Steam engines -- A new idea, maybe....

[Dream Relics]

Why do we use water for steam engines? Why not something else?

So, I have a question, or maybe it is an idea that i would like to share. So, sure, there are many finer points to the engineering of a steam turbine to make them work efficiently, and no doubt the designs have improved much since the invention of the steam engine. Also, as we all know, Nuclear reactors are just steam engines with a far more complicated and dangerous form of fire in them. Which once I realized that, I have to say, I was far less impressed with nuclear energy as a thing. But I digress. Here is my idea/question. Why use water for steam engines. The boiling point of water is 212 degrees and that is pretty hot. Hot enough that it requires burning fuel to achieve that temp. But there are lots of things with lower boiling points. Like Acetaldehyde for instance. It boils at 69 degrees. Or Methyl Bromide. It boils at 38 degrees. Seems to me one could set up a system that temp shields a condensation tank to cycle the substance back through in a loop and generate electricity with no fuel just using the ambient temperature of any given day if you use the right substance. For instance, with the Acetaldehyde, which is abundantly found in nature, one might be able to make a unit attached to a home, say for the AC, and when the temp rises above 69 the unit would kick on, power the AC and cool the house. Maybe part of it would need to run into underground pipes or something so the substance can cool and cycle back through again. I don't know know how it would work exactly, I am not an engineer, I am just a creative thinker. But this seems like a good idea. There seem to be some chemicals which boil at below freezing temps. Why can't we not build a turbine to run on those that would run in the winter? perhaps a large power plant that employs a variety of these different boiling point elements to just run on the ambient temperature even as the weather and temperature change. Someone tell me why this wouldn't work.

Bear in mind, I am not an engineer. So I am not asking for advise so that I can build it. That is never going to happen. I am more interested in just hearing whether or not this would work and getting the idea out there so that some of you smart people might build it if it is in fact a workable idea. I am just an idea person.

 

[russ_watters]

Why do we use water for steam engines? Why not something else?

Water makes a good working fluid because it holds a lot of heat when it is converted to steam and boils at a relatively low temperature.

But there are lots of things with lower boiling points. Like Acetaldehyde for instance. It boils at 69 degrees. Or Methyl Bromide. It boils at 38 degrees.

Lower boiling points aren't necessarily helpful. For any thermodynamic engine, larger temperature differences make for more efficient engines. Think of the temperature difference like the height of a dam: the higher the dam (the wider the temperature difference), the more energy available to harness and the more efficient the harnessing of it.

Seems to me one could set up a system that temp shields a condensation tank to cycle the substance back through in a loop and generate electricity with no fuel just using the ambient temperature of any given day if you use the right substance. For instance, with the Acetaldehyde, which is abundantly found in nature, one might be able to make a unit attached to a home, say for the AC, and when the temp rises above 69 the unit would kick on, power the AC and cool the house. Maybe part of it would need to run into underground pipes or something so the substance can cool and cycle back through again.

You could run a sort of inverted hydrothermal plant that way, yes. The energy would be free, but it would need to be an extremely large device to generate a useful amount of power. It would be very expensive (as regular geothermal power is).

 

[Dream Relics]

Okay. That makes sense. Thanks.

 

[billy_joule]

Google "organic Rankine cycle" for alternative working fluids used at lower temperatures.
Many common refrigerants are used in ORC systems.
https://en.m.wikipedia.org/wiki/Organic_Rankine_cycle
https://en.m.wikipedia.org/wiki/Ocean_thermal_energy_conversion

There is a phenomenal quantity of waste heat available from many industries but it's not economically viable to capture..rising energy prices and increasing effort to reduce emissions is slowly changing that.

 

[NTW]

It's an old idea, and one that didn't work, for the reasons stated above by russ_waters...

http://douglas-self.com/MUSEUM/POWER/ether/ether.htm

 

[Baluncore]

Or Methyl Bromide. It boils at 38 degrees.

Methyl Bromide was used as a fire extinguishing gas in WW2. It also destroys genetic material, so is now a restricted material. It is still used as a fumigant to sterilise soil, but only because it is the least dangerous agent available.

The major problem with water is corrosion, but additives solve that problem in closed systems. Water is the safest liquid available for use in external combustion engines. Everything else is more expensive, toxic or corrosive.