First you want to displace natural gas for fertilizer production. But yes, if the energy efficiency is good enough and the electrolyzer costs are very low, it would make more sense than electrolyzing hydrogen and then running that through Haber-Bosch. Remains to be seen if either of those criteria can be met.
Probably not just yet. I calculated downthread that the productivity is something like 16x less per unit area than a hydrogen electrolyzer, so that would need to be improved to make it cost-effective probably. Also they don't mention the energy efficiency, only the "current efficiency" so I would assume the energy efficiency is also poor. Sounds like there's much to be done still.
Probably refers to "Coulombic efficiency." Ie, it takes 4 electron transfers to turn 2 H2 + N -> NH4, so that gives you a conversion factor between Coulombs of electrons (1 Amp of current is 1 Coulomb per second) and number of NH4 molecules produced.
Yeah, someone would have to get access to the paper to see if they state the energy efficiency. I assume that b/c they don't mention it, it is abysmal. There's pressure to put good results into the abstract.
For hydrogen electrolysis they typically quote around 1 A/cm^2 current. One Coulomb is ~6e18 charges, whereas one mole is 6e23 molecules, so that makes about 1e-5 H atoms per cm per second. Of course making one molecule of ammonia needs 4 H atoms, so it works out to something like 16x lower productivity. I assume it's not competitive as is.
Only if there's enough wind shear between the aircraft and kite altitudes... kinda like dynamic soaring [0] but without the need for the aircraft to jump back and forth between the two regions. You'd probably need to use a sailplane or something to get high efficiency flight at low speeds. I think it would be super dangerous, b/c in order to access the strongest differential wind speed, the craft would need to be very low to the ground (just a few meters).
No-moving-parts is probably not realistic - the core would be much hotter than the radiating surface due to the thermal resistance of the shielding. More likely, you would have a working fluid to transfer the heat. Most nuclear reactors operate at much lower temperatures where TPV wouldn't be efficient or cost-effective. It's certainly possible to go much higher. Nuclear-thermal rocket propulsion tests ran with exhaust temperatures up to ~2200 C [0]. Whatever fluid is used, you would need to avoid radioactivity in it, b/c that would probably degrade the TPV. Also you would probably want to avoid having heat exchangers because each one incurs a temperature drop. So helium would fit that bill. That's what the "high temperature gas reactors" use [1]. IDK if helium-compatible plumbing/pumping could be made to work at >2000 C though.
Edit: BTW, there are radioisotope thermoelectric generators used for space applications primarily, but they are not true nuclear reactors - they produce short-range radiation that doesn't require much shielding. Nuclear reactors produce neutrons and gammas that require thick shielding.
Yeah, tungsten is far too expensive. Sounds like more realistically you would have molten silicon in graphite plumbing [0]. This article claims the self-discharge rate could be made 1%/day.
There's been talk of doing solar concentration -> hot object -> thermophotovoltaic converter. IIRC concentrated solar already exceeds 40% efficiency so it doesn't make sense to add the extra step, unless you are using the solar concentration to "recharge" a heat storage system.
I buy the argument that thermophotovoltaics can become cheaper on a $/kW basis than comparably-efficient fluid/mechanical heat engines. The power per unit area is intrinsically orders of magnitude higher than for direct solar, so even if these cells are pricier than regular solar cells, they have a fighting chance. Also, both the power density and efficiency increase with temperature, and in principle the operating temperature can be higher than that of a turbine (since the materials don't have to simultaneously withstand crazy mechanical stresses and reactive chemical environment).
Just to amplify on your point about lithium: the tritium production function is critical. Every fusion neutron needs to produce more than one tritium atom on average, so that the reactor is sustainable (there are inevitable losses & tritium also decays radioactively) or even making excess tritium (to bootstrap other reactors). This is challenging b/c even in the best case each neutron can produce maybe 2 tritium atoms, so there's not much margin. The lithium needs to comprise most of the material surrounding the plasma, limiting the fraction that's available for other functions (structural supports, heat shielding, cooling, plasma control & heating systems, sensors, etc).
Not at the moment. IMO the prospects aren't good, due to the low efficiency (high energy cost) and high equipment cost. There are start-ups working on it (ex: Prometheus Fuels [0], Twelve [1], Synhelion [2], honorable mention to Terraform Industries [3] which is targeting methane).
Everything that can be electrified, will be electrified, because it's more efficient. It seems like shipping & aviation are probably the hold-outs.
I used to think this was the way, especially for balancing the grid, but the more I read [4-7], the more it looks like batteries will be used for fluctuations <1 day, and demand adaptation for longer periods. Some of that adaptive demand may wind up being used for hydrocarbon synthesis, but I don't expect it to compete with fossil fuels for a long time, if ever.
"Information theory and statistical mechanics" by E.T. Jaynes [0]. Starting from a Bayesian/information-theoretic perspective, he derived statistical mechanics in just a few pages. This approach finally made stat-mech 'click' for me.
It's a paradigm shift. Entropy (in the Bayesian sense) is fundamentally a subjective quantity. Maximizing entropy is just minimizing the assumed information. When dealing with large complex systems over long timescales, the only pieces of information one is justified in assuming are the values of those quantities that are conserved globally by the dynamics (eg, total energy, total mass, etc). The "thermodynamic entropy" S is just the maximum entropy, given a certain set of conserved quantities (an "ensemble") -- it is therefore more or less objective, modulo the conserved quantities.
The second law of thermodynamics is just the information processing inequality: you don't have any more information about the future state of the system than you do about the present state. If anything, you have less, because if you have any information about the current state of non-conserved quantities, that information is not valid at other times (assuming you don't have complete information & the ability to fully simulate the dynamics). From this perspective, entropy does not generate the "arrow of time": the argument is symmetric in time.
Another from Jaynes worthy of mention: "Prior probabilities" [1] discusses the use of group theory to derive "non-informative" prior distributions by considering the set of transformations that result in equivalent inference scenarios. This is resolved another question I had when studying stat mech: "How come we start with an assumption of uniform probability in phase space as expressed in (x,p) coordinates -- if we transformed to a different coordinate system, it would look like a non-uniform distribution!" The answer is that we assume Galilean invariance. The concept applies outside stat mech as well.
