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Almost-Carnot Heat Engine

Rotary Screw Compressors & Expanders
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This is an idea for a closed cycle heat engine, where heat is removed while compressing, added while expanding.

Sart with the working fluid (a gas), at low pressure, and low temperature.

1) Cooled oil is sprayed in. This oil is both a hydronic fluid (for cooling), and a lubricant. It's temperature (at this point) is just slightly below the gas temperature.

2) The gas/oil mix goes through the first stage of compression, which is a flooded rotary screw compressor. Because of the presence of the oil (to absorb heat), the temperature rises much less than if the gas were compressed dry. This approximates the isothermal compression of the Carnot cycle.

3) The gas/oil mix goes through a gas/liquid seperator.

4a) The high pressure oil goes through a hydraulic motor, then a radiator, where it cools, and then back to step 2.

4b) The gas undergoes a second stage of compression, this time in a dry rotary screw compressor (or other, non-intercooled compressor). This approximates the isentropic compression stage of the Carnot cycle.

6) Hot oil is sprayed into the gas. The temperature of this oil is just slightly above the gas's temperature.

7) The gas/oil mix goes through a flooded rotary screw expander. The oil acts both to lubricate the screws, and to keep the gas from cooling as much as it otherwise would, if expanded dry. This approximates the Carnot cycle's isothermal expansion step.

8) The mix goes through a gas/liquid seperator.

9a) The low pressure oil goes to a heater, then a pump, then back to step 6.

9b) The gas passes through a dry rotary screw expander, or other non-interheated expander. This approximates the Carnot cycle's isentropic compression step.

goldbb, Jun 04 2009

Carnot Engine http://hyperphysics.../thermo/carnot.html
" As Schroeder puts it "So don't bother installing a Carnot engine in your car; while it would increase your gas mileage, you would be passed on the highway by pedestrians." [normzone, Jun 08 2009]

Real world Stirling Cycle engine description http://en.wikipedia..._versus_crank-angle
Just below the "Temp vs Crank-angle" header, it mentions "hysteresis loss" [goldbb, Jun 09 2009]

[link]






       Yes, and it does your homework for you, and will walk your dog, and solves global warming.   

       Seriously, though...   

       Although the first stage of gas compression is done with intercooling, it will inevitably be hotter than before it was compressed; it's not really isothermal.   

       Similarly, the second stage isn't truly isentropic, since heat will inevitably move from the gas to the compressor walls during the end of the compression stage (an undesirable a loss of heat at high pressure), and some of that heat will move back into the gas during the beginning of the compression stage (an undesirable gain of heat at low pressure).   

       The same two problems occur during the two expansion stages.   

       However, since it's so much more like a Carnot engine than other engines, its efficiency should be close to the theoretical limit.
goldbb, Jun 08 2009
  

       I keep reading this as Carrot.... I'm a little disappointed.
xenzag, Jun 08 2009
  

       Sorry zenzag, I think of heat engines when the weather's hot, and food engines when I'm hungry.   

       normzone, the important part to quote would be: "It is not a practical engine cycle because the heat transfer into the engine in the isothermal process is too slow to be of practical value."   

       That's due to the limited surface area between the working fluid and the heat exchanger, and the requirement that the heat addition (and heat removal) really and truly be isothermal.   

       In this design, the heat exchange is between the oil and the gas. If they can be sufficiently well mixed, there will be a high surface area, so lots of heat conduction.   

       Furthermore, I'm not trying to produce "true" isothermal heat transfer... I'm perfectly content with a limited amount of temperature rise during the intercooled compression, and a limited amount of temperature drop during the interheated expansion.   

       The latter means less efficiency (compared to a "true" Carnot engine), but greater overall speed and power.
goldbb, Jun 08 2009
  

       Would this be a "Car-not" engine then?
Gamma48, Jun 09 2009
  

       [goldbb] - but that part wouldn't make people smile.
normzone, Jun 09 2009
  

       I am puzzled why the hot and cold oil (reservoirs) are only "slightly" hotter and colder than the working fluid. The hotter the hot and (more importantly) the colder the cold the better...
madness, Jun 09 2009
  

       bigsleep, do rotary screw compressors and expanders have cams?   

       Gamma48, Hopefully, it would be a car-yes engine :)   

       madness, it's not about the oil temp, it's about the gas temp.   

       Suppose that the maximum temperature to which we can practically heat our heating oil is temperature X, and the lowest temperature to which we can practically cool our cooling oil is temperature Y.   

       The higher the peak temperature of our engine is, and the lower the bottom temperature of our engine is, the more efficient the engine will be.   

       Therefor, for maximum efficiency, it's better to get the gas temp as close to X as possible (using non-intercooled compression) before injecting the heating oil.   

       Similarly, it's most efficient to cool the gas (by non-interheated expansion) as much as possible, prior to injecting the cooling oil.   

       Actually... getting the gas temperature exactly as hot as the hot oil, and exactly as cold as the cold oil, will produce the greatest efficiency, but with a tradeoff of lower specific power.   

       Perhaps more importantly, we don't want to accidentally heat our gas (by compression) higher than X, or else we *lose* energy through what are called "heat transfer losses" or "hysteresis losses". For the same reason, we don't want to cool the gas (by expansion) below temperature Y.   

       By playing it safe, and compressing the gas until it's temperature is just below X, and expanding the gas until it's temperature is just above Y, we guarantee that there will be no hysteresis losses.
goldbb, Jun 09 2009
  

       Is nice your idea. Please contact me at dinamic@email.ro. Thank you
danm, Mar 21 2010
  

       don't forget us when you're rich and famous goldbb.
FlyingToaster, Mar 21 2010
  

       Sorry, I wrote wrong address, please contact at dinamica@emailro. Thank
danm, Mar 22 2010
  

       How would the oil become slightly hotter than the gas in step 6?
marklar, Mar 25 2010
  

       Your question is a good one.   

       Please again the author of proposal to contact me at dinamica@email.ro, or at frigomatic @email.ro.   

       Thank you Dan MIHALCEA.
danm, Mar 27 2010
  

       Your second email address worries me. I'm not sure this would be the best source of power for your invention.
marklar, Mar 29 2010
  

       The oil would get hot by being heated. The source of heat might be combustion, geothermal, solar, etc..
goldbb, Mar 29 2010
  

       I have never heard of a screw expander. I might also point out that a screw compressor (I'm assuming direct displacement compression here) would necessarily have frictional losses on the seals.   

       What advantage does this have over piston expansion/compression with massive (oil?) heatsinks which is the way pseudo-isothermal processes are usually approached.
sneakythumbs, Mar 30 2010
  

       The advantage is direct contact between the oil and the working fluid, which should allow a higher rate of heat exchange.
goldbb, Mar 30 2010
  

       Dear goldbb,   

       Please give me a example of engine which can to work after your ideia. If you have same specification, brochure, drawing, phots of such one please send at dinamica@email.ro.   

       Thenk you Dan MIHALCEA
danm, Mar 31 2010
  

       If you heat/cool the compressor/expander you will be able to keep the gas as a constant temperature.
marklar, Jun 19 2010
  

       Is a carnot a "care not" without the 'e'? Or is it a failed carrot?
ie - "I could'a been a carrot, but look at me now, I'm not even a carnot." (apologies to Marlon Brando)
xenzag, Jun 19 2010
  

       Dear goldbb,   

       Please again, give me a example of engine which can to work after your ideia. If you have same specification, brochure, drawing, phots of such one please send at dmihalcea2003@yahoo.com.   

       Thank you Dan MIHALCEA — danm, Mar 10, 2011   

       [edit, delete]
danm, Mar 10 2011
  
      
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