Stirling engines need large temperature differentials, but...

[udrcn]

I have read that Stirling engines need large temperature
differentials so that sufficient expansion and contraction of gas can
take place.

But what about using something other than air in the chamber? if
water was used, for example, wouldnt you just need to heat it up to
101C, at which point it would start to expand? Of course, that's a
simplification since the additional pressure would result in an
increase in the boiling point.

Can't the repulsive force of non oxygen compounds be used to reduce
the need for a large temperature differential?

OK, so this probably isn't possible, but anyone know why?

Thanks,
Amit

[dholmes7812]

The problem with useing water (Or steam as the case may be), is that to turn water from ambient (room) temp to 100deg c (boiling) takes a lot of energey. This is called SPECIFIC ENTROPY. By useing a gas as the working fluid (air, co2, nitogen, or whatever), you avoid this problem. That is the primary advantage of Stirling engines over regular steam engines.

[udrcn]

Daniel,

Thank you for thinking about this.

I agree with you that it would take substantial energy to take water from room temperature to >100C.

My thought went further though:
1) heat water to 100C
2) heat water further to 101C (water is now a gas; use this as the expansion cycle)
3) cool vapor to 99C (water is now a liquid; use this as the contraction cycle)

The difference between this and a steam engine would be that here the water would go from 99C to >100C every cycle, rather than room temperature to >100C.

(obviously the boiling point would change depending on the pressure but this is a simple illustration)

My impression of steam engines (memory of old movies + very limited knowledge + quick read at howstuffworks.com) makes me think that they release hot steam as exhaust rather than reclaiming it.

-Amit

[dholmes7812]

I'm in the same boat as you. I am simply an enthusist and hardly an expert on anything. You are correct in that most of the old steam engines worked on an open cycle, as opposed to a closed cycle like in the Stirling. A Stirling using steam as the working fluid would work, but keeping the tempeture very close to boiling would mean very low power(hense the title of this forum). Another problem is as the internal pressue rises, the vapor pressure of the water rises, so it takes even more heat to boil the water. Thirdly, at high rpm's, the travel of the piston could create shock waves in the steam, lowering the vapor point in some areas, and raise it in others. This would make for very uneven power transfer to the piston (imagine pockets of high and low pressure inside the same cylinder). Lastly, most of the high power Stirling engines use hydrogen of hielium as the working fluid. This is because the more mass the gas has, the more heat it will soak up and it will tend to hold that heat longer. At 3000 rpm, the piston and displacer are moving up and down the cylinder 50 times a second, so the gas has to heat and cool very very quickly.

[startruck1242]

Mr. Holmes you noted that high power Stirling engines use hydrogen of helium. what is this also i read that hydrogen can penitrate soild steel walls so they must use regenerators to keep consistent pressure. also have you heard of the use of Deuterium an isotope of heluim
I only addresed this to mr Holmes because of a post if any one else would like to comment as well it would be appreciated greatly

Thank you all for any help for a dumb College kid.

Zach

[me___145]

As far as using steam or water and steam as the working fluid;

Imagine a graph with energy input on one axis and expansion on the other. With a gas the temperature/pressure relationship is pretty linear, a straight smooth slope on the graph, which is good for making efficient Stirling engine. But with water, there will come a point on the graph where the water begins to boil and the line jumps. (or dip depending on which way round you're imagining the graph ) Before the jump, virtually no expansion is occuring, and after the jump virtually no change in temperature is occuring.

To avoid the non-productive parts of the cycle you'd have to have the "cold" side at just below boiling and the "hot" side just above boiling, and arrange the engine so that condensation would occur and drip back into the cold side.

Doing it this way would give you many of the drawbacks of a steam engine but with greater complexity and probably less power.

Engines which take advantage of phase change in a closed system such as the Newcomen steam engine were used in the 18th century, but were very inefficient.

(An asside; the regenerator is a heatsink with high surface area that sits between hot and cold sides to speed up the temperature change as the gas flows from one side to the other)

Using only steam (no liquid) at relatively high temperature would work, but nowhere near as well as helium. (Theoretically hydrogen is the best, but as another user said, containment issues make this problematic.

[kassouras]

I have tested recently, and operates well as an idea.
Have a look at YouTube, under the name BillExperim,
titled 'My first steam Stirling engine'.
It actually has about three times the torque of the same engine operating with hot air.

[vimnjicki]

I am planning a small (14T displacement) personal submarine and want/need an air independent power source to maintain battery charge. A Stirling engine is the most feasible, in theory, but I do have a question that relates to this thread. How big a temperature difference is necessary to generate substantial power? By substantial I mean kilowatts or even megawatts. Am I being ridiculous? An I asking too much? I hope not. Thanks for any responses...

[bptdude___2569]

The reason certain nations have done a very clever thing, did it with a Stirling design, was that they had to go beyond diesel/electric and skip the nuclear reactor level of operation.

The usual fuel seems to be clorine and sodium, two difficul to handle dangerous materials, combined in an intense hot burn, to produce a harmless salt that can be ejected into the ocean.

Personal sub? NO WAY !
You will have to think of something else for propulsion.

You have enough money to build a personal sub?
Hire me to design it!


The sub will be a stainless steel hull sailboat fully sealed in extra thick layer of fiberglass. The mail sail is really a flexable solar cell array, use the other sails as sails. The boat could also tow a large solar cell array on floating pontoons, which could also be used when the boat is submerged.

The solar cells make hydrogen, which powers the fuel cell, which powers the electric engine. There is also a towed array to extract flowing water into turbines to make electricity.

You can stay submerged for as long as compressed, superchilled hydrogen will last. Hydrogen storage will be a major component of the sub. The Stirling engine you would use is really the cryocooler to chill the hydrogen.

Since this has nothing more to do with a Stirling engine, I will stop.

[vimnjicki]

I appreciate the response to my, um, response, but my question still remains. How great a temperature difference is required to produce large amounts of energy? 500 degrees? 1000 degrees? Thanks again.