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.)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
The reason solar thermal is fading into obscurity is that in most places the solar energy doesn't arrive at the times when it is needed, and there is no good technology for storing heat in quantities and for the time necessary for it to be useful.
Where do you get these strange beliefs? Solar thermal is pervasive. Look up Ivanpah. You store it in countless ways, like molten salt, the earth, conversion, etc. But its broader prevalence doesn't have anything to do with my questions. I'm mostly asking how to best use the energy harvested in this scenario.
Products for residential combined PV and thermal already exist, most notably from DualSun. Here's a good recent paper looking at their solution in different climates: https://www.researchgate.net/publication/335954458_Analysis_of_a_Residential_Photovoltaic-Thermal_PVT_System_in_Two_Similar_Climate_Conditions
But my concept would be much more advanced, with a monolithic roof, not a bunch of ridiculous panels ruining the roof. But I'm mostly just wondering about applications for the thermal harvest.
You could use the hot water for the urinal your proposing.
Now you're on to something!
You can go on Craigslist in any city and find people giving away perfectly functional thermal solar collectors. Or I should say, trying to give away. Nobody wants them. This is what I mean by "fading into obscurity."
There is nothing wrong with these collectors, they work exactly as designed. However, for the reasons that Akos so ably lays out below, the physics is just against you in trying to collect usable energy in the form of heat from a rooftop. A better collector isn't going to change the physics.
For 40 years rooftop solar was a fringe niche, as tinkerers tried to get the formula just right to find a way to collect energy in ways that were usable at a cost that was reasonable. Rooftop solar didn't go mainstream because someone finally got the formula right. Rooftop solar happened when the price of PV panels declined, and when utilities were forced to implement net metering.
This is a solution in search of a problem, but setting that aside I would recommend reading about Thorston Klupp's home builds in Alaska and you'll get a sense of the engineering complexities of long-term hot water energy storage and use. They're interesting experiments, but light-years away from cost effective solutions.
I'm assuming you live somewhere it doesn't freeze or you'd steer away from water in your roof, so most of the time your roof will be too hot and when you have no use for the heat you will suffer an energy penalty dumping it 'somewhere'. I would advise saving the energy with airtightness and continuous insulation, not with thermal collectors.
I'm asking about how a home would best use the thermal harvest from whole-roof solar thermal collection, which would be several times a normal solar water heater setup. This isn't about whether to buy more insulation or airtightness. The roof is a given in this context, and my questions are how such a structure would best leverage it. Cost isn't relevant – I'm asking scientific/engineering questions.
These sorts of roofs are inevitable, since the energy is just sitting there for the taking, and cooling PV panels increases their efficiency. Only in the construction industry is stuff like this considered hard – the technology, materials, and manufacturing methods are trivial. Given the assumption of a modern monolithic roof design made of modern materials in a modern factory, of course it will have thermal collection built in – the energy is just there for the taking.
In order to be useful, the thermal energy must be able to be stored and the temperature regulated for given uses at given times like domestic hot water or hydronic heating. Those are the two primary uses for this hypothetical home roof solution in location unknown. Consider well the "scientific/engineering" challenges this poses. You'll find that photovoltaic roofs make more sense at the scale you're considering; it's easier to distribute, store, regulate and use intermittent and excessive electrical energy than solar thermal energy.
If all else is done right (airtight construction and insulaton) the primary energy demand of a home will be low enough to render the energy capture and storage discussion moot, especially when you haven't defined climate, home size, number of occupants or their lifestyle.
Your problem statement lacks specificity.
There seems to be a consistent pattern to the discussions you start.
- The way building is done right now is stupid.
- I've thought up a better idea.
- I'm not interested in counter-arguments or experience that don't support my position.
That seemingly inevitable trajectory makes them fairly pointless.
The Dunning-Kruger Effect:
Similar schemes have been attempted for decades. Here is the iron rule: for a solar thermal system to make sense, the value of the collected heat has to be high enough to justify the cost of the hardware needed to collect it. So a one-panel system in a frost-free climate like that of Israel may be justified for a residence. But the type of system you described falls outside of the dictates of the rule. You'll be gathering low-value heat at very high expense. Your heat will be at a low temperature, and will be gathered at the wrong times of year.
Because of the rule that the value of the collected heat needs to be high enough to justify the cost of the hardware needed to collect it, it is a waste of the world's resources to invest in collectors or piping as you propose.
Your setup is pretty close to rooftop pool heaters. A roof collector without any glazing will only get you warmish water. If you look at the specs here, any real delta T and the output drops off. You might get at best 120F water out of it in the summer time. In my climate, the collector won't produce any useful heat in the winter as the max water temperature will always be bellow house temperature.
So assuming you are in warmer climates has a 10000BTU/h heating. This is just a hypothetical number for say a PGH build.
Let say you want to store enough thermal energy to make it through 24h without much insolation but because you are talking about unisulated collector, you have only low temp heat so allow for a 10F temperature swing in your storage fluid.
So to make it through 24h, you would need around a ~3500gallon tank. I guess doable, but doesn't feel practical. If you want to account for longer spells the tank gets even bigger.
Cooling gets a significantly worse as you have only night time radiative cooling so the coldest water you get is near night time dewpoint, in dry climates you can get a bit more cooling but you are not likely to get much more than 15F bellow ambient:
To extract any meaningful cooling or heat from either option, you would need very large emitters, delta T is your enemy, so it might mean that you need both heated floors and ceiling.
Humidity control would have to be outside of this, in areas with high humidity, this setup won't help.
Now lets take the same house heat it with a mini split with COP of 3.5. That will need 20kWh of electricity to run. You are probably look at a 5kW solar array and a 15kWh battery. That feels like the simpler and cheaper setup.