[Gasification] More on ORCs -- food for thoughts

[Robert Taylor]

Hi Peter

Thanks for taking the time to write that. I must admit I'm in over my head
here, but your arguments sound convincing to me. The only part that sounds
iffy is whether concentrating troughs that are static and semicircular in
profile will be effective, as against parabolic ones that track the sun.
Also, for high efficiency you may want the collector pipe to run inside an
evacuated glass tube. But then you are no longer in the backyard-simple DIY
league.

Robert
>
> Hi Robert;
>
> Actually -- Kevin's old chart says it can't be done -- and in that case --
> it is correct.
>
> Each and every working fluid (in a heat induced liquid/vapor cycle)
> suffers
> from the same problem -- losses that can't be recovered due to latent heat
> of evaporation.
>
> The higher you "super-heat" the vapor -- the greater the mechanical energy
> that can be extracted -- for pay back.
>
> For butane (and or pentane) -- 350 F and over "shines"
>
> For water/steam -- you have to go 1100 F to find that same sweet spot.
>
> The other factor to consider is your delta T -- your condensing work
> "costs" sky rockets when that number is low.
>
> Simply put -- if you going to invest a lot of money in ORC prime mover --
> condenser -- solar collection -- etc -- you should try to get the bigger
> bang for your investment.
>
> Over all system efficiency is how you do the "bang" rating.
>
> OK -- a theoretical example:
>
> Now -- if I bought a 1000 gallons plastic cistern -- as they sell here for
> rain water vats -- cylindrical -- and split it lengthwise down the very
> middle -- so I would have two perfect 180 deg troughs -- and line those
> with reflective plastic (as they use for making Christmas decorations -- I
> would have a very effective solar concentrator that certainly would heat a
> centrally positioned tube to 400 F or plus. And for relatively little
> cost.
>
> The ORC working at those temps would probably produce at least 5 times
> more
> power (if not a lot more!!) from the same sized turbine -- than the low
> temperature operation you describe.
>
> The cost of a turbine -- condensers -- feed pumping mechanisms -- etc --
> etc -- of five times -- if not greater -- the capacities would far exceed
> the cost of that home made solar concentrator.
>
> A 1000 gallon rain vat costs $500 US -- 
>
> That is a round cylinder 5 ft diameter -- 6.5 feet long -- 
>
> You end up with 13 feet of trough when split.
>
> total surface area facing the sun would be: 104 ft sq.
>
> Now -- what is the solar energy per ft sq??
>
> And multiply that by 14% --
>
> Well -- using the Arizona example -- they state:
>
> 110,000 ft sq gives them 6,843,000 watts "heat"
>
> 104/110,000 X 6,843,000 = 6470 watts X 14% = 905 watts electric power
> "out"
>
> So now -- just add extra customized water vats as required ---
>
> Fun with math -- eh??
>
>
> Now -- do the same for your low temperature system -- but oops -- one
> problem -- no over all efficiency numbers to be found anywhere -- 
>
> That is very "suspicious"!!!
>
> By the way -- Kevin's charts go wrong at the higher heats because they do
> not factor in superheat -- always crunching for "saturated" state --
> though
> at greatly increased pressures.
>
> The trick is to boil the working fluid -- then super heat the vapor ---
> the
> energy being in the super heating then -- and thus -- at least
> theoretically -- "availalble" for conversion to mechanical energy.
>
> The higher your superheat -- the higher can be your system efficiency --
> no
> other way around it!
>
> The system you describe probably has over all efficiencies of a fraction
> of
> one percent -- ergo -- they do not publish that minor detail -- eh??
>
> Denial rules so supreme --- and always -- in these to modern times.
>
> People want "myths" -- not facts -- that plant better in their dream
> gardens ---
>
> The myth of ethanol from corn -- thrives in America's dream garden these
> days - -as but one other excample.
>
> You all plant myths -- not seeds ---
>
> Peter / Belize
>
>
>
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