[adrian_b]

Deuterium is also not renewable, even if it is more abundant than uranium.

The H1-B11 reaction would be a much better energy source than anything else, but for now nobody knows any method to do it. There is no chance to do it by heating, but only by accelerating ions, and it is not known how a high enough reaction rate could be obtained.

[rnhmjoj]

I'm curious, what are you considering for stating that deuterium is not renewable? AFAIK there's an essentially limitless supply in the form of HDO in the oceans[1] and there are cost effective methods[2] to isolate it.

[1]: https://en.wikipedia.org/wiki/Semiheavy_water

[2]: https://en.wikipedia.org/wiki/Girdler_sulfide_process

[adrian_b]

If you are able to say that there is a limitless amount of deuterium in the oceans, than you can say the same about the amount of uranium in the oceans, even if the amount of dissolved uranium is about one thousand times less.

Both the amounts of deuterium and of uranium in the solar system are finite and smaller than of the abundant elements. Moreover, the natural processes that create deuterium and uranium within a normal stellar system are slower than those that destroy them, so there is no chance of their quantities ever increasing.

Unlike using other chemical elements to make some stuff, using deuterium or uranium for producing energy destroys them without any means to regenerate them, so it is by definition a non-renewable process.

The hydrogen (protium) in the Sun is also non-renewable, but its quantity is enormous in comparison with the amount of deuterium existing on Earth (and the amount of energy that the Sun produces per proton is greater than the amount of energy that can be produced per deuteron).

Like deuterium is extracted from sea water, uranium can also be extracted from sea water, where it is one of the most abundant metals, except for the alkali metals and the alkaline earth metals. However the energy required for extracting uranium is significantly higher, due to its much lower concentration than deuterium (though deuterium is difficult to separate due to its similarity with the lighter isotope of hydrogen, while for the uranium ions much more efficient chemical reactions would be possible, which would bind uranium ions without being affected by the other dissolved ions).

[perihelions]

Then wind power is not renewable either! The saturation wind power potential of this planet (250 terawatts?), integrated from now until this planet ceases to exist, is a finite number—and it is actually a smaller number than this planet's deuterium resource.

[ben_w]

> Deuterium is also not renewable, even if it is more abundant than uranium

Technology correct, in that after around a hundred trillion years even the red dwarf stars will have stopped burning hydrogen.

But last I checked as yet there is no known way to harness the only (and even then merely suspected) infinitely renewable energy source: the expansion of the universe.

[adrian_b]

The amount of deuterium contained in a planet is a very small fraction of its hydrogen content.

The amount of hydrogen contained in a medium-sized planet like Earth is extremely small in comparison with the amount of hydrogen contained in a star.

The amount of energy that can be produced by fusion per deuteron is smaller than the amount of energy that is produced in stars per proton.

With all these factors multiplied, the amount of energy that could be obtained from all the deuterium contained in Earth is many orders of magnitude smaller than the energy produced by the Sun or by any other star.

Moreover, the energy obtained from fusion could never exceed a very small fraction of the energy received by Earth from the Sun as light, otherwise it would lead to a catastrophic warming of the Earth.

Nuclear fusion reactors are not really useful for solving Earth's energy problems. They could have a crucial importance only for the exploration of the Solar System and for providing energy for human bases established on Moon, Mars or other outer planets.

For Earth the only problems worth solving are how to make better batteries, including very large capacity stationary batteries, how to make other large capacity energy storage devices, e.g. thermal devices, and how to improve the energy efficiency of the methods used to synthesize hydrocarbons from carbon dioxide and water.

Making hydrocarbons at large scale from carbon dioxide would be the best way to sequester carbon dioxide, offering the choice between just storing the carbon in safe products (paraffin like) and using a part of the synthesized hydrocarbons for generating energy in a carbon-neutral way.

[ben_w]

I am already aware of those things.

On earth, there is an estimated 4.85×10e13 tonnes of deuterium; the energy density is 3.4x10e14 J/kg, giving a total yield of 1.649e31 joules. If you deleted the sun, this would be sufficient to maintain the current temperature of the Earth for ~9.5 million years: https://www.wolframalpha.com/input?i=%281.649×10%5E31+joules...

At "merely" the level of current human power consumption, this will last about 43 times longer than C3-photosynthesis, about 26 times longer than the oceans, about 5 times longer than before Andromeda merges with the Milky Way, and 6-3 times longer than when the Earth is currently expected to be absorbed into the outer envelope of the sun as it enters the Red Giant phase: https://www.wolframalpha.com/input?i=%281.649×10%5E31+joules...

Even if the sources I read giving those estimates are off by a factor of 10, deuterium alone, from earth alone, used as a total replacement for the sun, would still last longer than our species is likely to last before even natural evolution would have us speciate.

In the hypothetical future where we had a useful fusion reactor, the gas giants become harvestable, so the fact they're not on earth is unimportant. Likewise, on this timescale, every star in the nearest several galaxies — indeed, even absent novel technology and "merely"(!) massively scaling up what we've already invented, we already 'know'* how to get to places so far away that cosmic expansion is what would prevent a return trip.

As I said, it's technically correct that it is a finite resource. All I'm saying is that this is not a useful point on the scale at which we operate.

I expect it will be a useful point when we're star-lifting, but not now.

> Nuclear fusion reactors are not really useful for solving Earth's energy problems. They could have a crucial importance only for the exploration of the Solar System and for providing energy for human bases established on Moon, Mars or other outer planets.

I agree, however I also hope nobody makes a convenient cheap fusion reactor due to the proliferation impact of an affordable switchable source of neutron radiation.

> For Earth the only problems worth solving are how to make better batteries, including very large capacity stationary batteries, how to make other large capacity energy storage devices, e.g. thermal devices, and how to improve the energy efficiency of the methods used to synthesize hydrocarbons from carbon dioxide and water.

FWIW, I think that — if only we could cooperate better — a global power grid would be both cheaper and better than stationary batteries. Even just made from aluminium, never mind superconductors (and yes, I've done the maths). But we'd still need mobile batteries for transport, so that's fine.

The cheap abundance of PV power even today means I don't think we need to care much about making hydrogen electrolysis more joule-efficient.

> Making hydrocarbons at large scale from carbon dioxide would be the best way to sequester carbon dioxide, offering the choice between just storing the carbon in safe products (paraffin like) and using a part of the synthesized hydrocarbons for generating energy in a carbon-neutral way.

I suspect that carbon sequestration is unlikely to be a great win: there's a very narrow window close to zero loss/profit where on the loss side it's still cheap enough that people do it because it's a vote winner and on the profit side where it's not so profitable that people break photosynthesis a few hundred million years before natural processes do it.

* in the sense that Jules Verne "knew" how to get to the moon: the maths wasn't wrong, but the engineering was only good enough for a story

[DoctorOetker]

Do you have an opinion on using N2O (laughing gas) as an energy carrier?

2 molecules of N2O exothermically react to form 2 x N2 and 1 x O2 molecules, approximately the same composition as our atmosphere.

It is a very potent greenhouse gas, so quite disturbing on that front.

I've been making calculation for designing earth suits, where the suit replaces the home, internal showering, ventilation, heat recovery etc. Using N2O for heating looks rather promising because with fossil fuels one is forced to lose heat by inefficient heat exchange or forced to be exposed to the exhaust fumes; laughing gas decomposed is just warm atmosphere like air.

[ben_w]

I have no opinions about most chemistry. I do know that the heaviest recreational users develop issues due to it being neurotoxic, so that's worth considering.

I don't know if your designs are technical or world-building for a story? If the latter, I'd suggest https://worldbuilding.stackexchange.com as I've had good conversations there, if the former perhaps (but not as a recommendation because I'm not a chemist) https://chemistry.stackexchange.com would help?

[fragmede]

Just tie the end of an infinitely long string to the edge of the universe and have it pull on a generator to spin it.

[DoctorOetker]

I would certainly like to see serious critical analysis and calculations of such hypothetical setups by physicists.

Would such a setup slow down the local expansion (action and reaction)?

Since iron is essentially a nuclear ground state, a steel cable being lengthened seems like the least worse mass loss imaginable.