When he left a job at Google, Paul Lambert knew that he wanted to work on a new challenge that would interest him for the next decade. The answer: designing a better heat pump.Heat pumps now outsell gas furnaces in the U.S, both because they can shrink energy bills and because people are realizing that it’s also a meaningful way to cut their carbon footprints. (If everyone in the U.S. got a heat pump, it could reduce emissions from buildings by as much as 64%.) The new state rebates that are beg...