In hot summers, cities can overheat particularly badly due to the sealed construction. However, floods caused by heavy rain also cause damage running into the billions each year, particularly in urban areas. As a solution to both problems, researchers are now presenting a building facade that absorbs water when it rains and releases it again on hot days for evaporative cooling. This could benefit the urban climate, especially in densely built-up areas.
The glass facade of a skyscraper can get so hot in summer that you can fry fried eggs on it. In densely built-up areas in cities, temperatures can even be around ten degrees higher than in green parks. These effects occur because asphalt, concrete, and other man-made surfaces retain heat from the sun's rays, while green spaces provide a cooling effect through the evaporation of water. In the event of a downpour, sealed surfaces also increase the risk of flooding and water damage because they only drain the water superficially but cannot absorb it. As a result of increasing urbanization and building density, the risk of heat and flooding in cities is becoming increasingly worse.
Rainwater used efficiently
One solution to the problem could be to expand the sewage system so that it can better absorb the suddenly occurring amounts of water. However, this would entail enormous structural effort and is also not a good solution in times of scarce resources, as Werner Sobek from the University of Stuttgart explains. His team therefore looked around for other solutions and developed the first hydroactive building facade. "Hydroactive elements represent an effective façade solution for neutralizing the urban heat island effect with minimal use of resources," explains Sobek.
The basic idea of these so-called "HydroSKINs" is that the facade elements absorb and store part of the rainwater. On hot days, this water can then evaporate and provide the desired cooling effect. The core element of the HydroSKIN are two textile layers, which are kept apart by threads and are therefore well ventilated. The high air circulation promotes the evaporation of water and increases the cooling effect of the facade. The knitted fabric is surrounded on the outside by a water-permeable textile cover, which allows almost all raindrops to penetrate and at the same time protects the knitted fabric from contamination. A film on the inside drains the water into the lower profile system. From there, either stored in a reservoir or used directly in the building, it can reduce water consumption. On hot days, water is fed back into the façade element, where it evaporates and thus ensures the natural cooling effect.
High-rise buildings particularly suitable
The first HydroSkin elements are currently being tested on the world's first adaptive high-rise building on the campus of the University of Stuttgart. “The results are promising. We were already able to demonstrate a temperature reduction of around ten degrees through the effect of evaporation in laboratory tests. The first measurements on the high-rise building at the beginning of September indicate a significantly higher cooling potential," reports Sobeck's colleague Christina Eisenbarth.
As she explains, high-rise buildings have particular potential for using hydroactive facades - and not just because of their large facade area. On the one hand, with increasing height, the rain hits the facade as driving rain at an angle, so that from a building height of around 30 meters more rain can be absorbed via the facade than from a roof area of the same size. On the other hand, the high wind speeds intensify the evaporative cooling effect and a cool air flow is created that pulls downwards into the urban space. Another advantage of the hydroactive facade elements is that they are very light and can therefore be retrofitted to any facade in new buildings as well as to existing buildings.
"This facade system represents an artificial retention surface for rainwater retention and evaporation in the building facade, which is not only incredibly beautiful due to its visual and haptic qualities, but also represents a milestone in the adaptation of the built environment to the acute challenges of our time," summarizes Eisenbarth together.
Source: University of Stuttgart, specialist article: Civil Engineering Design; doi: 10.1002/cend.202200003