Viewing 15 posts - 1 through 15 (of 21 total)
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  • #1588
    nemmart
    Participant

    There was some interesting discussion on the NextBigFuture page. In particular, GoatGuy claimed
    that the required 80-90% conversion efficiency from charged alpha particle and X-Ray conversions were
    not achievable.

    Let’s take that claim at face value — if we assume a 40% conversion efficiency, is it still possible to
    build a generator based around a DPF? Could you build a biggger device whose fusion energy output is
    4x the input (input energy + 4x for fusion energy=5x input * .4 = 2x input, as is the current goal).
    Do the engineering challenges get dramatically worse to scale the device to 4x the input?

    #13291
    Breakable
    Keymaster

    There is no 90% efficiency requirement. Some of the . This is the energy flow diagram:
    http://fusionenergyleague.org/assets/device/lpp/fofu_flow.jpg

    Efficiencies are 81% X-Rays and 80% ION, which should be reachable considering it is high energy direct conversion. For example transformers can convert energy with 98% efficiency, but X-Ray to DC (or AC) technology still needs to be developed.
    Thermal is counted as a loss. If Thermal is used as energy source it might reduce efficiency required for other methods.
    If fusion output is greater due to some pleasant surprise or maybe some clever designs that are thought out during engineering it might be easier to build a production level Focus Fusion reactor.

    #13292
    nemmart
    Participant

    That doesn’t really answer my question. If we assume a 40% conversion efficiency for both X-Rays and alpha particles, is it
    still possible to engineer a DPF power source? i.e., fusion energy output equal to 4x input energy?

    #13293
    Joeviocoe
    Participant

    nemmart wrote: That doesn’t really answer my question. If we assume a 40% conversion efficiency for both X-Rays and alpha particles, is it
    still possible to engineer a DPF power source? i.e., fusion energy output equal to 4x input energy?

    Eric Lerner was asked, “What is the minimum conversion efficiency to be a practical power plant.” (paraphrased)…
    Lerner responded, “about 70% for both Ion and X-ray”… but he is still confident that at least 80% for both is possible.

    I really don’t think anywhere near as low as 40% is likely for direct conversions. Very few non-thermodynamic conversions are so poor.

    *Photovoltaics are 20%-40% because of the type of photons that sunlight produces… a wide spectrum of frequencies. The material that reacts by letting photons knock off its electrons, can only do so for photons of specific energy levels (quantum)… thus wasting a whole lot of potential solar energy.

    Most of the time, 40% conversion efficiency is because of thermodynamic limitations of Heat Engines… unavoidable.
    Which is why this whole project is designed for Aneutronic Fusion…. not D-T or D-D fusion where most of the energy comes out as Neutrons that would be used to boil water to run a steam turbine.
    Yes, it is much easier to get fusion ignition (because of the larger cross section)… but more fusion yield would be needed because of the losses downstream in the turbines. That also is the reason why costs would be high, and the physical footprint would be so large. (not to mention the radioactivity and danger of proliferation)


    So, to answer your question… no. But the assumption is highly unlikely.

    #13297
    Tim1
    Participant

    Even if both ion and x-ray efficiency are 80% you still have 10 MW of heat produced. You might be able to capture a few MW by running it through a heat engine. Not so elegant but it should work. Remember you have to dissipate the heat any way.

    #13298
    Tim1
    Participant

    Breakable wrote: There is no 90% efficiency requirement. Some of the . This is the energy flow diagram:
    http://fusionenergyleague.org/assets/device/lpp/fofu_flow.jpg

    Nice chart. What are the prospects of getting more than 66 kJ of gross fusion energy per shot? That would make conversion efficiency less of an issue.

    #13299
    zapkitty
    Participant

    Tim1 wrote:
    Nice chart.

    I believe that FF Sankey diagram has been updated:

    Edit: See the attached pdf for the more recent full size version.

    .

    Tim1 wrote: What are the prospects of getting more than 66 kJ of gross fusion energy per shot? That would make conversion efficiency less of an issue.

    Yes, a bit of an increase would be good… but if you up the gain too much then you run into the same problem as if you’d increased the pulse repetition rate: too much heat for the anode to handle.

    Attached files

    Sankey.pdf (564 B) 

    #13300
    Joeviocoe
    Participant

    zapkitty wrote:

    Nice chart.

    I believe that FF Sankey diagram has been updated:

    Edit: See the attached pdf for the more recent full size version.

    Ewww… I think I like the old one better. This new one is too cluttered. And doesn’t really have new information.

    #13301
    Breakable
    Keymaster

    Tim1 wrote: Nice chart. What are the prospects of getting more than 66 kJ of gross fusion energy per shot? That would make conversion efficiency less of an issue.

    I believe that the best answer is “we dont really know”. Without an extensive research and engineering we cannot really find out. There are some simulations worked on by Dr. Warwick, but they are far from complete.

    #13334
    vansig
    Participant

    Joeviocoe wrote:
    Most of the time, 40% conversion efficiency is because of thermodynamic limitations of Heat Engines… unavoidable.

    by the way, heat engine efficiency reaches its limit based on:
    (1) the high and low temperatures of the Carnot cycle, expressed as (1 – Tc/Th), where Tc is the absolute temperature of the cold reservoir, and Th is the absolute temperature of the hot reservoir; and
    (2) the practicalities of the creep limit of stainless steel (about 530C or 1000F) and the condensation of water vapour at atmospheric pressure. if Tc = 100C = 373K, and Th = 530C = 803 K, then 1 – Tc/Th = 46.5%

    but, the alpha beam conversion is a transformer. for those, 98% is routine, and up to 99.5% is in use in power generation and distribution networks today:
    http://www.indiastudychannel.com/resources/163504-Energy-Management-in-Distribution-Transformer.aspx

    and, while the onion still has to be developed, and we should justify the claim that we expect high efficiency from it, as well, the efficiency is expected to increase with xray energy, which will be up around 100 keV if i recall correctly.

    #13336
    nemmart
    Participant

    If the onion and ion beam capture hit the 80-90% targets then everything will be just peachy.

    But I’ve been following this project for a while and there are more twists and turns than a
    mountain road. Switch problems that had to be overcome, arcing issues, plasma impurities,
    and several redesigns of the cathode and anode…

    It’s not too difficult to imagine a scenario where the fusion works, but the engineering to hit
    the 80-90% conversion efficiencies fails. This could be for a lot of different reasons, maybe
    the ions comes out at different velocities and must be sorted and some energy is lost. Or
    maybe the ions come out in a spread out cone and not directly down the axis of the machine.
    Who knows. And I’m sure there are even more issues for the onion.

    To me, this seems like a significant risk to the project.

    So why not have a plan B in place? I would advocate that someone on the LPP team or who
    knows what parameters can be tweaked should be looked to see if there is an alternative
    configuration that produces more fusion power per shot. We know that a heat engine can
    achieve 40% efficiency and as per the earlier post:

    > fusion energy output is 4x the input (input energy + 4x for fusion energy=5x input * .4 = 2x
    > input, as is the current goal).

    I realize that using a heat engine will increase the cost of the energy produced by a FoFu significantly.
    But it should still be significantly cheaper than a natural gas plant. It would provide base load power
    and would still be a very compelling option.

    #13340
    zapkitty
    Participant

    nemmart wrote: If the onion and ion beam capture hit the 80-90% targets then everything will be just peachy.

    The high efficiency estimates are a result of the known physics.

    nemmart wrote: It’s not too difficult to imagine a scenario where the fusion works, but the engineering to hit the 80-90% conversion efficiencies fails.

    The efficiencies could be lower by 5-10% each, depending on how each side sorts out, and FF would still be a viable electrical power source.

    nemmart wrote: To me, this seems like a significant risk to the project.

    Not really. I’m not saying it’ll all be smooth sailing but the physics of both the ion beam and the x-rays are known quantities. It’s just that we are used to converting electricity into ion beams and x-rays rather than tapping them for power.

    nemmart wrote: So why not have a plan B in place?

    That would be fine… as long as the plan B doesn’t involve thermal-electric conversion.

    nemmart wrote: I would advocate that someone on the LPP team or who knows what parameters can be tweaked should be looked to see if there is an alternative configuration that produces more fusion power per shot.

    This seems to be a fairly common misconception… the power output of an FF unit is primarily determined by its pulse rate. Any increase in individual shot power can be approximated by increasing the pulse rate.

    And the maximum pulse rate is governed by anode cooling. The thermal output of an FF unit operating at 200 Hz has been estimated at ~7 MW thermal… doable, but pushing things a bit.

    A substantial increase in either shot power or pulse rate would both end up with the same result: a corresponding rise in temperature and a slagged core.

    nemmart wrote: We know that a heat engine can achieve 40% efficiency

    … errrrrrrrrrr… not quite. Those kinds of thermal efficiencies are achieved at the expense of very high operating temperatures. Temperatures in the thousands of degrees C.

    The most anyone has ever postulated for the output of an FF cooling stream is at ~700-800 degC… i.e. maybe 20-25% thermal-to-electrical conversion efficiency. That’s game over for a thermo-electric FF, I’m afraid.

    … still would be a great x-ray source, though.

    #13343
    nemmart
    Participant

    zapkitty wrote:

    I would advocate that someone on the LPP team or who knows what parameters can be tweaked should be looked to see if there is an alternative configuration that produces more fusion power per shot.

    This seems to be a fairly common misconception… the power output of an FF unit is primarily determined by its pulse rate. Any increase in individual shot power can be approximated by increasing the pulse rate.

    And the maximum pulse rate is governed by anode cooling. The thermal output of an FF unit operating at 200 Hz has been estimated at ~7 MW thermal… doable, but pushing things a bit.

    Mhmm, well, I probably should have written “more fusion energy per shot” rather than “fusion power”.

    Initiating each shot requires 100 KJ from the capacitors. If the conversion efficiency is low (say 40%) then you won’t get enough energy from each shot to recharge the capacitors for the next shot. And the shots/sec is irrelevant. A plan B designed around a lower conversion efficiency would require far more energy from the fusion reactions, something like 400 KJ per shot instead of the planned 66 KJ.

    #13345
    Tim1
    Participant

    nemmart wrote: We know that a heat engine can achieve 40% efficiency

    … errrrrrrrrrr… not quite. Those kinds of thermal efficiencies are achieved at the expense of very high operating temperatures. Temperatures in the thousands of degrees C.

    The most anyone has ever postulated for the output of an FF cooling stream is at ~700-800 degC… i.e. maybe 20-25% thermal-to-electrical conversion efficiency. That’s game over for a thermo-electric FF, I’m afraid.

    … still would be a great x-ray source, though.

    Actually ultra supercritical steam turbines run at ~600 degC and supercritical CO2 turbines could have an efficiency of 50% and run at ~550 C. Their main problem is cost. Also CO2 turbines are in the research phase.

    #13346
    zapkitty
    Participant

    nemmart wrote:

    The thermal output of an FF unit operating at 200 Hz has been estimated at ~7 MW thermal… doable, but pushing things a bit.

    Initiating each shot requires 100 KJ from the capacitors. If the conversion efficiency is low (say 40%) then you won’t get enough energy from each shot to recharge the capacitors for the next shot. And the shots/sec is irrelevant.

    The question is core temperature and so the results will be the same whether you increase the shot rate or the energy per shot.

    The final set of beryllium electrodes will be quite a bit smaller than the old copper and current tungsten designs. Less mass and closer to the plasmoid. Increase the temperature too much [em]and the anode is going to be slag.[/em]

    nemmart wrote: A plan B designed around a lower conversion efficiency would require far more energy from the fusion reactions, something like 400 KJ per shot instead of the planned 66 KJ.

    That would, of course, mean switching from an aneutronic to a neutronic process. Switching from pB11 fuel to D-T fuel… from hydrogen-boron to deuterium-tritium .

    And that just means that the anode will not only be slag but the entire FF core will be highly radioactive for centuries to come. Not very good for the FF project.

    The temptation of D-T… the low-hanging but extremely radioactive fruit of the fusion fuel tree.

    Interesting aside: EMC2’s Polywell project recently seems to be trying to use the prospect of D-T fuel to lure in investors… which seems odd since D-T’s intense high-energy neutron flux would quench the HTSC coils a Polywell power generator would need to function.

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