What makes the H-bomb so efficient?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
3l
Posts: 1866
Joined: Sat Jul 07, 2001 7:51 pm
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by 3l » Sat Nov 26, 2005 9:10 pm

Hi folks:

Without going into too much detail which I've done in other posts.
Simply remember a H-Bomb only fuses 2% or maybe at best 10% of the fuel load the rest turns into fallout. The longest time of thermonuclear burn is measured in microseconds.
Using an H-Bomb as a heat source would be like using 20 billion dollars in crisp 100 dollar bills as fuel for a wood furnace.
If it worked would you be at this forum? I don't think so!

Happy Fusoring!
Larry Leins
Fusor Tech

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by DaveC » Sun Nov 27, 2005 2:56 am

Steam type power plants operate typically in the 30-45% thermal efficiency range. Using 33% efficiency as the "easy math" figure, then the plant consumes 3 times as much fuel energy as the electrical energy it produces.

Therefore if the assumed fusion-thermal process operated successfully at the temperatures of a typical supercritical boiler approximately with 1050 F (566 C) superheat (which may be too hot for a high voltage process) , then it would take 3 KWhr of heat energy to produce 1 KWhr of electricity, and etc.

Thus you need a fusion output of some 300% of the desired electrical output.

If you were getting some of the energy directly from charged particle deceleration, the efficiency for that portion of the process could approach 100% without violating any thermodynamic principles.

Dave Cooper

User avatar
Richard Hull
Moderator
Posts: 12210
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: What makes the H-bomb so efficient? A: (with some math)

Post by Richard Hull » Mon Nov 28, 2005 4:43 pm

This all goes back to satisfying the bean counters and the engineers in the REAL WORLD power biz.

Everyone who really believes that JET or any fusion reactor has produced 50kw more energy out that was put in please raise your hand!!

Fusion doesn't stand a chance by any measure of the current planning on ITER or JET or whatever! So, the upshot is now that with steam, 300% out of a fusion system IS BREAKEVEN!

Thus, you might say that until you hit about 1000% the fusion power system would not interest a real "going concern" in the power biz to swing through its current infrastructure to create new.

It is important to remember the current "longest run" times (fractions of a minute) of the finest fusion efforts, all at under unity and compare that with say, some of the many late 1960's fission reactors that have never stopped producing multi-megawatts, 24-7-365 for the last 40 odd years. We aren't even at the distant vision point with fusion yet, by any methodology.

ITER is just the next carefully planned and executed mistep in a long running fusion boon-doggle.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Retric
Posts: 75
Joined: Tue Sep 06, 2005 5:48 pm
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by Retric » Mon Nov 28, 2005 8:48 pm

I think we have had this discussion a few times. But, to clarify I never
said that JET produced 50kw more energy out than was put in. I was
trying to use simple numbers to explain what efficiency meant for
electricity generation. We can pick an arbitrary “breakeven” and
useful numbers but adoption is going to be the real test of success. If
we want real numbers: at it’s best Jet gave us:

22 MJ of fusion energy in one pulse
16 MW of peak fusion power
A 65% ratio of fusion power produced to total input power.
http://www.jet.efda.org/pages/history-of-jet.html

As it’s only producing 16 MW of power over short periods of time
there is zero electricity recovery going on. However, the fusion
power ratio means that for an input of ~25MW of electricity you get
~16MW of heat from fusion AND ~25MW of heat directly from
electricity AND some heat from fission (all those neutrons activate
stuff some of which decays).

At this point we need to go to theory. (Which Mr. Hull seems to hate.
Where / are you an engineer?)

Anyway, from a TD perspective fusion operates at an extreme
temperature and can give you insane efficiency levels, but working
with proven systems we can reasonably assume 40% efficiency.
Now assuming the fission heat counters the heat that is lost and not
directly used in our power plant we have (16 + 25) MW * .4 = the
potential to generate ~16.4 MW of electricity. Which just happens to
be the same efficiency as the ratio of fusion power produced to input
power, but that’s simply an odd outcome based on the 65% fusion
power level and would not show up under any other efficiency level.

Solving (X+1)* .4 = 1 for X gives us 1.5 so “breakeven” is 150% if the
input power is also converted to heat, which is true. Solving X *.4 = 1
for X gives us 2.5 or 250%, so if we ignore the heat generated by
electricity (which is stupid IMO). Now in the real world power plants
could as high as 45% AND you can use low-grade heat for other
processes. (I like the idea of heating northern city’s via underground
pipes to get rid of winter snow and ice and eliminate heating costs,
but that gets into insane power requirements. 1GW would keep
~1km^2 clear but you would probably create a huge updraft...)
Anyway, cheep heat for local chemical plants or homes seems like
the best use of such energy.

Moving back to the real world there are significant power
requirements to process tritium and other losses associated with
basic plant upkeep, which alter the picture somewhat.

Anyway, moving back into theory. Jet and other Tokamak designs
are limited in efficiency based on their size. The math is complex but
if you double the size of the facility you more than double efficiency.
There are several breakeven points but a large enough Tokamak
needs no input energy to sustain fusion temperatures other than the
minimal losses associated with cooling the coils.

In 15-20 years we can look at ITER and see things work out, but the
basic math says that the larger you build the system the easer they
are. At this point the largest problem is probably getting a sufficient
supply of tritium, but a lithium blanket seems like a reasonable
solution to that problem. Jet produces 22 MJ of energy from less
than .05 grams of tritium so assuming a reasonable operating
efficiency most of the other problems are probably solvable.


PS: JET operated for up to 60 seconds at a time an ITER should
operate for over 1000 seconds (16min 40 sec) at a time. Once again
scaling the system produces vast improvements in operating
conditions.

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by DaveC » Tue Nov 29, 2005 12:48 am

We need to keep our heads above water, doing these "theory" excursions in Thermodynamics.

First - very high temperatures, do NOT produce "insane" efficiencies... they only allow Carnot efficiencies to approach 100%. And "all" it takes to get to a theoretical 99% efficiency figure is a high temperature for absorbing the energy of about 100 times the low temperature heat rejection temperature.

If you are working with a heat rejection apparatus (aka heat exchanger) ...at say 127 C or 400K, then the high temperature part of the system is a measly 40,000 K... about 3.5 eV. Piece of cake. eh?

Except for the fact that all known materials are in plasma form at these temperatures and thus the apparatus for extracting this energy has to be able to work with plasmas.

That said, we are not any closer than before to having anything we yet know how to build work at these temperatures or at this efficiency.

Also, I am sensing that there tends to be either a bit of indifference or inadvertency in the mixing of energy and power in discussing Fusion device outputs. With pulse type devices, this is a very common problem. Failing to properly integrate the input energy over the device's entire cycle, leds often to outragious and physically impossible claims of efficiencies... i.e.: > 100%.

I think all on this net are more or less aware of these pitfalls, so there's no need to preach on this subject.

Where there is a need to remain alert, is when we are adding up thermal energy..."heat".... without regard to temperature. Here you can get a real headache... because it is entirely possible to move more low grade heat than you put into something... because along the way in the process, mechanical work, electrical energy, mass, etc.... was converted into low grade heat. The heat pump is the obvious example... moving more heat than the equivalent elecrical or fuel energy that was in put.

The amount of work that a low temperature device can do.. approaches zero, as the high temperature of the device approaches the temperature at which heat is to be rejected.

The amount of heat that such a device can pass, while doing almost zero work... increases without limit, as the high temperature approaches the low temperature limit.

So... where all we need is heat, we should use devices that pass through.. heat. Where we need work, we have to worry about how "hot" (temperature) the heat is, and hence the Carnot efficiency.

But we need to always remember the Carnot efficiency is the theoretical limiting efficiency... the bogey... not the achievable day in -day out practical operating efficiency... We do well usually to approach 75% of Carnot efficiency with most devices.

Hope this is useful..

Dave Cooper

Retric
Posts: 75
Joined: Tue Sep 06, 2005 5:48 pm
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by Retric » Tue Nov 29, 2005 8:13 pm

Sorry, by "insane" I meant 50+%, which is outside what any other
thermal to electricity system has done. 1300k to 300k could be ~75%
efficient and is inside normal temperature ranges, but when we are
talking about 42,000,000 Kelvin > 300k your not really limited by the
Carnot Cycle's limit of 99.999%. I was pointing out that there are
other systems that you can use that are not what people think of as
"thermal" systems. AKA Solar cells are limited by the Carnot Cycle
just like steam systems, but Solar cells don't touch the "hot" side of
that system.

Anyway, JET operated in bursts that lasted up to 60 seconds and
ITER should hit 1000+ seconds so the there is less need to focus on
"burst" power vs. power over time.

Check out http://www.jet.efda.org/documents/wesson/wesson.html
for a good look at JET’s past. It looks at the history of JET vs it’s
best operating conditions so there is a lot of graphs at the lower
operating conditions. Most of the time was spent on R&D in a non
fusion burn so that they could avoid activating the materials. On page
167 you see where they look at the energy used to increase the
plasma temperature vs. maintain the plasma temperature in which
case the Q turns out to be .9 vs. .6 when you integrate to get net
power input vs. net heat output. But they stick with the “real” value of
.6 for the reasons you stated. Anyway, this is so much better than the
1950’s starting value of Q=10 ^ - 12 that I think there is real hope that
they are on to something.

As to net power efficiency I think that we are going to be limited by
the use of beryllium systems but it can take fairly high heat loads so
40% thermal efficiency is not out of the question. ITER is so much
larger than JET that they are almost at ignition vs. breakeven.

PS: At 75% heat > electricity efficiency JET could operate as a power
plant. But, I used 40% efficiency because there are well-tested
systems that operate at that level instead of trying to think of what a
good theoretical limit would be.

User avatar
Richard Hull
Moderator
Posts: 12210
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: What makes the H-bomb so efficient? A: (with some math)

Post by Richard Hull » Tue Nov 29, 2005 10:33 pm

Since it has been real soon now for 50 plus years, we can all wait, I am sure, for this new iteration to show us the wonderful fusion future we have awaited.

Much as Dave noted engineers don't work off theory alone they work off of practicalities and realities coupled with the limits of material science and a load of ever present bean counters, bosses, overseers, etc. This is the first real layer in getting things from burst mode pulsed operations showing some distant promise to public usability. All this wonderment can be stopped at any layer beyond engineering for any number of reasons that relate to political, economic or other issues not having a thing to do with the glorious theory, the realizable science, the empirical proof of concept, the engineering, material science or even the intial bean counting. Many hurdles exist; way past the glory of JET or the ITER, regardless of operational reports.

Fission is a mature, well understood technology which has proved itself as a useful power source over many years and yet it lay dormant, not due to theory, science, engineering, bean counting or any of the other physical concerns associated with reason or logic. Fusion has much more of an uphill battle having never converted the first joule of fusion to a watt of juice.

Still, we are asked to empty our pockets and hold our breath, yet once again.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Retric
Posts: 75
Joined: Tue Sep 06, 2005 5:48 pm
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by Retric » Wed Nov 30, 2005 7:03 pm

I agree that politics will play a large role in the future of fusion but the
current view that fusion is a clean power source should help things
along. There are several independent programs that are activity
inspecting fusion and a vary positive outlook.

ITER should have a Q >= 10 which is well within the “useful” energy
range ~4x electricity out to electricity in. There will be issues with
contamination of the plasma due to sputtering from the walls. But,
containment time keeps going up. “The world record for pulse
duration is held by the TRIAM-1M tokamak in Japan, which ran for
more than three hours. However, the machine performed with an
injected energy of only 110 MJ and at a very low current, temperature
and density.” http://physicsweb.org/articles/world/17/1/6/1 (Controlled
fusion: the next step January 2004 IMO it's a good look at the wide
range of reactors being tested.)

“As of April, 2005, there are 104 commercial nuclear generating units
that are fully licensed by the U.S” So, I don’t think of Fission as a
dormant power source. It’s currently a heavily subsidized power
source that’s not directly competitive with coal power due to the risks
of contamination and the cost of dealing with waste. I agree that there
is a NIMBY problem with fission in the US but overall it’s much more
adopted than wind, solar, and hydro combined.

Anyway, I don’t expect fusion to reduce the price of electricity.

We have at least 100 years of coal and 1000 years of Fusion energy
on tap, so we are not “running out of energy.” With 40% efficient
sterling solar systems we have access to cheep and unlimited
energy right now.

http://pesn.com/2005/08/11/9600147_Edis ... est_solar/

The reason why I am interested in fusion is as a power plant for large
space ships, which is probably not going to happen till long after I am
dead. (I am 25 now, which means I am thinking well over 75 years on
that one.) Even if it takes 300 years we will soon enough reach for
the stars.

Building huge super conductors is a lot cheeper now than they where
30 years ago, computers are a lot cheaper, as are devices for the
remote handling of activated materials, and the world is a lot richer
now than it was 30 years ago. Right now Bill Gates could privately
fund ITER with the money he is giving to charity, in 50 years I expect
a wider range of people capable of privately funding such research.

Anyway, from the technical side things are looking good and from the
political side I am reasonably hopeful so I think the future is looking
good. With more funding we could probably see a working power
plant in 20 years, but even still once ITER demonstrates a useful
level of power gain people will have a hard time calling fusion a pie in
the sky dream. So, I expect funding to continue.

DaveC
Posts: 2346
Joined: Sat Jun 30, 2001 5:13 am
Real name:

Re: What makes the H-bomb so efficient? A: (with some math)

Post by DaveC » Wed Nov 30, 2005 11:56 pm

I think that these discussions are interesting and almost always useful. They would be more so, IMHO, were they kept more on the analytical plane, as opposed to the conjectural plane.

The long awaited promise of the controlled, non-explosive fusion device, is still.... being awaited, as Richard has so patiently pointed out to us.

It seems to me most likely true, that the required investment into a new technology is directly related to not only its sophistication and hence difficulty, but also to its economic potential. The bigger the payoff, in all probability, the bigger the investment needed to achieve it.

So the enormous potential payoff of "fusion" could well require 10X what has been put into it already, before it yields its secrets and becomes availble to help the common man.

But whether it can or will ever do this... is the BIG question that we labor to answer. Today, I think we are still in the "Reasonable Experiment Mode", which may be all we can expect at this point in time. The people who are stitching together economic justifications for the purposes of getting funding approvals, are more or less forced to play the game, or.. give up.

The work only becomes an embarassment in my opinion, when it is mis-represented in the technical community. That should stop.

The study of "mortar" while not particularly intesting, and would hardly win any architectural awards, is downright important to all building practices. Some of these fusion experiments, may just be "a study of mortar"... and not architecturely interesting, yet.

Nice discussion, thanks to all.

Dave Cooper

Post Reply