More than 40 percent of the world’s people face water shortages, and that number will rise in the years ahead, the United Nations says. Oceans are a source of clean water. Saudi Arabia and other nations have been withdrawing water from the seas and purifying it for human use by forcing the water through a membrane. It’s an energy-intensive process, and consequently, an air pollution culprit. Now the US Department of Energy has a new idea — concentrated solar power, or CSP. CSP isn’t new. Several solar farms already boast arrays of hundreds or thousands of mirrors that bounce sunlight onto a small collecting point. The heat from all that sunshine is transferred to a medium such as an oil or molten salt. The superheated medium is then pumped to a boiler to heat water, make steam and drive electricity generators. The energy department’s idea is to use solar-driven steam boilers not to drive generators, but to distill sea water. It would be cheaper and cleaner than current methods. The department is offering to fund tests of the idea. Now from the big to the microscopic: Researchers from Northeastern University and the Lawrence Livermore National Laboratory have borrowed from the human body to show a different way to purify seawater. In the body, water is filtered by proteins called aquaporins. Aquaporins are less than a billionth of a meter is diameter, forcing water molecules to pass through one at a time and thus screening out larger particles. But aquaporins are too fragile to use in an industrial filter. So, the scientists replicated them using carbon nanotubes, which are almost as small but stronger than steel. These engineered tubes — 50,000 times narrower than a human hair — carry a negative electric charge, which repels salt and other similarly charged molecules. Also, the tubes’ sides are smooth, unlike aquaporins, which speeds the filtering process six-fold.