Researchers at ETH-Zurich are borrowing from biology to cool buildings in a novel way: by making them "sweat." They've developed a permeable polymer mat that can be spread across rooftops to absorb moisture like a sponge when it rains, locking it inside. But when heated to a certain temperature by the sun the material becomes hydrophobic and pushes the water out. Just as humans expel heat from the body by imparting it to sweat on our skins that then vaporizes to carry the heat away, so would these "sweating" buildings remain passively cool by imparting their heat to water evaporating into the atmosphere, shaving up to 60 percent off air conditioning loads.
Reads good in theory, I like to see a roof top over year in the South, where all the buildings that stay wet, tend to turn green. But, it may work too. ;)
I wonder if the potential for mold and bacterial growth has been addressed. The abstract didn't mention it and my university doesn't carry a subscription to the journal it was published in, apparently.
At any rate, articles such as this one are why I haven't completely written off PopSci. Unfortunately they're almost completely buried in the other tripe PopSci publishes.
If the gel could absorb some liquid to kill the bacteria (something that wouldn't evaporate off, high BP) then it could continuously prevent growth while staying on the roof. That or a periodic antibacterial wash. We also have to consider the roof heating up enough to evaporate the water, possibly killing bacteria/algae.
The amount of water needed to cool a roof in a common area would require a building to hold several tons more.
There is no way a city in Texas could benefit from this idea. One day of hot summer sun could boil off 1000 gallons of water. Then you get into the ability to fix roof leaks and so forth. It is just as easy to make a well ventilated attic and proper insulation above the ceiling.