Centuries before the laws of thermodynamics were described, no calculations were needed to know that buildings in hot, sunny regions had to be painted white to lower the temperature inside. Finally, in the 20th century, the invention of air conditioning brought the definitive solution for cooling buildings, or so it seemed until the climate change emergency reared its head. According to the International Energy Agency, air conditioning and ventilation equipment today account for 10% of global electricity consumption, which for the United Nations Environment Program “will be a disaster for the planet.” In order to reduce this demand, researchers are working on more sustainable systems, some of which hark back to the old concept of painting or covering buildings—an age-old idea, but with 21st century technologies.
The problem with air conditioning equipment is not only that the production of energy to power it likely emits greenhouse gases, but also that, unlike passive solar heating, the law of thermodynamics means that cooling buildings to below ambient temperature generates heat that must be expelled outdoors, where it contributes to global warming. In other words, in the case of air conditioning, the local air is the sink for the heat generated. In contrast, passive radiative cooling solutions (“passive” because they work without additional energy consumption and “radiative” because they emit infrared radiation) are based on the idea that heat escapes from the Earth without heating the air. While near-infrared radiation, with a wavelength close to visible light, is absorbed by air molecules, these scarcely interact with the mid-infrared, for which the freezing emptiness of deep space acts as a sink.
Covering materials for sustainable cooling
Radiative cooling is relatively easy to achieve at night, which is why researchers are currently developing solar panels that can generate electricity after the Sun goes down. However, it is more complicated to do this in broad daylight when sunlight heats up the materials, a goal that until a few years ago was considered unattainable. It was first achieved in 2014, when a team from Stanford University published the design of a covering material made of layers of silicon dioxide and hafnium dioxide that reflects 97% of sunlight and demonstrates cooling to almost 5 °C below the ambient air temperature. The following year, researchers from the University of Technology in Sydney obtained a combination of polymers with a thin silver film that reflects almost 100% of the light in full summer sun, maintaining a temperature 3 °C lower than ambient and 11 °C cooler than a nearby roof painted white.
For Aurore Julien, an expert in environmental design at the University of East London, these investigations open a very promising path for sustainable cooling: “I believe that cool roofs, and more generally the careful considerations of emissivity of materials and finishes, can be a very important and effective way to support the passive cooling of buildings,” she tells OpenMind. “This type of consideration is becoming and will become a more normal part of building design.”
Subsequent research has tried to improve the technical qualities of these solutions. At the University of Colorado in 2017, a group of materials scientists created a translucent and flexible plastic sheet embedded with tiny glass spheres that reflect infrared radiation for better heat dissipation. The result is a material that reduces the temperature by up to 10 °C, close to the theoretical limit of daytime radiative cooling. Equally important is that these solutions are suitable for commercial use and at an affordable cost, which in this case would be achieved at a price of approximately half a dollar per square metre.
Paints for roofs and facades
In addition to coverings, another field that has been little explored so far is that of paints that can be applied directly to roofs and facades. This is the case with the latest approach to date, published by researchers at Purdue University in Indiana. They have developed an acrylic paint with a calcium carbonate filler that reflects 95% of sunlight—normal white paints reflect around 80%—and lowers the temperature by almost two degrees. Although the performance does not reach those of polymer coverings, the great advantage, as the authors write in the journal Cell Reports Physical Science, is that this system offers “the benefits of convenient paint form, low cost, and compatibility with commercial paint fabrication processes.”
However, for a variety of reasons it may not always be desirable to paint surfaces white. In another recent study, scientists at Columbia University have designed a two-layer coating whose top layer can be adapted to absorb different wavelengths of visible light so that the desired colour is visible. This material generates a cooling effect of between 3 °C and 15 °C below that of conventional paint of the same colour. According to the authors, it is a “simple, inexpensive and scalable” solution.
All these approaches offer innovative and sustainable cooling solutions. However, for Julien, an impediment to their widespread implementation is that not all of them will be valid in every climate. “I do believe these technologies are extremely promising in hot climates,” she says. However, “they may not be suitable in the context of a very variable climate or a building where much heating is required during the winter season, as much insulation would be needed on the roof, reducing the impact of the cool roof,” she explains. And since the majority of the world’s population lives in temperate regions, it can be difficult to find the balance between the benefits of passive cooling and the demand for heating.
For such cases, Julien thinks that it may be necessary to combine these systems with others: “The dynamic systems such as for example those using a combination of evaporative cooling and radiative cooling by spraying water on the roof at night are more suitable for climates that also require lots of heating in the winter.” The expert notes that further development and experimentation will be required to determine the usefulness of these innovations and their optimisation for different climatic conditions.
At the end of the day, the biggest obstacle to radiative cooling becoming widespread is that it will have to face down a powerful competitor with a head start of more than a century. “The issue generally with the use of AC is that it is a simple, cheap and effective way to cool a building,” sums up Julien. “They are readily available and familiar systems, and building owners are confident that they can maintain or replace these systems.” Therefore, the expert suggests that a true transition towards more sustainable technologies may only occur when limitations are imposed on carbon emissions or the energy consumption of buildings. “If regulations are in place, this will help in developing new technologies,” she concludes.