Just as mammals can reduce their body temperature by sweating, so a coating of heat-sensitive hydrogel could 'sweat' to cool buildings.
Wendelin Stark and his colleagues at the Swiss Federal Institute of Technology in Zurich produced a 3-millimetre-thick layer of thermoresponsive hydrogel. When heated to roughly 32 °C, the gel undergoes a phase transition from a wet state to a dry state, and releases water. With this water could go much of a building's heat.
Miniature model houses (pictured) with roofs coated with the substance were up to 20 °C cooler than uncoated models when exposed to simulated tropical midday Sun. The authors estimate that this translates into annual savings of 220 kilowatt-hours of energy for a detached house. A brief 'rain' recharged the hydrogel with water.
Adv. Mater. http://dx.doi.org/10.1002/adma.201202574 (2012)
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Hydrogel makes buildings sweat. Nature 489, 180 (2012). https://doi.org/10.1038/489180c
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DOI: https://doi.org/10.1038/489180c