De-carbonising electricity generation

1. We'll need both wind and solar, plus hydro, tidal, small hydro, and wave power, and any legacy nuclear.   The reason we'll need both wind and solar is that the wind blows at night, when the sun isn't shining, and also that they are negatively correlated.  In summer in high latitudes, the wind drops off, but solar is surprisingly strong.  In winter, the sun in high latitudes is dim, but the winds tend to be strong.  Together, wind and solar can produce much less variable output than either on its own, except in the tropics.
2. We'll need 4 hours of storage.  4 hours will be enough to handle the morning and evening peaks in demand, and to do some short-term smoothing of the random fluctuations in wind, though geographically diversified wind farms will help with this.  But 4 hours will not be enough to take wind and solar to more than 90% of total generation.  We'll need long-term storage for that.
3. We'll have built-in generation overcapacity.  Remember, that wind and solar variability can mean not just too little generation but also too much.  Up to now, when it's potentially too much, the grid operators curtail output of wind and commercial solar farms.  They're also getting the ability to curtail output of rooftop solar too, in many locations.  In future, this output will not be lost but used to generate green hydrogen.
4.  Storing hydrogen is hard.  Its atoms are very small and easily escape through the interstices between larger atoms, meaning the using hydrogen for seasonal storage will be much less effective than using methane.  It also makes pipelines brittle.  Green methane can be made from hydrogen using the Sabatier method, with a small additional loss of energy.  Methane is already routinely stored for months with minimal losses; there are already methane (natural gas) grids in most mid- to high-latitude developed countries, where seasonal storage will be needed; gas peaking plants will not need to be retro-fitted to handle hydrogen; methane is much easier to store and transport than hydrogen; and it can be used for heating, though electric heat pumps may be cheaper and more efficient.
5. It's possible, though I am not convinced, that we will need some small percentage (10- 20%) of nuclear in the grid.  But we are a long way from 80% wind and solar penetration in most global grids--the most recent data have the global average at 10%.  Setting aside legacy nuclear, new nuclear, if it happens, will tend to be smaller plants, because the giants are just so expensive.  I've talked about SMRs, micro nuclear and the great potential for hydrogen-boron fusion reactors, but these are all at least 5 to 10 years away from them being successfully deployed.
6. Micro-hydro, tidal power, wave power and electricity generated from incinerating/gasifying rubbish will all be handy additions to the grid, because they  are uncorrelated with wind and solar.  Even waves, which are driven by wind, can be created by winds hundreds of kilometres away.

OK, how does the actual transition happen?

1. We could leave it to the market.  Wind and solar are much cheaper than new coal and gas, and comparable to existing coal and gas in most places on the globe.  As existing fossil fuel power stations age, they will be replaced by renewables plus storage.  This will take decades, however, so, since we want to substantially reduce emissions from power generations as quickly as possible, we'll need to give this process a nudge.
2. We could set targets for renewables.  Each utility/electricity producer will be required to achieve an annual/quarterly percentage from renewables, rising steadily over time, aiming for 100% within a decade.   Those utilities which exceed this target will earn carbon credits, those utilities which don't reach this target will have to buy credits.  This carbon penalty will encourage utilities to build out more wind and solar farms, as well as other renewable supply, including nuclear.
3. We could run a reverse auction where individual power stations would offer to close down for a fee.  Let's say a decline of 10% a year in emissions from electricity generation is required.  Each year, the government would offer a lump sum payment to the power stations which would close down, choosing those with the lowest offers until the total reaches 10% for that year.  Conditions could be applied to any deals: redundancy payments to workers; requiring the output of the power station to be replaced with new "firmed" output from renewables; making sure the new wind and solar farms are located close to the communities where coal power stations are ceasing to operate.

Source: Our World in Data

Micro nuclear reactors

I've already talked  about the small nuclear reactors NASA has developed for use in space and on Mars, called KRUSTY.  These will produce 1 kW of power, with the scaled-up version producing 10 kW.   I've also mentioned small modular reactors, here.

Ex-SpaceX engineers are developing a micro, portable nuclear reactor that can produce 1 MW of electricity, 100 times more than the scaled up version of NASA's KRUSTY nuclear reactor, designed to portable on the back of a lorry and with safety features allegedly making it much safer than the behemoths that catastrophically melted down at Chernobyl and Fukushima.  Since the company is still in the process of acquiring patents, there aren't many details.  

For me, one of the interesting aspects of this development is that SpaceX seems to have a division designing small nuclear reactors for use on Mars.  And on Starship?  No wonder SpaceX's latest video update about Starship shows it journeying to Mars without deploying solar panels!  

Using helium as a coolant certainly reduces the risk of explosions.  No mention of how nuclear waste is to be disposed of, though.

It's intriguing to see lots of private sector companies producing new designs for nuclear fission reactors.  The old super large designs seem to have got stuck in an expensive cul-de-sac.  Nuclear reactors will be very useful right here on Earth, if they can be made cheap enough and safe enough, as a complement to renewables in our electricity grids.  Lots of competing designs and companies might get us there quicker than large government bureaucracies, with both nuclear fission and fusion.

From New Atlas

California company Radiant has secured funding to develop a compact, portable, "low-cost" one-megawatt nuclear micro-reactor that fits in a shipping container, powers about 1,000 homes and uses a helium coolant instead of water.

Founded by ex-SpaceX engineers, who decided the Mars colony power sources they were researching would make a bigger impact closer to home, Radiant has pulled in US$1.2 million from angel investors to continue work on its reactors, which are specifically designed to be highly portable, quick to deploy and effective wherever they're deployed; remote communities and disaster areas are early targets.

The military is another key market here; a few of these could power an entire military base in a remote area for four to eight years before expending its "advanced particle fuel," eliminating not just the emissions of the current diesel generators, but also the need to constantly bring in trucks full of fuel for this purpose.

Those trucks will still have to run – up until the point where the military ditches diesel in all its vehicles – but they'll be much less frequent, reducing a significant risk for transport personnel.

Radiant says its fuel "does not melt down, and withstands higher temperatures when compared to traditional nuclear fuels." Using helium as the coolant "greatly reduces corrosion, boiling and contamination risks," and the company says it's received provisional patents for ideas it's developed around refueling the reactors and efficiently transporting heat out of the reactor core.

Radiant joins a number of companies now working on compact nuclear reactors, and a smaller number focusing specifically on portable units, which would include the floating barges proposed for mass-manufacture by Seaborg. It'll be a while before we see one up and running, but a clean, convenient, low-cost, long-life alternative to diesel generators would be very welcome.