News Brief

Air Conditioners Are Heating Up Our Cities

Waste heat from air conditioners contributes to higher nighttime temperatures, in turn increasing cooling demands, according to a new study.

If waste heat from air conditioning units were used for beneficial purposes, Phoenix would save roughly 1,200 MWh per day, say researchers.

Photo: Todd Morris License: CC by 2.0
It turns out when you left the door open during the summer, you weren’t “cooling the whole neighborhood” as your mother reprimanded; you were probably heating it. New research from Arizona State University suggests waste heat ejected from air conditioners is raising nighttime temperatures—at least in very hot and dry cities—contributing to an even greater need for cooling.

In a computer simulation of 10 days of extreme heat across the Phoenix metropolitan area, researchers found that the waste heat put out by air-conditioning systems did not have a significant effect on air temperatures during the day when it was typically already above 106°F. However, during the night, when temperatures dropped to around 80°F, heat expelled from the indoors warmed the city air almost 2°F in some locations.

With extreme heat projected to increase this century, this positive feedback loop could prove a public-health concern or put further strain on electrical grids; in Phoenix, energy used for cooling already sometimes rises to half of the region’s total electrical consumption.

The study’s authors note the potential for capturing waste heat for purposes such as heating water. That could save Phoenix roughly 1,200 MWh of electricity per day as well as help mitigate the urban heat-island effect (see Rural Areas Feel Heat from Cities) and global warming.

By coupling a Weather Research and Forecasting Model with data on buildings and energy use in the city, the modeling used in the study, “Anthropogenic Heating of the Urban Environment due to Air Conditioning,” accounted for both the size and shape of buildings as well as climatic factors like wind speed.

Published July 16, 2014

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