tumadre
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High temperature propane decomposition
I'm considering building a concentrated solar heat engine, and from Google searches over the past 5 years, I've decided that I'm not going to waste my
time with water as a heat fluid.
My natural inclination is to use propane/isobutane and duplicate the turbine from proven geothermal designs, or straight up buy a replacement rotor
and cast & machine the rest of it myself to save cash, however the higher temperatures involved merit turbine redesign. (radial inflow seems to be
standard)
I am well aware that propane will decompose around 900-1100 kelvin, and 700 kelvin seems to be a great temperature to let it run at.. Not to mention
copper is a great catalyst.
My question is: is this an issue at 700-800K?
I have not done a full investigation on the power density of the collector, but it seems to me that pumping propane though standard 6 mm sintered
copper heat pipes is the way to go, at a .5mm wall thickness, 200 psi and 750 Kelvin gives me a 5:1 safety factor as far as hoop stress goes.
However I have no real life data on how pure the propane needs to be to avoid heat pipe deterioration or outright destruction. if sintered heat pipes
offer too much surface area then i can can use grove types, or straight up copper pipe.
I do not want to use smooth wall pipe, because it does not offer enough surface area without resorting to extremely creative collector designs.
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bquirky
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Location: Perth Western Australia
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Hi, I cant offer any intelegent suggestions. but it sounds like a very interesting project.
Im curious
if your concentrator is able to get up to 700K what benefit will propane give you over water ? lower viscosity ?
And how would the copper pipes degrade ?
can you describe your project alittle further ?
regards.
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tumadre
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in a nut shell, water is a terrible heat fluid for Rankine cycle heat engines.
Propane has three times the molecular mass, so radial inflow turbines need only spin 1/3rd as fast.
Mercury is another alternative heat fluid. additionally, it is simple to build a radial inflow turbine for a propane system, high side runs at
700-800K and 2-3 MPa, while the gas exits the turbine at half the temperature and 1/3rd the pressure, requiring only a single rotor for good
efficiency, if you were to use a radial inflow turbine with water you would need to cascade 3 of them.
Axial turbines and water require playing games with the steam so part of the steam condenses in places where it won't cut holes in the blades, thus
releasing heat into the rest of the steam to extract more energy out of the system, and the RPM is 50K for a small system, whereas you could use the
aluminum blade out of a vacuum cleaner for a low temperature propane system.
Concentrated solar systems have always neglected to fully account for the thermal resistance from the collector to the working fluid, thus I am
interested in directly exposing heat pipes to said sunlight.
Propane will decompose to ethylene, and oxygen present will generate carbon monoxide at those temps.
I am concerned that the 800K is not hot enough to allow propane/carbon monoxide to reduce any copper oxide that will clog up the heat pipe sintered
copper wick.
I'm not sure how to interpret the coefficients in the attachment, and their data only goes to 4 atmospheres, so i have to extrapolate to 20
atmospheres.
[Edited on 19-1-2010 by tumadre]
Attachment: CombInst2005a.pdf (429kB)
This file has been downloaded 1022 times
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