Cheap, Easy, And Invisible DIY Solar Thermal?

I'm planning to build a home in northern Arizona—the warmer part, probably. It's a climate where winter highs might regularly reach the 50's (F), but winter lows can get cold enough for a great deal of snow on occasion. I think the cold-but-not-too-cold winter weather should make it perfect for solar thermal.

A while back someone here (sorry, it's been a bit and I've forgotten who!) recommended putting coils of black PEX on the roof to use as a cheap solar panel. I'm thinking of doing something like that, but hopefully even cheaper: I want to put the PEX under the roof (but touching it), and use the metal roof itself as the solar collector. This would save me some money on the PEX, and it would look nicer, I think.

Design Constraints

PEX Heat Tolerance

The primary difficulty with a system like this, as I understand it, is that PEX doesn't like temperatures over 180 F. And while a roof may not get more than 100 degrees F above ambient temperature, in my neck of the woods the record highs thus far have all been around 105 F—and presumably they will only get hotter. 100 degrees hotter than 110 degrees F is 210 degrees F which sounds like it would be too hot for the PEX.

Of course metal pipe is another option—and probably it would do better with the heat, in spite of the need for connections where leaks might occur—but I've not found a kind of metal pipe that seems sufficiently cheap. (I'm curious if folks think they know of one, though). Also there is the issue of corrosion with dissimilar metals, which would require some kind of barrier between the two metals…which, given that the roof will move around a bit, would need to be quite robust and short of doing something expensive like having a copper roof with copper pipes under it I'm just frankly not sure how to couple metal pipes to a metal roof without also severely limiting the heat transfer between the roof and the pipes.

Thus the PEX.

From what I have read PEX is usually happiest if temperatures stay below 180 F. However this document from plasticpipe.org seems to be saying that there are PEX pipes made for higher heat applications. Class 5, meant for high temperature radiators, seems to be the most robust vs high temperatures, though Class 4, meant for underfloor heating and lower temperature radiators seems pretty decent as well. Both are rated to accommodate 100 hours of temperatures of 212 F within their expected 50 year lifespan, and Class 5 is expected to withstand a cumulative year at 194 F, and 10 years at 176.

Roof Temperature

I've yet to find a really good resource on exactly how much hotter than ambient a dark roof will get when incidence is low. This blog (?) lists a few figures, with the hottest being dark asphalt which gets 75-105 F over ambient temperatures. I've seen other people mention 100 degrees F as a theoretical maximum. I'd like this number to be more firm (anyone have experience here?) but for now I'll assuming that when incidence is very low and the sky is clear, a dark metal roof will be 105 degrees above ambient.

Roof Tilt

One thing that may help reduce the times when the PEX gets too hot would be a decently tilted roof. This is only really practical at some more central latitudes where the sun is nearly overhead in the summer—and Arizona should qualify nicely. A south facing roof tilted at about 55 degrees should reduce solar intensity by 28% at the solstice and 19% on August 10*. One of the record heat days in my town was August 1, though all the others were June or July, and presumably August will be more into the mix on any year the Monsoon fails, which could happen more. Still, a 19% reduction in solar intensity in the summer may give me the wiggle room I need. 19% less intensity should mean that rather than heating the roof to 105 degrees above ambient, August sun will only heat it to 85 degrees F above ambient, which on a worst-case-scenario day when the August temperature is 110 F (Five degrees higher than the current record high), would mean the roof would be 190-195 F for about three hours, which should be no trouble for Class 5 PEX.

*This according to some trigonometry and a formula I found for solar intensity loss due to changes in incidence. See this thread for … well I didn't get to detailed, but a bit more than here.

Uncertainties

As I mentioned above, there are a few parts of this plan that rely on less-than-verified fact.

How much hotter than ambient will the hottest metal roof I can find get? I wonder if companies marketing metal roofs for northern climates have these numbers.

How do I make sure the metal will touch enough of the PEX for heat exchange to happen? I am a bit leery of tapes holding up for the 50 or so years I'm designing the system for (ideally this should work for as long as the roof does). Seemingly there are forces holding standing seam metal roofs down and squishing the pex a bit so I would hope it would work but…it might require something else to hold it flush and I'm not sure what that would be.

Pros

-It's cheap! If it works.

-PEX under a metal roof is invisible. Not that I mind looking peculiar but… as far as resale value goes, it is probably better to have it invisible.

-It's also a bit more fire-safe than having panels on the roof, I think.

Cons

-Compared to glazed solar thermal panels it won't heat as well in the winter. I'm hoping that this won't be a big deal in this climate—I still need to run RETScreen and see—but it may be a concern.

-Roof installers probably haven't done anything like this before, and there's some chance it could void the warranty.

-Roof penetrations may be a bit tricky. The devil is in the details here, but ideally this system would not penetrate the roofing membrane at all, or failing that, it would need to be taped in a way similar to that of windows…and that tape would need to tolerate these higher temperatures.

Summary

So that's the idea. I'm very curious to hear feedback from folks. I've not gotten into how I'd store this heat—I'm thinking a large low-heat heat bank for space heating, which should maximize the amount of heat the system can provide.

It seems like a simple way to save a lot of money over the life of the home…so simple that I'm not really clear why more people aren't doing it. But that said, I know that 95% of the time there's a reason people aren't doing the thing I've just thought of. I mean, I can't be the first person to play with an idea like this. So I'm curious if there's some glaring issue with my plan, or more concerns that would need to be addressed.

Thank you all in advance for your help!