Excess heat from air conditioners causes higher nighttime temperatures

May 15, 2014 by Rhonda Olson
Excess heat from air conditioners causes higher nighttime temperatures
An Arizona State University research team found that releasing excess heat from air conditioners running during the night resulted in higher outside temperatures, worsening the urban heat island effect and increasing cooling demands. Credit: freeimages.com

(Phys.org) —A team of researchers from Arizona State University has found that releasing excess heat from air conditioners running during the night resulted in higher outside temperatures, worsening the urban heat island effect and increasing cooling demands. 

"We found that from was maximum during the day but the mean effect was negligible near the surface. However, during the night, emitted from systems increased the mean air temperature by more than 1 degree Celsius (almost 2 degrees Fahrenheit) for some urban locations," said Francisco Salamanca, a post-doctoral research scientist at Arizona State University's School of Mathematical and Statistical Sciences.

The research is presented in the paper, "Anthropogenic Heating of the Urban Environment due to Air Conditioning," published in the March 6 issue of Journal of Geophysical Research Atmospheres.

Salamanca, the lead author of the paper, is joined by Matei Georgescu, an assistant professor in ASU's School of Geographical Sciences and Urban Planning and a senior sustainability scientist in the Global Institute of Sustainability; Alex Mahalov, The Wilhoit Foundation Dean's Distinguished Professor in the School of Mathematical and Statistical Sciences at ASU; Mohamed Moustaoui, an associate professor in ASU's School of Mathematical and Statistical Sciences; and Meng Wang, a graduate student also in ASU's School of Mathematical and Statistical Sciences. All five authors are affiliated with ASU's College of Liberal Arts and Sciences and the Global Institute of Sustainability. 

The paper focuses on the anthropogenic contribution of air conditioning systems on air temperature, and examines the electricity consumption for the rapidly expanding Phoenix metropolitan area, one of the largest metropolitan area in the US. Phoenix is located within the semiarid Sonoran desert and because of its harsh summertime conditions makes considerable use of air conditioning systems. 

To keep people cool, air conditioning (AC) systems can consume more than 50 percent of total electricity during extreme heat and put a strain on electrical grids. Cooling demands for rapidly expanding urban areas like Phoenix are likely to increase considerably during the next several decades. To address future energy needs in a sustainable manner, the researchers determined it was essential to study current AC demand and assess AC waste heat.

For this investigation, researchers used a physics based modeling system to evaluate the impact of heat emission from air conditioning systems on air temperature. This physically based dynamic approach has the advantage of taking into account both urban scenarios, such as the size and shapes of buildings, and climatic factors, such as temperature and wind speed, when the energy consumption is calculated. This approach is necessary given the inherent feedbacks associated with AC systems. For example, hot summer nights will lead to increased air conditioning demand, which in turn will output additional waste heat into the environment, leading to further increase in AC demand, resulting in a positive feedback loop. 

The authors simulated a 10-day period, covering ten extreme heat days from July 10 to 19 of 2009. They used the non-hydrostatic version of the Weather Research and Forecasting (WRF) model coupled to the Noah land surface model to analyze the contribution of AC systems on air temperature. To evaluate the ability of WRF to reproduce the near-surface climatology, eleven weather stations maintained by the National Weather Service and the Arizona Meteorological Network were used. 

This research shows that release of waste heat raises the outdoor temperature and as a result increases the electricity consumption needed for cooling. The analysis of this feedback required using a building energy model and an atmospheric model that were dynamically coupled. The atmospheric model supplied the building energy model with the outdoor air temperature, outdoor air humidity and the boundary conditions for the temperature calculation for building walls and roofs. The building energy model then provided the atmospheric model with the heat fluxes associated with the energy consumption of AC systems within the buildings. 

They found that the effect of the AC systems was more important during the night due to the limited depth of the urban boundary layer. The effect is stronger from late afternoon to early morning. A smaller quantity of excess AC systems heat ejected during the night can increase the air temperature more compared to a greater quantity released during the daytime when the hot sun is beating down.

"Our work demonstrates one Celsius degree (almost 2 F) local heating of urban atmospheres in hot and dry cities due to air conditioning use at nighttime. This increase in outside in turn results in additional demands for air conditioning," Salamanca said. "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, 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 on a city scale and providing a real example of urban climate mitigation."

With regard to economic impacts, it is estimated for the Phoenix metropolitan area that successfully reducing the temperature with this strategy would result in at least 1200-1300 MWh of direct energy savings per day alone.

According to the US Department of Energy, summertime extreme-heat days are projected to become more frequent and intense as a result of climate change, presenting significant challenges for the energy sector and electric grid. 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.

Explore further: Future heat waves pose risk for population of Greater London

More information: "Anthropogenic Heating of the Urban Environment due to Air Conditioning." F. Salamanca. Journal of Geophysical Research: Atmospheres Accepted manuscript online: 9 MAY 2014. DOI: 10.1002/2013JD021225

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User comments : 13

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MP3Car
3.4 / 5 (5) May 15, 2014
Wondering why this heating effect is "negligible" during the day? It's putting out more heat during the day, as mentioned in the article. Did I miss something????
Waaalt
5 / 5 (7) May 15, 2014
Wondering why this heating effect is "negligible" during the day? It's putting out more heat during the day, as mentioned in the article. Did I miss something????


The sun.

It's less significant because it's a much smaller percentage of the total.
runrig
4.6 / 5 (11) May 15, 2014
Wondering why this heating effect is "negligible" during the day? It's putting out more heat during the day, as mentioned in the article. Did I miss something????


The sun.

It's less significant because it's a much smaller percentage of the total.

Plus during the day the heat will escape vertically. At night, a surface inversion is likely, except in cloudy/windy conditions, above roof tops and "cap" in heat. A kind of heat island effect.
TegiriNenashi
1.7 / 5 (11) May 15, 2014
Next experiment: place thermometer in the AC vicinity. Write an article about the observation record proving "robust" temperature increase. Witness media going mad making all kinds of silly claims about hurricanes, tornadoes, heat waves, fires, and diminished bio-diversity.
Uncle Ira
4.6 / 5 (9) May 15, 2014
Next experiment: place thermometer in the AC vicinity. Write an article about the observation record proving "robust" temperature increase. Witness media going mad making all kinds of silly claims about hurricanes, tornadoes, heat waves, fires, and diminished bio-diversity.


@ Tegin-Skippy. I don't think you are thinking that out right. Ol Ira don't think you going to learn much by just measuring the heat right beside the AC. To know anything good you need to measure the allover air temperature. And the AC heats aren't what caused the hurricanes, they start out over the warm big water over by Africa. I really don't think by itself the AC causes them other things either, not by itself. You would probably have to add in some other heats to get them going.
Tom_Andersen
2.1 / 5 (10) May 15, 2014
AC efficiency is much higher at night, and electricity is cheaper, so the only real effect this has is to alter temperature records to the benefit of CAGW.
Huns
5 / 5 (4) May 16, 2014
This would seem to be a good justification to advocate for more ground-source heat pumps. These work by burying 100s of feet of tubing in the earth (either vertically in a few deep wells, or horizontally in a grid-like pattern) to perform heat exchange. The average temperature at the heat exchanger, being somewhat deep underground, remains fairly stable throughout the year.

With an air-source (conventional) heat exchanger, you have to transfer heat to the hot outside air in the summer and transfer heat from the cold outside air during the winter (unless you use a furnace.) The air is almost always the wrong temperature for what you want to do! With a ground-source heat pump, the heat exchanger is usually cooler than the outside air in the summer and warmer than the outside air in the winter. So, it lends itself to higher efficiency.

Unfortunately, the tech is so new and there are relatively few companies producing products for it. Bad for cost and innovation. We need more companies!
adam_russell_9615
5 / 5 (1) May 16, 2014
We should build houses underground to save energy.
Eddy Courant
1 / 5 (4) May 17, 2014
Air Conditioners, now there's your AGW.
Caliban
4 / 5 (4) May 17, 2014
Air Conditioners, now there's your AGW.


And right you are, EC.

Just like it says in the study's published title: "Anthropogenic Heating of the Urban Environment due to Air Conditioning".

Let's see here...heat added to -or more appropriately, moved around in- the atmosphere by air conditioning, so that people(the "Anthro" part of AGW) can stay cool. So, no problem with relocating ambient heat from one place to another -even though part of that is, in fact, the "Global Warming" part of AGW. And this is also joined by the waste heat generated mechanically by the air conditioner pump frictionally, electric resistivity of the electrics in the air conditioner, the waste heat of electrical transmission losses due resistivity for the power supply to the home electric supply and thence to the AC unit itself, and, finally the waste heat and GHG emissions produced in the minining, transport, and production of the electricity being delivered to that one of millions of AC units.

AGW!
Caliban
4 / 5 (4) May 17, 2014
Hey EC,

Don't you just love being right --especially when it's for all the wrong reasons?!

And, for myself, I'm very happy that we were able to be in agreement about this.

Aren't you, too?
PinkElephant
4 / 5 (4) May 17, 2014
@adam_russell_9615,
We should build houses underground to save energy.
Perhaps J.R.R. Tolkien was even more of a visionary than he's normally given credit for ;)

@Caliban,
If you're going to bait trolls (an odd choice), at least try not to confuse them beyond their already confused state. The notion that AGW is driven to any significant extent by heat emissions from human technology is already enough of a common fallacy, without you giving it a helping hand.
Maggnus
3.7 / 5 (3) May 18, 2014
@adam_russell_9615,
We should build houses underground to save energy.
Perhaps J.R.R. Tolkien was even more of a visionary than he's normally given credit for ;)

@Caliban,
If you're going to bait trolls (an odd choice), at least try not to confuse them beyond their already confused state. The notion that AGW is driven to any significant extent by heat emissions from human technology is already enough of a common fallacy, without you giving it a helping hand.
Just ask Alchem!

As an aside, I note that Caliban did extend his analogy beyond the simple mechanics to the production and delivery of the electricity for the appliances, so he was more right than wrong.

Still, given the stunningly obtuse mindset of the bulk of denierville, your caution has import.

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