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Why are underground parking garages hot? (law.harvard.edu)
68 points by soundsop on July 6, 2013 | hide | past | favorite | 59 comments

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."

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

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 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.

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.

>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.

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.


"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. ..."

The heat is substantial, although it may not be enough to explain it here. My own personal garage, at least, has an elevated temperature for many hours after I park. You have a ~400 pound block of steel heated up enough to boil water, that's going to contain quite a lot of heat.

Don't think about the car's movements inside the garage. Think about the fact that this big lump of metal has been running around for half an hour getting hot before it decides to use the parking garage to cool off. This is a massive amount of heat that persists for hours.

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.

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

This building:


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.

>* 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.

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!

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?)

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.

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

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.

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.

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.

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.

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.

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

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.

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/...

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..., 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...)

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.

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.

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



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.

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.

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.

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".

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.

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.

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.

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

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.

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.

Concrete is a very good insulator.

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.

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.

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

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.

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.

As a radiography student I remember being impressed that something like 99.9% of the energy used trying to make x-rays off a tungsten target were actually just turned into heat. Then a pretty tricky engineering problem arises trying to dissipate it. Rotating anode, bigger anode, fancy liquid bearing, air cooled, water cooled, massive fans etc. And still I would run into the error message about too much heat!

Actually ALL of the energy is lost to heat.

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

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!

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.

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

I think you did not post any maths or contribute anything useful to an open discussion.

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.

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.

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.

Are you serious?

Yes, quite:




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.

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.

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.."

It sounds more crazy than cute to me.

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.."


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