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Paint that retains heat in winter: the illustration shows how the coating should look

Photo: Team led by Wang Fuqiang / dpa

Houses can be painted in any color. What they all have in common is that once applied, the color often remains the same for a long time. However, a Chinese research team has now developed a coating for houses that changes color depending on the temperature. In summer, it cools the building, and in winter, it warms it. The researchers were inspired by the animal world – more precisely, by chameleons. The results were published in the journal "Nano Letters".

The new coating for roof and exterior walls reflects about 30 percent of the incident sunlight at temperatures of more than 93 degrees Celsius. When heated to about 20 degrees Celsius, the surface began to change from dark to light gray. In winter, the temperature of the then dark gray paint is more than four degrees above the ambient temperature.

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Chamaeleo namaquensis

Photo: ingimage / IMAGO

Model: Chamaeleo namaquensis

"Buildings consume about 35 percent of the world's energy, 60 percent of which is used for heating and cooling to create comfortable indoor temperatures," the study authors write. It is true that paints have been around for a long time that passively cool a building by reflecting most of the sunlight; this makes it possible to save costs for air conditioning systems. However, these coatings also reflect sunlight in winter, which leads to an additional cooling effect and thus to a higher heating requirement. The research team therefore set out to develop a coating material that cools in summer and warms in winter.

The team led by Fuqiang Wang from the Harbin Institute of Technology in Harbin used a chameleon with the scientific name Chamaeleo namaquensis, which lives in desert regions in South Africa and Namibia, as a model. It has a light dandruff skin during the day to reflect as much sunlight as possible. In the evening, the color changes to a dark brown to absorb the remaining radiation and store the heat for the often cool nights.

The decisive factor for the new coating is the chemical compound crystal violet lactone, which is used, for example, as a coloring component in thermal papers. In combination with the substance bisphenol A, crystal violet lactone changes color depending on the outside temperature through a chemical reaction, especially in the range between 20 and 30 degrees.

Not yet suitable for everyday use

Tests showed that at an ambient temperature of 10 degrees Celsius, the temperature of the white cooling paint was 10.5 degrees, that of the coating 19.2 degrees. If the ambient temperature exceeded 30 degrees, both had something of the same temperature.

This was also noticeable in the building. In tests in winter, the interior temperature of a small building with the new coating was 1.2 degrees higher than in the identical house with cooling paint. The team of authors assumes that their invention can lead to savings of up to 20 percent of the annual energy requirement in mid-latitudes compared to white cooling paint.

The researchers have now been able to show that it is in principle possible to use the color changes for coatings. However, it may take many years before it becomes a purchasable product. Paints that passively cool a building already exist.