Air Conditioners Heat Up Metro Phoenix Nights

Published: Monday, June 19, 2017 - 7:42am
Updated: Monday, November 6, 2017 - 12:10pm

Parents tell their kids, “Shut the front door, I’m not paying to cool the entire neighborhood.”

But research shows that, during Phoenix summers, we do pay to heat the neighborhood — to the tune of 1 to 2 degrees Fahrenheit — thanks to waste heat vented by our air conditioners.

“But then we need to actually, again, cool even more — use air conditioning even more — to compensate for [the] increased temperature, right, that we just increased using air conditioning. So, it’s really [a] vicious feedback loop,” said Alex Mahalov of Arizona State University’s School of Mathematical and Statistical Sciences (SoMSS).

A few degrees mark the difference between Phoenix’s June and July average high temperatures. Mahalov said comparing those two months’ energy bills can provide a rough sense of how much 1 to 2 degrees can affect your pocketbook.

Other contributors to the 2014 Journal of Geophysical Research: Atmospheres study included lead author Francisco Salamanca and co-authors Mohamed Moustaoui and Meng Wang, also of SoMSS, and Matei Georgescu of ASU’s School of Geographical Sciences and Urban Planning.

All five hold appointments with ASU’s Global Institute of Sustainability, and they continue to explore sustainable solutions, such as using heat exhaust to run a water heater, which has yet to prove cost effective.

Mahalov said the effects of air conditioner waste heat are more pronounced at night.

“During daytime, you have this convection, and heat just kind of goes up, so it doesn’t stay near the ground. During nighttime, we have this so-called stable boundary layer, and so heat is kind of trapped near the ground," Mahalov said.

The stable boundary layer is a relatively cool section of air that lies just above the cold ground. It often forms as the ground radiates heat into the air above, which, lacking other heat sources such as the sun, stratifies into stable layers.

Because sufficiently high overnight temperatures correlate with higher rates of sickness and death, anything that adds heat to Phoenix’s nighttime air — already boosted by urban heat island effects and the heat-blanketing effects of monsoonal humidity — is cause for concern.

“Minimum nighttime temperature [is] already high; we don’t really want to have another 1-2 degrees, right?” said Mahalov.

Factoring in air conditioning waste heat also enabled the team to improve on existing computer models, which tended to underestimate urban nighttime temperature.

Such models enable scientists to ask many kinds of “What If?” questions, and not just in the Valley of the Sun.

“Because it is a physics-based model, it also can be used, not just for Phoenix, but for other metropolitan areas,” Mahalov said.

That could prove important in the coming years: Mahalov said that experts expect demand for air conditioning globally to accelerate, driven by higher installations rates in countries like India, China and other emerging economies.

“What I’m saying is, this problem is not going away; in fact, it is going to be more important with time.”

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