Solar collectors(metal and fabrication content)

I played around with some things about 20 years ago, just piddling during a winter, and found that painting a common red brick black in color raised it's temperature 9 degrees when compared to a red brick placed next to it on wood blocks in the direct sun. In Mother Earth News at the time, the statement was made that a dark green color was actually the most heat absorbent color to use. That's about as deep as I got into it.

RJ

I think it was that dark green was almost as good as glossy black and a lot more attractive. At least that is how I remember it.

Dan

Finding black body info. is hard , I think it's because solar people want the edge from proprietary info. Something turns copper or was it aluminum black , maybe sulfuric acid? Don't know if it will hurt the pipe , but its best to stain it somehow. Maybe anodize it flat black. I tried engine paint once and it didn't last long , but then I was focusing about 25 suns on it and what ever water boils @ 300 psi. That was one scary experiment. Try looking for redrock web site from alt.thermal it has tons of info. in it.

I can't say the process (because I don't know), but we work with a product that uses IR. Preventing reflections, which are bad, is important in the equipment we make. Not reflecting is of course another way of saying it absorbs IR.

One of the many aluminum formulas available from the anodizing shop we use is much superior to others at absorbing IR.

-- WS mostly in m.s -

If we place on wood blocks directly in the sun two equal size samples of bare steel and bare aluminum it is found that aluminum gets way hotter. Although the bare aluminum reflects more heat than the bare steel, the absorbed heat is very poorly radiated back out so it gets bloody hot. The steel reflects less heat but radiates back out that absorbed heat far more efficiently with the result that the temperature rise is much lower. If our two samples are painted with the same paint all over they each get the same temperature. The color of the paint determines how much heat is reflected back. The type of paint determines how much heat is radiated out: Color determines reflection/heat input. Material determines radiation or heat output. Practical applications: 1) If you use in house or shop hot water radiators it is best to paint them a light (white) color as this will minimize absorption of heat from out of the room. For the highest heat output (radiation) to the room you select a paint or clear surface finish over the paint that has the best radiation property.

2) If you got one of those aluminum campers and you paint the aluminum with a clear varnish you retain the good reflection of the aluminum color and cash in on the much better heat radiation of the surface film of the lacquer. HTH

Surely cheaper to get a convector radiator and paint it matt black? (620mm X 1600mm rad. ~ £40)

My (commercial) 4x10 collectors use 3/8 tubing for the 10' direction, but a dozen or more in parallel, all fit into 1" (domestic) or 1.5" (pool) headers at the ends. If you're thinking of a single serpentine tube, 3/8 will waste serious pumping energy. You'll also want to think about layout for getting the air out initially, and draining the water out for freeze protection. Or plan to use a heat exchanger at the other end, and antifreeze.

Also remember that steam is serious power - if your pump stops and your system doesn't thermosyphon, it _will_ exert awesome force somewhere... And you can't just switch off the sun to solve the problem...

I'd say it wouldn't be difficult to equal or exceed commercial efficiency, but it would be a lot of fiddly work with some learning curve issues. Glazing... Insulation... Keeping moisture out of the box... You'd have to be doing it for the love of the project...

I built an exchanger as the heart of my system. Five parallel 50' lengths of 1/2" copper for "indirect" DHW output, five more 50' lengths of 5/8" copper for input (from solar, propane, and/or wood), all in a tank of "dead" water for heat storage. The input size was required to handle the output of my boilers using reasonable pump power (solar electricity is expensive); the output size may be overkill, but it can flash heat 3GPM at 70F rise to within a couple degrees of the storage tank temperature.

Loren [first post with new "Agent" newsreader - apologies if something unexpected happens!]

Thanks, good tip on that web site lots of usefull stuff.

For hot water you really should look into batch heaters; basically the bottle out of a water heater in a solar collection box. Trying to heat the water on demand requires gobs of collector and they are going to reach inconvenient temps when the water isn't flowing. It's unfortunate but true, that most people use hot water when the sun is inconvenitly placed in the sky: early morning and evening as they are gone to work during the best times. Unless some one is at home during the day to do laundry etc or your life style can be modified of such this is a serious issue. Another reason to look into batch heaters as they can store some of the energy.

Way back when I was looking hard at solar heating the number one absorber was a coating of "black chrome", not that I ever really figured out what that is. It was available in peal&stick though that was nearly as effective as a direct coating. About any flat black high temp paint was cost effective: we're talking stove, barbecue and header paint. The secret, as with all coatings, is to keep it thin.

Oh yea, three layers of glazing was what I came up with as the max useful number of glazing layers. Pass three layers you block more energy from getting in than you keep in with the extra insulation.

I have a hard time believing this; it takes more energy to raise the temperature of a block of steel than an equal size block of aluminum. The heat capacity of steel is about half that of aluminum per gram but its density is about triple so it would take half again as much energy input to the steel block to raise its temperature the same amount.

No, the vast amount of heat transfer from a radiator running at a reasonable temperature will be via convection not radiation. Color will have very little to do with efficiency. The insulating properties of the finish will have a lot more to do with the heat transfer than the color.

This is indeed more complicated, but the difference in heat capacity may be less important in practical terms.

Considering tubing for pipes in solar collectors, one would expect the strength/weight ratio of the two metals to reverse the effect - aluminium tubes would be thicker, equalising (or more) the total latent heat.

What would be nice to know is the rate of heat transfer across the inside tube wall, into whatever the working fluid is - especially if it is a large effect.

Most things have the same absorption as emissivity. That is they tend to radiate heat and absorb heat equally. But some things will absorb heat at shorter wavelengths and not radiate as well at lower wavelengths. I know that copper oxide is one of these. So it works well for a solar collector.

Glossy black paint will absorb more than flat black paint at reasonable angles of sunlight hitting it.

Dan

Energy vs temerature. Larger heat capacity will slow the approach to equilibrium but, in and of itself, will not effect the final equilibrium temperature.

Further, water has one of the highest specific heats of any known material. The heat capacity of the water in the system will far outway that of the metal.

Ted

Thanks to all that replied both here and via e-mail.

Loren, it sounds like you've built what I'm headed for, do you have any pictures or plans? I'll have to use a glycol system because I'm in Vermont, so I'm very interested in your heat X design. How big is your "dead" water tank? did you build it ? I'm thinking about a 400 gal tank with a coil from the panels and one to heat the boiler loop(D hot water comes off the boiler). I'll still use oil but the base temp will be higher.

I would think the issue would be moving the heat in to your working medium: the water. If that is so the ideal choice would be the material with the greatest thermal conductivity for the required thickness to meet your mechanical goals: all most certainly copper. The only reason I can think of that might invalidate that idea is if you for some reason are wanting to use your tubing as thermal mass. Perhaps in a small collector system that's going to rapidly draw warm water off in large volumes compared to the systems capacity.

hunting. Regards. Ken.

You would need thick wall tubing but that would reduce the overall conductivity.

If you compare the heat capacity of (say) a foot of copper tube to the heat capacity of the water in it for typical copper tubing (~1/16" wall), you find that the thermal mass of the water is 1.67 times the thermal mass of the copper for 3/8" ID tube and 2.34 times for 1/2" ID tube and gets even larger as the nominal size increase.

Also of note: Some years ago I read a paper on non fan cooled heat sinks for semiconductor devices. It was found that black anodized aluminum was considerably more effective than painted or bare.

The thermal conductivity of copper is ~1.7 times that of aluminum. This may not be terribly significant especially in a fairly thin wall tube.

I would think that the most important characteristics would be the emmisivity of blackened (oxidized?) copper compared to black anodized aluminum and the comparative ease of joining copper compared to alluminum.

Ted

A stupid question. Would you benefit from making the internal surface of the copper pipe black?

In absolute terms, yes. In practical terms, no. The effect would be way too small to be economical and that is assuming all else is equal, which it never is.