Air to Water Heating Solar Panel?

[paborralho]

Hello, I'm new here, and would like to hear some advice.

I've been thinking of making another kind of Solar Panel for Water Heating. The idea is:

• 1-Solar Panel: an empty box, with black metal sheet as backgroung (so far nothing new). Just for Heating the Air. Like the ones for heating your room.
  
• 1 Ventilator conected to a thermostat
  
• 1 Little Radiator (from a motorbike for instance). The hot air from the panel is forced through the radiator.
  
• 1 Insulated Tank  for hot water keeping .
  
• 1 Small WaterPump connected to the same thesmostat, forcing water flow from the water tank through the radiator, and back to the tank.
  
  

So it goes like this: ther panel heats the air, the radiator transfers heat from the air to the water.

The advantages I see are:

• to maximize the heat absortion in the panel surface
  
• avoid the cost (biggest as I see it) of the water piping inside the panel
  
• minimize the water circuit size (and soldering, and losses)
  
• you can increase the size of the panel putting another one in series with the first
  
• the radiator and tank can be inside the house, thus no water freezing problems.
  
  

My hope is that a radiator can work equally both ways for this sistem to work efficiently. There's absoluting nothig revolutionary in this idea but so far I coudn't find someone that has done it before.

Hoping to hearing your comments

Paulo Borralho

[wooferhound]

What are you trying to do ?

Heat water or air ?

[juddley]

Back in the late 70's I "scavenged" excess "waste" heat from 10 Huebsch gas dryers in a commercial laundry to preheat 250 gallons of water for the main boiler.

Problems I encountered and observations:

1. Radiator..... get a refrigeration condenser coil to start with... any auto or motorcycle radiator just wont take any long term pressure without leaking. For a small unit the condenser from a tiny window unit AC should be the right size.
   
2. Water Circulating pump..... I gave up and finally just moved the tank to the attic so I could use gravity heat rise. (Could be modern circulating pumps are more reliable.) With your design concept it should be fairly easy to locate the radiator lower than the tank since you are ducting "hot air"... (no pun intended)as the conduction transfer medium.
   
3. Insulation is going to be critical as well as smooth runs.. no right angles or turbulence inside air "pipes" (ductwork) as this will waste heat.
   
4. Avoid circulating air too fast.... allow time for absorption in the radiator before recirculating air back to solar heater. Will require some fairly "smart" control or maybe a multistage fan control High (full sun) Med (partly cloudy)...etc. This will likely require monitoring both water and air temps.
   
5. Air needs to recirculate as a closed loop... may need to use some desiccant to remove moisture and avoid "fogging" inside solar collector.
   
6. Flapper doors... block airflow everywhere unless fan runs.... this prevents re-radiating your stored heat and heating the solar collector. I might even consider a couple of small solenoid operated flapper valves to block air flow when no solar heating avail.
   
7. Very tight insulated box around radiator... lots of duct tape (or duck tape) everywhere... (explanation here http://www.ducttapeguys.com/duckvsduct.html )
   
   

On a tour of an off-grid home in SantaFe NM last year I saw a "ducted" solar air system used to heat a "Storage" wall which radiated the heat back at night. They used 6" duct and a bathroom exhaust fan controlled by a thermostat.(collector was on roof) About as simple as you can get.

Just my thoughts,

Juddley

[juddley]

Hi Woof,

His first line is "...making another kind of Solar Panel for Water Heating..." so I assumed he is using heated air circulated through a radiator to then heat water... I can see the advantage, especially if he is in a freezing climate. The radiator and all water plumbing would be indoors avoiding need for drainbacks or secondary heat exchange/transfer fluids.

Juddley

[paborralho]

I'm sorry i didn't attach diagram before, hope its easier with this one.

The objective is to heat water. Air is the "media" that will transfer the heat from the sun to the water.

Working with air seems much easier than water (especially if you are working on the roof). I have now one solar panel, (much more tradicional) on the roof, besides of 2 water tanks (hot and cold) and lots of piping. Once or twice i've had problems with losses and its scary as the damage could be big if all the water came out. On the other side, losing a couple of dm3 of air presents no danger.

Another advantage of this system could be that you can take some of this hot air to heat rooms.

Thanks for your interest!

Paulo

[paborralho]

Hi Juddley!

Thanks you for you comments.

- Water pump: I've had a "washing machine water pump" working for an year (solar to floor heating), and it broke one freezing night, otherwise it would be still working. In this case I dont think I could use one because it means too much flow. I was thinking of a aquarium pumb - some can be regulated, the problem is it must be submerged and I dont know how long it can work between services.

I'll try the gravity thing for the begining but I don't trust much on it. I think it is too slow and losses begin as it needs more heat to work, rigth?

• 3 stage fan control: Good idea! I'll think a lot on some simple circuit for that , maybe with 2 or 3 thermostats, like 40°C, 50°C, 60°C, and resistances in series with the ventilator.
  
• Flapper doors: these could be mounted inside the  solar panel, right?
  
  

I think the eficiency of this system relys on the radiator, ventilator regulation and insulating.

(Pressure is no problem in my case, it is tank level)

I kept thinking on that Storage Wall, it's a good idea, too. Or a storage "bunch of rocks". Good thing is that you can heat almost anything with hot air.

Thanks again,

 Paulo (Chile)

[paborralho]

Air Speed - is Ventilator Speed Control useful?

Thinking better, it seems to me that ventilator speed would be no problem, since too much speed would only lower the air temperature if water is circulating too fast. The opposite - too slow air - may restrict the maximum energy that can be transmitted to the water. So I wold say that the ventilator can work always at its maximum speed.

Is this right?

Paulo

[juddley]

Hi Paulo,

Nice drawing by the way, It is exactly what I had envisioned from your description.

First, A washing machine pump?? WOW that can be as much as 17gpm.... thats a lot of water flowing. I have tried running them at low rpm using small 12v motors and you don't get much useful flow out of them. I think they rely heavily on centrifugal force.

So if I understand, your system tank isn't pressurized??

So here is my best effort at an explanation of thermodynamics involved. The greatest exchange of heat (heat transfer) occurs with the greatest temperature differential. If the air circulates too fast then the temperature "equalizes" in the system this lowers the temperature differential between the "absorber" (your small radiator) and the air temp. Not good. you want to take the highest temp air you can out of he collector and run that across the absorber.

Same holds true for the water circulation. If you circulate the water too fast then the water temperature equalises throughout the tank and absorber. This causes a higher absorber temperature and lowers the transfer rate. If you allow gravity to circulate the water then you will get the coldest water out of the bottom of the tank and that gives the greatest temperature differential. Keep the radiator as close as possible to the tank and make sure supply's are as straight as possible and no smaller than the dia. of the tubing in the radiator. Use ball or gate valves and not washer type supply valves. If you don't have any system pressure you can get good cheap pvc valves used in pool construction.

You asked...... "Air Speed - is Ventilator Speed Control useful?"

Temperature differential becomes more of an issue as you approach equilibrium of water and air temp. So if the air is circulating too fast you will get an air temp about the same as the water temp. Circulate the air slower and the top of the collector gets very hot then you can get a few more degrees of water temp:-)

You asked...."Flapper doors: these could be mounted inside the solar panel, right?"

Yep, thats where I'd put them. Would flatten out some aluminum Coke (or Beer)cans to get light thin sheet stock.

I ran your idea past a friend who does HVAC designs and he suggested... use a small 12v solar panel + muffin fan to circulate the air. I think the fan would have to be in the "cold" (return air) side as you have drawn. His thought was that if there is sun the air circulates. Muffin fan might melt in the Hot (top) side.

I also might suggest you consider using corrugated roofing painted black in the collector and circulate the air above and below the metal sheet to get better solar to air absorption. This is used in almost all heat absorption air panels I have seen.

Your idea of using the same air to provide space heating would work if you would wait until you get the water temp where you want it then divert excess heated air to space heating..... however... your control system suddenly got a whole lot more complicated. Also you would then need to filter the air to keep dust out of the collector which will reduce its efficiency. Also if the moisture level is high in the structure then collector fogging and residual condensation in the system becomes a problem.

Another thing you may want to consider.... depending on age and source of engine type radiators is they have high lead content. Very bad if the hot water is sanitary and probably not good for you bathing in it either. Back when I did the Laundromat system we still had lead plumbing in service in many homes in US...LOL "The good ol days...."

Keep me posted on your progress....

Juddley

[feral air]

Photo resistors are pretty cheap and you should be able to make one vary the fan's speed...why not?

That or you might be able to pull a temp. sensor from a junk computer. A temperature sensor would probably be the better way to go as long as you could keep it from oscillating too much.

I dunno, just tossing out ideas.

[divebuddy]

Great design. I was also thinking of a solar fan, a 12 volt water pump would also ensure it only works when there is solar activity. Dont forget a check valve in the pipe to the heater.

[fcfcfc]

Hi:

The biggest difference between air collectors and liquid collectors is collection efficiency. Apples to apples, liquid collectors are more efficient than air. The hotter the air in the collector the more heat you will loose back out the glazing, which is your only "true" loss. Remember in space heating mode, heat lost to the inside of the house is not a loss, just to the outdoors. There are so many factors to determine the right way to design your system, its almost to complicated for this mode of communication. Where you are, how many in your household, any unusual DHW loads, orientation, access, distance between the collector and the storage, distance between the storage and the backup storage, etc.., etc.. DHW by itself isn't to bad, but add apace heating and it gets allot more complicated. You should be able to get an 80 gallon preheat SDHW system installed for 3.5K to 5K, all good quality components and no bother after the fact. If however you are just looking for something to try and tinker with, than an air/liquid "mix" might be a good place to start...

.....Bill

[paborralho]

Hi Bill

there are 2 main points why I started to think in this manner instead of putting water pipes inside the panel. One of them was efficiency. In the usual solar panels (at least the homebrew ones) water pipes are separated, say 8 ou 10cm (copper is really expensive these days). This space, in my oppinion is one of the main reasons not to get the 100% efficiency. On the other hand: air inside the panel gets hot, unless there is no air, right? that is another loss, as I see it. And the hot air tends to go up, away from the metal surface. So, wherever I look, I see "hot air". I'll try to get most of it to make hot water. I may be wrong, when we put our hands in the dirt, we find many new unexpected things. So I do hope it will get some efficiency. And it would be lot cheaper and easily upgradable (more m2 of panel).I would be happy with some 1kw/m2. Do you think it is possible? I dont even know what's the usual efficiency. Once got that with a policarbonate panel, but had problems with water losses.

By the way, english is not my languagge, what is DHW loads?

I'll not try any space heating, I already have plenty of glass to the north (I'm in South Hemisphere), so I'll just heat water.

Thanks for your comments, I'm now preparing the radiator from an old propane water heater.

Paulo

[fcfcfc]

Hi:

If DHW (Domestic Hot Water) is the goal than I would go liquid collector. Buy a copper collector plate (Absorber) and build the box or get a used flat plate collector, usually easy to find with a little patience. Just use two pumps with a flat plate exchanger and an 80 gallon electric water heater as a preheat tank (not connected). That will give you the easiest system in a freezing climate. Use 50/50 prop glycol in the collector side with an expansion tank, fill valves etc.. There are alot of schemitics out there. That will give you the best results...

.....Bill

[desertcoyote]

I built an ran a similar system for 5-6 years. I used a mechanical thermostat in the "hot-box" for start-stop the blowers, and a second thermo on the output from the air-water heat exchanger to enable the circulation pump. I used these large surplus air handlers with back-to-back 3'x4' radiator heat exchangers. The pump would go on and off so that only premium hot water water was pumped. I was able to get as much as 70F temperature differential across the heat exchanger. I had 250Gal of storage and commonly was able to heat all of it up to 105F.

It worked well but finally decommissioned it, thinking I had a better idea... just no time to implement better idea yet.

I used EBM-Pabst Reverse impeller blowers that pushed air into the panels and pulled air through the heat exchangers. These (AC-type) are very reliable and never gave any problem. The main problems were the bimetallic type thermos would get memory and would need to be re-tweaked. and that DC brush type circulation pump got old and became a maintenance hassle, dirty commutator new brushes... PITA.

Need to carefully consider the blower CFM / Back pressure characteristics of the heat exchanger. Propeller blowers do not have the muscle to push the air, and squirrel cage  blowers are horribly inefficient. These inverse impeller was my best comprise, however they do reside in the air stream, so when a thermo sticks, the motors get bathed in 180F air.  

Note: carefully select the heat exchanger also, One of my earlier models used a surplus aerospace aluminum heat exchanger that became corroded and clogged after maybe one week. Hot water is fairly corrosive.

[paborralho]

Thanks,

Obviously there´s a lot to learn from others experience. I´m considering no water pump at the begining, just gravity, with a smooth oneway valve. Is this too bad? My radiator doesn't have vertical loops, I expect no air will be trapped.

[paborralho]

Hi Guys,

Don´t know if the same happens to you. The summer will pass (south hemisphere) and this thing won't be ready...

Am starting the radiator part,

radiator: from propane water heater

fan is 220v, from very old computer or printer (from the 80´s ?)

metalic box is from 2 PC computers box,

waterproof base is a simple iron-zinc sheet, shaped by hand, ready to receive any condensation.

This box will be put inside the roof under the solar air heater, with good insulation.

Note: in the photo of the naked radiator you can see a change: the outgoing tube was cut and lifted to avoid vertical loop. There is still a piece of the older output. I won´t put any water pump, unless it is needed.

Hope I´ll have time to the solar panel part soon.

Bye

here´s the comments on the radiator

[paborralho]

Hi,

don't lose time reading, pics speak for themselves

electronics are good, but they need current. Had an old iron, and took the thermostat out. In almost all electric equipment the thermostat cuts when it reaches certain temperature. Is solar panel we need to get things working when it's hot. After some analisys of the mechanism i discovered the bimetal plate. These things are pretty accurate for what we need. I figured that if is was inverted, it would work fine.  And it worked! Besides, calibrating was very easy, cause it as a screw for that. . I'll put this one inside the solar air heater to start the ventilator. I'll calibrate it for 55ºC or maybe a little more for the summer and some 45º in winter. What do you think?