Half a croissant, on a plate, with a sign in front of it saying '50c'
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alcohol/vapour piston engine

A mechanical engine driven by a fluid with a low boiling point
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Water boils at 100deg.C. Ethanol boils at <80deg.C. Why can't we construct a low pressure piston engine powered by expanding ethanol vapour, the exhaust vapour to be collected, condensed and fed back into the ethanol reservoir?

The heat needed to raise the temperature to >80deg. C. can be commonly found (geothermal, solar, industrial exhaust gases) or generated cheaply, and the engine (constructed of modern materials) put to useful work.

If the unit is designed well, sealed, and the temperature is kept low, the chances of fire should be minimal.

Such engines existed around the turn of the 20th century using naphtha. Could someone please - intellectually - revisit this technology with 21st century experience?

I also understand that there are fluids which boil as low as 65deg.C. (ethanol/benzene or cyclohexane), but I don't know the downsides of using this fluid as a working vapour.

As for Carnot, what does it matter if the piston is as big as a dinner plate and the stroke is small, that's what cogs are for. The one question to keep in mind is - can this engine do more work for a lower cost than competing systems?

andresier, Feb 13 2008

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       As you pointed out yourself, it is going to be very low pressure. Also, the higher the pressure gets, the higher the boiling point of your fluid becomes.
DanDaMan, Feb 13 2008
  

       This is esentailly a Sterling (closed-cycle) engine, and as such is Baked.
8th of 7, Feb 13 2008
  

       Why alcohol? There are non-inflammable liquids that boil at any temperature you like from about -273°C upwards.
MaxwellBuchanan, Feb 13 2008
  

       Umm, yes, I thought about alcohol before I filed for the patent for the water version of this (it runs at below atmospheric pressure, so lower boiling point). I think it's a good idea, please build one and help save the planet, and I being serious.   

       Your idea is better, because if it doesn't work, you can drink the alcohol, I just get to drink water.   

       "very low pressure", sure it's low pressure, but it's energy doing nothing, examples are the heat from the radiator in your car, the heat from CPU on the computer you are reading this on...   

       "Why alcohol? There are non-inflammable liquids.." I could be wrong, but when I looked at those, they all seem to be expensive, poisonous, corrosive or flammable except in lucky cases, where they are all four.   

       Of the projects I have heard of that seem to be going somewhere, most seem to have opted for paraffin.   

       // expensive, poisonous, corrosive or flammable//
Alcohol - three out of four ain't bad.
AbsintheWithoutLeave, Nov 21 2009
  

       ok, I should have added "not fun to drink" although after hearing the old story of the soviet army shoe polish/bread trick some people would be inclined to drink paraffin..   

       In a closed system, your pressure is going to be defined be your temperature, and vice versa, working fluid doesn't enter into it (significantly). If you start with alcohol, a certain amount will enter the atmosphere even below 80 degC, it will then increase pressure and with it boiling point on a more or less linear scale until you stop adding heat. Exactly the same thing happens with water, even below 100C. Whatever energy you can take out will depend on how far you can cool the fluid (thus lowering the pressure) on the far side of your piston/turbine/etc. This can only be decreased to ambient temperature (slightly above, anyway). It is thus the difference between working and ambient temperatures that defines your energy out. The greater this difference, the more energy you can pull out.   

       The advantage of an open loop system is you can dump your working fluid to ambient temperature and pressure much more simply, but it is still the difference between working and ambient temperature that defines your energy out.   

       I'm not saying you can't use these very low heat sources for power, but the energy out is not going to be worth the cost of the system to harvest it.
MechE, Nov 21 2009
  

       ... but the energy out is not going to be worth the cost...
Wait: If I have low cost ice storage from night non-peak electricity then I get another 30 degrees diff, and a lot of energy stored in the zero degrees area (317 kbtu in 1 meter cube to be exact). So why not?
pashute, May 04 2010
  
      
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