Hi all – I’m working on a concept roof that would be quite different from standard roof structures. My core question is: If an entire roof area served as a solar water heater / solar thermal collector, what would be the best uses for the collected thermal energy?
The roof would be a single, seamless structure made of high-strength fiberglass, chlorinated PVC, basalt fiber reinforced polymer, or similar, maybe an inch thick. Dense, narrow piping would be embedded in the board. It would either be a separate assembly of plastic piping laid into the board during the molding, or the channels would be formed in the board itself with a 3D printer or additive process. The working fluid is either water or propylene glycol. In the water scenario, the roof could be a hot water tank, just like some direct solar water heater systems. Solar PV panels would snap into place over the board, so the thermal collection here will be somewhat less punchy per sq ft than a normal system with nothing covering it.
So assuming we had a roof like this, saturated with solar thermal collection, what would you do with that heat? Hot water is straightforward. What else? I was thinking that in the winter it could warm the roof penetrations, vertically. For example, the chimney, vents, and skylight tubes spearing down into the house could have warm water sluicing down them, countering the bridging. (Even with lots of insulation, those penetrations bridge across their interiors, right?) Reversing the flow would cool these bridges during the summer.
What about warming or cooling the house overall? I haven’t done the math yet, and my thermodynamics is rusty. Would you expect any big wins from liquid heating/cooling of the walls, roof, or other structures? The climate would normally be hot, so a full-roof solar thermal collector is going to cool the roof, taking the heat to… well, anywhere we want.
It seems like in some cases we’d want to be able to just dump the heat, maybe through a radiator off the side of the house. In the Southwest, it would be a win just to get the heat off the roof and dump it in such a way that gets it away from the house. Would you expect this kind of liquid-cooled roof to be as effective at cooling the home as an all-white “cool roof”? (Those white roofs have dramatic effects in Arizona, slashes your AC bills. The roof under discussion can’t be white since it’s going to be covered in snap-in PV panels and silicon is dark.)
What else could we do with a “water roof”? Would you think about storing the heat in a more sophisticated way, like salts or commercial style solar thermal technologies?
If we used this roof as a hot water tank, that would cripple our cooling effect right? (Unless a lot of hot water was being used at a given time, causing the system to cycle cooler water up to the roof.)
There are various ways to move the working fluid, either active pumps or passive mechanisms like thermosiphons. That’s not an issue. I’m most interested in what you think the best uses and management of the heat would be. I’m also stuck on the energy balance issue – will taking the collected heat into the home in the form of hot water or glycol offset the cooling effect on the roof? That heat has to go somewhere, and I wonder if sticking it in a hot water tank, via a heat exchanger, ultimately must warm the interior as much as the interior was cooled by the initial thermal collection up top. Will there always have to be some heat dumping outside?
(The roof will have foam insulation attached or adhered underneath, ergo above the attic space, likely R20+, ending in radiant barrier foil. This typically makes roofs hotter, which I suppose helps us. Another R30+ will be on the floor of the attic, probably mineral wool.)
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