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Brayton cycle solar generation.

  1. Dec 5, 2009 #1
    I've been trying to wrap my head around this one for a while. What are the drawbacks to using a brayton cycle solar generation setup?

    Here's the setup. It seems a car turbo is the cheapest and best suited so that's what I'll prototype if the idea gets that far.

    Atmosphere => Compressor => Solar array => turbine => Exhaust to atmosphere.

    Benefits:

    Uses the atmosphere as the heat sink.
    No high temperature fluids. (low specific heat capacity for air.)
    Nothing environmentally controlled or hazardous. (such as ammonia or refrigerants)
    Cheap off the shelf components.
    Simple design. (No circ pumps or feed pumps.)

    Problems:

    High RPM will need to be geared down.
    May not have reasonable efficiency at achievable temperatures.


    Here are some links to bring you up to speed if you need em.

    Brayton cycle:
    http://en.wikipedia.org/wiki/Brayton_cycle

    A similar design that uses solar to improve efficiency:
    http://ec.europa.eu/research/energy/pdf/solgate_en.pdf


    Anything else y'all can thing of?
     
  2. jcsd
  3. Dec 7, 2009 #2
    air has such poor heat transfer properties. The transfer of the energy from the heaters to the drive pressure air would probably yield poorer results vs fluid/vaporization style closed system

    dr
     
  4. Dec 7, 2009 #3

    Mech_Engineer

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    Science Advisor
    Gold Member

    Well air has a genrally low specific heat, but this just means it heats up more for a given thermal flux and isn't necessarily a drawback. Air is also a poor thermal conductor, so you would have to rely on a pretty large heat exchanger to transfer heat into it (more of a design consideration than a problem).

    Overall the system is feasable, but as with all solar systems it will be limited by the amount of solar flux you are collecting. Concentrating a usable amount of solar energy could be the most challenging part of the system, especially when paired with losses in the compressor/turbine, and heat exchanger.
     
  5. Dec 7, 2009 #4
    Planing on using large parabolic trough reflectors so the heat exchanger will be large. I have no idea what kind of temperature/flow-rate I'm gonna get, but I'm hoping for an overall efficiency of >10%. Guess I'll start putting pen to paper and planning a prototype.

    May even use a regenerative heat exchanger, but that's all dependent on the pressure.
     
  6. Dec 7, 2009 #5
    I looked into similar power cycles for spacecraft power, and the one thing that really killed me was the power needed to run the compressor. I thought it would be small, but it ended up making my design infeasible due to size constraints of the spacecraft.
    Just an FYI.
     
  7. Nov 20, 2010 #6
    i have been involved in the same idea. i find that the only real hurdle is that an automotive turbocharger (or even a large industrial diesel engine turbocharger) are limited to very low pressure ratios (2.1 to the most), and the reason for it is simple, in gas engines you cannot overboost or you get preignition, and even in diesel you don´t need much more. Brayton cycle based t/c need an 8 at least to begin doing their thing (check the efficiency formula for Brayton). remember regular gas turbines (land,or air use) are always near 15 in their pressure ratios.
    my goal is 1KWe. and i have no problem in being suplied with enough thermal energy from my coleagues who are already storing > 40 KWH thermally, so i can go on around the clock.
    does anyone have a clue on which way i should go?
     
  8. Nov 23, 2010 #7
    Hello Chayced.

    Not sure if this of any help but it works.

    We have had it going but no one seemed interested so we switched over to internal combustion.

    Its uses the same design as for the pistonless IC engine. However the air compressor becomes the the suction/compressor found in fridges.

    The refill tank also becomes Co2 gas compressor to orice and expansion chamber as in "Eienstein" fridge, this occuring by rising water level from turbine discharge and the compressor becomes vacuum pump drawing cool Co2 back into the combustion cylinder which is instead heated.

    Working temperature begins at minus 10* celsius.

    References to heat/pressure Co2 are aplenty on the web.

    Power output is suppled by California University. 9 bar pressure at flow of 1 litre per second generates 720 watts. Increase in either pressure or flow increases wattage output.

    I am no engineer just old bush folk and happy to supply all as I have given it over to all as Open technology.

    Sorry about the combustion example, will draw again in low heat turbine configeration should your interest be such.

    Cheers

    Peter
     

    Attached Files:

  9. Nov 23, 2010 #8
    i am very interested in the turbine configuration. bottom line, i am interested in generating electrical (around 1KWe) from thermal solar energy. all inputs are greatly appreciated.
    rgds
    eduardo
     
  10. Nov 23, 2010 #9
    Hello eduardo,

    Should around 1KWe be all your seeking, no compresser is needed, ambient cooling will cut back on costs by and be far simpler to make.

    The refill tank accepts hot Co2 from exhaust, which in fact speeds the DaS Valve refill.

    New incoming water from the turbine lifts the held water in return tank forcing the hot Co2 into cooling line which feeds back into the shorter cylinder being heated.

    2 litre flow per second at 9 bar pressure produces 1,440 watts. 9 bar pressure being minimum pressure required to obtain full operation of turbine generator.

    Cheers

    Peter
     
  11. Nov 23, 2010 #10
    thank you Peter, i have to take a good dive into your idea, let me try to figure it all together and i´ll be back

    rgds
    eduardo
     
  12. Nov 23, 2010 #11
    this is my actual setup. it has a garrett t/c meant for a large Toyota engine(amongst many), the heater (heat exchanger where i get via a graphite conductor the heat from a molten glass tank. the actual exchanger is an steel portion of an auger, meant as a drill for laying fence posts on farms. this is inside a cylinder where the compressed air is passed in helix) . i´d like to include a couple of photos and a drawing, but somehow i couldn´t mannage. if anyone is interested, tell me how it´s done, or give me email to annex them. it is part of an integrated household energy system, 1 KW electric, plus a.c., plus hot water, all out of thermal/solar, and thermal storage. my rig supposes a compressor, the heater, a regenerator, the turbine and a cooler (warms the water for the household).
    i am stuck, because even if i spin the turbine with an air jet to maybe some 10000rpm ?? i don´t get any boost whatso ever. garrett claims the surge line is above 32000rpm, and they sure seem to be right.

    eduardo
     
    Last edited: Nov 24, 2010
  13. Nov 24, 2010 #12
    Hello eduardo,

    Are you using a gas turbine and does the gas recycle through hot cold, or go to atmosphere.

    Cheers

    Peter
     
  14. Nov 24, 2010 #13
    no it is supposed to be a closed air circuit
     
  15. Nov 24, 2010 #14
    the bottom line is " Solar Thermal", all your needs for a household (electricity,a.c. and hot water). but: no PV and no electrical storage batteries, the a.c. is absortion.
     
  16. Nov 24, 2010 #15
    Change the gas and add cooling tank. Gas bubble cooling water through and recollect.
     
  17. Nov 24, 2010 #16
    so you say i can still use my automotive t..c.?, and i only have to switch to say CO2 and add the recollector?
    what about the compression ratios? and the very high rpms?

    eduardo
     
  18. Nov 24, 2010 #17
    Hello eduardo,

    Picked up virus since last contact real difficulty getting back. E-mails still getting through. pmdastech@gmail.com if you hear no further.

    T..C fine just add collector. No compression ratios. no problem high RPM

    Note earlier atachments (hopefully) Co2 pressure/temperature scale. Example Co2 +10* Celsius 20 bar pressure Co2 +30* Celius 70 bar pressure. Pressure force going to T..C will be 50 bar.

    Tried again to attach no go. Can you send email adress.

    Cheers

    Peter
     
  19. Feb 10, 2011 #18
    ----------------------------------------------------------------------------

    i am trying to come up with a setup that can produce a mechanical (shaft)output around 1KW.
    i have an automotive turbine (the turbine side of a regular turbocharger), and this should drive both: it's compressor and the abovementioned shaft output.
    i removed the compressor part, because it would never give me sufficient compression ratio that could make sense in a Brayton cycle configuration (the original compressor is meant to give very little boost, since otherwise pre-ignition would occur, diessel TCs don't go very much higher), from various literature i see that i will need something over 8 as compression ratio. i am considering designing, and building a centrifugal compressor, with forward looking vanes(to get higher pressure output).
    i calculate i need to handle some 50 #/minute of air.
    since i must use a dynamic compressor (no positive displacement options), i ask...
    am i in a right track?, has someone looked into something similar?, is there something already available?

    tks
    eduardo
     
  20. Jul 17, 2012 #19
    does anyone have some new thought on Brayton Solar?
     
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