I was wondering, in an idle moment, about what the ambient temperature in a city such as New York (thinking Manhattan Island) would be if they didn’t use any air conditioning.
I reckon it’d be noticeably cooler outside, as the air conditioning systems change the hot air inside buildings into cool air, and dump that heat outside along with the heat energy used to change the temperature.
How much do they all add to the outside ambient temperature?

Spiny Norman said:

I was wondering, in an idle moment, about what the ambient temperature in a city such as New York (thinking Manhattan Island) would be if they didn’t use any air conditioning.
I reckon it’d be noticeably cooler outside, as the air conditioning systems change the hot air inside buildings into cool air, and dump that heat outside along with the heat energy used to change the temperature.
How much do they all add to the outside ambient temperature?

Heat island effect.

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0450(1968)007%3C0575%3AOOTUHI%3E2.0.CO%3B2

(NYC, 1967)

https://en.wikipedia.org/wiki/Urban_heat_island

I was thinking heat island as well.

“Every year in the U.S. 15% of energy goes towards the air conditioning of buildings in these urban heat islands. According to Rosenfeld et al., “the air conditioning demand has risen 10% within the last 40 years.”“

(From the wiki article.) You could do order-of-magnitude calculations using that, I guess. 15% is very significant.

https://en.wikipedia.org/wiki/Urban_heat_island

Spiny Norman said:

I was wondering, in an idle moment, about what the ambient temperature in a city such as New York (thinking Manhattan Island) would be if they didn’t use any air conditioning.
I reckon it’d be noticeably cooler outside, as the air conditioning systems change the hot air inside buildings into cool air, and dump that heat outside along with the heat energy used to change the temperature.
How much do they all add to the outside ambient temperature?

The other contribution to the urban heat island is motor vehicles.

I did a 3-D fluid dynamics computation once of the plumes of waste heat given off by air conditioners of tall buildings in Brisbane. One thing I noticed is that, since all the air conditioning heat sources are located on the tops of tall buildings, a large fraction of it tended to blow away even in light winds. By contrast, the heat generated by motor vehicles is generated deep within urban canyons so tends to accumulate.

But for Brisbane, that’s the heat generated by air conditioning used to cool buildings, which is different from the New York case.

Isn’t a large part of the heat island effect due to passive absorption and re-radiation of heat by buildings and paved areas?

Generally speaking, yes. Also humidity increases the effect because the moisture in the air traps more heat than dry conditions (if I remember my high school geography correctly). My understanding is that New York is very humid in summer.

Fanks, most interesting everyone.

Spiny Norman said:

I was wondering, in an idle moment, about what the ambient temperature in a city such as New York (thinking Manhattan Island) would be if they didn’t use any air conditioning.
I reckon it’d be noticeably cooler outside, as the air conditioning systems change the hot air inside buildings into cool air, and dump that heat outside along with the heat energy used to change the temperature.
How much do they all add to the outside ambient temperature?

Obviously this would be a hard one to model closely, probably depend on geometry etc, but I’ll have a crack at some OOM BOTE calcs.

About 6% of electrical power in the USA goes on AC.
source https://energy.gov/energysaver/air-conditioning

Now, people in NY are not using their AC in winter, so I’m going to make a rough estimate that in summer that average is twice as high, 12%.

This is a fucking amazing map by Vijay Modi showing the areal density of energy consumption!
http://www.huffingtonpost.com/2012/02/02/new-york-city-energy-interactive-map_n_1249856.html

In the heart of Manhattan there are large areas with annual power consumption of 8000 kWh per square metre!

Sorry for the exclamation points!!!

Now, obviously power consumption will vary throughout the year, with heating in winter and AC in summer, but let’s say that the summer power consumption is about average, which means on a summer’s day, 21 kWh per square metre per day of power is consumed in mid-town Manhattan, of which maybe 2.5 kWh is air conditioning. I will estimate the average coefficient of performance of the airconditioning to be 3, so the amount of heat pumped will be 7.5 kWh per day per square metre.

This is significantly higher than the amount of solar energy received by New York on a typical summer’s day, which will be more like 4.8 kWh per day per square metre.

However, note that if the temperature inside is stable, heat is seaping back into the building (through the walls and windows) at the same rate as it is being pumped out. This may seem a paradox but it must be true.

Nonetheless, all of the energy being consumed by the airconditioners (2.5 kWh per square metre per day) ultimately winds up as heat.

(Come to think of it … ALL of the electricity consumed by mid-town Manhattan (21 kWh per square metre per day) will end up as heat. Everything ends up as heat.)

Bottom line, though: the amount of heat caused by air conditioning is about half of the amount of energy being received by the sun in summer in mid-town Manhattan. How much this contributes to the temperature would depend on how fast that extra heat can escape due to hot air rising etc.

dv said:

Spiny Norman said:

I was wondering, in an idle moment, about what the ambient temperature in a city such as New York (thinking Manhattan Island) would be if they didn’t use any air conditioning.
I reckon it’d be noticeably cooler outside, as the air conditioning systems change the hot air inside buildings into cool air, and dump that heat outside along with the heat energy used to change the temperature.
How much do they all add to the outside ambient temperature?

Obviously this would be a hard one to model closely, probably depend on geometry etc, but I’ll have a crack at some OOM BOTE calcs.

About 6% of electrical power in the USA goes on AC.
source https://energy.gov/energysaver/air-conditioning

Now, people in NY are not using their AC in winter, so I’m going to make a rough estimate that in summer that average is twice as high, 12%.

This is a fucking amazing map by Vijay Modi showing the areal density of energy consumption!
http://www.huffingtonpost.com/2012/02/02/new-york-city-energy-interactive-map_n_1249856.html

In the heart of Manhattan there are large areas with annual power consumption of 8000 kWh per square metre!

Sorry for the exclamation points!!!

Now, obviously power consumption will vary throughout the year, with heating in winter and AC in summer, but let’s say that the summer power consumption is about average, which means on a summer’s day, 21 kWh per square metre per day of power is consumed in mid-town Manhattan, of which maybe 2.5 kWh is air conditioning. I will estimate the average coefficient of performance of the airconditioning to be 3, so the amount of heat pumped will be 7.5 kWh per day per square metre.

This is significantly higher than the amount of solar energy received by New York on a typical summer’s day, which will be more like 4.8 kWh per day per square metre.

However, note that if the temperature inside is stable, heat is seaping back into the building (through the walls and windows) at the same rate as it is being pumped out. This may seem a paradox but it must be true.

Nonetheless, all of the energy being consumed by the airconditioners (2.5 kWh per square metre per day) ultimately winds up as heat.

(Come to think of it … ALL of the electricity consumed by mid-town Manhattan (21 kWh per square metre per day) will end up as heat. Everything ends up as heat.)

Bottom line, though: the amount of heat caused by air conditioning is about half of the amount of energy being received by the sun in summer in mid-town Manhattan. How much this contributes to the temperature would depend on how fast that extra heat can escape due to hot air rising etc.

Nice breakdown, deevs. I would suspect the the percentage of power that is consumed by A/C in a town like Manhattan would be far higher than 12%. Closer to 30 or 40% would be my guesstimate.

In the heart of Manhattan there are large areas with annual power consumption of 8000 kWh per square metre!

———

If that’s a 80 story building, then each floor would consume 8000/80 = 100 kWh per square metre. Remember that (at least central) Manhattan has heated footpaths, too…

(I am reminded of Marilyn Monroe and that white dress.)

Michael V said:

Remember that (at least central) Manhattan has heated footpaths, too…

(I am reminded of Marilyn Monroe and that white dress.)

I think that’s largely the result of the enormous centralised steam system that provides heat to the city during Winter, and to a lesser extent, cooling in Summer. Check out the New York City Steam System (which also explains the up-draught Marylin’s dress was riding.

31° inside

cool change outside

turns on kitchen and bathroom ceiling fans

hot air rises

up it goes

Tau.Neutrino said:

hot air rises

less dense air

ChrispenEvan said:

Tau.Neutrino said:

hot air rises

less dense air

I’ve seen those less dense air balloons over the city early in the morning.

…and stay out!