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Best dark metal for solar absorbance. Answered

What's the best metal for absorbing sunlight? ie, darkest. It needs to be immersed in ethanol, which will dissolve pretty much any paint etc. I've tried steel with some kind of enamel or anodised layer, but the eths started eating that too. Also whatever it was seemed to reduce the heat getting into the ethanol. And are there ways of making metal darker, chemically or through heating? I've been recommended getting something rusty and using a rust converter, which turns the oxidised layer into a phosphate, which is dark and might work well. Also.. are there other characteristics I should be paying attention to in order to maximise the amount of heat transferred to the fluid, like conductivity, emissivity, etc? Cheers.


So I went ahead with the recommendation of black on the outside.
I'd still like to try having the metal inside the eths but it's proving too much of a hassle to seal it properly. I got some Quicksteel metal epoxy but there's always one more leak...

What I have now is a copper pipe hammered almost flat, sealed top and bottom (minus a small tube at the top), painted black and housed in a clear plastic sleeve. Any exposed metal I've wrapped in inner-tube rubber and gaffer, but even with that I'm still getting heat loss so I'll go a bit more hardcore with bubble wrap or foam or something.
Inside the tube I have the eths.

With this in a 700 cm2 (70 Watt) parabolic trough and starting from air temp the ethanol rapid boils after about 18 minutes. If I shadow it for 5 minutes it only takes another 5 to start boiling again.

So, basically I'm going to call this a complete success.

Tomorrow I'm going to hopefully be given an old overhead projector, from which I'll take the Fresnel lens to use as a collector. I was going to make a Cassegrain dish but since this is still just the prototype I'll just take the easy way out.

Then it's just the tracker and the light pipe and I'm done.

Awesome- glad to hear the idea worked out eventually. I assume the tracker/light pipe is going to let you collect sunlight to a stationary boiler over the course of the day? I'm interested to see how that works out- will you be focusing sunlight into the light pipe?

That's the plan, yes. The pipe's opening, (I'll be using a pretty fat pex pipe with liquid paraffin as the waveguidand plugged with glass), sits at the lens's focus. The other end feeds into a copper tube for dropping the heat into a steam engine / distiller / pasteuriser / oven. I won't be trying out applications just yet, I'll leave that till I get back to Europe in a few weeks and hopefully hook up with some kind of alt energy / tech centre or community or whatever. There's an interesting one in Portugal... I'll hopefully have something tracking by the end of the week, touch wood, in which case I'll post some pics here.

Why not darken the ethanol itself, using dye, dissolved pigment or stirring in carbon powder or soot?

Yeah, I thought of that but for some reason didn't investigate. I will. I'm not sure how well it will boil without a hot surface tho?

You want it to boil? Or just evaporate more quickly? Boiling doesn't require a surface, just heat.

Definitely boil, rapidly if possible. The problem with just pumping heat into the fluid itself is going to be leakage. I can double glaze with clear plastic, but the heat loss is still pretty large. If I have metal within the liquid it will get hotter than the boiling point and the ethanol in contact with it will boil, if I heat the fluid uniformly by the time it all gets hot enough to boil it will be losing a lot of heat through the outer surface. If that makes sense. I could well be wrong about this and will try blackening the eths as soon as I can. Yesterday tho I tried a bit of thin black plastic instead of the steel and it didn't boil, I think I need something with a bit of thermal mass.

If the black object is inside a clear container of ethanol, it probably isn't receiving as much heat as you think- while it is pretty much transparent to visible light (so looks clear/colourless), I believe ethanol will absorb infrared radiation, so you are already heating the mass of the liquid rather than just the absorber. Glass (and probably clear plastic) are also good at absorbing/reflecting infrared radiation (cf. greenhouse effect) so I don't think this design will give you the localised high temperature you are looking for. If you need it to boil you are probably best building a tube boiler, as per lemonie's suggestion- thin copper tubes painted black on the outside, as close to the focus of your reflector as you can. Also, good luck with boiling ethanol and concentrated sunlight, I hope you keep your eyebrows!

The transmittance of glass stays at about 75-80% for a good amount of the sunlight spectrum (up to about 1500 nm). I couldn't find IR data for PET or similar plastics, but up to 800 nm it's about 80%. I know that when I tried just a black copper pipe with eths inside the outside of the pipe was hot as hell, painful to the touch, and the eths still wasn't boiling. It seems I would still need to insulate the copper, which means getting all the problems of reflectance and absorbance from the plastic or glass. And yes, eyebrow retention is definitely a non-trivial design consideration.

Why not use copper, blackened on the outside? Paint it if it's not going to get above 100oC.
You're biggest concern is getting the closed-loop to flow in the right direction and power (whatever).


That was the original idea; running the eths through a black pipe, and would've been somewhat simpler than what I've got now. Unfortunately tho I just couldn't get enough heat into the fluid, there was too much of it and the copper shed too much heat back outwards. I have to use a compound parabola so I'm limited to a ratio of about 4:1 with the width of the parabolic trough and its focal range. So now I'm going with a small amount of eths isolated from the rest with clear plastic on the outside to let the light in and something black in the ethanol to absorb the heat.

Clear plastic and hot ethanol are not going to work well together, glass would be preferable. Try some dye perhaps (kiteman)? L