Facilities managers looking for ways to reduce air conditioning costs might find inspiration from recent research into passive cooling designs.
In one study, researchers at Drexel University in Philadelphia took a systematic look at the ingenuity of architects of old to see how their use of windows and other design treatments helped keep rooms cool in the days before air conditioning.
The researchers looked at porches, roof overhangs, awnings, large windows, high ceilings and exterior wall materials like wood and brick in old houses in San Antonio.
"Historic homes were designed for times when there was no HVAC," said Antonio Martinez-Molina, an associate professor at Drexel and the lead researcher on the study, published last year in research journal Energy & Buildings.
Since people are used to having the AC on, the cooling effects of design features like awnings and window placement for cross ventilation won’t be enough by themselves to make the rooms comfortable in hot weather. But they help, said Martinez-Molina, whose study was summarized by Drexel in Tech Xplore.
"Taking into consideration modern-day expectations, most [of these old homes] are not comfortable buildings — but they perform way better than we expect," Martinez-Molina said.
The study confirms what people tend to assume when it comes to passive cooling features: they work.
For example, shading devices can reduce indoor temperatures and cooling loads without causing a corresponding increase in humidity, and high ceilings, by design, lead to cooler indoor temperatures. Wall materials with a high thermal mass, like brick and limestone, are also good at lowering temperatures, according to the Drexel team’s research.
Another study, published in Advanced Materials Technology, says that passive cooling design principles won’t lead to lower AC costs right away but it could lead to new cooling products soon.
Akhlesh Lakhtakia, an engineering science and mechanics professor at Pennsylvania State University, collaborated with scientists in the U.S. and China to develop a type of porous plastic sheet that, if used as a wall liner, can reduce indoor room temperatures by the way it diffuses air through small holes in the material.
The researchers created the sheets, about a twelfth of an inch thick, and infused them with random-sized air pockets. The air pockets create a cooling effect by scattering light at different angles.
The sheets are effective at high daytime cooling because they reflect both visible and short-wave infrared light, not just the short-wave infrared light that other passive radiators reflect, said Lakhtakia, whose research was summarized by Penn State in Tech Xplore.
"The sheets would be an inexpensive, effective addition to … siding and roofs to … supplement air conditioning units," Lakhtakia said.
The researchers found the sheets were able to lower the temperature in an enclosed space by 8.4 degrees Celsius, or about 14 degrees Fahrenheit.
To conduct their test, the researchers enclosed a thermometer in a box created out of the sheets and placed the box outdoors in the sun. The air pockets reflected on average 96% of the visible and infrared light, cooling the temperatures in the box to 65.3 degrees Fahrenheit from 80 degrees outside. A cardboard box used as a control also had a cooling effect on its inside temperature, but only to 75.2 degrees.
The experiment was conducted a second time, this time inside, with similar results, although the interior temperature didn’t drop as much as it did outdoors.
“Room temperature has a significantly higher temperature than cold outer space [so] the radiative cooling effect was lessened inside the lab than in open air,” Lakhtakia said.
The sheets can be expected to have a useful life of several years, after which they can be recycled into new sheets.
"The sheets can be easily ground up, recycled and powder sintered on an industrial scale to reuse on another building," Lakhtakia said.
Should manufacturers create a commercial market for the sheets, the new product can give facilities managers an additional option for bringing down their AC costs by adding the sheets as wall liners.
"The passive daytime cooling requires no energy or electricity and can be applied to buildings in communities experiencing increased daytime temperatures year over year due to climate change," Lakhtakia said.
