Keeping buildings cool with colour

A new type of exterior paint and colour changing windows hold the promise of buildings that can cool themselves without using energy, and perhaps even generate electricity along the way.

These engineering concepts are aimed at reducing the amount of solar energy being absorbed by buildings as heat. That heat would normally have to be removed by  air conditioning, which represents a substantial amount of the electricity consumed by buildings.

Scientists at Purdue University have developed a white paint that reflects 95 per cent of sunlight that falls on it, which can actually result in the surface it coats being cooler than the ambient air temperature — by about 1.7 C in their experiments.

Of course, any white white paint reflects more sunlight than darker colours, but this paint does it more effectively than any developed in the past. The key is that it contains particles of calcium carbonate — basically chalk — that can reflect wavelengths of light other paints can’t.  

Sunlight is a combination of many colours of light — which we see in the rainbow — and invisible infrared and ultraviolet light. The colours represent different wavelengths of light, with blue being shorter than red.

Previous versions of solar reflecting paints could reflect 80 to 90 per cent of light that falls on them. But any light that is not reflected will be absorbed and its energy would heat up the building. The scientists were able to improve reflectivity by adding particles of different sizes, which would reflect more wavelengths of light. 

A researcher uses an infrared camera to compare the cooling performance of white paint samples on a rooftop. (Purdue University photo/Jared Pike)

One key principle in the idea is that some of the light this paint reflects goes right back into space, where it came from. Certain longer wavelengths of light aren’t absorbed by the atmosphere. They fall in what are called “sky windows,” that effectively use the whole universe as a heat sink. 

Again, this new paint was able to shed light so effectively, the surface it coated became cooler than the air around it. This is a simple form of passive cooling which could reduce the need for air conditioning.

Windows that self-tint and generate solar power

The second experiment at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) involves a technology that could lead to the development of windows that can change colour to block sunlight and also act as solar panels to generate electricity. 

Sunlight shining through building windows is literally the greenhouse effect, and is responsible for most of the unwanted heat in our buildings during summer months as all that solar energy is trapped inside. 

The new experimental windows are infused with materials called perovskites that have both crystal-like and liquid-like properties. A thin film of these is embedded in the window. As the film is heated by sunlight, the crystalline form of the perovskite changes, which changes its colour. Different temperatures will produce different colours, and thus more or less light filtering.

Passive houses, like this one in Calgary, are built to the highest standards of energy efficiency. These technologies could be part of similar designs in the future. (Dave Will)

The light filtering would reduce solar heating and thus air conditioning needs, which would save energy. But in addition to tinting windows, perovskites also have photovoltaic properties, and the researchers demonstrated in previous work that these could be used to produce electricity, similar to the way that ordinary solar panels work.

The amount of electricity produced by an individual window would likely be small, but big cities have a lot of windows. You could imagine that deployed widely these solar windows might represent a real contribution to the energy supply.

A member of the NREL team suggested a prototype energy-generating window could be just a year away.

Building energy efficiency could help climate goals

We tend to think of our energy future as vast fields covered with wind farms or sprawling solar panels. But there is another source of energy available — and that is the energy we save.

Both of these technologies are examples of how relatively simple changes to building designs might help achieve our climate targets by reducing energy demand. The U.S. Energy Information Administration reports that 20 per cent of global energy consumption was from residential and commercial buildings, and that’s expected to rise, especially in the developing world.

Making buildings more energy efficient through reflective paints, windows, insulation and other construction techniques, especially in countries where new construction is increasing, can go a long way to limiting energy use and meeting our climate goals.

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