Why are underground parking garages hot?

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."

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.

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:

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.

>* 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.

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.

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.

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.

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.

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.

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'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.

engines.