steam alternative

[mekennedy1313]

Are we locked into steam? Would an ORC cycle using a compliant scroll
compressor modified as an expander be of interest? The technology is
readily available, inexpensive and suitable for experimentation.

[Ken Helmick]

All come down to the working fluid.  Bill Lear spent a fortune on "Learium", it never panned out.  To date water is still the best working fluid anyone has found for a Rankine cycle.

[jamie clarke]

The scroll compressor would still work in a steam cycle.

I am for the scroll compressor in general.

[Ken Helmick]

Maybe a scroll compressor would work.  By nature they have pretty tight tolerances.  Depending on superheat and thermal expansion it might bind up...this is why pistons are undersized and fitted with expanding rings.

[Jamie Clarke]

Can good material matching stop it seizing from temp difference?

Sent from my iPhone

[Mike Stone]

> Can good material matching stop it seizing from temp difference?

Probably not.

The coefficients of expansion you see in manuals are averages. Any given piece of steel will probably perform within about 10% of the book value for that composition.

Low cost steels aren't all that precise in terms of composition either. They're basically the sausage of the metal world.. mixed together from tag ends of other stuff and then adjusted to be within reasonable distance of the necessary specs. The variations don't matter for the ways the metal is generally used, and if you want better performance you need to buy a better material.

Metals that match the way you're thinking are expensive, and even then, just choosing a good material doesn't solve all the problems.

The idea that objects expand and contract uniformly as they heat and cool only applies for very slow changes in temperature. If you get a thermal gradient in an object, all bets are off. Then you have stresses from different amounts of expansion in different parts of the object, and the thing will flex.

[Russell Philips]

There should be an Open Source ORC group.

along with an Open Source Stirling group.

[Mark Norton]

You are welcome to start up such a group, though I personally think that a Sterling engine is much harder to build and have it do useful work.  There are people who disagree with that, however.

- Mark

[Ken Helmick]

The usual reason for organic working fluids is the low heat of vaporization, folks figuring that if it is easier to boil, it makes for a better engine.

 

Of course, Sadi Carnot said that theoretical maximum efficiency is equal to 1 - (cold reservoir temperature / hot reservoir temperature), which obviously means that potential efficiency rises as you increase the temperature of the working fluid...so a fluid that vaporizes more readily isn't naturally your best bet.  More to the point, organic fluids tend to break down well below the temperatures at which many steam systems idle along, so efficiency is not a strong suit.

 

Another problem is that these low boiling point fluids usually ALSO have low specific heat content, maybe from 1/4 to 1/2 half that of steam, which means you need to pump a lot more fluid to produce the same work...leading to much higher feed pump losses than a steam plant sees.

 

Another issue is that the Mollier tables for such fluids shows that quite often the fluid goes into superheat when the exhaust releases, which amounts to a lot of wasted energy unless a very exhaust heat exchanger is fitted.

 

Then there is the cost, water is kinda cheap, even if you splurge on demineralized, deionized and buffered feed. If you spring a leak and lose your water, it is readily replaced.  Organic fluids, much less so.  And the safety factor, unless the steam is so hot that it will scald your lung tissues, a little steam vented into the working area just tends to soothe the sinuses; a lot of these organic chemicals qualify as a Hazmat condition. 

 

Admittedly there is a place for organic working fluids, this being when you have a LOT of lower level heat that you want to exploit, usually said heat being a by-product of some industrial process although there has been a lot of ongoing work in OTEC applications.  These systems will not be very efficient, so they start to pay as size goes up and scale factors become favorable.  The technical challenges are much higher to prevent working fluid losses and decomposition.  I don't really see this being suitable for the average backyard workshop unless maybe you have an old Ofeldt Naptha launch sitting around and are just dying to run it...and I admit the idea of boiling gasoline in a tank and piping it to an engine makes me a tad nervous...

 

Ken

-----Original Message-----
From: Mark Norton <markj...@gmail.com>
To: open-source-steam <open-sou...@googlegroups.com>
Sent: Wed, Dec 14, 2011 4:02 pm
Subject: Re: steam alternative

[nick raaum]

Sorry to jump in on a whim on this, but I wanted to mention a possible alternative approach to having a low boiling temperature working fluid, but while still using water .  I attached a quick schematic. Its an idea I had a while ago and am not certain of its validity.  In a nutshell you boil the water after the condenser at sub atmospheric pressures and create a very low pressure steam cycle. This process could  be done with low cost, non concentrated solar heat.  To obtain respectable efficiencies the saturated steam is then superheated with the input of another fuel. 

The advantage of this cycle would be that you could derive the heat of vaporization (an hence the majority of the energy input) with low quality heat from the sun. The system would also never really operate above atmospheric pressure and would probably be pretty safe.   The glaring disadvantage though of course would be the low energy densities (pressure differentials less ~10psi).   Assuming the cycle is even possible, a very different type of steam engine would have to be developed to run cost effectively on those very low differential pressures.  Anyways just a wild idea I had kicking around, that seemed somewhat relevant to this discussion.


Nick

---

To: open-sou...@googlegroups.com
Subject: Re: steam alternative
From: ken...@aol.com
Date: Wed, 14 Dec 2011 21:49:40 -0500