Steam engine at room temperature

[guss]

Let's say we have a room in which it is around 20° C and at normal pressure.

There is a substance that is a liquid, but when the temperature increases to 25° C or so, it rapidly expands, whether that is by boiling (conversion to gas), or just by expanding as a liquid.

First question: does a substance like this exist? As in, where by increasing the temperature only a few degrees, it undergoes a rapid expansion (similar to water boiling)? I know that this can be achieved through depressurization, but I want to have it expand at normal pressure to keep it simple.

If this is possible, would it also be possible, then, to make a "steam" engine that runs off of this fluid instead of water? (the engine would presumably recycle its fluid over and over again, instead of releasing it into the room/atmosphere)

Thanks!

 

[Mech_Engineer]

A heat engine's efficiency is directly related to the hot and cold-side temperatures of the system. So taking for example a system that operates between 20 degrees C (293 K) and 25 degrees C (298 K), the theoretical maximum efficiency (Carnot efficiency) that system can attain is 1.68% (and in practice it will be much less than that). This is why increasing you delta T is generally a good thing.

More reading:
http://en.wikipedia.org/wiki/Carnot_cycle
http://en.wikipedia.org/wiki/Rankine_cycle

 

[guss]

A heat engine's efficiency is directly related to the hot and cold-side temperatures of the system. So taking for example a system that operates between 20 degrees C (293 K) and 25 degrees C (298 K), the theoretical maximum efficiency (Carnot efficiency) that system can attain is 1.68% (and in practice it will be much less than that). This is why increasing you delta T is generally a good thing.

More reading:
http://en.wikipedia.org/wiki/Carnot_cycle
http://en.wikipedia.org/wiki/Rankine_cycle

I still don't understand, though. So, why can't I simply have a substance, that when heated a few degrees, greatly expands and does something like drive a waterwheel type thing? Is it because even though it can expand, it doesn't give a great enough force to drive the wheel?

I realize this probably violates conservation of energy, but I'm trying to understand why.

 

[Studiot]

So, why can't I simply have a substance, that when heated a few degrees, greatly expands and does something like drive a waterwheel type thing? Is it because even though it can expand, it doesn't give a great enough force to drive the wheel?

What makes you think it can expand?

If it is totally free to expand as much as it wants it can do no work.
If it expands against some constraint it does work against that resistance, but its expansion is more limited - the greater the resistance the less the expansion.

You can't have it both ways.

 

[guss]

If it expands against some constraint it does work against that resistance, but its expansion is more limited - the greater the resistance the less the expansion.

If this substance is heated 5 degrees, and is in an enclosed container, it is going to build pressure, right? Let's say one wall of the container can move. It is going to continue to apply force to this wall until it reaches equilibrium.

In a more extreme example, let's say we heat a substance 1 degree and it's pressure increases from 1 to 50 atmospheres (possibility of the substance being realistic aside). Wouldn't the force on this wall of the container be massive?

In the above scenario, would the substance take more energy to heat up, or what?

 

[Studiot]

or what?

Exactly.

I am trying to shine the light of reality on fantasy or wishful thinking.

Let's say one wall of the container can move

What do you mean?

This is an engineering forum remember so be precise and state some engineering.

 

[guss]

Exactly.

I am trying to shine the light of reality on fantasy wishful thinking.

I know what the reality is already, I'm just trying to understand why. I've got it now.

What do you mean?

This is an engineering forum remember so be precise and state some engineering.

Let's say our container is a pipe with one end sealed. On the other end, there is a cap that forms a complete seal, yet can move up and down inside the tube (depending on the pressure).

Just an example I was using, sorry for not using correct terminology.

 

[Studiot]

correct terminology

I'm not talking about terminology I'm talking about engineering/physics.

Yes, you have described a piston in a bore of a cylinder.

But what resistance to movement does it offer?

That question is the whole crux of the issue.

If it is totally free to expand as much as it wants it can do no work.
If it expands against some constraint it does work against that resistance, but its expansion is more limited - the greater the resistance the less the expansion.

 

[Studiot]

I'm off for my beauty sleep now so I will leave you with this thought.
I suggest it is a good idea to draw this out or yourself.

Imagine two identical cylinders with the open end upwards.
They both contain identical amounts of fluid, capped by a weightless, frictionless piston.
One piston carries a 1kg weight, the other carries no weight.

Both are started in equilibrium and heated equally.

The fluid in the cylinder where the piston carries no weight will expand and push the piston up until it is again in equilibrium with the air pressure.

How much work will have been done in this expansion?

The fluid in the cylinder where the piston carries the 1kg weight will also expand, pushing up its piston.

How much work is done by this expansion?
Will the expansion be the same as in the other cylinder?

 

[Mech_Engineer]

Regardless of the fluid being used, your system's efficiency would be limited to less than the Carnot efficiency. It might be possible to build a system that ran between those two temps, but it would be pretty inefficient (like less than 1%).

 

[russ_watters]

You could pressurize a refrigerant and use it for this purpose, but as said, the efficiency will be pretty low. Of course, if your input heat is free (for example, solar heat), maybe you don't need to care (though you will need a compressor that doesn't kill all of your efficiency).

Properties of R-22: http://www.engineeringtoolbox.com/r22-properties-d_365.html

I think the root of your confusion may be in the processes that make up a heat engine. I'm guessing you don't realize that the pressure needed to drive the turbine comes from a pump: when you boil your working fluid, you don't get free pressure, you just get a constant pressure phase change. This is a common source of confusion.

 

[Nik_2213]

You may want to google (etc) for schemes to recover power from thermal differences between deep and surface ocean water. There's only a dozen degrees to play with, so efficiencies are inherently low. The trick is to use large heat exchangers and a compatible working fluid. Chlorofluorocarbons were a good fit until the ozone layer problem came along. Now, ammonia, di-ethyl ether(*) or supercritical CO2 might suit...

(*) IIRC, diethyl ether is the working fluid in the 'nodding duck' toy.

The other option, IMHO, is the Stirling cycle. IIRC, miniature engines are available that happily run from the heat of your hand. But, given the low energy density available, getting *power* out of them is very limited. Think wrist-watch clockwork...

Uh, if you care to forego vaporisation of the working fluid, IIRC, you can build a Stirling cycle engine using a resonant liquid piston arrangement. Again, though, efficiencies are very low...