So science supports the Earth being compatible with powering 100 billion of us. 1.7 petawatts, can we actually produce all that power though?
Power production increase from today
We produced and used (primary energy) 20 terawatts of power in 2021 on average, according to wikipedia. Admittedly often a controversial source, but sometimes ok for hard figures.
Our best estimate for the power needs of 100 billion people with good productivity is 1.7 petawatts. So we need about a 85 fold increase in power production for a 12x increase in population, from 8.1billlion today to 100 billion. This assumes all 100 billion being "productive", which is a political question we'll look at in a future article. But for our numbers this doesn't matter. The less people we have that aren't very productive, the less power they also consume. Basically the power need of 100 billion of equivalent productive people should stay the same regardless of how many people we need to achieve this.
Lets round that 85x to 100x for the sake of simplicity.
100x more power, how?
Our current power production sources:
Source: https://en.wikipedia.org/wiki/World_energy_supply_and_consumption
We need 100x. This simplifies things somewhat.
Renewables
Hydropower can be ignored, as a reasonable guess would be that there is no way to produce 100x the dam power that we currently have. Short of projects like Atlantropa. (which in itself might be a good idea, but would probably massively effect global weather patterns)
Atlantropa, Gibraltar dam
Other renewables is a more difficult question.
Can we increase wind power be 100x? A reasonable guess would be no. We also don't want to make "power production" that in reality consumes more resources than it produces (= is not profitable). So at this scale lets say that wind power also rounds to zero.
On Earth solar power, how about that? 100x current levels? Probably yes. Actually putting numbers to this would be extremely involved though. But a reasonable guess would be that about 100x current levels might be right on the limit of feasibility without causing major climate issues. Utilizing current desert areas etc. for solar power is probably ok. Cutting down trees or removing farmland isn't. That's a few % of what we need. Very good start, but not enough to make a dent yet.
Bioenergy scalability is also difficult to estimate. Ultimately this is a competitor to on Earth solar power though. But on the scale of 100x current levels, lets say this is also at the extreme limits of what can be achieved. Plant life growth rate being the limiting factor. Overall plant growth rates can be significantly increased by increasing the average temperature and atmospheric CO2 levels. But being able to do that is not realistic on a "short" time period like what we're looking at here. But bioenergy, another couple of % in the bag.
Geothermal is limited by the total global heat output of the Earth of 42 terawatt. Lets say we could capture a completely unrealistically high 10% of that. 4.2 terawatt. That's 0.25% of our needs. Basically zero. Theoretically we could of course extract more heat temporarily, cooling part of the Earths surface while doing so. But the scale, cost, and power limits just make this a non starter from a global power perspective.
Oil & Gas
Time to look at some of the current big players. Can they scale to 100x of current levels? https://en.wikipedia.org/wiki/List_of_countries_by_proven_oil_reserves shows that we currently have 50 years of proven oil reserves. If we 100x our needs, that means we use all of that in half a year. This is of course not a reasonable way to analyze the situation. We would need to look at discovery rate in relation to speculated oil price for more accuracy. But in the overall scheme of things, oil is not going to be able to sustain 100x current use for any amount of time (say 500 years).
How about gas then? A reasonable guess would be that it's very close to oil. https://en.wikipedia.org/wiki/List_of_countries_by_natural_gas_proven_reserves numbers seem to agree. 50 years of current proven reserves. Again, ok for current sustainable use. But not going to be anywhere close to enough for long term power at 100x current use.
Coal
Not looking good so far. We have a meager few % of what we need. Can coal help? According to iae.org total global consumption is 8 terakg per year. While the worlds proven coal reserves are 1petakg according to Wikipedia. 125 years at current consumption rates and prices. So over a year at 100x. This might start to be in the region of feasible for a few years (prices will increase with demand and more of the global coal will become economical to use). But for say 500 years? No. Coal is also out.
Nuclear
Ok, we still need at least 90% of our 1.7petawatt demand. Can nuclear do it?
Current Uranium fueled fission power plants use about 70,000 'tU' (tons Uranium probably, what kind of a unit is that? :D ) per year, while total known reserves are 8,000,000 'tU' extractable at $260 per kg according to world-nuclear.org. So about 114 years at current levels. This can likely scale much more than coal by increasing cost. A 5x increase in price seems to scale to approximately 10x the amount of available uranium. But as nuclear currently provides 4% of total power, this means we would need to scale uranium production by 2250x to cover the missing 90% of our target power. Unlikely to be workable. Maintaining or even slightly increasing the current % numbers might just be feasible though.
Not with current tech
So we might be able to reach 10-15% of our power needs with current energy production tech. That's obviously not good enough. We're not giving up just yet though.
Close term Future Tech
Thorium fueled fission is easily within our reach with some prototype reactors already in existence. But we have a fuel quantity issue with this also. Current global reserves are estimated at 6,000,000 tonnes. Thorium mining hasn't been prioritized though, so maybe the actual reserves could be say 10x that? Additionally almost all Thorium is a useful isotope, as compared to 0.7% for Uranium. So we basically get more potential power per tonne of Thorium reserve than tonne of Uranium reserve. If we could scale Thorium production to about 20x of the current Uranium production rate, this could be feasible for a short period of time, say 50 years or so. Very helpful, but not a sustainable solution. It could provide power for 100 billion of us while we iron out the tech we need long term though!
Long term Future Tech
Three main power production future technologies come to mind here. Direct fusion nuclear power, off Earth solar, and bomb type indirect fusion power.
Of these obviously only bomb type fusion power is the only one we can build right now. But lets take a closer look at all of these.
Direct fusion
Several types of direct fusion power are being researched and developed currently. Good for us! The limiting factor for fusion isn't available power sources, but the technology needed for it to work.
The old school magnetic confinement continuous plasma fusion concept has been around for a while, but is probably the most unfeasible of them all. Might be workable, but even after decades of work it still faces significant engineering challenges. And overall just seems like a very inefficient way to get positively charged nucleus to fuse.
High power ignition inertial confinement fusion as tested by NIF seems very promising. But laser tech or some other engineering solution is needed to make this tech feasible. The research on that is currently in what I would call a critical stage. They have shown positive power out from the fusion pellet control volume. Which is a huge achievement. And brings with it the potential to get good data and significantly speed up r&d. Not something that we can count on as being feasible though. But we will likely have a accurate estimate within 2-5 years.
Kinetic ignition inertial confinement fusion also seems promising as being developed by First Light Fusion. But still much too early to know if this will be workable or not based on public information.
There are several other fusion startups with their own type of power production planned. But initial guessing by me is that they either aren't based on good ideas, or they are much too early in the development cycle to be able to say if they will be workable based on public information.
Off Earth Solar
This at first seems like a very viable concept with good potential for 'limitless' power. But. No-one is seriously planning this yet. And there are very significant issues in power transmission down to Earth. For a space based civilization this isn't a issue. But on our short time scale, and our "goal" of 100 billion productive people on Earth this likely isn't going to help us. Long term though, likely to be extremely viable.
Indirect Fusion (H-bomb)
Instead of wasting fission on direct energy production in fission plants, lets instead use it to ignite inertial confinement fusion. This is probably the only viable way we have of currently of reaching 1.7 petawatt in a sustainable way in the short term, without any new tech needed.
Well ok, some new tech is always needed. But in this case it's just a question of engineering, scale, and cleanliness. Almost no science needed.
Basically we would do this with either very big piston engines powered by "internal combustion" of hydrogen bombs, or water boiling turbine equivalent. This actually isn't as crazy as it sounds. Well ok, a hydrogen bomb powered turbine might be crazy. But a large piston engine not so much. Effectively the same thing has been looked at by NASA with project Orion in the 1950s and 1960s. It's surprisingly feasible to push a plate with a atomic bomb without destroying it. Pushing a plate is approximately the same thing as a pushing a piston. Very steampunk. And also very doable.
And as a huge bonus, this also has the potential to reach very high efficiency in regards to useful energy output / total energy produced.
This is however not being worked on at the moment either. So a argument could be made that this is would be far off. But technology wise the biggest problem would be series producing the smallest possible hydrogen bombs in a big enough quantity. And some serious calculations on the availability of materials would need to be done. Still likely to be mostly a engineering challenge. And engineering is easy.
Conclusion
Yes we will need future tech to be able to produce the power that 100 billion productive people will need.
That's a big bummer. But we're already making good progress on some of the viable solutions. And there is even at least one solution we could build today if we wanted to!
So basically, yes. Yes we can most likely get to 1.7 petawatts with a high degree of certainty, and be able to produce the power needed by 100 billion productive humans.