Wild idea? Anyone do combined water / electric solar?

A wild thought that I have had, don't know if it makes sense or not, but I wonder if anyone has ever thought of doing a combined water / electric panel?

It seems like space for solar panels of any sort is a finite resource - we have a roof that looks like it might be ideal for panels, but it's only so big, and I suspect the same would be true of others. I also hear that electric panels have very low conversion efficiency rates, and that their performance drops as temperatures go up. It would seem like putting a water collector on the back side of an electric cell would capture the some of the energy that the cell didn't, plus possibly help cool the electrics. Even if you didn't get as much water heating as you did with a pure water panel, it would seem like an improvement on the total, and let you get two kinds of energy out of the same amount of panel space...

I could also see a potential advantage in winter that running the water panels after a snow storm would help clear the electrics off so that they could start producing again sooner - either doing normal operations, or running them "backwards" as a radiator surface instead of as a collector...

Anybody know if there is a company doing this, or if there is some reason why it wouldn't work, why not?

Gooserider

Well, I still think that as long as PV is not so strong per sqft as to be able to provide 100%+ of everything you might want in terms of heating, DHW, pool heating, etc. there would still be an interest in combining applications in the same surface - and I could still see at least some variant of the forms you mention being able to be laid down on top of a liquid collector of some sort...

Just off the top of my head, what about a metal roof with collector pipes under it, and a solar sheet rolled out on the top of it. From what I've read, the first part would be an effective collector if it were a black roof, and the solar film would give you that.

I don't claim to be expert on the way these things work, but my understanding is that current PV stuff only converts 1-2% of the light that hits it to electricity, and most of that is the "high energy" light up near the UV end of the spectrum... OTOH, the liquid panels have a fairly high conversion rate, much of it from the lower end stuff on the IR end - which seems to me like the two would make a good combination, with the PV using the upper end of the spectrum, and the liquid using the lower end, and keeping the PV efficiency up at the same time by providing cooling...

Not disagreeing with you about the future of PV (I don't know what that will be) but just saying roll out or shingles wouldn't necessarily be incompatible with liquids any more than a panel would be.

Gooserider

Webmaster said:

I think you are exactly right, that eventually these things may be the actual sheathing and roofing, or even built into siding panels. Why not? It's mostly sand, after all!

Year ago I installed some solar roll systems, including one on my dads house for a pool. This was rubber EPDM which came on a giant spool and plugged into manifolds by hand pressure only. He had a large flat and black roof - worked like a charm! That was 1982 or so.

Pools (and thermal solar in general) work best when vast amounts of water are run through the panels with a small temperature rise. Getting a few degrees out of it each time through is better than heating it way up hot.

BTW, here is a current PV flexible type.....
http://www.brunton.com/product.php?id=256

Click to expand...

Might be a flexible product, but hardly affordable... Their featured product is ~$50 / watt! OTOH, the SolarFinder estimate for our house was $9 / watt, and I felt that was excessive - they were talking an 11 year payback with all the rebates and such, plus a $53K upfront cost - granted that's for a much bigger system (7 kW vs 4.5 Watts), but still...

I'm thinking they need to get the price down to more like $4-6 / watt, so that the payback is more reasonable.

Gooserider

Redox said:

The current benchmark seems to be $4/watt. I have found new panels for slightly less than this, but not much. Nanosolar is claiming $1/watt, but it's going to be a while till we see these available to the common man.

Click to expand...

WOW! Big Difference... I'm wondering though if we might be looking at different things? Is that an "installed price" or just for the bare panels themselves? The $9 / watt price I was quoting is apparently a "system price" for grid tied solar, with installation and all the other added equipment needed to tie into the regular power system... Presumably I could knock that down a lot if I did my own work (assuming one can get the panels in house size quantities at a reasonable price...) although my roof is a bit "scary" seeing as how it's a 12/12 pitch - however that would presumably be good for angle once I got the system up there...

Telco, I'm also thinking in terms of doing a solar / wood boiler heating / DHW setup - my idea about the PV / Liquid panels isn't so much to improve the panel efficeincy, as it is to get more "work" out of the same patch of roof. Cooling the PV panels might not actually be a major result, I'm not sure what temps they normally run, but I'd be trying to get that same 180* water you were mentioning in order to feed a storage tank. Use the tank (with wood boiler supplement) to heat the house and DHW plus do a bit of pool heating in the summer since it's there and not being used otherwise.

As to the evac tubes, I saw a blurb in a paper that was pointed at from one of the other threads in the GR comparing the outputs of flat panels to evac tubes on a house with both types installed. They found there wasn't a big difference in the two, not enough to justify the cost difference, and in some circumstances the panels actually did BETTER - it seems the "innefficiencies" of the flat panels is an advantage if you have heavy snow or frost, as the heat loss melts the covering off the panels sooner, getting them producing output while the evac tubes were still covered, resulting in a higher total. Since I'm a cheapskate flat panels sound better to me, especially here in New England where we get plenty of snow...

Gooserider

Telco said:

I agree, the flat panels are a better all around way to go. The evac tubes are neat though, and if you had a mixed system you'd be able to get heat any time there was light. Heh heh, if you have to have a street light up at your place (remember my uncle out in the country had a sodium tube light in his backyard that the power company put in) then it would be heating the water 24x7. All the evac tubes need is light, doesn't have to be sunlight.

The only thing I see as a problem with using water to cool the solar panels and usable heat for the house would be eventually the water would be hot coming in already, and would just get hotter under the panel, perhaps making power output worse since the electric panels make less power if they get too hot. Now as an alternative, you could just use the heat from the electrical panels for pool heat, by either pumping pool water directly through the panel passages, or setting up a heat exchange loop so as to not pump pool chemicals through the panel cooling setup. It would take a lot longer to warm 20-30 thousand gallons of water than there would be sunlight during the day. You'd also need some sort of heat tempering setup for a direct feed, otherwise you'd be feeding scalding water into the pool. Probably the best way would be a closed loop, with the heat exchanger inside of a small metal box beside the pool with a solar powered pump to move water through the box.

I'd not try using the electric panels to cool the water storage for the DHW, as you want that to get a lot hotter, and it would not be long before you'd have 160-180 degree water being pumped TO the electric panels. Course, you'd also need a lot fewer solar water panels if you weren't heating the pool with them, so it works out pretty good.

Click to expand...

Well the evac tubes can't produce more heat than they have light energy falling on them (basic physics) so the amount of heat you'd get from non-daylight illumination would be negligible. Probably take more to circulate the water than you'd possibly be able to gain.

As to the mix of DHW and PV panels, I guess the question would be what kind of temps do the panels normally operate at, more specifically what temps do the back side of the PV collector see? This is something I don't know, but could potentially find out. I'd suspect that since the panels are black, almost anything they don't convert into electricity is going to be captured as heat, which has to be shed, presumably either by radiation or convection... Considering the temperatures that a roof can hit, I'd expect that the normal panel operating temps would be quite high, possibly well over the 180 mark - IF that is the case, and again, I don't know, then even 180* water might provide a lot of cooling potential - even hot water can absorb more thermal energy than air. (Look at the difference in the size of the water jacket on a liquid cooled engine and the size of the cooling fins on a comparable air-cooled engine) It would be a case of needing to circulate the water fast enough to keep the net temp rise to a reasonable level...

In terms of doing the whole house and pool thing, I would probably think in terms of having the solar be one loop dumping into the main storage tank via a heat exchanger, and have a seperate heat exchanger loop for the pool, with the house heat, wood boiler and DHW on a third loop that either included the storage tank as part of the loop if it were a closed (pressure) system, or did a heat exchanger into the tank if the tank was open. (I haven't put anything on paper as yet, since it's just a fantasy at present, but I can see the layout in my head..)

Gooserider

Got me in the wrong place - I'm north of you up in MA - the nanny state! We have central AC, I would probably not bother with it, but the GF insists on it, and uses it a fair bit about 2-3 months out of the year. I figure we are heating about 6-7 months, so that is FAR more of a priority.

Right now we don't have any sort of heating on the pool, but it's also used less than we might otherwise, as it's a bit too "refreshing" (that's spelled C-O-L-D!) for most of the season. If we could warm it up 10-15* it would be a lot nicer....

I figure that even with flat panels, it should be plenty easy to get water hot enough for heating the house and DHW. If one did radiant, you only need about 120* in the storage tank, and you only need about 120* minimum for DHW - both easy to get with a flat panel, and even if the panels only did a partial pre-heat on the storage, it would still presumably mean a lot fewer firings on the wood boiler or other backups.

Gooserider

Telco said:

I'd suggest checking at Northern Arizona Wind and Sun's forum for more specifics on how the PVs operate, that's what they do there and there are some sharp cookies over there. It's like this place, only for solar power instead of wood heaters. Everything I've read there states that heat hurts power production, so I'd not want to run anything that might pump warm water TO the PV. This is why I suggested using the pool water. It is a large mass of cool water, so will be able to absorb a lot of heat without potentially pumping 140-150 degree water to the panel. As the DHW tank gets hot, it will absorb less and less heat from the loop, which means warmer and warmer water will go to the PV panels. That heat will be wasted, and will lessen your power production and perhaps cause overheat damage to the panels. You'd definitely want a way to automatically cut water flow off if the incoming water reaches X temp. The water temps can get VERY hot too, as I've read accounts where people using solar water heaters that work well in the winter are having problems with overheating in the summer, and are having to develop strategies to deal with it.

Unfortunately I can't yet offer direct experience, and it drives me nuts sometimes. All I've got is what I've read, but I've read a lot and can link to a lot of those places when necessary.

Click to expand...

Thanks for the link, I will have to check them out... I totally understand the point you are trying to make, and sort of agree with your logic - the question is one of what are the relative temperatures involved? If the PV's just sitting on the roof w/o any water behind them would normally run at say 150* (or less), then I absolutely agree, pumping 180* water to them would be a BAD idea.... OTOH, if the PV's would normally be at 200* (or more) then they might be really "happy" to get cooled by 180* water... I just don't (at present) have any idea what the normal operating temps of the PV panels would be, so that is probably one of the first things I'd be asking.

I'm sort of resistant to the idea of heating the pool directly for a couple of reasons...

1. Right now we don't do ANY pool heating, so it doesn't work as an "energy saving" method (and wouldn't no matter how we started heating it) and it would be harder to sell the GF on the notion of pool heating on it's own as opposed to "we are generating heat that we can't use any other way, so lets not waste it...

2. Currently we have the pool shut down, w/ all plumbing drained, filter dismantled, etc. for about 8 months out of the year. Even with heating, we would probably only extend the season a month or two (just be more comfortable using the pool during the times it was open) unless I spent really BIG sums to try and build an enclosure for it (and then pay the gov't the increased property taxes, etc...) So there would be no benefit to the pool heating plumbing for most of the year.

OTOH, if I set up to do DHW / radiant heat production, I would get year round benefit on systems that we would already be using (and paying for) regardless, and then be able to use the swimming pool as a "dump zone" for the excess heat produced during the summer...

Gooserider

pcampbell said:

4/watt is just the panels right?

How is the efficiency of the grid tie panels vs 12/24v panels?

Click to expand...

I don't know how the efficiency compares directly, but some of the stuff I've been reading says that there is more power produced by the grid tie systems. I don't know if that is because the panels in a grid-tie system are better, or if it is because the electronics needed to produce the appropriate clean 60 cycle AC is more efficient - I suspect it's more the latter, as you avoid the entire mess dealing with the battery bank. I haven't actually heard that there is an actual difference in the panels themselves.

For the on-grid user, everything I've heard seems to suggest that it is better to go grid tie, the only thing that bothers me is that the current designs don't work if the grid goes out. IMHO unless you have a very unreliable grid connect, I don't think the power-failure issue is enough to justify going to the expense and hassle of putting in and maintaining a battery bank, but it is something that would really annoy me if we had a long outage with a bunch of unusable juice sitting on the roof... I'd want to see if there was some way to have a small setup that could "kick-start" the grid-tie system so that it would function like a generator - something like either a UPS or a small (say ~1kW) gas generator - It would only work during daylight obviously, but might be a way to keep the critical stuff going - i.e. keep the freezer cold and run the furnace long enough to keep the house warm...

Gooserider