Air conditioning obviously makes homes in Phoenix cooler, but A.C. units make the city itself even hotter, according to new research.
A recently published study led by a researcher from Arizona State University found that the heat emitted by A.C. systems makes the air temperature hotter -- in some cases more than two degrees hotter.
Smithsonian magazine, which had access to the article published in the Journal of Geophysical Research, quoted the following passage from the researchers:
During the night (from 08 pm to 05 am), the effect of the AC systems was significant, increasing the mean 2 [meter]-air temperature between 1 °C [1.8 Fahrenheit] and 1.5 °C [2.7 Fahrenheit] for most of the urban area.
According to the abstract, the effect on the air temperature was negligible during the day, as the temperature increases were measured at night.
The abstract says heat released by A.C. units "exacerbates the nocturnal urban heat island." (In science-speak, the abstract says, "The AC systems modified the thermal stratification of the urban boundary layer promoting vertical mixing during nighttime hours.")
According to the Environmental Protection Agency, the heat island effect can make a temperature difference of up to 22 degrees in urban areas.
Similar studies on the heat expelled by A.C. units came with similar results in Tokyo and Madrid, as researchers in those cities found A.C. units causing about 1 or 2 degree Celsius increases in temperature.
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UPDATE May 14: ASU's School of Mathematical and Statistical Sciences was kind enough to send us a copy of the entire journal article. Here's part of the authors' conclusion:
Our work demonstrates one-Celsius degree local heating of urban atmospheres in hot/dry cities due to air conditioning use at nighttime. This increase in outside air temperature in turn results in additional electricity demands for air conditioning. Sustainable development and optimization of electricity consumption in cities would require turning 'wasted heat' from AC into 'useful energy' which can be utilized inside houses for various purposes including, for example, in water heaters. Implementing this mitigation strategy would achieve several objectives: successfully reducing the urban heat island temperature by one-Celsius degree at night, reducing AC electricity consumption on a city scale and providing a real example of urban climate mitigation. With regard to economic impacts, it is estimated for Phoenix metropolitan area that successfully reducing the urban heat island temperature with this strategy would result in at least 1200-1300 MWh of direct energy savings per day alone.
Climate change projections predict significant increases in the frequency, intensity and duration of summertime extreme heat events presenting significant challenges for the energy sector and electric grid (US Department of Energy 2013). Reliable methods are needed for forecasting energy demands that can help to inform and assist in the future planning of sustainable energy needs of rapidly growing urban areas. Comparison with observational data has demonstrated that the presented physics based modeling system is an effective tool for assessing urban cooling requirements in semiarid environments. In ongoing work, we are evaluating AC electricity consumption for different scenarios of urban expansion and under summertime extreme weather conditions. These studies are needed to develop reliable projections on the future cooling needs of rapidly urbanizing regions.
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