Solar is no doubt the coolest.
Hydro and wind are also very neat, going directly from mechanical to electric via generator, without a steam-turbine.
There is also a very cool fusion-category based on dynamic magnetic fields, that basically form a magnetic piston which expands directly due to the release of charged particles via fusion and then captures the energy from that moving electric field by slowing it back down and initiating the next compression.
A fully electric virtual piston engine in some sense, driven my fusion explosions and capturing straight into electricity.
Feels so much more modern than going highly advanced superconducting billion K fusion-reactor to heat to steam to turbine.
Yes! That is super cool tech. If I remember correctly, only about half of the fusion reaction energy was produced as charged particles though. The other half was free neutrons which are notorious for not interacting with the EM field.
I love the idea, it is such a cool direct energy capture method, but it is inherently inefficient.
I’d love to be proved wrong. I did a quick search and couldn’t find the company I’m thinking of, so I’m going off memory.
There are different fusion reactions, one example would be ²H-³He fusion used by Helion.
²H-³He is aneutronic, so doesn’t produce chargeless particles (every clump of stuff is either an electron or contains a proton). It is also an easy to achieve fusion reaction with good energy yield, with the downside that we don’t have ³He. Helion therefore has to split their fusion into two steps, producing ³He via ²H-²H fusion in a breeder-reactor and then fusing it in their energy-reactor. The first step would then emit neutrons and not really produce energy, the neutrons here could be used to further breed fuels.
Not having neutron emissions is quite useful because it allows you to make your fusion generator a lot smaller and safer around people, so it’s certainly something you want to avoid for far more valuable reasons than improving efficiency.
If we get very good with fusion we could also use the much harder to achieve ¹H-¹¹B reaction, which produces some neutrons but at very low energy (0.1% of total energy output), and is effectively aneutronic for safety concerns (neutrons have low penetration power and don’t really activate material, so can’t be used to breed say weapons-grade fission material). ¹H and ¹¹B are common so require no further steps to produce them.
There might still be directly-to-electricity pinch-fusion approaches that use neutronic fusion, I tried looking for any but didn’t find an example. We’ll see what ends up being done in practice, but close to 100% energy utilization is at least possible using pinch-fusion.
On the other hand, the losses in heat-conversion are inevitably huge. The higher the temperature of the heated fluid compared to the environment the higher the efficiency, but given that our environment has like 300 K we can’t really escape losing significant amounts of our energy even if we use liquid metal (like general fusion) and manage to get up to 1000 K. The losses of going through heat are <environment temperature>/<internal temperature> (carnot efficiency), so would still amount to 30% energy loss if we manage to use 1000K liquid metal or supercritical steam to capture the fusion energy and drive a turbine. In practice supercritical steam turbines as used in nuclear plants hover around 50% efficiency at the high end.
The magnetic field in pinch-fusion interacts with the (charged) particles directly, which are emitted at (many many) millions of K. Therefore this theoretical efficiency will be at over 99.99%. In effect in heat-based fusion we loose a lot of that energy by mixing the extremely hot fusion results with the much colder working fluid.
They can’t control which particles fuse though. The Helion energy reactor still has the particles for deuterium to deuterium fusion. 50% of the time that gives your tritium+p and 50% is He3+n. I don’t know the preference of each fusion event in their reactor, but not all events will produce charged particles.
Fair point, though there are ways to change the probabilities of fusion paths, just not ever fully to 0.
Reaction probabilities scale with reactant concentration and temperature in ways we can exploit.
I tried to find some numbers on the relative probabilities and fusion chains, and ran into The helium bubble: Prospects for 3He-fuelled nuclear fusion (2021) which I hope is a credible source.
This paper contains a figure, which gives numbers to the fusion preferences you mentioned.
Paraphrasing the paper in chapter “Technical feasibility of D-3He fusion” here, first we see that up to 2 billion K, the discrepancy between ²H-³He and ²H-²H fusion grows, up to about 10x. ²H-²H reactions will either produce a ¹n (neutron) and a ³He, or produce an ¹H and an ³H, with the ³H then (effectively) immediately undergoing the much more reactive ²H-³H producing a neutron too.
In addition to picking an ideal temperature (2GK), we can also further, for the price of less than a factor 2 increase of pressure, use a 10:90 mixture of ²H:³He, or even more. This will proportionally make the ²H-²H branch a factor 10/90 ≈ 11% as likely as the ²H-³He correcting for reaction crossection.
Past that, reactivity goes about with the square of pressure and the inverse of ²H concentration, so another 10x in fusion plasma pressure would net another 100x decrease in neutron emission at equal energy output.
Given how quickly fusion reactivity rises with better fusion devices, we can probably expect to work with much higher concentrations than 10:90 when the technology matures, but 10:90 at 2GK would still have about 1/100ᵗʰ the neutrons per reaction and less than 1/100ᵗʰ per energy produced compared to fully neutronic fusion like ²H-³H.
The problem is solvable, but there is definitely a potential for taking shortcuts and performing ²H-³He with much higher neutron emissions.
It’s why photovoltaics are so cool. Direct electricity generation without having to spin magnets in circles like neanderthals.
“Direct” (from energy created by a massive nuclear fusion reactor in space).
I mean to be fair, “add hydrogen to space, and wait” is a pretty simple reactor.
Solar is no doubt the coolest.
Hydro and wind are also very neat, going directly from mechanical to electric via generator, without a steam-turbine.
There is also a very cool fusion-category based on dynamic magnetic fields, that basically form a magnetic piston which expands directly due to the release of charged particles via fusion and then captures the energy from that moving electric field by slowing it back down and initiating the next compression.
A fully electric virtual piston engine in some sense, driven my fusion explosions and capturing straight into electricity.
Feels so much more modern than going highly advanced superconducting billion K fusion-reactor to heat to steam to turbine.
Yes! That is super cool tech. If I remember correctly, only about half of the fusion reaction energy was produced as charged particles though. The other half was free neutrons which are notorious for not interacting with the EM field.
I love the idea, it is such a cool direct energy capture method, but it is inherently inefficient.
I’d love to be proved wrong. I did a quick search and couldn’t find the company I’m thinking of, so I’m going off memory.
Kind of, it’s more complicated.
There are different fusion reactions, one example would be ²H-³He fusion used by Helion.
²H-³He is aneutronic, so doesn’t produce chargeless particles (every clump of stuff is either an electron or contains a proton). It is also an easy to achieve fusion reaction with good energy yield, with the downside that we don’t have ³He. Helion therefore has to split their fusion into two steps, producing ³He via ²H-²H fusion in a breeder-reactor and then fusing it in their energy-reactor. The first step would then emit neutrons and not really produce energy, the neutrons here could be used to further breed fuels.
Not having neutron emissions is quite useful because it allows you to make your fusion generator a lot smaller and safer around people, so it’s certainly something you want to avoid for far more valuable reasons than improving efficiency.
If we get very good with fusion we could also use the much harder to achieve ¹H-¹¹B reaction, which produces some neutrons but at very low energy (0.1% of total energy output), and is effectively aneutronic for safety concerns (neutrons have low penetration power and don’t really activate material, so can’t be used to breed say weapons-grade fission material). ¹H and ¹¹B are common so require no further steps to produce them.
There might still be directly-to-electricity pinch-fusion approaches that use neutronic fusion, I tried looking for any but didn’t find an example. We’ll see what ends up being done in practice, but close to 100% energy utilization is at least possible using pinch-fusion.
On the other hand, the losses in heat-conversion are inevitably huge. The higher the temperature of the heated fluid compared to the environment the higher the efficiency, but given that our environment has like 300 K we can’t really escape losing significant amounts of our energy even if we use liquid metal (like general fusion) and manage to get up to 1000 K. The losses of going through heat are <environment temperature>/<internal temperature> (carnot efficiency), so would still amount to 30% energy loss if we manage to use 1000K liquid metal or supercritical steam to capture the fusion energy and drive a turbine. In practice supercritical steam turbines as used in nuclear plants hover around 50% efficiency at the high end.
The magnetic field in pinch-fusion interacts with the (charged) particles directly, which are emitted at (many many) millions of K. Therefore this theoretical efficiency will be at over 99.99%. In effect in heat-based fusion we loose a lot of that energy by mixing the extremely hot fusion results with the much colder working fluid.
They can’t control which particles fuse though. The Helion energy reactor still has the particles for deuterium to deuterium fusion. 50% of the time that gives your tritium+p and 50% is He3+n. I don’t know the preference of each fusion event in their reactor, but not all events will produce charged particles.
Fair point, though there are ways to change the probabilities of fusion paths, just not ever fully to 0.
Reaction probabilities scale with reactant concentration and temperature in ways we can exploit.
I tried to find some numbers on the relative probabilities and fusion chains, and ran into The helium bubble: Prospects for 3He-fuelled nuclear fusion (2021) which I hope is a credible source.
This paper contains a figure, which gives numbers to the fusion preferences you mentioned.
Paraphrasing the paper in chapter “Technical feasibility of D-3He fusion” here, first we see that up to 2 billion K, the discrepancy between ²H-³He and ²H-²H fusion grows, up to about 10x. ²H-²H reactions will either produce a ¹n (neutron) and a ³He, or produce an ¹H and an ³H, with the ³H then (effectively) immediately undergoing the much more reactive ²H-³H producing a neutron too.
In addition to picking an ideal temperature (2GK), we can also further, for the price of less than a factor 2 increase of pressure, use a 10:90 mixture of ²H:³He, or even more. This will proportionally make the ²H-²H branch a factor 10/90 ≈ 11% as likely as the ²H-³He correcting for reaction crossection.
Past that, reactivity goes about with the square of pressure and the inverse of ²H concentration, so another 10x in fusion plasma pressure would net another 100x decrease in neutron emission at equal energy output.
Given how quickly fusion reactivity rises with better fusion devices, we can probably expect to work with much higher concentrations than 10:90 when the technology matures, but 10:90 at 2GK would still have about 1/100ᵗʰ the neutrons per reaction and less than 1/100ᵗʰ per energy produced compared to fully neutronic fusion like ²H-³H.
The problem is solvable, but there is definitely a potential for taking shortcuts and performing ²H-³He with much higher neutron emissions.
Thank you for this researched, thought out answer. I really appreciate the time you put into it. Super interesting topic and I’m glad to learn more!