It's always fascinating to me that, no matter how many interesting new ways to release lots of energy we develop, we are still stuck with the same method for converting it to electricity: release the energy as heat, use heat to make steam, use steam to drive generator.
The reason just that this is a simple process for which a steam turbine can achieves 90% of the thermodynamic optimum. To my knowledge, the only reason people consider alternatives is to reduce capital costs. You're still capped by thermodynamics though.
That's what I was wondering: if heat -> steam -> turbine is close enough to the theoretical upper limit and you get diminishing returns by other means.
Still, it seems very indirect. Like generating solar power by using a parabolic mirror to heat water instead of photovoltaic panels... but of course I just found an example of doing that too: https://en.wikipedia.org/wiki/Parabolic_trough
Which doesn't make sense for power generation since there will always be some percentage of neutrons produced by any type of fusion reaction that can only be useful for generating steam.
To entirely skip the steam cycle portion is to intentionally make a much less efficient design.
For space-constrained, high-value, applications where economics don't matter that much, such as a submarine, that would make sense, but otherwise...
Helion's fuel mix produces just 6% of its energy as neutron radiation, and if you harvest it you'll lose a third of that. As long as you have enough net energy, collecting that 4% might not make financial sense.
With fuel costs insignificant, your cost per kWh is mainly capital cost. Let's say it's all capital just to keep it simple. I don't know how much the input energy will be but if your choice is between, say, generating net energy of 50MW without a turbine or 54MW with a turbine, then you would skip the turbine if it adds more than 8% to the capital cost. I suspect Helion has done this calculation in detail.
It does seem to hinge on the cost of fuel, I have some doubts about whether they can secure a fuel supply so cheap as to skip out on that extra 4 MW, even after factoring in the cost of a small steam turbine installation.
Deuterium costs several thousand dollars/kilogram. But even though it is just one part in several thousand of the hydrogen in water, there's enough deuterium in your morning shower to provide all your energy needs for a year.[1] Cost of deuterium is definitely insignificant.
Helion's other fuel is helium-3 which they'll make themselves by fusing deuterium. So the helium-3 cost will directly depend on the capital cost of the reactor producing it.
(This may be the same reactor, both generating electricity and breeding He3. Or they may use dedicated He3 breeders, and minimize the D-D reactions in the generators.)
Nitpick: You’re always going to get some parasitic D+D reactions because it’s reaction cross section is appreciably higher than D + 3He below 50 keV, and non-negligible even after the crossover point.
You could always add the liquid metal blanket if you want to eke out the extra 10% (or if you want to generate tritium). But it's not worth the complication in an early prototype.
Helion is planning to use a different fusion reaction, one where the bulk of the energy will be coming out as charged particles, not neutrons.
However, D+T fusion is the only type of fusion that we have been able to sustain for any significant amount of time with reasonable energy inputs. What Helion is planning to do is completely unexplored and requires some major scientific advances.
Well, we do have some other strategies. Hydro and wind just turn the turbine mechanically, they don't heat up water to make steam. And photovoltaics create electricity directly using an effect that won Einstein the Nobel prize and began the age of quantum mechanics, so that's about as advanced as they come.
