Solving the Vicious Cycle of Air Conditioning

In hot, dry Phoenix, air conditioning is for much of the year a necessity for public health. But with all those machines spewing hot air outside to provide cool air inside, what happens to the outdoor temperature? That’s the question researchers set out to answer in a recent study, and what they found was pretty surprising.

Over 10 days, excess heat from air conditioners running during the night resulted in temps two degrees higher than they would have been, worsening the urban heat island effect and adding to cooling demands. (During the day, when the average high was 106 degrees F in the period studied, there was no effect.)

Francisco Salamanca, a postdoctoral research scientist at Arizona State University’s School of Mathematical and Statistical Sciences, says he had previously created a model of how this effect might work and wanted to validate it with data from a city. Living in Phoenix, smack dab in the middle of the Sonoran Desert, didn’t hurt his motivation.

The researchers simulated a 10-day period in July 2009, taking into account the mixing of hot and cold air, wind variations, and weather patterns. (The results were published in the Journal of Geophysical Research: Atmospheres.) During extreme heat, electricity for air conditioning can consume 50 percent of a grid’s total electricity. In big American cities, which can have the electrical generating capacity of larger countries, that’s a lot of juice.

Does two degrees really make a difference in the environment? Even a small rise in temperature on hot days and nights can have a life-threatening impact on some populations, including the homeless, the elderly, and infants. There’s also an economic cost. Salamanca said, “An increase of two degrees for the Phoenix metropolitan area represents around 1,200 megawatt-hours of extra electricity consumption each day to maintain our residences cooled in summer.” Stretched out over an entire summer, the economic effect is sizable.

There may be ways to turn that extra heat into something useful. Waste heat could be recaptured and used to heat water for homes. The researchers also note that in polluted areas, waste heat might have the benefit of reducing the concentration of pollutants near the ground (because heat rises).

Salamanca is working on another study to evaluate air-conditioning electricity consumption for different scenarios of urban expansion under extreme summertime weather conditions. “Extreme heat events present significant challenges for the energy sector and electric grid and may dominate summer months across the U.S. by the mid-21st century,” he says. “Reliable projections on future cooling needs are essential to inform urban planners and energy providers.”

In the meantime, Salamanca recommends that people raise their thermostat in the summer to a warm but tolerable 80 degrees. “You will save money, you will reduce the emissions of carbon dioxide into the atmosphere, and at the same time, you will reduce the impact of air conditioning systems on the air temperature,” he says.