repetition of ideas from the article serves no purpose. it's science, not a popularity contest.
I haven't been seriously thinking about it, like I said it was just a fancy, not a seriously considered notion, and anyway it nevertheless resulted in a healthier response: I void driving, and thus parking, and thus parking in concrete structures.
Doing the right thing for the wrong reason is not an effective strategy.
I'm not particularly bothered if I'm "wrong", its just a cute thing to say .. "oh, its so radioactive in here, lets get out.."