There's a great sci-fi novel, The Windup Girl by Paolo Bacigalupi, set in a post-oil world where tightly wound springs are used to store energy, as we do now with clocks. One of the subplots deals with a character trying to develop a material that can be wound tighter and more compactly without breaking. Or explosively unwinding. I'm not an engineer or physicist, so I've wondered, what is the theoretical limit for storing energy in a spring. Sci-fi stories usually do a bit of handwaving with nano/bio/quantum-tech to make magic materials, but what is the real science? Cars and laptops powered by springs sounds plausible (and awesomely steam punk) to me.