A team of researchers at the University of Pennsylvania has taken inspiration from the natural cooling system of elephants to develop a porous, water-retaining cement tile. When attached to buildings, the tiles keep interiors cool through evaporating water.

The African elephant has one of the most efficient cooling systems in the animal kingdom. Its dark gray skin, which can be up to 4cm thick, is wrinkled and crisscrossed with cracks. It stores water in these cracks, which it absorbs during bathing. The water in these chambers evaporates, creating evaporative cooling that keeps the elephant cool for an extended period.

The material of the tiles is permeated by a planned network of cracks. The honeycomb-like structure also ensures even distribution of water over the entire surface. Image: University of Pennsylvania.

Researchers at Penn Engineering have examined this process of 'cooling without sweating' more closely, applied it to buildings, and developed a cement tile that exhibits similar properties to elephant skin.

The researchers published the results of their research in the study 'Elephant‐Skin‐Inspired Porous Cementitious Tiles with Programmable Crack Networks for Passive Cooling', which appeared in Advanced Materials.

Porous tiles with special crack networks

Developing a tile for cladding the exterior surfaces of buildings that can store and gradually release water sounds more trivial than it actually is. Conventional building materials are unsuitable for this purpose, as raindrops bounce and bead off them. They then run directly down the surface and cannot be stored to create a cooling effect through evaporation.

The scientists came up with the idea of deliberately introducing cracks into a material to create a network of tiny channels. These cracks develop a capillary effect that can draw water across the surface and hold it there.

As a suitable material, the scientists used ordinary Portland cement, which they mixed with diatomaceous earth (DE), a soft sedimentary rock composed of the shells of fossil diatoms. The resulting material was used to cast thin slabs. These were partially enriched with water and dried under precisely controlled conditions.

During the drying process, stresses build up in the material, which are released. This results not in random cracks but in planned crack networks. The cracks are designed to absorb water quickly and distribute it via their connections across the entire surface of the tile. This also ensures that water is not lost too quickly upon impact with the cement tile, the researchers write.

This works even when the tiles are attached to inclined surfaces and even against gravity. The honeycomb-like structure of the tiles helps with this. The hexagonal geometry ensures that water moves sideways in a zigzag pattern. Thus, it cannot simply flow downwards. Consequently, it remains evenly distributed in the tile for longer and can evaporate there. The researchers were able to demonstrate a cooling effect for a duration of up to 20 hours.

High passive cooling performance

The researchers determined the exact effects of the cooling experimentally. They subjected the tiles to constant infrared radiation and watered them regularly. The temperature under the tiles remained constant at 32 °C. Under conventional plaster, the temperature was 42 °C under the same radiation, and even 52 °C with crack-free plaster. The researchers assume that buildings clad with cement tiles can be kept about six to 11 °C cooler compared to those with conventional plaster. 

With this technique, buildings can be cooled naturally, according to the scientists at Penn Engineering. The energy required for active cooling systems for interiors can thus be significantly reduced. The effort required to create a passive cooling system based on cement and DE is relatively low. The mixture can be applied with spray guns to large panels, which are then used to clad buildings.

The researchers envision the use of intelligent irrigation systems that supply buildings with water from the outside in fixed cycles during dry periods. The operation of these irrigation systems could be controlled in real-time using weather data, for example, to consume as little water as possible with optimal cooling performance.