I received a useful comment on my post from A Flash in The Cosmic Pan from Serban Tanasa asking if nuclear fusion would dramatically alter the calculation I made there. The gist of the post was that the sizes of interstellar civilisations are ultimately limited by the fact exponential growth in energy demand outruns cubic growth in stellar energy availability, and this can help explain the Fermi paradox as well as giving insights into our own far future. Serban believes that being able to add fusion energy to meet this growing demand breaks this relationship and thus undermines the conclusion. As promised in my response, I am now going to lay out in more detail why I don’t think it will do so.
As an aside, this is precisely the sort of interaction I’d like to encourage here. I’m not aiming to make ex cathedra pronouncements to a passive audience - I am testing ideas by seeing what feedback they elicit. As I have mentioned elsewhere, I am writing a book and I would much rather discover counterarguments to what I am saying before publication than afterwards!
On to the main subject - why I don’t think fusion will help keep up with exponential energy demand.
Nuclear fusion works by combining lighter elements together to form larger ones, and the change in nuclear binding energy is then liberated. This is a very energy dense source of power, but difficult to harness because the electrostatic repulsion between nuclei has to be overcome by very high impact velocities, i.e. very high temperatures. This means one of the biggest hurdles is containment of the extremely hot plasma required.
A second difficult is to extract useful energy from the reaction. The plasma is far too hot to be allowed to come into contact with any material, and thus has to transfer the energy of the fusion reactions in some other way. The easiest fusion reaction from a containment perspective is the Deuterium-Tritium (D-T) reaction, which is the focus of current research. Most of the energy produced by this in given off in the form of fast neutrons, which must be absorbed by a reactor cladding and converted into heat to drive a conventional thermal power plant.
To compare this with solar energy, we must consider the Energy Return on Energy Invested (EROEI), which as its name implies is the ratio of useful energy produced to the energy that must be put in to generate it. The reason we do not yet have viable fusion reactors is that this number is less than 1 for all existing fusion experiments. On the other hand, solar photovoltaic and solar thermal systems already have an EROEI above one and can be profitably operated. Let us assume that technological breakthroughs give us fusion with an EROEI greater than 1, and try to speculate on how this might compare with solar in space.
The issues that solar energy has on Earth of intermittency do not apply in space; outside low orbits around Earth there is no real ‘night’ to be concerned with and there is also no worries about cloud cover. Solar energy can be harvested through photovoltaic cells, or through thermal plants that concentrate sunlight to a single points. I will only consider the latter, as it makes the comparison simpler.
Both fusion and solar thermal plants require the construction of a thermal power plant, to convert the generated heat to useful energy - typically electricity. To generate the heat though, all a concentrated solar plant in space requires is some free floating mirrors to reflect sunlight on the target. A fusion plant requires the machinery of confinement and some means to capture the reaction energy. It should be clear that for a given energy output, regardless of what form fusion reactors ultimately take, simple mirrors require far less energy input than a confinement mechanism - both in terms of their initial manufacture and ongoing operation. In an exponential scenario, solar therefore always wins. For any amount of power available to install new power generation, more solar would be installed than fusion power, thus the growth rate will be higher, all other things being equal.
Serban points out that the EROEI of solar gets worse as you get further from the host star - this is true, but it is not as bad as first appears. For the same thermal plant, all you need to do to have the same energy available at a greater distance is to build more mirrors and reflect more sunlight to compensate for its weakness. This likely gets impractical very far out - in the Oort cloud of a solar system you may well be better off with fusion - but because the best EROEI will be found in the inner solar system, that is where the fastest growing civilisation will be.
Going back to my original argument - the large scale behaviour of an extraterrestrial civilisation, as seen from a distance, will be governed by the ergovore i.e. by its fasting growing component, and as explained above solar will always be a faster growing component than fusion. Fusion likely will occur in such civilisations, but won’t change the ultimate outcome of exponential demand growth exceeding cubic supply growth. It may be the case that, when this critical point is hit a slower growing fusion economy takes over from the fast growing solar one - but this is likely to be quite a disruptive event at the very least, and in any case will stop the progressive modification of more and more stars, meaning further growth will be far less visible over interstellar distances. I will come back to this is a later post, where I will revisit this and other objections to the ergovore model.
Fusion definitely has a role for humans, in scenarios where a low physical footprint and low mass are important. If we can master the technology on Earth it will transform our existence. It will also be useful as a means of propulsion, especially if we wish to attempt interstellar travel.
No Gold Rush
Above I have shown why fusion will not be the main driver of the solar economy, at least not until after a rapid expansion of solar energy has occurred. However, there is occasional talk from space advocates and the media of Helium-3 being a useful fusion fuel for Earth, one that is found on the surface of the Moon or in gas giants, and that this resource could be the economic driver of future space exploration.
Helium-3 fusion is aneutronic, which is an advantage from the point of view of safety and also makes it easier to extract energy from the plasma as it does not escape in the form of neutrons - but even if it is economically useful there is no real case for mining the isotope from space. Helium-3 a decay product of Tritium, which is bred for D-T fusion reactors - in future the neutron emissions from such reactors are expected to regenerate their own fuel supply. If such fusion is economical, why not generate Helium-3 using reactors rather than mine the Moon or Jupiter?
This talk given at the 40th COSPAR Scientific Assembly, which analyses the economics of mining the Moon for Helium-3 gives an ‘optimistic’ concentration of the material on the surface at 20 parts per billion, and the authors do not consider such mining operations as being likely to be profitable, and they did not appear to consider the alternative of manufacture on Earth.
It is also worth noting that fusion fuel, whilst plentiful, is finite - and this matters on the scale Kardashev type 2 or larger civilisations. Even if industrial proton-proton fusion were possible (allowing us to use by far the most abundant isotope of hydrogen as fuel) and perfectly efficient, yielding 27MeV of usable energy for every 4 protons converted to Helium, there is only around 10^20kg of hydrogen in the Earth’s oceans. To provide a solar luminosity worth of power it would all be fused in around 5 years. A Kardashev 2 civilisation running only on solar power can keep up that level of output for billions of years.
Other icy bodies would yield similar amounts, and then its on to mining gas giants. If all of Jupiter’s mass of 2x10^27kg were hydrogen, and it were all available for perfectly efficient fusion, you would get about 100 million years out of it. This is more or less in the territory of artificially engineering a second star in our solar system though, which if it happens at all would be a much slower process than enclosing our Sun with a Dyson swarm of solar collectors.
Fusion is Good
I am very much in favour of research into nuclear fusion and hope that it can dig humanity out of the energy and climate problems we have created for ourselves, so I don’t want to be taken as anti-fusion. The argument here simply addresses the narrow matter of whether fusion will be a significant economic driver of a large scale solar system economy, and whether or not it will be the deciding factor determining the trajectory of an interstellar civilisation, and I believe we can answer ‘no’ to both questions.