[Gasification] More on ORCs -- food for thoughts

[Peter Singfield]

OK -- junk the politics -- back to hard "tinkering"

At 11:16 PM 2/3/2007 +0800, you wrote:
>Who's saying "it can't be done" now? ;-)
>
>The boiling point of propane, or whichever working fluid is used, is well 
>below 0 C. According to their website, it is the parasitic load of the 
>feedpump that makes conventional systems uneconomic below temperatures in 
>the range you mention. By eliminating the feedpump, they lower the minimum 
>useful working temperature. At least read their explanation before rejecting 
>it out of hand!?
>
>The advantage of the lower temperature being that (for solar applications) a 
>glazed box collector is going to be simpler and cheaper to build and 
>maintain, and less vulnerable to extreme weather, than any kind of 
>concentrator. And for other heat sources, that there's just more waste heat 
>around at lower temperatures.
>
>Or is your objection that below about 350F the temperature gradient from 
>ambient is too small to do a useful amount of work? However, they claim 
>their demonstration unit will produce 1 kW of electricity from a temperature 
>gradient of 189 -74 = 115 F (64 C).
>
>We shall see. Perhaps.
>
>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