5 ms·

This is surprising to me. I would have thought the energy output per ton would be way higher. Do you have a source that explains why it is so low?

bognition 5y ago
It's pretty easy to forget that the sun is a ball of gas. It not very dense (roughly 1/4 the density of the earth). So it's extremely unlikely that a specific hydrogen atom will undergo fusion during the lifetime of the sun. However the sun is also ENORMOUS. So in the aggregate there is a lot of fusion going on and a lot of energy is being produces. However its not super dense. From wikipedia: https://en.wikipedia.org/wiki/Sun#cite_ref-power_production_density_84-0 https://en.wikipedia.org/wiki/Sun#cite_ref-power_production_... The large power output of the Sun is mainly due to the huge size [...] Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core,[77] which is about the same power density inside a compost pile
- CRConrad 5y ago
  So it's low in terms of "amount of energy in relation to amount of hydrogen in the star"? But wouldn't much of the un-fused hydrogen be eligible to fuse at some later time, if for instance convection streams pull it down into the deeper (=higher-pressure) zones of the star? So it seems to me the relevant measurement ought to be the ratio of energy per hydrogen atoms fused, not hydrogen atoms total. (Certainly in a reactor, where presumably all or most of them should be.)
- ZeroGravitas 5y ago
  A song about the sun being a ball of gas, for those that find it hard to remember: The Sun Is A Mass Of Incandescent Gas https://g.co/kgs/g8rKyh https://g.co/kgs/g8rKyh A follow up song, correcting the incorrect physics of the first one: Why Does the Sun Really Shine? https://g.co/kgs/2QKBdw https://g.co/kgs/2QKBdw
- mannykannot 5y ago
  Inside the sun's core, where the fusion occurs, the density is considerably higher - 150 g/cm3 vs. 5.5 g/cm3 (average) or 13 g/cm3 (inner core) for the earth.
spaetzleesser 5y ago
That’s why a fusion reactor would use different fuels than a star.
- adgjlsfhk1 5y ago
  no? hydrogen to helium is by far the best.
  - xoa 5y ago
    >no? hydrogen to helium is by far the best. Not necessarily for humans. Stars get to be gravity-mass containment/fusion pressure automoderated, but there is nothing known even in blue sky to manipulate gravity beyond sheer mass. So humans have to deal with containing via electromagnetism and relatively little isolated surrounding matter, which means we have to deal with both ignition thresholds and the issue of neutrons. In stars neutrons provide heat/energy by sheer virtue of hitting stuff as they pass out from the core, and obviously radioactivity isn't a concern. But it's definitely an issue for a power plant, where neutrons mostly represent wasted energy since we don't have any good way to harness them for useful work, plus they'll activate surrounding materials over time which is also an irritation. So the ideal fusion may be aneutronic, which then faces tradeoffs of fuel availability vs ease of production. For example terms of plain ease of engineering/compactness etc, helium-3 would probably be the best fuel. But helium 3 is also super ultra rare and effectively nonexistent naturally on Earth. We'd have to get it from space, or breed it from neutronic reactors which obviously isn't ideal (might be worth it as a compact fuel for far future spacecraft though). On the opposite side hydrogen-boron fusion requires much much higher ignition energy, greatly complicating actually building a working plant, but the fuels themselves are readily available. There are other potential ones between, or maybe we just suck it up and deal with neutron activation (still a lot less radioactivity than in fission plans), which could be moderated with more careful selection of construction materials. At any rate, we're definitely not yet at the point where we can really say what the "best" fusion cycle will be, and it's perfectly possible we'll even end up with multiple (He3 for vehicle/more portable reactors, H-B or D-D for massive stationary ones say).
    - 5y ago
pixl97 5y ago
If it wasn't so low the universe would be dead. We see this is large blue stars. They burn through their fuel supply in millions of years and years and explode with the force of a billion suns. Meanwhile small stars can burn for billions and billions of years because they are conservative with their hydrogen supply.

bognition 5y ago

It's pretty easy to forget that the sun is a ball of gas. It not very dense (roughly 1/4 the density of the earth). So it's extremely unlikely that a specific hydrogen atom will undergo fusion during the lifetime of the sun. However the sun is also ENORMOUS. So in the aggregate there is a lot of fusion going on and a lot of energy is being produces. However its not super dense. From wikipedia: https://en.wikipedia.org/wiki/Sun#cite_ref-power_production_density_84-0 https://en.wikipedia.org/wiki/Sun#cite_ref-power_production_... The large power output of the Sun is mainly due to the huge size [...] Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core,[77] which is about the same power density inside a compost pile

CRConrad 5y ago
So it's low in terms of "amount of energy in relation to amount of hydrogen in the star"? But wouldn't much of the un-fused hydrogen be eligible to fuse at some later time, if for instance convection streams pull it down into the deeper (=higher-pressure) zones of the star? So it seems to me the relevant measurement ought to be the ratio of energy per hydrogen atoms fused, not hydrogen atoms total. (Certainly in a reactor, where presumably all or most of them should be.)
ZeroGravitas 5y ago
A song about the sun being a ball of gas, for those that find it hard to remember: The Sun Is A Mass Of Incandescent Gas https://g.co/kgs/g8rKyh https://g.co/kgs/g8rKyh A follow up song, correcting the incorrect physics of the first one: Why Does the Sun Really Shine? https://g.co/kgs/2QKBdw https://g.co/kgs/2QKBdw
mannykannot 5y ago
Inside the sun's core, where the fusion occurs, the density is considerably higher - 150 g/cm3 vs. 5.5 g/cm3 (average) or 13 g/cm3 (inner core) for the earth.

spaetzleesser 5y ago

That’s why a fusion reactor would use different fuels than a star.

adgjlsfhk1 5y ago
no? hydrogen to helium is by far the best.
- xoa 5y ago
  >no? hydrogen to helium is by far the best. Not necessarily for humans. Stars get to be gravity-mass containment/fusion pressure automoderated, but there is nothing known even in blue sky to manipulate gravity beyond sheer mass. So humans have to deal with containing via electromagnetism and relatively little isolated surrounding matter, which means we have to deal with both ignition thresholds and the issue of neutrons. In stars neutrons provide heat/energy by sheer virtue of hitting stuff as they pass out from the core, and obviously radioactivity isn't a concern. But it's definitely an issue for a power plant, where neutrons mostly represent wasted energy since we don't have any good way to harness them for useful work, plus they'll activate surrounding materials over time which is also an irritation. So the ideal fusion may be aneutronic, which then faces tradeoffs of fuel availability vs ease of production. For example terms of plain ease of engineering/compactness etc, helium-3 would probably be the best fuel. But helium 3 is also super ultra rare and effectively nonexistent naturally on Earth. We'd have to get it from space, or breed it from neutronic reactors which obviously isn't ideal (might be worth it as a compact fuel for far future spacecraft though). On the opposite side hydrogen-boron fusion requires much much higher ignition energy, greatly complicating actually building a working plant, but the fuels themselves are readily available. There are other potential ones between, or maybe we just suck it up and deal with neutron activation (still a lot less radioactivity than in fission plans), which could be moderated with more careful selection of construction materials. At any rate, we're definitely not yet at the point where we can really say what the "best" fusion cycle will be, and it's perfectly possible we'll even end up with multiple (He3 for vehicle/more portable reactors, H-B or D-D for massive stationary ones say).
  - 5y ago

pixl97 5y ago

If it wasn't so low the universe would be dead. We see this is large blue stars. They burn through their fuel supply in millions of years and years and explode with the force of a billion suns. Meanwhile small stars can burn for billions and billions of years because they are conservative with their hydrogen supply.