Dual hot air and water panel?

[The master plan]

I've seen the hot air panels and the hot water panels. What do you guys think of a dual panel constructed with a coil of pex behind the black absorber for heating water.

I've constructed a couple of panels out of black metal used for hot air collectors. They have achieved air temps in excess of 150F.

I am thinking the pex could stand up to the temp because the water would be coming out at temps of 120 max, the systems wouldn't be high pressure and finally the pex wouldn't be exposed to sunlight.

Copper would be more ideal, but the price for copper tubing is very high right now. The panels would be used to heat my garage in the winter. Air could be blown threw them like a traditional hot air panel to heat the garage and the pex would be collecting hot water to store for cloudy days, nights(radiant floor heat) and/or domestic hot water preheating.

I have 34 feet of south facing wall on my garage and am intending to use that to the fullest extent possible. There are a few trees on the neighbors property, but they loose the leaves in the winter. Summer sunlight will still be good but would provide some relief to the panels to prevent overheating in the summer.

Things to work out would be how much 1/2" pex per 4'X8' panel...I think you could use a steel mesh grid to tie the pex to and provide air turbulence to help scub heat off the back of the absorber plate.

Any ideas, thoughts?

Below is a short video of the metal that I have. I have a roll of this stuff. Still has the protective plastic on the black.It's not the painted stuff...this is the real deal. Its black coated aluminum one side and uncoated the other, was told they don't make it anymore.

The video isn't showing full temps as it was sitting in the sun for only a bit.

solar collector metal - YouTube

[Daox]

I can't see the video (blocked at work), but I don't think combining the panels would be a great idea. If you want a hot water panel make a hot water panel. Having it heat air and water will just result in less hot air and less hot water.

[AC_Hacker]

Originally Posted by The master plan

I've seen the hot air panels and the hot water panels. What do you guys think of a dual panel constructed with a coil of pex behind the black absorber for heating water.

I don't know if anyone has actually tried this exact approach...

There has been work done on combining PV and air heating or water heating. This happens to be a very favorable combination, because the higher the surface temp gets on a PV panel, the lower the efficiency, therefore harvesting thermal energy will increase the yield of electrical energy. So there is a kind of additive process going on here.

Here's a link to this approach.

In your case, you'll be gathering the same kind of energy (heat) using two different methods in the same collector, so there will be a kind of subtractive process.

I think that you'll lose maximum temperature from, but the total amount of heat you'll gather could well be greater.

I think that if you had a lot of room to erect panels, you'd be better off to use panels that were specific to each purpose. But I don't believe that to be your case. 34 feet sounds like limited real estate.

Solar panels of any kind are not 100% efficient, and if you could build yours so that the water-heating mode could scavenge some of the wasted heat from the air-heating mode, and vice-versa, so much the better.

It sounds like you are considering suspending your PEX behind the black aluminum surface... There are radiant floors that use this method also, but their efficiency is notoriously low. You'll get much more heat when your PEX is in direct contact with the hot surface.

* * *

Another way you could think about this is to go hot water all the way...

You might be totally amazed, as I was, how many perfectly good copper hot water collectors there are for cheap or free (check Craig's List in your area... give it some time). You would be hard pressed to build one any better or cheaper.

Or you could build your collectors to take full advantage of water heating, and store the water in an over-sized, well-insulated tank, located inside your garage. That way, any thermal leakage through your tank insulation will be heating the garage.

Under normal circumstances, if the tank temp dropped below garage air temp + 10 degrees, you wouldn't be able to use the heat directly, but as I recall, you were interested in building a homemade heat pump and could then harvest heat all the way down to around 40 degrees. That could be a lot of heat.

-AC_Hacker

[Ryland]

I agree, it sounds like you will be disappointed with the output of both the hot water side and the hot air side because the collector is not going to collect any more heat, so you will be asking it to give more output without having any more input.

AChacker is correct, that cooling PV is really the best way to go and used solar hot water panels tend to sell for scrap value if they are used, they often need new wood if they have wood sides but replacing that part is going to cost you the same if they are used panels or you build them your self.

[strider3700]

somewhere I remember reading about a water collecting panel that was mostly air heating panel with a small copper pipe area that the water ran through. I don't know if they collected the hot air as well or just used it like a giant oven to help rapidly heat the water in the small copper xchanger. I'll have to see if I can find it again.

[Solar Mike]

Air heating panels should have sufficient air flowing through them so the heated air is only a few degrees (say 10) above the incoming cold supply air. This gains max efficiency from the panel and is directly at odds with combining heating water at the same time. The higher temperatures required to heat any water will make the panel very inefficient.

Perhaps another way is to build a Hot water heating panel with a large alloy or copper finned absorber as normal, but for the glazing use a double layer of 1mm polycarbonate separated 50mm apart. This space has your air flowing through it, the heat it will gain will be the losses from the water heater absorber that normally gets wasted radiating into space. This should have minimal effect on the water heating part.

Cheers
Mike