A recent article by researchers at Arizona State University reported that the heat emitted during the night from air conditioning systems increased the mean air temperature at two metres above the ground by more than 1°C for some urban locations with warm nights. This adds to the already significant “heat island” effect where energy use in all kinds of equipment and appliances may raise air temperatures by as much as five degrees.  During the day, due to greater thermal mixing of the air column, the effect of air conditioners on outdoor air temperature is negligible.

In areas with warm nights this increased air temperature causes air conditioning systems to work even harder, using more energy, putting out even more heat and warming the air even more.

Air conditioners are a form of heat pump and it is unavoidable that they move heat from inside the building being cooled to somewhere else, usually the air outside of the building. The researchers suggest that the warming effect of air conditioners might be reduced or eliminated if the waste heat is redirected to some other location. One possibility is to use it for heating hot water.

Our article on the GE GeoSpring™ Hybrid Water Heater (17 February 2012) has been one of GallonDaily’s most popular articles ever (see http://www.geappliances.ca/appliances/waterproducts/Geospring.pdf for a Canadian brochure). In cities which regularly experience warm nights, this water heater could be one way of effectively reducing the outdoor temperature and energy use impacts of air conditioning systems. The researchers calculate that using the heat from air conditioners for another productive purpose instead of dumping it into the urban environment would save more than 1200 Megawatt hours of electricity per day in the City of Phoenix, Arizona, alone. A heat pump based air conditioning system which directly discharges heat from the building into a hot water heater or other system which can make use of the unwanted building heat would further increase energy savings.

An abstract of the article (free) and the full version (subscription or payment required) can be found at http://onlinelibrary.wiley.com/doi/10.1002/2013JD021225/abstract