Inventors have tried to use hydrogen fuel cells as a cleaner way to create commercial electricity, but they've always been limited by the cost. That's beginning to change, however, thanks to a California start-up called Bloom Energy. Its Bloom Box about half the size of a shipping container generates electricity using solid oxide fuel cells, which provide juice by oxidizing a fuel source. In the case of the Bloom Box, that fuel source is natural gas, though the company hopes to substitute cleaner sources in the future. Silicon Valley companies like Google and eBay are already using Bloom Boxes for greener backup power, at a cost of about $800,000 each.