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Blue is cool for roofs

USA: A colour developed by ancient Egyptians, and routinely used on ancient depictions of gods and royalty, could be very effective in keeping buildings cool.

Previous studies with Egyptian blue, which is derived from calcium copper silicate, have shown that when it absorbs visible light it then emits light in the near-infrared range.

Now a team led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory claims the pigment’s fluorescence can be 10 times stronger than previously thought.

Measuring the temperature of surfaces coated in Egyptian blue and related compounds while they are exposed to sunlight, Berkeley Lab researchers found the fluorescent blues can emit nearly 100% as many photons as they absorb. The energy efficiency of the emission process is up to 70% (the infrared photons carry less energy than visible photons).

The finding adds to insights about which colours are most effective for cooling rooftops and facades in sunny climates. Though white is the most conventional and effective choice for keeping a building cool by reflecting sunlight and reducing energy use for air conditioning, building owners often require non-white colours for aesthetic reasons.

Berkeley Lab researchers have already shown that fluorescent ruby red pigments can be an effective alternative to white; this insight on Egyptian blue adds to the menu of cooling colour choices. Further, they found that fluorescent green and black colours can be produced with yellow and orange co-pigments.

Berkeley Lab scientists measured the temperature rise above air temperature observed in full sun for five pigment-coated samples, each 75mm square. The white and black samples show low and high temperatures. Photo: Berkeley Lab

In addition to its cooling potential for buildings, Egyptian blue’s fluorescence could also be useful in producing solar energy. Used on windows tinted with the blue, photovoltaic cells on the edges can convert the fluoresced near-infrared energy to electricity.

Substantial research over the years from Berkeley Lab’s Heat Island Group has found that reflective roofs and walls can cool buildings and cars. This reduces the need for air conditioning and mitigates the urban heat island effect. By reflecting the sun’s rays back to space, these cool materials also release less heat into the atmosphere, thus cooling the planet and offsetting the warming effects of substantial amounts of greenhouse gas emissions.

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