When you’re trapped on a boat, you can easily make fresh water, right? Simply let the sun heat up and evaporate salt water. Then trap the steam, condense it on a plastic surface and collect the fresh water. The liquid even gets sterilized in the process. So why can’t people around the world who lack clean drinking water do something similar?
Turns out, desalinating or sterilizing water with solar energy is way harder than Hollywood makes it look. The process is super inefficient and way too slow to be practical. "The average yield is only about 1 cup per day," says the U.S. Air Force survival guide, even when you’ve got eight hours of sun and plenty of water. But engineer Hadi Ghasemi, at the University of Houston, is trying to change that. He and a team at the Massachusetts Institute of Technology have developed a cheap material that desalinates water efficiently and fast using solar energy. And the secret to the new technology was sitting right on their desks: the graphite in pencils.
A simple solar still, and even more expensive versions with mirrors and lenses, heats up the entire water surface before it starts to evaporate, Ghasemi says. That takes time and wastes energy. "Why do we need to heat the bulk of the liquid to get steam?" Ghasemi says. "Why not concentrate the solar energy at ‘hot spots?’ " Then all the energy goes into creating steam. The trick to creating these "hot spots" is having the right material, he says. And that’s where the graphite in pencils comes into play.