If gates were serious, he would bankroll the construction of new generation nuclear powerplants, along with restoring our spent fuel recycling capacity. After Three Mile Island, the US all but abandoned nuclear power; the rest of the world kept moving forward with better and more efficient designs. Just reprocessing spent fuel rods saves on fuel costs, and reduces the amount of high level waste that needs to be stored. Even then there are reactor designs that can utilize that waste to produce power. The obstacles are not technical, they are political.
Next best is the mother of solar power: hydroelectric power. Combine sunlight, weather, and gravity, and you can generate an amazing amount of power with a well placed dam. The Columbia river hydroelectric dam system is a perfect example of this concept.
The holy grail of power generation though is fusion power, or more specifically, direct conversion of fusion to electron flow, without intermediate energy conversion steps.
And finally we come down to coal. Coal fired plants have been engineered to wring as much power as possible out of existing direct fired designs, as well as cleaning up the combustion products. What would be even better would be destructive distillation of coal, and burning the product in a combined cycle plant. That way, you can generate power, and reclaim valuable industrial products in the process. This isn’t new technology; “coal gas” or “producer gas” was used for lighting and heat throughout the 19th and early 20th centuries.
Well, he maybe watched the weather channel once..