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Need help with a new solar engine

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Posted by Morris Dovey on September 18, 2010, 5:51 am

Well, at least it's new to me - and although I'm working in a solar
context, it's just another (external combustion) heat engine.

My starting point is a solar trough like that shown at


For the sake of discussion, let's replace the copper target pipe with
one made of unobtanium which, as everyone knows, can withstand infinite
pressure but is horribly expensive.

For part 1 of my project, I want to completely fill the pipe with water
and cap both ends with unobtanium fittings.

As the seventh photo on the web page shows, the trough can heat the
target to 384C - well above the boiling point of the water inside the
pipe. I've been thinking about turning the mirror 90, so that my trough
will be 8' wide and 4' long - to push the temperature up to near 575C...

My understanding is that above ~373C the physical properties of the
liquid and vapor phases are identical and that, in fact, any discussion
of 'liquid' and/or 'vapor' is meaningless. Can anyone verify and/or
improve my understanding?

And, most important of all, if my pipe contains some volume /V/ of
water, does anyone know how to calculate pressure /P/ as a function of
temperature /T/ so I can think about using something less expensive than

Morris Dovey

Posted by dow on September 20, 2010, 1:50 pm

-373C would be 100 degrees below Absolute Zero. I guess the minus was
a typo.

According to my Rubber Bible, the critical temperature of water is
374.1C. Above that temperature, there is no distinction between liquid
water and gaseous steam. There's just a "fluid" phase. The critical
pressure, is 218.3 atmospheres, so you could use any vessel that can
withstand that pressure to hold the water up to its critical
temperature. I'm afraid I don't know how to calculate the pressure at
higher temperatures. I don't think it would be easy.

For Carbon Dioxide, the critical point is at 31C and 72.9 atmospheres.
It's quire easy (I've done it) to seal a few drops of liquid CO2 into
a glass capillary tube. At temperatures below 31C, the meniscus
between the liquid and gas is clearly visible. If you gently warm the
tube through 31C, the meniscus abruptly vanishes. It's sudden and
quite startling.

Good luck. Be sure to protect yourself from possible explosions.


Posted by Morris Dovey on September 20, 2010, 3:53 pm
 On 9/20/2010 8:50 AM, dow wrote:

The minus is actually a tilde (squiggle). My bad, I should have said
"about" or "approximately".

Up to that point it appears fairly easy to approximate. It does seem
strange to me that water hasn't been studied to death and every possible
equation (however convoluted) published...

<geek alert!> Oooooh - I think I'm going to have to pester friends until
I can watch that for myself. That tube sounds like something every high
school science teacher should have!

Would you care to sell 5 of those to me? I'd like to give 4 away...

Thanks. An acquaintance over on news:alt.energy.homepower translated 217
atm to a little over 3200 psi and assures me that's well below the burst
strength of the 0.049 inch wall 1/2 inch diameter (seamless) stainless
steel tubing he's working with (on a very different project).

I've been working with air as the working fluid in fluidynes, and the
Ideal Gas Law makes behavior conveniently predictable - and at constant
volume PV =nRT is really P = kT, a simple linear relationship; but when
I looked at the vapor pressure curve for water in the immediate
neighborhood of the critical point, the relationship is (at least
approximately) exponential - which means that for a relatively small
change in temperature I can expect a relatively large change in
pressure. That's exciting, because it means that I may be able to effect
a very large improvement in efficiency by doing little more than
changing from one commonly available working fluid to another.

I'm going to be distracted by the capillary tube image all day - it
doesn't take much to amuse some people. :)

Morris Dovey

Posted by dow on September 20, 2010, 8:15 pm
So it is! I wasn't fully awake when I looked at it this morning.

It has been extensively studied. However, under high pressures it
doesn't obey nice simple laws. Really, the only way to describe its
behaviour is with ad hoc graphs or tables of values.

Normal liquid water is mostly empty space. The H2O molecules interact
with each other strongly via hydrogen bonding. Essentially each
hydrogen is, on average, half-way between two oxygens, and doesn't
really "know" which one it belongs to. Each oxygen is surrounded
tetrahedrally by four others, with a hydrogen half-way along each
bond. So the structure, which is also that of low-pressure ice,
resembles that of, say, diamond, but with much bigger spaces.

The orderliness, and the average space size, increases as water cools.
That's why ice floats, and water expands as it cools just above its
freezing point.

At pressures of about 1000 atmospheres, the structure breaks down and
the density increases dramatically. There are several (at least six)
different high-pressure forms of ice that are denser than liquid
water. Also, at high pressure, the freezing point rises, well above

Liquids are said to be incompressible, but they aren't. Water is much
more compressible than most liquids.

I used to know a lot about this stuff...

My high-school chemistry teacher had one. Later, when I was a research
student in a lab with easy access to glass-working equipment, solid
CO2, and liquid nitrogen, I made one for myself. Just take a few
inches of glass capillary, as used in thermometers, seal one end, then
cool it in LN and shove some solid CO2 into the other end. It's quite
easy, using a fine wire as a tool. Then, with the CO2 down at the cold
end, immersed in LN, heat and seal the other end of the tube. Let it
warm up, and you'll see the CO2 melt, which of course it won't do at
atmospheric pressure. Warm it to 31C, and you'll see the critical
point effect.

I have no idea what happened to that thing.

It's considerably less than the pressure in a fully-charged scuba

Posted by daestrom on September 20, 2010, 11:20 pm
 On 9/20/2010 11:53 AM, Morris Dovey wrote:

Actually, it has.  One group of folks that really need to 'get a life'
is the International Association on the Properties of Water and Steam...


The math they have is daunting, but it's empirical and derived from
countless laboratory observations.

:-)  It does sound like a neat 'high-school physics' experiment.

If you heat water much beyond the critical point, it starts to act like
an ideal gas.  So you'd have to stay near the top end of saturation
curve, just below the critical point.

But when you cool it and it condenses into liquid, you have an
incompressible liquid, so I'm not sure how that's going to work in your


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