I like aluminum, too - but its thermal conductivity makes it a non-starter for this application. Wood, on the other hand is a near-perfect material (for a lot of reasons) - except for the vulnerability of that portion exposed to weather.
Where it makes sense to use aluminum (for the absorber/heat exchanger and the trim that protects seals from UV) I already do use aluminum.
One side of the channel would be indoors (in the heated volume) and the other side of the channel would be outdoors. No matter how much insulation was fit inside the channel, the channel itself would still constitute an unacceptable loss mechanism.
Construction would be _easier_ if it weren't wood, but the performance wouldn't even be comparable unless very much more expensive materials were used.
That's been a concern for me from the very beginning - and I'm sure that by now there're truckers who make jokes over coffee about this over-protective so-and-so in Iowa. To their credit (and my amazement/admiration) panels have been arriving at destination without even minor dings or scratches.
I've found sources of stainless foil that offer thicknesses up to 3mm (which isn't really "foil" to me), so the skin can be made thicker if that turns out to be necessary, but it really isn't my goal to add any mechanical toughness - just to improve long-term weather resistance.
Exactly so - during daylight hours there will be some continuous dry airflow, becoming "huge" airflow during winter months. Over the past 7+ years I haven't seen any problems with painted panels, and expect that behavior won't worsen with stainless steel "paint".
To the extent possible, this is already being done in the panel itself - and I've made a point of making clear to customers that attention to structure detail is one of the reasons to have installation performed by a pro.
Galvanic action is a concern, and I'm expecting s/s to behave reasonably well in this regard. Lew has suggested silicon bronze instead of s/s, and this will be one of the aspects I'll need to investigate further.
Most plastics, including vinyl, don't seem to hold up well in the long term to UV exposure. Primer+paint extends their lifetimes decently, but makes product longevity a function of maintenance quality - and I'm attempting to remove that dependence.
When I finally figured out what you wanted to do, my thoughts ran to residential windows. The Pella folks (also Iowegians) make windows with wood cores protected by aluminum overlays. I think the aluminum is brake/roller formed and the wood inserted. They might have some methods that work, or could at least move you further down the path.
Seems to me if you had an L or U shaped piece of anodized aluminum sheet that would cover the front and sides of the box, and lap over the back slightly, you would no longer be at the mercy of glue holding it in place. You would only have joints at the corners then. If you needed fastners they could be installed at the back and the front under the panel. You should be able to get a neutral color that could be painted to renew. Moisture that got in could get out through the corners. Or be baked out when the sun hit it.
Anderson does the same thing with a preformed plastic, which seems to hold up. I know they make plywood with that plastic laminated to it, but you would still have edges to contend with. It is also spendy...I bought a piece of trim for a window installation for $20 per sf!!!
I've seen what ocean air and salt spray does to steel, wood and aluminum, and it isn't pretty. I understand that seaside cottages have to be repained every couple of years to keep deterioration at bay. By my observation, galvanized seem to hold up best, it developed a layer of rust and then quit. Other stuff just kept pitting.
Yuppers - I have one of their windows directly behind my display, and I've looked at their stuff (and talked with a few of their people). I like their products, but I'm trying to go beyond everything they seem to have done.
I'd like to avoid the need to form sheet stock - my sense is that the job can be done with foil, although 0.002" may be a bit on the light side.
Ouch! I need to avoid spendy - and although plastics can look good, I'm just not confident that they'll hold up well.
I have, but I keep telling myself that if I keep my aim high enough, I'm less likely to shoot myself in the foot. Actually, I'm just trying to find a really decent starting point - from which I can make improvements as I find 'em. At the moment, stainless steel or silicon bronze "paint" looks like it might make a good starting point.
I have the same problem. This is how I "wait" for spring thaw. :)
Note that there are numerous bronzes in addition to silicon...manganese, aluminum, phosphorous, lead etc. I'm not suggesting silicon is not good, merely advising and suggesting research into the characteristics of the various alloys.
Too bad you can't hot dip galvanize your frames :)
Good suggestion, and that research is already underway...
Hmm. Lemme see - first dip in a concentrated copper sulphate solution, then do a light copper plating (think baby shoes or tree leaves), then plate gold over the copper - how's that?
Yup. The problem isn't really much different than house trim. If people would keep it painted, it'd last for a really long time - the problem is that they don't. That leaves me with the option of either blaming them for failures or doing what I can to eliminate the possibility.
Metalized mylar works pretty well when new. The guys I've talked to about using it in parabolic troughs have been unanimous about not expecting it to last longer than two or three years. The polycarbonate mirror I used looks lite it'll do better than that - but since I started on this effort, I've been looking at mirror-polished s/s foil on a plywood or Delrin substrate.
I've talked to local plastics distributors and haven't yet received any encouraging news - and if I can arrive at a good bonding method, I think a non-corroding meal surface would really be more sellable.
I've given some thought to thermoforming the panels and foam filling the result, but I'm not very confident that they'd last much longer than painted wood. Given that, I think I'd prefer to stick with wood.
There's a nearby outfit that manufactures a sprayable roof coating that they claim is even better - but they didn't think it would last appreciably longer than primer+housepaint on the panels.
I probably haven't done enough of this. So far I haven't heard of anything that seemed worth the cost.
Already done - and I'd guess that if anyone could coax epoxy into doing the job, it'd be Lew. :)
To minimize the conductivity issue, how about an aluminum frame (L channel) that would "clad" the box on two sides? This should get the aluminum out of the heat-path. You could add threaded bosses to the underside of the channel, counter-bore the wood frame, and through-bolt the aluminum frame to the wood frame with plastic bolts.
This also avoids the lamination issues others have noted, as there isn't a need for a bonding mechanism between the wood / cladding anymore.
And that's exactly what good builders look for, as you are, materials that obviate that necessity. In the case of house trim, I use Hardi type products exclusively.
Not enough info available for anything more than a guess, but it seems like boxes of that size could easily be made of something like exterior "aquapanel" instead of wood.
Serious response to lighthearted comment: Solar heating technology has advanced to the point where a conventionally-built structure can be (_has_ _been_) 100% solar heated through several winters in an area where this morning's temperature was below 0°F. Ten years ago, not even I believed it could be done - and even (especially?) I know it can be done better still - hence this discussion.
Leaving aside all AGW possibilities (because I have no first hand knowledge of the subject) and all political crappola, there are obvious real and significant financial and social benefits to be had from a technology that keeps people warm and comfortable without the need for them to pay and pay and pay.
Please don't quit. :-|
You're not actually all that far from the current design - except that the L doesn't extend all the way from the outermost edge to the structure surface. It's actually a 1/8 x 3/4 x 3/4 aluminum angle - used to secure the glazing, shield the glazing-to-box sealant from UV, and hide the glazing's edge.
If that angle were extended inward past the structure skin, there would be a problem with removing the glazing for cleanout, and with replacing the glazing in the (unlikely) event of damage. I think a two-part solution is appropriate, so I'm after just the flat side portion that can extend from under the angle to a point inside the structure wall.
As suggested, any of a number of materials might be used for that protective extension, and even though aluminum is easy to work, I have reservations about its longevity.
Thus far I've carefully avoided any through penetration of the walls. I'll need to think about this a bit. Much will depend on the reliability and durability of available adhesives.
Aluminum exterior door threshold, often has a rubber/vinyl/silicone (or some other unknown to me at least) seal inserted into a bead in it. Imbed that in the house, and overlap your angle piece over the seal. You could probably get it extruded in eight foot lengths with the right supplier.
On Sat, 9 Jan 2010 18:06:44 -0800, the infamous "Lew Hodgett" scrawled the following:
Hey, I just had a great idea. Let' all ask that fine Nobel Prize Winner Algore for some carbon. He can just pluck it out of the air, it's so thick out there. Then we can make lots of carbon fiber while saving the world!
REDUCE, REUSE, RECYCLE
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