> If I need steady state gigawatt scale power in a specific location, Nuclear is the only green option.I don't believe that is true: one way to produce electricity is a thermal engine driving a generator, but for a thermal engine you need both a cold heat bath and a hot heat bath.
Those 2 heat baths could be externally delivered (a stream of ice, and a stream of steam, say) or one of the 2 heat baths could be chosen as the local environmental temperature heat bath.
Historically the local environmental temperature heat bath was selected for the role of the cold heat bath, and the hot heat bath was heated by say burning fuel (fossil or nuclear; and I am ignoring the chemical and mechanical energy terms of internal combustion engines).
If you could source a cold heat bath, one could select the local environments as the hot side heat bath instead.
Above the tropopause the atmosphere has become a lot more transparent for thermal infrared radiation, and thats why it is a lot colder up there, its in better thermal radiation contact with the CMB (the temperature of dark space), very close to the absolute zero point for temperature.
It is not a scientific challenge but a "mere" engineering one, to create a robust, all-weather aerostat where the "cable" transports mass (presumably, but necessarily a refrigerant) symmetrically up and down (in a loop) heating the upper layers of the atmosphere (puncturing the CO2 blanket), while cooling ground level environment. That large temperature difference persists day and night, winter and summer. So it is a form of green baseload energy generation, which helps cool the planet, and runs 24/7 reducing dependence on oil countries or places like Russia for nuclear fuel.
Depending on north/south lattitude, the height of the tropopause differs a bit.
You wouldn't want to risk such a contraption (some lightweight ~12km vertical zeppelin housing the up and down paths) falling on populated areas, but luckily 90% of the world population lives close to a coastline, so just anchor it further away from the cost than it is tall, if it falls over, at least it can't reach populated areas on land. Another upshot of coastal chimneys is that the sea is a very heavy thermal mass, so you won't run out of thermal energy that fast, the cold mass flow that comes down can be used to freeze water, desalinating it. During a transition period where conventional fossil / nuclear power plants still exist such ice or ice slurry could be pipelined to the "cold" thermal baths of such power plants, greatly improving the electric yield for the same amount of fossil / nuclear fuel.
There is just embarrassingly little research in this direction, to solve such an engineering challenge.