Air to Water Solar Heater - How I did it

[paborralho]

This is the second attempt, last one had too big photos.

Working at Last

If lack of beauty is a sin I am a sinner when DIY. But you can always do it prettier.

The thing is that it works. It is mainly done with scrap and it supplies hot or warm water to bath and washing.

Air is easily heated by the sun, easier to transport, and ducts are made with waste. Ventilators are everywhere.

Exchanging heat is also very easy with a radiator.

Let's see the pictures, I think they are self explained.

this picture is an improvement to an old one. Here, the water pump is an Aquarium air pump that simply "bubbles" into the rising hot water.

I tried before with just gravity, result: Really Hot Water (60 Celsius), but slow flow.

If you dont believe this works, try blowing with a straw into a 1/2" pipe filled with water. Here you don´t need gallons per minute.

Dimensions:

solar panel: 3.6m2

tank:120lt

ventilator: one 11w 220v + four 1w 12v pc fans

radiator: from car air heater

The Panel

For the panel I used the very roof of my house, it is 0.3mm iron, zinc coated, painted with ordinary black spray (no glow)

All glued with silicone. Quite heavy to do it all alone, just my dog watching.

"Mano! Porque estás täo longe?"

as you can see I glued the glass to the roof. Now I think It would have been easier to do the complete box and put it up. It was cheaper this way.

I also made holes in the metalic roof to allow the air to flow up and down.

This box is closed, air circulates in a closed loop. This way you avoid condensation.

I have no pictures on the inside, it is a wooden box, witch rear panel looks like this:

In This Panel I made 2 holes 10x10cm and put this in and out made of thin metalic plate:







The Turbo

This is the second turbo I made. The first one was with a radiator I took out from

a propane water heater.

Now you'll see the 5 blower and radiator that makes the heart of this:

This radiator I found in a car junk (10 bucks), it must be from the heating system.

I first proved to see if it had losses. It can surely handle 1.5m of water pressure,

but you cannot connect it to regular domestic water pressure.




4 12v Fans from Computer Power Supplies, they are easy to find, low wattage, can be connected to a Photovoltaic Solar Panel.

I also had an old fan 220v, 11watt. I put all these inside a box, like this

This is the air pump connection, a needle into the rubber pipe. It can only be done

if you have no pressure in your tank.

This air pump is the ones used in aquarium and can't do more than 1m of water pressure.

So it must be inserted no more than 1m above the water lever.

I was surprised to see how much water it pushes. you must also have an opening on the top of the tank.

I could have done some other pipe inside the turbo to heat this air before joining

it to the water, but air is only 1/800 of water density....

The Thermostat

this is a transformed thermostat from a clothes iron, you must invert the bimetal plate, It goes inside the hot air box. I can even regulate the starting temperature. You would laugh if you saw it.

And that's it, I have no calculations of how many watts I am taking from the sun,

but in sunny days the water gets pretty hot. Now autumm has started And I still have

hot showers in the night. In the morning the temperature of the water in the tank is

almost enough to shower(35 Celsius more or less), but the house is colder, so I prefer to

shower at night. In cloudy days it only warms the water.

This house has lots of solar light as you see in the first photo, and it has another

solar panel to heat the floor: a typical panel with copper pipes. Now that I

confirmed that this system works I will improve this panel with a radiator and a

4.5m2 air panel.

In deed I started thinking of this air to water system because inside this solar to

floor panel the air heats a lot even if the water comes out only warm. I think most

of the losses are due to the hot air. In an air to water system you keep the air

temperature lower.

Another advantage is that the whole water system is far from the outside, thus it

will never freeze.

I hope you understood my english.

Thanks for the comments to my first posting, last year when I was thinking of this.

Hope you enjoy it, and helps you doing something like this. Make comments.

Paulo

"One day the sun will extinguish, make your solar panel today"

[Capt Slog]

I'm glad that you resized all this and reposted, it's very interesting to see.  You've obviously made the best use of what you had, which I think is great, it means that you don't have to wait for so long for it to start to pay for itself.  As someone commented on one of my posts "When life gives you lemons, make lemonade".

I'm impressed enough to consider building one myself.

I would be a bit worried about the holes in my roof, and I wondered if you'd considered making the air ducting part of it on the inside instead?  You could make the box on the inner face of your steel and paint the outside black.  The box ends would have to be cut to match the corrugations of your roof or you could use something like this foam strip to make the closures.

http://www.greenhousemegastore.com/prodinfo.asp?number=PC%2DCL%2DH

The air bubble pump is a good idea.  Have you had this running for some time and has it encouraged algae to grow?

[hvirtane]

Hello,

in my opinion your design idea is exactly the correct one. I've been advocating the design principle to circulate air instead of water for many years, too. It is much cheaper to build and more reliable to operate than circulating solar hot water on your roofs...

Please tell more about air pumps. Are there available other suitable cheap pumps?

- Hannu  

[Ungrounded Lightning Rod]

If you put the radiator as high as possible in the loop you might find you don't need the fans.  By cooling the air near the top of the return duct you'll make it heavier, driving the air by convection.

Downside is you have to pump the water farther.  B-)

Of course if the water tank is in the attic you can leave the radiator near the bottom of the return duct (and at or below the bottom of the water tank).  Then you can keep the fans to circulate the air and let convection circulate the water.

Putting foam insulation around the plumbing between the radiator and the tank will help water circulation by convection, as may a "thermal break" fitting - the "energy saving" insulating fittings you can get to go where the flexy-pipes hook to the water tank, to interrupt thermal conduction through the pipe into/out of the tank.

By the way:  Your English is doing just fine.

[ghurd]

The $6 household fish tank air pumps work quite well, and use very little power. Some only 1W.

My experiments were surprisingly good.  They move a lot of water for a little power, at very low head.  

It works better with an air stone to make tiny bubbles, instead of an open hose and large bubbles. I simply dropped the air tube and air stone 18" down the 3/4" pipe.

It sounds like this used a needle to get into the riser pipe, which would make small bubbles.

I would like to find a cheap efficient 12VDC air pump that would last more than a month.

G-

[BigBreaker]

You can connect the intake of the air pump to the top of your water tank and make the system closed.  The air will be very humid, perhaps an issue for some air pumps, but the air will be as hot as your water - an advantage.

There was a suggestion that an air stone will make the pumping action more efficient.  I do not think this is the case.  The displacement of water by air makes the riser column lighter than the water pressing in from the bottom port regardless of bubble size.  Perhaps the issue is at the exit of the riser?  A better exit port might be necessary with big bubbles.

[paborralho]

I would say my problem with this is too much water flow, as the sun goes weaker water starts to get colder. I'll put another thermostat to check water temperature. I must found one or make some electronic one, which wont be hard once the pump is ,as you said, low power.

Thanks for commenting

PB

[paborralho]

Thank you!

This was the cheapest one I found. It is a membrane pump, not centrifugal: two valves, one piston, moved by a magnet near a coil. I think this one can function for several months before I will have to replace the rubber membrane (bought two just in case). I also saw one operated by batteries (for emergency). If you buy one it is important to check what's the highest water head it can do. There are some models with more than one "out".

You're right, working with air is so much easier, and you can use any materials.

Thank you for commenting!

[paborralho]

Thank you,

I also saw and liked the Lemons phrase. It is so simple and yet so difficult to follow sometimes...

Air on inside to avoid hole in roof: Yes, in the beginning I had it just on the inside, and with no glass on the outside. I feel that if you take both lower and upper air, you will get almost the double of the energy. If (you also have mettalic roof and) you make it just up to the top it will be easy to prevent rain from getting in.

other thing is that I made the holes on the upper part of the waves, where water will not get if it filters.

Algae? good question... do not know. It is a copper tank. I heard that copper has some antibiotic characteristics. Water comes from a well, no chloration, it could be more alike to grow that kind of things, maybe some one else here should talk about this.

Thank you for your comments

Paulo

[paborralho]

this is the needle, something passed to the photo

[paborralho]

Maybe I am not understanding the whole thing. the radiator on top would avoid any gravity  water flow (as I had it inittialy - but was too slow). It would also have been difficult to use the air pump - you cannot let air get in the radiator. If I put the radiator inside the panel it will be more likely to freeze in winter. Experience tells me that natural convection is not enough, so much for the water as for the air. It is much too slow.

What is "thermal break"?

Bye, thank you for commenting.

Paulo

[scottsAI]

Paborralho, Nice job.

Please note the purpose of the glass above the metal is to insulate.

The Glass is not a good insulator, the still (dead) air beneath is the insulator.

The R value of the dead air space between the glass and collector increases as the distance increases peaking out at 5/8 inch = R2, further increases adds no more to the R value due to the increase in natural convection, R value stays at R2 even for large spaces. R value is improved by dead air space.

Moving air above and below the metal should not double system efficiency, moving more air will improve efficiency so the effect is noticed by doing both. Cold outside temperatures you will see a dramatic drop in system efficiency when moving the air beneath the glass. On cold days with no wind it may work fine, windy days not so good. Heat exchanger efficiency is improved by increasing the surface area which using both sides of the metal would seem like its doing. The metal is so thin I do not think this is what is going on., moving the air faster below only should give the same result. (with cold day improvement).

Green houses use polycarbonate sheets (corrugated), usually lower cost than glass.

Have fun,

Scott.

[GaryGary]

Hi Paulo,

Nice job -- very creative!

I'd agree with Scott on running air on both sides of the absorber.  The data I've seen says that for the same total airflow, you will get more heat (more efficiency) out of a backpass collector than one that passes air on both sides.  

Some baffling to make the airflow sweep the full surface of the absorber might also improve performance:

http://www.builditsolar.com/Experimental/AirCollectors/AirCollectors.htm

Just for information, there is another scheme for using an air collector to heat a hot water tank.  Its described in Solar Air Heating Systems:

http://www.builditsolar.com/Projects/SpaceHeating/SolAirHtSysBook/SolAirHtingBk.htm

Basically they take the outer sheet metal shell off a standard water heater, then remove the insulation.  Then they take aluminum bands and fit them around the tank -- the bands have their edges up so they get into the airflow path.  They probably did 20 ish of these bands from the top of tank to the bottom.  The band are clamped tightly to the tank.  I think that they used some kind of heat transfer compoud between the tank and the bands to increase thermal conductance.   After the bands are in place, they reinstall the outer sheet metal cover.  So, in use, the air from the collector flows in the space between the tank and the outer sheet metal shell.  The bands pick up heat from the airstream and transfer it to the tank.  Not sure which scheme would work best, but just thought you might be interested.

Gary

[paborralho]

Hi Gary and Scott

It's great to hear from people who have lots of experience!

You say: "you will get more heat (more efficiency) out of a backpass collector than one that passes air on both sides." Is this true at any circumstances? It should depend on the outside temperature and wind exposure of the panel, right? I believe you because you have investigated it. But one question keeps bouncing in my head: if the upper air does not circulate it will get hotter. Doesn't this increase the energy loss through the glass?

At this moment it is not very difficult to close the holes I made (a couple of hours). And I could put some fins too, to direct the air.

I also thought of making the radiator with the tank itself, but I had it already insulated, and it would have been more complicated, the tank is heavy, I don't have much space up on the roof.

Thank you, I'll read the paper carefully.

[GaryGary]

Hi,

Yes, it does depend on outside air temperature and wind.

I guess that one rough way to think about it is that for the convection/conduction losses out the glazing, is that the R value of an air film varies from about 0.65 for a still air film down to about 0.15 for breezy conditions.   So, under breezy conditions, for a collector with air passing between the glazing and the absorber, the total R might be as low as (0.15 inside + 0.15 outside) = 0.3, the corresponding backpass would be 0.65 + 0.15 = 0.8.   But if you had still air outside the collector, you still have the 0.65 R still air film on the outside.  So, the backpass R glazing might vary from R 0.8 up to R 1.3, and the front pass might vary from about R 0.3 up to R 0.8.  This is kind of a rough way of looking at it, but it does give a rough idea of the potential for more loss through the glazing on the front pass.

As you say, the outside air temperature makes a big difference as well.  In the extreme, for pool heating, collectors that are not glazed at all have better efficiency than glazed collectors as long as the air temp is higher than the pool water temp.

I'm not sure how you would figure out which is best for your situation.  I guess you could measure the output both ways -- it does not take too much instrumentation to do this:

http://www.builditsolar.com/References/Measurements/CollectorPerformance.htm

Let us know how it comes out.

Gary

[ghurd]

Could the needle be moved higher up the pipe?

G-

[paborralho]

The higher you move it the more bubbles it will make (less water column). I suppose it will increase flow. Anyway I now found that the flow was too much for the sun: water wasn't heating more than 35°C. I got back to thermosiffon. I think when summer comes (that 'll be in December around here)  I'll put the new "adapted washing machine thermostat", which will turn on the air pump when water gets more than 45°C.