In order to make a low-temperature-gradient turbine for generating
power, I need a gas that has a fairly high boiling point (as gasses
go).  Something along the lines of Freon or Ammonia would work, but
I'd like more information than I have (not much) and more options than
just those.

I need at least 1 gas that is commercially available, and can have its
boiling point vary between the temperatures 50F to 100F or so,
depending on the pressure its at.  Compressability is important.

Thus, the goal: find a safe fluid to put in a closed system that I can
increase or decrease the pressure of for the purpose of manipulating
its boiling point.  I have to be able to make the boiling point range
somewhere between about 50F-120F (exactness is not important), and a
wider range would be nice.

Accurate phase-change diagrams would be extremely helpful as well.
Here's a (poor) example of what I'd like for any suitable gasses:
http://www.cem.msu.edu/~mantica/cem383/co2pd.gif

If you want some background information on the general thing I'd like
it for, plug "otec" into google.  It's an alternative power generation
system, and I'd like to play around in the field, but I don't exactly
live near the ocean nor have a few million dollars to spare.  Thus, I
want to make a poor-man's version thereof.

Oh, and if it does something nasty like corrode rubber, poison living
things, or strip superheros of their special powers, that's a definite
downside.

I hope this isn't a wild-goose-chase.  Happy hunting.

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Request for Question Clarification by mathtalk-ga on 10 Jul 2003 08:50 PDT

Hi, db8soothsayer-ga:

Given that this is to be a "poor man's" experimental setup, perhaps
you should specify the range of pressures that you believe are
feasible.  Many "gases" can be pushed to condensation somewhere in the
range of temperatures given, if only enough pressure is supplied.  Is
10 atmospheres in your ballpark?

regards, mathtalk-ga

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Clarification of Question by db8soothsayer-ga on 10 Jul 2003 13:51 PDT

well, what I was thinking for the setup was using a pump to get the
initial gas charge into the system and to flush out the ambient air,
and then shut the pump valve, but have a reservoir w/ a piston on some
sort of linear actuator (solenoid or hydraulic probably) to fine-tune
the pressure by changing the volume (PV=T and all that).  The pressure
I'm capable of attaining of course depends on how good the pump and
the piston are, and how much my materials can withstand.

Given that I plan on using schedual-80 PVC and flexible copper tubing,
I think 20 atm is a reasonable pressure, but of course, even lower
would be easier and therefore more desireable.

Funny, spellcheck says both "gases" and "gasses" are correct.  Ain't
english krazy?

Upon further thought, reducing pressure below 1 atm is also an option.
 I don't know if room-temperature liquids under reduced pressure might
be easier than room-temperature gas(s)es under increased pressure. 
It's something to look into, though.

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Request for Question Clarification by mathtalk-ga on 10 Jul 2003 19:43 PDT

Hi, db8soothsayer-ga:

I can identify for you a readily available substance which exists in
both liquid and gaseous phases throughout the temperature range 50F to
100F.  The pressure at which it condenses in the middle of this range
is below 10 Atmospheres, and it is not corrosive to rubber or copper. 
While this substance is not particularly toxic, you should be aware
that in concentration almost any substance, even water or oxygen, will
be either poisonous or at least a suffocant.  Therefore your setup
should be prepared to contain (for proper disposal) whatever
liquid/gas substance is chosen.

I have concentrated on identifying a single chemical compound, and in
consideration of the list price offered, I don't feel that an
exhaustive list of all "options" would be feasible.  In particular you
might be interested in exploring a mixture of substances; without
knowing more about the role that phase change plays in your design it
is merely my speculation that something of this kind might be of
greater utility than a "pure" material.

You said, "Compressability is important."  Of course all gases are
compressible, so I take your comment to relate to compressibility of
the liquid phase.  However I cannot ascertain exactly what your
requirements are for compressability.

Subject to those caveats I can provide a phase diagram for you
throughout the given temperature range 50F to 100F.  Please advise if
this will be acceptable as an answer.

regards, mathtalk-ga

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Clarification of Question by db8soothsayer-ga on 15 Jul 2003 18:19 PDT

By the way, I apologize for the delay in the clarification.  I had to
go out of town for a bit.

I understand the dangers inherent in experimentation with gasses.  I
plan on using a well-ventilated area, so I don't think asphyxiation is
a problem.  I would prefer to simply not have to worry about using a
noxious gas, though.  If need be, I can wait until I have access to
vacuum hoods (when I go back to school) to do any actual work on this.
 I'm mostly concerned with figuring out the feasibility and cost of
this project before I start buying anything.

I've raised the list price because, on further thought, the question
probably deserves a little more effort than I offered to pay for.

I briefly looked into mixtures but I realistically do not have the
equipment to mix gasses as well as (I imagine) that would involve (if
at all).  From what I've learned, a mixture would likely be a better
choice in terms of performance, but I just want a "proof-of-concept"
system for now.

As for compressibility, it just occurred to me that it was a somewhat
silly statement.  Feel free to ignore it.  :-)

A phase diagram of an acceptable substance in that temperature range
would be a satisfactory answer.  The resources you used to find the
information would also be appreciated.  With no real idea of the
amount of research this question required, I'm going to trust your
judgement on how much information to provide for the price I'm
offering.

Thank you for your time and help.

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Clarification of Question by db8soothsayer-ga on 22 Jul 2003 10:38 PDT

Hello? anyone there?

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Request for Question Clarification by mathtalk-ga on 22 Jul 2003 12:52 PDT

Hi, db8soothsayer-ga:

I have references to some research journals from the 40's that I'll
need some time to check out at a library.  I'm still here but don't
want to post an incomplete answer.

regards, mathtalk-ga

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Clarification of Question by db8soothsayer-ga on 22 Jul 2003 16:39 PDT

ok, just checking.  It sounds like you're really making sure you give
me the best answer you can.  I appreciate that.  Take however long you
need.

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Request for Question Clarification by mathtalk-ga on 22 Jul 2003 18:20 PDT

Actually, let me clear up the point about compressibility.  Initially
you said that compressibility "is important" but later (after I
commented that I took this to be a requirement on the liquid phase of
the fluid), you said "Feel free to ignore it."

I had selected a liquid that was between 1 and 2 orders of magnitude
more compressible than water, not really knowing precisely what your
design requirement was.  My impression that compressibility was a key
characteristic was reinforced by the examples of freon and ammonia you
started with.

Freon's compressibility is of course key to its use as a refrigerant;
ammonia as I'm sure you know was used in a similar manner in an
earlier "era".  However your design is not for refrigeration, but "a
low-temperature-gradient turbine for generating power".

Obviously a steam-driven turbine is not "low temperature", but it it
may be worth pointing out that such systems are not typically "closed
circulation" designs, either.  The use of a more expensive "polluting"
fluid would argue for as nearly closed a circulation as is practical
(where water is cheap and harmless in small doses).

So I'm trying to rethink your requirements.  If you have any further
light you can shed on these design characteristics, it will be of
great assistance to me in providing the best possible answer.

regards, mathtalk-ga

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Clarification of Question by db8soothsayer-ga on 25 Jul 2003 15:35 PDT

eh, how to explain this...

Ok, my basic idea is to use a difference in pressure between two
points, A) a solar evaporator (high temperature), and B) an air-cooled
condenser (low temperature), to cause fluid flow, to drive a small
turbine to power an electric generator.  This in its basic form is a
Rankine engine.  Pretty much all commercial power plants use the
Rankine cycle to produce electricity -- they use water/steam as the
working fluid.

The problem with using water to drive a rankine engine is that it
generally needs quite a high temperature to vaporize the working
fluid.  Thus, only a high-temperature heat source can be used (burning
coal/oil, nuclear decay, etc).  It is possible to use a fluid with a
lower vaporization point than water, and lower the temperature
required of the heat source, down to, say, that provided by sunlight.

I want to make a very small powerplant, using sunlight on a dark
surface as the heat source for the evaporation, and the cooler air in
the shade as the cooling for the condenser.

Basically, its like an air conditioner working in reverse: instead of
electricity coming in to cause cool in one area and warm in another,
it uses a cool area and a warm area to produce electricity.  In that
sense, yes, I do need a refrigerant.

In terms of compressibility, the easier it condenses and evaporates,
the better it is, as long as the range for the phase change can be
brought around the temperature of a black surface in summer sun
(reasonably between 100 and 140 F).

I have to go away for the weekend, but i'll be back on sunday.