Much debate about energy centres on replacing coal power stations with something cleaner, yet that is not really the hardest energy problem. We already know how to do renewable electricity generation. Reducing cost and finding more efficient ways of storing energy to work around intermittency are engineering problems that can be solved in time.
Once you solve those problems, short-range transport can easily be fixed, as can long-haul overland transport. Get as many people as possible into electric trains, trams and trolley buses, and convert cars over to electricity wherever practical. In South Africa we have some big practical problems such as persuading business that rail freight is reliable, and re-architecting cities for public transport, but these are problems that can be solved with the political will to do so.
Ships can potentially convert over to wind: sailing ships were pretty fast by the time they were replaced with steam and with modern materials and weather prediction, we should be able to do better. We can eliminate some of the larger cargo requirements (oil: we won’t ship that around as a fuel in a clean energy world; ores: better processed closer to source if you care about energy). There is in the meantime a variety of technologies that can be used at least as partial (hybrid) solutions to reduce the carbon footprint of shipping; once these develop further, we could see a return to real sailing ships, but without the huge crews required for 19th-century technology.
The much harder problem is where air travel is the only option. Air travel requires energy storage of high density both in volume and weight. Hydrogen has great energy density in terms of weight, but not volume. Electric aircraft are impractical because batteries or fuel cells have too low an energy density, which cannot be offset by the much higher efficiency of converting stored electrical energy to motion, as compared with converting heat to motion.
We aren’t likely therefore to see electric intercontinental planes built the same ways as jet aircraft. That doesn’t mean however that there aren’t other form factors that could work. For example, several experimental electric helicopter projects are active. Current prototypes are only good for short flights (up to 6 minutes in one case) but the key to these designs is minimal weight, making longer flight times viable with bigger batteries. This seems to me a more promising approach than that of helicopter giant Sikorsky, who have replaced the engine of an existing design with an electric motor in a design that’s yet to fly. It’s unlikely that we will see an electric helicopter capable of flights of an hour or more any time soon, but it’s interesting nonetheless that it’s possible to do this with something bigger than a toy.
Yet another possibility is to power airships with hydrogen. You can store the hydrogen in the gas bag (possibly along with helium to reduce the fire risk) and, as you burn it, drop ballast to keep the weight constant (in a form that will not cause damage as it falls, e.g., some of the water created from burning the hydrogen). I haven’t done the arithmetic on the volumes needed and how long a trip this would work for but I’m not the only one to have thought of the idea.
There are many exciting possibilities for new energy technologies out there. Let us not limit our imagination by thinking that burning stuff we can’t replace has brought us to the pinnacle of civilization, and it’s all downhill from here on.