How about using galvanized steel pipe?
 

...in a collector?
Copper obviously has its advantages. Pex seems to me like it would suffer from lower conductivity and the risk of damage with high temps in summer, etc.
You can buy a 10' 1/2" piece of electrical conduit for like $1.60 US.
(pardon my non-SI units, but that's how they're sold here)
Seems it would be far superior to Pex in conductivity. I know the zinc coating will probably eventually flake off or corrode, but could you use some corrosion inhibitors (small quantity of the ones you put in your radiator - toxic, I know...) to improve its life?
I have a group of students I'm prodding to develop and build a low cost, effective solar heater for my classroom. We can cut these easily, attach galvanized sheet metal to them pretty easily, bend them if needed, and use rubber automotive heater hose and hose clamps to make connections. If it works great for a few years and then we have to replace the pipes, wouldn't break my heart in the name of experimentation. I can always use metal tubing on other projects (frame for my car or misc. stuff)

I did some searching and didn't come up with much, so I'm curious what you guys think. I don't want to finance copper pipe.

I say give it a shot and see how it works. I too thought that pex would suffer from low conductivity, but the panels weren't all that horrible according to Gary's testing on builditsolar.

I would be wary of this plan for a number of reasons, first off conduit is very thin galvanizing, it will even start to rust at times if just located in a damp location, it is also very very thin.
Steel is also a poor thermo conductor, not as bad as pex but no where near as good as copper so difference in temp between the outside of the steel and the inside of it where the working fluid is is going to be greater then with copper, there for you are going to have more heat that can radiate off of it and be lost, these are all things that have to be looked at when designing soar hot water panels as the more heat you can get out of a square foot the less you are going to spend on insulation, glass and building the frames.
Another thing to think about with interconnects is that good solar hot water panels can reach 200-240 degrees and because of this I've seen radiator hose brake down really quickly, a system we worked on a few years ago needed a flex hose to allow for a small amount of movement we first we used a piece of PEX but that softened, swelled, burst and spewed $100's worth of coolant all over the place and we ended up buying high pressure hydraulic hose as it's not only designed for high pressure but heat as well.

Good points about the thin galvanizing and the cost per square foot including insulation and cover, etc.
My radiator hoses in the car are exposed to 215+F temps often and seem to last quite awhile, under much greater pressure than a gravity system, but for the quantity we are using, I agree - some high temp hydraulic hoses are a good idea and won't add significantly to the cost.
Still really want to give it a try.... but I do suspect that the largest difference between steel and copper in terms of thermal conductivity will result after the inner walls become corroded faster than copper ones do.
Will have to present these pros and cons to the kids. Due to cost (and the adventure of just trying it!) I bet we will go with the galvanized pipe and see how it holds up.

You can paint the outside of the steel with some good rust-resistant paint.
Something like flat-black engine paint maybe? (So it can take the heat).

Regrading heat transfer from the back-plate, I'm wondering how hard it would be to solder the pipe onto a flat back-plate made of a similar metal? (And then paint it).


If you aren't hooking up to the domestic water supply, no need to worry about contamination by the anti-freeze. They put some pretty good rust inhibitors in that stuff these days. So, the steel conduit parts might last a good long time..

If you wanted to make it simple, you could use large OD conduit and pump air into the collector.. Less problems with rust. :)

Or, forget the pipes and use a flat sheet metal collector under a glass lid..
Blow the room-air into the bottom and take the output off the top..

;) about the air pumping and rust!
My ideas for attachment of the sheet metal collector is to bend then drill bolts through, then tighten to clamp it down against the pipe. Labor intensive, but no need to use the tack-welding methods they use at sheetmetal shops to connect. I wonder how solder would work...May try that.. could even do it after it's clamped.
I'm going to try to pick up some sheet metal tomorrow and try out a small sample piece with the kids and see what we all think of how to attach.
Flat black engine paint is a good idea.
My friend is using an air-heating system and he likes it, but he wants to upgrade to water to transfer thermal E more effectively, so I think I will not go that route.

if you are going to weld or braze galvanized just remember what burning zink and breathing it will make you really sick.
Also it's not the outside that I'm worried about rust but the o2 that is dissolved in the water that is going to brake them down from the inside.

Hi,
The thin wall electrical conduit is amazingly inexpensive stuff -- it does seem worthwhile to try and figure out a way to make good use of it.

If you used the conventional parallel riser type of collector, one feature that would allow you to use a lower temperature connection technique to connect the risers to the manifolds would be to separate the manifold area of the collector from the riser/absorber area with thin insulation, and then omit or paint out the glazing over the manifold area of the collector -- this would make the manifold are run cooler, and allow more latitude in the kind of connection you used.

There are supposed to be corrosion inhibiting additives that can be added to open hydronic systems that allow the use of iron pumps without rust/corrosion problems -- maybe the same additives would protect metal conduit. This is not an antifreeze product, but a corrosion inhibiting product.
You might try asking the people who make the Taco pumps about this -- they are pretty good about answering emails on technical issues.

I don't think that the lower conductivity of the iron would be a problem. Its far better than PEX, which does fairly well. A significant portion of the thermal resistance from the collector fins to the fluid flowing in the pipe is the slow moving layer of fluid (boundary layer) just inside the pipe wall -- once the pipe wall resistance gets below a certain point the boundary layer thermal resistance will be the largest part of the resistance, and increases in pipe wall conductivity won't help much.

If you are looking for a cost effective collector, you might also consider the MTD type collector:
Modified Trickle Down Solar Heating Collector

Gary

Thanks, Gary!
I will check out the MTD idea; I think that's what we are doing.
**edit - I misunderstood - we're doing a gravity drainback, not an MTD - looks cool though.**

We started construction today on a 3' x 6' panel and it seems to be going well. Will get some photos up next week after I bluetooth them from my phone.

well, I've failed in the photo update department. The panel is going well and will hopefully be completed this coming Wednesday and we'll be able to test soon thereafter and see how it works. It's looking pretty good thus far.