Using water to cool non-domestic air-conditioning systems could have reduced outside air temperatures by as much as 1.5°C during a heatwave in Hong Kong, researchers have found.
The study shows that water-cooled air-conditioning units are not only more energy-efficient, but also relieve the anthropogenic “urban heat island” effect, which sees cities have greater ambient temperatures than the countryside.
“We cannot ignore the effect of air-conditioning systems on the city environment,” said Yi Wang of the University of Hong Kong.
As air-conditioning units cool us indoors, they expel heat outdoors. The amount of heat displaced can be enough to increase outdoor air temperatures measurably, contributing to the urban heat island effect.
And of course, if it gets warmer outdoors, even more air conditioning is needed indoors.
Not all air-conditioning systems are the same, however. Many rely on air to cool their condensing units, but those that are more energy-efficient use water. One method of water-cooling is known as direct cooling; it involves seawater being fed into the buildings that house condensers. Another type is centrally piped, with a tall cooling tower where air ascends amid a water cascade.
Wong and colleagues wanted to explore how these water-cooled systems affect the urban heat island in Hong Kong, where a subtropical climate has led to air conditioning accounting for some 30% of electricity consumption. Using a meteorological model, the researchers investigated the impact on the heat island had non-domestic consumers used one of the two water-cooled systems, compared with the baseline case of air-cooling. The team performed the simulation for conditions matching 23-28 July 2016, when Hong Kong experienced extremely high temperatures.
The results suggested that, had water-cooled systems been used, outdoor air temperatures would have fallen by 0.5-0.8°C during the daytime, and as much as 1.5 °C between 7 and 8 pm. Wong believes this demonstrates the importance of exploring the effects of air conditioning beyond the immediate environments of the systems.
“Existing studies have concentrated much more on single-building energy-efficiency using new air-conditioning systems,” said Wong. “However, the climate within or around every building is connected to the overall city environment. A city has tens of thousands of buildings, [a] large number of streets and many other facilities, and the environment has impact on each individual building.”
The researchers now plan to extend their study by exploring the effect of air conditioning on urban air quality, and on temperatures in other parts of the world.
They published their findings in Environmental Research Letters (ERL).