How can
air conditioning heat city streets by up to +8°C?
When we
talk about air conditioners and their external nuisances, I often hear people
say that "they give off a little heat," as if that weren't a problem.
However, according to some studies, it would seem that the heat emissions are
actually relatively low.
A study
published in 2020 estimates that air conditioning all of Paris during a heat
wave would only increase the air temperature by +0.25 to +0.75 °C in the city
center. This seems insignificant.
But this
study relies on a modeling tool (the TEB model) that uses a very simplified
geometry and assumes that the air in the street around buildings forms a
homogeneous block. This model is useful for conducting a study on a
metropolitan scale, but not for understanding what actually happens on a single
street.
To study
local heat transport on a street, it is necessary to use a computational fluid
dynamics (CFD) tool and analyze the building height/street width ratio (H/W).
A study
published in 2025 shows, for example, that on a very narrow street (H/W = 5),
heat emissions from air conditioners can locally cause an increase in air
temperature of 2 to 10 °C, depending on the wind.
Yes, you
read that right: we're talking about +10 °C.
By
calculating the average temperature of the air layer in which pedestrians walk
(between 0 and 2 meters), the study estimates that they would be exposed to
+8°C due to heat emissions in a very narrow street. This is absolutely
enormous.
These
results are significantly higher than the average values obtained by urban models used at the
scale of an entire city and are far from the 0.75°C modeled by the TEB. They serve as a
reminder that the impact of air conditioning on public spaces is highly
dependent on the street's geometry at the microclimate level.
Of
course, all of this is only valid when the outdoor units are oriented towards
the street, as overheating becomes almost negligible (1°C) when they are
installed on the roof. Air conditioning saves lives, but its development must
not come at the expense of the quality and comfort of public spaces. Our
historic European cities have streets that must retain their full value and
utility for commerce, tourism, and the quality of life they offer. Do we want
to sacrifice the quality of life in public spaces to preserve our housing?
Images:
- Left:
Air temperature in a narrow street (H/L = 5) under the effect of facade air
conditioning (Yujie Zhao et al., 2025)
- Right:
Narrow street in an air-conditioned historic center
Sources:
- TEB
Model (Vincent Viguié et al., 2020)
- CFD
Model (Yujie Zhao et al., 2025)
Sem comentários:
Enviar um comentário