Where warm water comes into contact with surfaces, limescale builds up over time. We are familiar with this effect in the home from kettles, washing machines, etc. It is particularly important in thermal power plants, where deposits significantly reduce efficiency. As a possible solution, researchers have now developed a lime-repellent coating that uses microscopic grooves to prevent lime crystals from depositing. The skin of sharks served as a model.
We can regularly observe the effect on our kettle: Over time, an increasingly thick layer of white limescale forms inside the device. If the limescale covers the heating elements, it will take longer for the water to get hot. With a little vinegar or another descaler, the deposits can be dissolved again and the kettle regains its efficiency. On an industrial scale, however, calcification represents a major problem: Especially in thermal power plants, which are used to generate electricity, for example, the efficiency losses lead to higher fuel requirements, higher costs and more emissions. Current methods for descaling them are often expensive and require the use of environmentally harmful chemicals.
Avoid efficiency losses
“Limescale deposits can lead to enormous energy losses, at least two percent of the total world energy production per year, as the efficiency of heat transfer and flow performance decreases,” reports a team led by Julian Schmid from the Swiss Federal Institute of Technology in Zurich. This means that millions of tons of additional hard coal have to be burned. The researchers have now developed a possible solution to the problem: a coating that prevents the accumulation of lime crystals.
Although numerous research teams have already worked on such surfaces, no effective material that could be used on an industrial scale has yet been created. “This is partly due to the lack of understanding of how micropollutants settle and adhere in dynamic aqueous environments,” explain Schmid and his colleagues. Before they set about developing a corresponding surface, they examined in detail how individual growing lime crystals, the surrounding water flow and the surface interact on a microscopic level.
They came to the conclusion that instead of a solid coating, a soft surface material is probably most effective against limescale deposits. The team therefore chose a polymer hydrogel that is flexible thanks to a high water content. To prevent the lime crystals from accumulating, the microstructure of the material must be finer than the diameter of a single lime crystal, as the preliminary analyzes showed. “We varied the surface structure of the material to achieve the greatest efficiency and carried out the crystal experiments with this optimal structure size,” reports Schmid.
Natural-style coating
The surface of the coating is based on a natural model: It has microscopically fine grooves, similar to shark scales, which use the same trick to prevent dirt or algae from accumulating. Lime crystals have little grip on appropriately coated surfaces and are largely removed by the water flowing past. This means they cannot form a single layer.
“With this coating, up to 98 percent of limescale deposits can be prevented,” write the researchers. “We assume that these results provide important information for the design of lime-repellent surfaces.” The coating could also be applied on a large scale using common industrial processes. In addition, the hydrogel is biocompatible and could therefore offer a more environmentally friendly alternative to previously common decalcification methods.
Source: Julian Schmid (ETH Zurich) et al., Science Advances, doi: 10.1126/sciadv.adj0324