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Which low boiling fluid to use (that won't corrode metals and plastics)? Answered

I'm having a bit of a headache trying to chose and procure a working fluid to use in my solar device. It needs to be low boiling, but as my device so far is involving copper, aluminium and maybe some plastics and rubbers, it would also need to be as non corrosive as poss. Cheap and plentiful are also important, not so much for the prototype, but when these things are ultimately used, they need to be easy to assemble. The four options I'm aware of so far are: Methanol boils at 64.7 °C, I thought it would be easy to get but is turning out quite difficult and maybe expensive, also apparently eats through every material I plan to use. Ethanol Boiling Point: 78.4 °C, easier to get but still corrosive. Hexane Boiling Point: 69 °C, not corrosive (?) but not at all easy to get or cheap. Water Boiling Point: 100 °C, which may be a bit too high. The fluids will be boiling in a non-pressurised but sealed environment, so no atmospheric water will be able to get in or out. Help?


so what did you decide on? Im designing a similar solar generator. Is it a viable effective system? Thanks

Pressurise it slightly, and use butane or propane (I believe the fuel in disposable lighters is a mix of the two?).

I want to avoid internal pressure as there's an axle leaving the container and I don't want leakage. Otherwise, good idea. I've gone with methylated spirits, as the bottle says it's actually 95% ethanol. I don't know what the other 5% is, but it shouldn't alter the boiling point too much. I hope. Now I'm going to paint some copper black, fill it with flammable liquid, and leave it in the sun to see what happens...

Considering that the latent heat of evaporation is quite a bit lower for ethanol (846 kJ/kg) than methanol (1100 kJ/kg), how much more energy would it actually take to boil, given the differences in boiling points (78 C to 64 C) ? Like, how much energy to boil 1 litre of each in 1 minute?

Ok, so, doing the maths I get the following numbers:

To heat 1 litre of methanol (789 grams) from 40 C to 54 C and then boil = 916.59 (kJ?)
given a vaporisation heat of 1100 and a heat capacity of 2.51.

To heat ethanol from 40 C to 68 C and boil = 740.65
with 846 and 2.44 respectively.

So it's quite a bit easier to boil eths than meths, even given the higher boiling point.
Well that's good news, since ethanol is a let easier to get, cheaper, and less corrosive.

it works, beyaches.

I would have thought ethanol OK, it's hardly "corrosive" but might have a solvent effect on some plastics. You've got a bit of a problem here with the low-boiling, because until you've built it you don't really know how low-boiling you need. There's also heat capacity, vapourisation and such to take into account. Non- pressurised while sealed is going to be a headache too. You could just use a separate tracker - run off solar-electric? L

Ethanol is starting to look more attractive, yes. I should have that bit of the device finished in the next few days, I got the last two bits today from the hardware store. Man new copper pipe is expensive... The 'non-pressurised while sealed' shouldn't be a problem, I'm just going to attach a sort of bladder to the top where the vapour can recondense. It'll inflate with the gas, but isn't elastic so there's no restorative force. Hence no significant pressure increase. The tracker is the whole point, I'm trying to develop something which can be lashed together from mostly scrap so people with no money can still have solar energy...

To track properly like an astronomical telescope, you need constant-speed motors, digital control or both. To test this you'd do as well to manually track it yourself for an hour, leaving the mechanics for when the main device is perfected. And easy way to do this is simply to mount a tube on the side of the thing - pointed directly at the sun you get a circular shadow, not directly and the shadow isn't. L

The tracking system basically works by boiling a fluid. The buoyant force of the gas bubbles provides the mechanical force to reorient the collector. So I need something which won't boil at room temperature, but will as soon as I put some sunlight on it. The fluid will be in a black copper pipe within a parabolic trough, but I'm not sure how much heat I'll be able to get into the fluid and how quickly, I think 100 C isn't going to be easily achievable.


9 years ago

If it is going to be non-pressurised, what will the phase change be doing? You can do sensible work with pressurised low-temperature-boiling liquids, for instance in my second-favourite heat engine the Minto wheel, but I'm not sure what you will achieve without pressurisation.

Could you post a brief explanation of what mechanism the tracker is going to use?

Acetone is low boiling point, but eats through stuff... Cooling systems used to use ammonia, in fact one man intended to build a much more effective steam engine before realizing that it'd be deadly... What's the project out of question?

. I'd go with water. Cheap, safe, and you shouldn't have any problems getting it to boil.

Water is definitely the most practical, but the lower the boiling point the more responsive and efficient the device is going to be. (I'm using the boiling of a fluid as the basis of a tracking system for the collector).

> but the lower the boiling point the ... . harder it is to condense. It's a double-edged sword. . > the basis of a tracking system for the collector . I'm not sure I understand, but why would you need a phase change for tracking?

Harder to condense is a good thing in this instance. I'll post full details of the design on Instructables (hopefully in the form of an instructable) once I've got it working, which should be in the next couple weeks.