The race to find affordable sources of drinkable water has two new entrants, one of which should gladden the hearts of steampunk enthusiasts.
Most of the water on Earth is far too salty or polluted to drink. As groundwater supplies are depleted, supplying clean water to a growing population will be one of the major challenges of the 21st century, and climate change will make it harder. Current solutions are either too expensive or too slow to be universally appropriate.
Steam generation relies on an old idea for water purification. When water is turned into steam, it leaves impurities behind. If the steam is collected and cooled, it makes drinkable water. Solar steam generation uses the Sun's heat to turn water into steam, making it more environmentally friendly and cheaper than other energy sources, but heating the water affordably is still a challenge.
"I think there are many, many materials that can be used in solar steam generation, but wood really stands out in terms of performance as well as cost," Hu said in a statement.
The researchers floated a variety of wood species on top of a dish of salty water and focused light on them. Water was drawn into the pores and turned into steam by the heat, leaving a vacuum that was soon filled with more water. Charred wood outperformed graphite and carbon nanotubes used in previous studies. In particular, where other materials' pores quickly became jammed up with salt, the wood channels self-cleaned. Hu and co-authors attribute the success to wood's straight microchannels, its attractiveness to water, and poor heat conductivity.
But not all woods are the same. Pine and poplar easily outperformed the other species. Given the ubiquity of pine, that's good news. At a light intensity of 10 kilowatts per square meter (around ten times the intensity of direct sunlight), carbonized poplar blocks wasted less than 14 percent of the input energy. The production rate was 12 kilograms per square meter per hour (2.6 pounds per square foot per hour), an important figure given the frequently slow production rates of low-cost alternatives.
Another paper, in the same edition, took a more high-tech approach, using an “ionic analog” to photovoltaic cells to split water into protons and negatively charged hydroxides. These recombined as pure water. The process is currently much less practical than Hu's version, but could have a wider range of applications beyond water purification.