Vastly improved solar panels and battery technology. The efficiency of solar panels has remained essentially unchanged since it became widespread (occasional solar panels on houses, in calculators, etc) in the 1980s. I am not a scientist, but even I can tell it is possible for them to collect far more energy than they currently do. It's like the difference in capacity between a CD, a DVD, and a BluRay DVD. As far a s solar panels go we are still at the CD stage. When we make the leap to DVD and BluRay with solar panels, we will be able to power our homes and cars and store that energy with no need for gasoline or other fossil fuels. Imagine a car with a solar panel integrated into the roof and a battery that enables the car to go 500+ miles. All you'd need to do to recharge it is park it in the sun for 1-2 hours.

I think a Dyson sphere will always be in the realm of science fiction, but the sun sends a lot of energy here and virtually all of it is wasted.
