Why do steam turbines use water?

I've been reading a lot about turbine engines and it occured to me that water isn't really the best substance to use. For example, ethanol (alcohol) boils at a temperature of ~20C less than water. Wouldn't it take less energy to boil the ethanol and get nearly the same steam pressure? Obvious downsides would be the safety aspects, but overall it seems like it could be overcome for such a nice energy savings.

I'd imagine it'd be a hell of a lot more expensive to fill them with alcohol than water. Don't know about the chemical ramifications ie how much energy would be released etc, but think the cost would be a major factor

regards
flc

Originally posted by funlovincriminal
I'd imagine it'd be a hell of a lot more expensive to fill them with alcohol than water.

That's assuming you don't condense and recirculate it though.

EDIT:
Because the energy savings would make up for the initial cost over time, if you recirculated.

[edit on 8/22/2004 by shbaz]

I would imagine that the high volatility of ethanol would make it a poor candidate for high-pressure, super-heated steam applications. Super-heated steam from water is dangerous enough without having to worry about a flamming explosion. Also, ethanol evaporates at lower temperatures than water, so condensing and reclaiming it may be quite difficult.

Perhaps the difference in specific gravity between ethanol and water makes ethanol less efficient in a steam phase as well?

During my apprenticeship (Im a union pipefitter) we learned that one of the major benefits of steam power is that the yield is limitless. What I mean is that the pressure will not stop building. The power output capabilities we learned about were nothing shy of incredible.

Originally posted by who
During my apprenticeship (Im a union pipefitter) we learned that one of the major benefits of steam power is that the yield is limitless. What I mean is that the pressure will not stop building. The power output capabilities we learned about were nothing shy of incredible.

Yeah, the invention of the safety pressure relief valve saved a lot of lives because of that fact.

Also, although ethanol is a dangerous chemical, surely there are others with lower boiling points than water that would make good candidates. The fact that they have lower boiling points would bring a lot of benefits. They would condense easier and pressurize faster with a lower amount of heat, making a closed circulation system easy.

[edit on 8/22/2004 by shbaz]

With a closed cycle power plant where you both boil and condense a working fluid, a lower boiling point is actually a disadvantage. It also means a lower condensing point at a realistic pressure. It is much easier to cool something to 100C than to 20C to condense it.

Heating is no problem, it is the condensing that is the problem.

Also water is pretty non toxic, and the cheapest working fluid imaginable.

This is not to mention that the machinery needed to capture and re-condense the ethanol, or steam for that matter, without using too much of the energy derived from the steam, would probably be prohibitively large.

Originally posted by Warpspeed
With a closed cycle power plant where you both boil and condense a working fluid, a lower boiling point is actually a disadvantage. It also means a lower condensing point at a realistic pressure. It is much easier to cool something to 100C than to 20C to condense it.

Heating is no problem, it is the condensing that is the problem.

Also water is pretty non toxic, and the cheapest working fluid imaginable.

Thanks, that pretty much explains it. I don't know what I was thinking saying a substance with a lower boiling point would be easier to condense. In low temperature environments it might still help save energy, but otherwise you're right.

Originally posted by taibunsuu
This is not to mention that the machinery needed to capture and re-condense the ethanol, or steam for that matter, without using too much of the energy derived from the steam, would probably be prohibitively large.

Depends on the application. Lots of steam engines require condensers - most notably mobile ones.

Originally posted by shbaz

I don't know what I was thinking saying a substance with a lower boiling point would be easier to condense.

No doubt you were thinking to try and improve something, which is the kind of thinking engineering always benefits from. )

Originally posted by taibunsuu
This is not to mention that the machinery needed to capture and re-condense the ethanol, or steam for that matter, without using too much of the energy derived from the steam, would probably be prohibitively large.

Depends on the application. Lots of steam engines require condensers - most notably mobile ones.

What I mean is a device to capture all the outgoind steam and recondense it requires a lot of the energy captured by the expanding steam.