Retractable sunshades have long been used to adjust the amount of sunlight an area gets during the day, but these are usually driven by motors. Taking inspiration from the humble pine cone, engineers at ETH Zurich have now developed a shading system that can open and close automatically – no electricity required.

In order to release seeds at the most opportune time, pine cones open their scales in warm and dry weather, and remain closed and sealed up when it's cold and wet. The natural mechanism behind this is clever – the scales are made up of two connected layers with fibers running perpendicular to each other that lets them contract as the air around them dries, which pulls the cone open.

In the past, this mechanism has inspired scientists in different ways. One team developed clothes with pores that open and close in response to warmth, to keep the wearer at a comfortable temperature. Other studies developed memory materials that would coil or curve as the humidity changed, and even robots that could use the bending and straightening cycle to crawl.

The ETH Zurich team, led by Chiara Vailati, built on the concept to create a prototype sunshading system that would move to provide maximum shade at midday and tuck away in the early morning, evening and night. To do so, the system consists of planks made with two layers of different wood – spruce and beech. These wood samples have been cut and joined so the grain of the spruce layer runs in one direction and that of the beech wood runs perpendicular to it. That mimics the pine cone structure, allowing it to bend in response to humidity.

To maximize the speed of that reaction, the researchers arranged these planks in pairs, with one mostly vertical and the other suspended about halfway up the first. When it's humid, such as overnight or in the morning, the pairs of planks stand straight and relatively flat against each other. But as the day progresses and heats up, drying out the air, the planks bend, so the lower one pushes the upper plank outwards to be almost horizontal, thus creating more shade underneath.

The team says this system could be used to make a kind of roof or window covering that opens and closes automatically, entirely under its own power. As well as reducing the use of electricity to cool a building, the system is made of environmentally-friendly materials, and has relatively low installation and maintenance costs.

The research was published in the journal Energy and Buildings, and the team demonstrates the mechanism in the video below.

Source: ETH Zurich

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