When you're leading the department's clean-energy and efficiency programs, it's fair to assume you're passionate about clean energy. That might be an understatement for Danielson. "I'm so interested in clean energy, it's pretty much what I talk about at work and over beers and elsewhere," Danielson, 36, says. "It's honest to say clean energy is the core of my life." Danielson joined the Energy Department in May 2009 after cofounding the clean-energy investment practice at General Catalyst Partners, a Boston-based venture-capital fund. Danielson says his office is critical right now. "We are at a truly unique time and place when it comes to clean energy," he says. "We have these two mega-forces that we need to take advantage of now, or we could really see the opportunity pass us by." Those two forces are the natural-gas boom and growing renewable-energy development. Danielson, a California native, earned an undergraduate degree in materials science and engineering from the University of California (Berkeley) and a Ph.D. in the same discipline from the Massachusetts Institute of Technology. While at MIT, Danielson started the MIT Energy Club, where he collaborated with Ernest Moniz, Obama's new Energy secretary. At that time and until he left to head the Energy Department, Moniz was director of MIT's Energy Initiative.