11 ms·
This is somewhat tangential to the specific issue here, but I was surprised by this: At the center of the Sun, fusion power is estimated by models to be about
by mannykannot 8y ago
This is somewhat tangential to the specific issue here, but I was surprised by this:
At the center of the Sun, fusion power is estimated by models to be about 276.5 watts/m3. Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap, and is lower than the power density produced by the metabolism of an adult human.[1]
So practical fusion power requires producing much higher power densities than are found in the Sun.
[1] https://en.wikipedia.org/wiki/Solar_core#Energy_conversion https://en.wikipedia.org/wiki/Solar_core#Energy_conversion
- mikeash 8y agoThat also shows why fusion power always centers around exotic isotopes like deuterium and tritium rather than plain hydrogen. The reaction paths needed to fuse plain hydrogen are so improbable that they don’t happen fast enough to be useful.
- kamaal 8y agoIs this an another way of saying that the only way to fuse to Hydrogen is to create a structure as large and with gravitational features that of a star? Then a star/solar energy is the only way you will ever have large scale fusion energy generation and harvesting.
- DennisP 8y agoNo, it's saying that large-scale fusion using hydrogen fuel requires a star, because hydrogen fusion reactions are very improbable. That reaction only happens when four nuclei collide at once. Fortunately we have other fusion fuels that are much more likely to fuse, including deuterium, which makes up 1/2500 of the hydrogen atoms in the ocean. Most projects target the easiest fuel, deuterium-tritium; while tritium isn't naturally available on Earth, it can be bred from lithium.
- wbl 8y agoNope! The p-p chain only has pairwise interactions. The issue is the p+p->d step is very disfavored, thanks to the weak step. D+T is all strong interaction.
- bjelkeman-again 8y agoCould that be explained with less jargon? I’d like to understand it.
- ravar 8y agoThere is a chain of individual steps that need to happen to create the final product of hydrogen fusion, helium. One of the crucial steps is the production of deuterium from two protons. This step is incredibly slow and limits the rest of the chain. However if you skip the slow step and start by fusing deuterium and tritium you can have a much faster and efficient reactor.
- DennisP 8y agoThanks, that was interesting. Apparently the five-minute video I got my statement from led me astray.
- pfdietz 8y agoIndeed. After a D nucleus forms in the sun, it reacts (with a proton, to form 3He and a photon) within a few seconds. The 3He+3He reaction is also slow, but not as slow as that first step.
- simonh 8y agoI suppose the universe doesn't owe us a cheap, easy, unlimited source of energy. A lot of people seem to think that it's inevitable we will achieve amazing, magical feats of technology given sufficient time and effort that are unthinkable today. Maybe. But it's also just as possible that we already know most of the general parameters of what is possible in the universe, and the peak practical feats of technology that will ever be open to us are not all that far off from what we have now. How can we tell?
- narag 8y agoNuclear tech is not even a century old. The idea that we can't do any better in a thousand or a million years seems so improbable to me that I can't take it seriously. There is some kind of extrange pessimism around that mindset: that we will be extinct much sooner, or that the Sun will die and thus Humanity, as if there's no obvious way to escape that apocalypsis.
- peterashford 8y agoI think that significant swathes of human kind throughout history have assumed that their current state of existence is the peak and final form of humanity. While it is always logically possible that this is the case, it has never yet been true.
- rectangletangle 8y agoAgreed, the accepted models in physics are laughably incomplete. There is still a tremendous amount we still don't know. Although it's possible we've peaked, I have doubts. It is also a self fulfilling prophecy to declare that we have reached our technological apex, and not striving to progress further.
- Koshkin 8y ago> do any better in a thousand or a million years From what I can tell, there is a much bigger chance that we'll do much worse - and much sooner.
- 8y ago
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- curtis 8y ago> Despite its intense temperature, the peak power generating density of the core overall is similar to an active compost heap... I've seen this comparison before and it also seemed quite surprising to me. But in hindsight maybe it shouldn't: The sun is already billions of years old and it hasn't burned out yet. Another way to think of it: A larger compost pile has to radiate more heat out of the same surface area as a smaller heap because of the Square–cube law [1]. Now imagine a compost heap more than 800,000 miles in diameter. [1] https://en.wikipedia.org/wiki/Square%E2%80%93cube_law https://en.wikipedia.org/wiki/Square%E2%80%93cube_law
- gattr 8y agoAh, one of my favorite astronomy-related trivia. It also shows how a supernova can (briefly) shine as brightly as its whole host galaxy (i.e. some 10-11 orders of magnitude stronger than an average dwarf star - like you wrote, those basically "smoulder").
- thomasahle 8y agoIt continues like this though: > The low power outputs occurring inside the fusion core of the Sun may also be surprising, considering the large power which might be predicted by a simple application of the Stefan–Boltzmann law for temperatures of 10 to 15 million kelvins. However, layers of the Sun are radiating to outer layers only slightly lower in temperature, and it is this difference in radiation powers between layers which determines net power generation and transfer in the solar core. So if a m^3 of sun core was moved to a powerplant it would generate a lot more energy.
- thaumasiotes 8y ago> So if a m^3 of sun core was moved to a powerplant it would generate a lot more energy. Would it? Or would it expand into basically normal hydrogen and helium?
- sova 8y agoProvided you could keep the plasma contained in magnetic fields, it would certainly generate plenty of free neutrons to provide heat for substantial power generation.
- voidmain 8y agoIf you let it cool, it would stop fusing. The fusion power wouldn't magically go up.
- sova 8y agofusion halts harmlessly when conditions are unmet
- thaumasiotes 8y agoHalting harmlessly is the opposite of "producing a lot more energy".
- 8y ago