Imagine a solar panel without the panel. Just a coating, thin as a layer of paint, that takes light and converts it to electricity. From there, you can picture roof shingles with solar cells built inside and window coatings that seem to suck power from the air. Consider solar-powered buildings stretching not just across sunny Southern California, but through China and India and Kenya as well, because even in those countries, going solar will be cheaper than burning coal. That’s the promise of thin-film solar cells: solar power that’s ubiquitous because it’s cheap. The basic technology has been around for decades, but this year, Silicon Valley–based Nanosolar created the manufacturing technology that could make that promise a reality. The company produces its PowerSheet solar cells with printing-press-style machines that set down a layer of solar-absorbing nano-ink onto metal sheets as thin as aluminum foil, so the panels can be made for about a tenth of what current panels cost and at a rate of several hundred feet per minute. With backing from Google’s founders and $20 million from the U.S. Department of Energy, Nanosolar’s first commercial cells rolled off the presses this year. ..