
Why are underground parking garages hot? - soundsop
http://blogs.law.harvard.edu/philg/2013/07/05/stupid-geophysics-question-of-the-week-why-are-underground-parking-garages-hot/
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bretpiatt
3rd comment on the original post explains with physics, "J G

July 5, 2013 @ 10:10 pm

3 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."

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brudgers
If it is using 1j/g/degree K, then it is doing it wrong. It's a question of
Psychrometrics
[http://en.wikipedia.org/wiki/Psychrometrics](http://en.wikipedia.org/wiki/Psychrometrics)

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...](http://www.engineeringtoolbox.com/enthalpy-moist-air-d_683.html)

Latent heat is why auditoriums get hot, when filled with people.

~~~
Tloewald
The linked document shows humid air having an enthalpy of 75kJ/kg at 25C, so
that explains why humidity increases perceived heat and increases the energy
needed to heat up humid air, but the heat itself is coming from car engines,
and the humidity from tailpipes. On the whole, i don't find underground
carparks notably humid, but they are warm.

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abalone
Have you ever been in a boiler room? Those are underground too, but we aren't
surprised that they're hot.

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.

~~~
coldtea
> _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._

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.

~~~
chebucto
Ventilation is a major factor, at least on hot days.

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.

[http://www.mobilecalibrationservices.com/Ventilation_for_enc...](http://www.mobilecalibrationservices.com/Ventilation_for_enclosed_parking_garages.pdf)

"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. ..."

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Eduardo3rd
What an interesting problem!

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.

This is a great example of why it is important to work with people in other
disciplines when trying to solve a problem! I can't tell you the first thing
about javascript or web hosting, but I've got heat transfer locked down.

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dnautics
I don't know the answer to this question, but there are architects who have
attempted to solve this question.

This building:

[http://www.jcvi.org/cms/sustainable-lab/construction-
update/](http://www.jcvi.org/cms/sustainable-lab/construction-update/)

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.

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mistercow
>* Much of the thermal mass in the garage consists of cars, which have
recently driven in from the hot surface and are therefore hot.

>...

>* 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.

~~~
lostlogin
Exactly. I often used to ponder how effective a heater a PC is with decent
graphics card running under load. I could heat my room in winter with just
myself and the computer. With the computer off, I was cold. So I suspect that
a 2ish litre car is somewhat better a heater, even long after its been turned
off. The neighbourhood cats sure seem to love my car after I've been parked up
for a while!

~~~
mtdewcmu
I think the PC as heater question is easy to solve. Just add up all the watts
and assume that it all becomes heat. A cpu and graphics card can easily be
over 100 watts each, and a 100 watt light bulb gets pretty hot. (I had this
conversation with another computer guy at one time, and he didn't think all of
the watts became heat. I'm not an engineer or physicist, but -- where else
could it go?)

~~~
YokoZar
You're right that it basically will all go to heat. The only real exception is
if light from the display goes out the window, or if sound from the speakers
escapes the room.

~~~
mistercow
Some of the heat will also radiate out the window as infrared.

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robotcookies
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.

~~~
gergles
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
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.

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josephers
I think hot cars & their waste heat makes sense. Each car's engine is
essentially a furnace.

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.

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dfc
The author compares parking structures to holes in bare soil, caves and/or
house basements which have very little concrete between the house foundation
and the soil. Is it appropriate to compare these underground environments with
a underground parking structure which has much more concrete insulation?

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.

~~~
mdturnerphys
Concrete isn't very good insulation. There isn't much difference between a
concrete wall and the stone wall of a cave.

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EGreg
I think the concrete conducts heat into the garage. Also the cars are hot when
they drive in, and the sheer number of cars also expends the heat into the
space. As they leave, new hot cars come. So over time the place itself heats
up.

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.

~~~
afterburner
Yes, was gonna mention the heat coming off the car itself. The engine and body
still have to cool from combustion and the sun/roads/outside air.

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sriramiyer
Au contraire, I've found underground parking lots to be cool. Much cooler than
their above ground cousins.

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.

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brudgers
I would start looking at it as an issue of thermal capacitance. The concrete
changes temperature slowly, once the mass is heated it stays warm a long time.
To mitigate hydrostatic pressure, the backfill around an underground garage
will be porous - i.e. not in full contact with the heat sink of the earth.

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.

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moreentropy
I'd blame the building's electrical and HVAC systems. Steam pipes, AC
condensers, engines, pumps etc.

~~~
halisaurus
Adding to this, we are questioning the basement temperature in an _urban_
environment. A city like Boston or New York has electrical and steam pipes
under every street, surrounding these basements with a constant heat source.

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akiselev
Something to think about on this topic: 70-80% of the energy used to move your
2 ton block of metals, composites, and flesh is lost to heat (depending on
engine efficiency and other factors like regenerative breaking). There are of
course other factors like large commercial AC systems that converge above the
garage and other openings for air flow, but it takes a hell of a lot of energy
to move a car and any heat not sucked away by the air moving through the
intake is dumped into the garage. On a hot day, any cooling systems in the car
are also prone to be less effective so you're "carrying" more heat into the
garage than you would on a normal day.

~~~
ErsatzVerkehr
Actually ALL of the energy is lost to heat.

~~~
tjgq
Unless you finish your trip at a greater altitude than where you started. ;)

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jasonzemos
There's a lot of comments citing only engine and exhaust heat- don't forget
about significant heat dissipating from the brakes as well. Brake heat is even
more pronounced in underground train stations like penn station. Heck, there's
even heat in the gas tank from fuel that pumped through the rails and got
returned!

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AUmrysh
Wild guess: Cars generate a lot of heat, most of the power in an internal
combustion engine is lost as heat (they're something like 30% efficient). The
engine block and body absorb the heat when operating, and when the cars are
shut off (not to mention running when parking/exiting), the heat radiates out
into the air.

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andrewcooke
metapost/summary (to save you reading rest of this page): copy and paste some
random selection of the reasons given in the article, prefixed with "i think".
do not post maths

~~~
dfc
_I think_ you did not post any maths or contribute anything useful to an open
discussion.

~~~
andrewcooke
why? surely criticism of something is useful - it shows that something is
wrong.

in contrast, vapid repetition of ideas from the article serves no purpose.
it's science, not a popularity contest.

------
pacomerh
I'm trying to remember if I parked underground when the parking was empty. I
always thought the reason was because there's no windows/ventilation.

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seclorum
I've always thought it was because concrete is slightly radioactive,
personally. Rightly or wrongly so, its been yet another reason to avoid the
oil-debt and walk, or cycle, to work.

~~~
ErsatzVerkehr
Are you serious?

~~~
seclorum
Yes, quite:

[http://hps.org/publicinformation/ate/q1543.html](http://hps.org/publicinformation/ate/q1543.html)

And:

[http://en.wikipedia.org/wiki/Environmental_impact_of_concret...](http://en.wikipedia.org/wiki/Environmental_impact_of_concrete)

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.

~~~
mturmon
Reading your comments, you vacillate between taking radioactive heating from
concrete seriously, and not.

Doing the right thing for the wrong reason is not an effective strategy.

~~~
seclorum
Concrete _is_ radioactive, I'm reminded of this fact when I go into a concrete
parkhouse and feel the heat, and I don't particularly care if its hot due to
radiation or hot due to heat energy from cars being stored - either way, the
darn places are uncomfortable and I avoid them.

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.."

~~~
ErsatzVerkehr
It sounds more crazy than cute to me.

~~~
seclorum
Well, next time you walk into a stinking hot concrete bunker, say to yourself
"I wonder if I know what the word radiation means, hmmm.."

------
malandrew
engines.

