One of many challenges in facing a future of higher temperatures is how to keep buildings cool without expending more energy. Inquisitive and creative minds continue to find solutions to this problem.
The answers can start with roofs, so often out of sight and out of mind. Roofs play a large role in the interior comfort of buildings of all sizes. Projections for a warmer planet have led to the development of what are called “cool roofs” and “smart roof” technologies.
In its simplest form, a cool roof is one that reflects rather than absorbs heat. Instead of using dark colours, designers are beginning to specify light colours to increase solar reflectance and reduce thermal absorption.
An ultra-white paint based on a solution of barium sulphate particles, developed at Perdue University in the U.S., may be the whitest paint ever. According to the journal Applied Materials and Interfaces the cooling effect of this coating could reduce air conditioning costs by as much as 70 per cent. It is now under development for commercial use and could cost no more than conventional paints.
“Overall, our BaSO4-acrylic paint shows a standard figure of merit of 0.77, which is among the highest of radiative cooling solutions while providing great reliability, convenient paint form, ease of use, and compatibility with the commercial paint fabrication process,” says project lead, nanotechnology engineer Xiulin Ruan.
Not only does the new ultra-white paint keep roofs cooler during the day, it also keeps them cooler at night, which can allow pent-up structural heat to escape.
Another solution to cooling roofs is as old as time itself. Trees.
Heat waves this summer have drawn attention to the fact that many homes and small buildings in urban areas have little or no tree cover whatsoever. These vast landscapes form what are called “heat islands” made up of roofs and roads that absorb and significantly magnify the heat effect of the sun. Anyone walking on hot pavement has experienced firsthand the heat drop when stepping into shade.
“The tree is such a fundamental and often overlooked piece of geoengineering,” Robert McDonald, an urban ecologist at the Nature Conservancy in Arlington, Va., told the Wall Street Journal. He notes that poorer neighbourhoods in particular have 30 per cent fewer trees and are 7 F hotter than affluent areas.
While these might be suitable options for regions with year-long summer climates, areas with distinct seasonal changes like Canada can at times benefit from heat absorption in order to reduce interior heating costs.
The answer may lie in what are being called smart roofs, those that can be selective as to when to absorb heat. The challenge, however, is to find a solution that overcomes any chemical instability and flammability issues.
Scientists are working on this. Jan Kosny and Bill Miller have experimented with Phase Change Materials (PCM) that can both absorb and release thermal energy. By sandwiching inorganic PCMs between sheets of aluminum foil in combination with an attic heat exhaust system, Kosny and Miller claim the roofs perform well in winter conditions while reducing interior attic temperatures by 20 F during the summer.
Tests conducted at the U.S. Department of Energy’s Oak Ridge National Laboratory have demonstrated how PCMs melt and absorb heat as the temperature increases without a significant increase in its own temperature. As the ambient temperature around the liquid PCM decreases, it solidifies again and releases the stored heat.
Meanwhile, scientists in Zurich have developed a roof surface incorporating a thermo-responsive hydrogel that sweats to cool buildings. And in Portugal, conventional roofing tiles are being reimagined as smart tiles using a similar optical structure seen in smart windows. This allows infrared light to pass through when below a predetermined temperature threshold but blocks infrared passage when above that threshold.
Adopting more passive alternatives to reduce the energy demands for cooling will become increasingly important over time. Innovations in roofing and increased tree shade literally offer top-down solutions.
John Bleasby is a Coldwater, Ont.-based freelance writer. Send comments and Inside Innovation column ideas to firstname.lastname@example.org.