Enlarge / Bubble wrap isn't just for stress relief. (credit: George Ni)
To boil water using the Sun, we typically burn fossil fuels carrying several-hundred-million-year-old solar energy that was extracted from underground at great expense. It's kind of Rube-Goldbergian. We're fortunate that the Sun's heat isn't strong enough to boil the oceans (or us), but extracting the Sun's energy at a significant scale is tricky.
The usual solution, as many magnifying-glass-toting children already know, is to concentrate sunlight and increase its intensity. Solar thermal plants, for example, use massive arrays of mirrors to focus sunlight and generate electricity. All that extra equipment gets pretty expensive-especially if you need the mirrors to track the Sun's position across the sky.
So how do we engineer another way? In the past, researchers made clever designs to concentrate the heat generated by lower-intensity sunlight into small volumes of water. This heat consequently created higher localized temperatures. While they managed to boil water with this method, they weren't able to ditch optical concentration completely.