Talk:Fusion energy gain factor

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An anonymous editor has slapped three templates on this article, Expert, expansion, and confusing. It would help to have a little more detail on the problems perceived. I already am an expert on the subject, and I find the presentation clear albeit succinct. I'm not sure where expanding it would make it much clearer. Please help me here. --Art Carlson 10:05, 30 April 2006 (UTC)

[edit] Why isn't Q > 5 enough?

I don't quite understand the high Q value necessary. Wouldn't a fusion reactor be successful if it could deliver more electricity to the net than what it is consuming?

Let's assume that most of the energy that the reactor needs is used for the heating of the plasma. Let's call the required power P_heat. If this power is taken as electricity from the grid, the amount of electricity needed for heating is P_{el_heat} = P_heat / ɳ_heat.

The electricity output would be, according to the article, P_elec = η_elec(1-f_ch)P_fus.

Now, what is wanted is that the output electricity is larger than the input electricity:

P_elec > P_{el_heat}

insert the definitions of P_elec and P_{el_heat}:

η_elec(1-f_ch)P_fus > P_heat / ɳ_heat

or

(Q = ) P_fus / P_heat > 1 / ( ɳ_heatη_elec(1-f_ch) )

which is the same expression used in the article save for the recirculation of electricity.

Put in the numbers in the article, and the result is Q > 1 / (0.7*0.4*(1-0.2)) = 4.46 ≈ 5. Won't this mean that a fusion reactor with Q > 5 will deliver more power than it uses? Of course you need to deliver more than that to have some energy to sell, but wouldn't, say, Q=10 be enough? --Maccer83 12:07, 14 May 2006 (UTC)

A fusion reactor will be successful if it produces electricity at a cost that is competitive with the alternatives. The Q needed to be successful in this sense will depend on the cost of the fusion plant and the cost of the alternatives. With the assumptions of the article and Q=22, f_recirc is 0.20 and you need to produce 25% more electricity than you plan to sell. With Q=10, f_recirc is 0.44 and you need to produce 78% more electricity than you sell. You're right that this could, in priciple, be OK. If a source of energy is cheap enough, it could tolerate a recirculating power fraction of even, say, 99%. The 20% figure is a soft limit that gives you an idea of when it starts to hurt. It is commonly used in studies, but other assumptions can be made. --Art Carlson 19:24, 14 May 2006 (UTC)