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dbc

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Reply with quote  #1 
Here is a concept to run a hot air collector fan off a 12V PV panel:

2) Concept 02 - DC fan, 100219.jpg 

The PV panel could be mounted next to the collector.  The whole apparatus could be outside, which would allow very short wiring runs.

I've got 4 of these Kyocera panels, sitting idle in my defunct off-grid system (batteries died after 11 years).  I don't know if one panel is sufficient for a right-sized 12V fan, but I could wire 2 in parallel if necessary.  Thinking of a heater fan from a pickup truck or van as a starting point.  The fan could go in a purpose-built box, with suitable in/out ducts.  The relay and fuse could also be outside in a separate compartment or box.  Only thing running into the building would then be the air ducts.

I would probably mount the PV panel vertical (no tilt) to minimize the chance of snow build-up.  Wouldn't want the snap-disc to call for air flow while the PV panel was blocked.

I also don't know if the unregulated PV output would cause problems for the fan or relay.  Adding a voltage regulator would be an unwelcome complication.

gbwillson

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Reply with quote  #2 
Don-

I love the idea of a solar powered heater. Powering a 12v fan or fans directly from the panel or panels certainly simplifies things. It also has the advantage of being self regulating in that the fans will spin slower or faster as the sun and heat output dictate. 

I have had this idea to power a ZP with enough 12v fans in parallel inside the intake manifold along the entire width of the intake slot. If I come across a PV panel rated for about 100 watts or so I likely give it a try. DigiKey electronics has a huge selection of 12v fans with different CFM, wattage, and pressure ratings. 

On paper it would be possible to achieve the high CFM numbers a large ZP heater needs. It might be something where in the designing of the heater, an easy access panel could allow you to add, subtract, or adjust the configuration of the fans in the intake manifold. 

Sounds like fun...

Greg in MN
dbc

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Reply with quote  #3 
Greg - That idea occurred to me also.  I did some research last year on Allied Electronics site (I sort of know someone there; used to order quite a bit from them back in my working days).  DigiKey probably has an equivalent range of choices.

What I couldn't tell is if the static pressure rating would be adequate for a zero-pass.  The smaller DC fans (sometimes referred to as computer fans) come in 80mm, 92mm, and 120mm sizes.  That translates to 3.15 in., 3.62 in., and 4.72 in.  It sure would simplify things if they could inside the intake manifold.  Kevin did that on his 4x10 tube collectors (which had 7 in. deep frames), and I believed they worked well for him.  I think Bert tried it a few years ago on a tube design.  As I recall, he thought the airflow was too low and added another larger single fan on the output side.  I haven't read that thread for a while, so I'm sketchy on the details.

I also thought about mounting a row of fans on the back, facing forward with a curved piece of sheet metal in the manifold to direct the airflow into the screen gap. That would require an extra housing to enclose the back of the fan assembly - added complication.  For whatever reason, I set the whole idea aside, but I'm still curious about it.

Backwoods Solar is offering the Vikram Eldora 100P panel for $135, the best price I've seen for new with a waranty.  It's a 100W, 12V (nom.) panel.  Backwoods has been around a long time, since when most systems were off-grid.  They still largely focus on the off-grid market.  I attached a data sheet for the Vikram panel.

Disclaimer - I have no affiliation with Allied or Backwoods Solar; just sources I would buy from.

Late Edit:  I remembered one of the reasons I didn't pursue using a row of DC fans was that I only had 2 5/8 inch depth to work with inside the collector.  It might work better with a 3 5/8 inch internal depth, which would be available on a track frame with externally-mounted back sheet (per my other recent hare-brained thread in this section).  The 80mm and even the 92mm fans would theoretically fit, although the 92mm wouldn't leave much room for a mounting plate.  Another unknown is optimum distance between the fans and the edge of the screens.  Seems like you wouldn't want too much of the fan to be closely blocked by the screen frames.  Maybe a venturi of some sort?

 
Attached Files
pdf vikram100pv4datasheet.pdf (1.38 MB, 3 views)

dbc

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Reply with quote  #4 
In the absence of any best-practices details for a row of DC fans in a ZP collector, I would probably start with something like this:

4) Deep frame with 80mm DC fans, 100519.jpg 

The intake plenum would be longer than what you would typically have with just the intake duct.  I came up with a somewhat arbitrary 16 inches.  The first 9 inches would be similar to a normal plenum, with just enough room for the intake port.  (You could have 'mini back screens' sitting against the back, although I didn't show them for simplicity.)

The row of fans could be mounted to a plywood bulkhead, which filled the entire interior cross-section.  This bulkhead would need to be attached somehow to the sides with a piece of angle or something.  I didn't show that detail either.  You'd also have to manage the DC wiring, but that wouldn't be too hard with a J-box on the back (have that already for the snap-disc).

On the downstream side of the fans, you could have another sub-plenum, where the pressurized air could mix and fill the enclosed volume, and push into the screen gap.  I drew this sub-plenum at 5 1/2 inches wide.  I have no idea if this is optimum, maybe it could be less.  It just didn't strike me right to put the fans right up against the absorber, since part of the fan aperture would be blocked by the screen frames.  Now if you had frameless screens, or even just a frameless front screen, you could probably move the fans closer to the screens without blockage.

I'm thinking about re-building my 2x16 collector as a one piece, using a frame like Solar Dan designed a couple years ago.  if I combine this with the 'deep frame' modification (polyiso back-sheet behind the track frame), I could enlarge the screen gap to 1 3/8 inch, and try this DC fan setup too.

Another unknown is whether a 2 ft. high collector leaves room for enough fans.  i figure there would be room for 6 ea. 80 mm fans (3.15 in).

All of this would probably be next year, if I'm ambitious enough and have the time.


gbwillson

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Reply with quote  #5 
Don-

I think you may be able to get away with bigger fans, either by angling them slightly or recessing them into the back foam. Some sort of smooth transition directing air into the screen slot would help too, being the fans are more or less directly pushing air into the slot.

And for that matter, since your collectors are permanently mounted, do you need to have them so thin? 

Greg in MN
Irishvoyageur

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Reply with quote  #6 
I am finishing up on a long narrow solar-powered SAH (zero pass with 3 alum screens) mounted below the windows of my porch. I use a 7 inch automotive fan with a 60 watt panel mounted inside the collector. I have the option of adding a snap switch, but right now, I see no need. If the sun is powering the fan, the collector is warm. My estimated air flow with the panel at full sunlight is about 110 cfm. If I powered the fan directly with a fully charged 12 volt battery, it would be about 190 cfm. However, the fan noise increase is substantial and my initial BTU calculations show a lower BTU output (less efficient). I have not added an intake filter yet, so the air flows will be lower than my initial testing. Initial testing yields a delta T (outlet temp-inlet temp) of about 60F at an airflow of 110 cfm and about 30F at 190 cfm. I will provide more details, including construction photos soon.

Attached Images
jpeg IMG_4203.jpg (855.19 KB, 22 views)

gbwillson

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Reply with quote  #7 
Nice looking collector build Irish!

Nicely done making your SAH look like it is part of the house rather than something stuck to the side of your house.

It might be a bit early yet to get an accurate picture of the CFM output due to the still high angle of the sun. It will get better as the sun sinks in the sky, and the addition of snow on the ground in front of the collector. Placing the solar panel inside the glazing really adds to the integrated look. Remember though, that twin wall glazing cuts out roughly 18-20% of the sun light reaching the PV panel. An externally mounted PV panel would have provided more power to the fan. But I think the sleek look of your build is worth the reduction in fan output. 

Speaking of CFM's, the fan will be dependent on the sun, and early and late in the day, will have a very slow fan speed. And while there may be enough sun to power the fan, the interior of your collector box will be an ice cube for several minutes while the fans are turning slowly. So you may end up blowing frigid air for a few moments, especially in the morning. You may need a certain fan CFM to open the back flow valve. A DeltaT of 60˚F seems a bit high. Being a porch, with all the glass, will likely offset such a temp gain. BUT, if you see output temps in excess of 120˚F, you are not only wasting heat, but you will begin to breakdown some of the collector components over time. But isn't having too much heat output a really nice problem to have in winter? You can always open one of the windows.[wink]

Again, great looking collector! I look forward to seeing some of the BTU numbers as we transition into the colder months.


Greg in MN,
Where a blizzard bringing up to 40" of snow are forecast later this week to parts of the state! Winter's Coming!!!


Irishvoyageur

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Reply with quote  #8 
I have the option of adding a second solar panel that will be mounted in the window of my porch should more power be required or desired. I suspect more power will be needed once I add an air filter just before the fan.  I built a small pilot screen collector last year and mounted it in a window of my porch. It was powered by a 10 watt panel and used a 4 inch PC fan.  It never delivered cold air. By the time the solar panel provided enough power to power the fan, the collector was warm. I hang a double layer of shade cloth over the inside of the porch windows  during the winter for passive solar heating. Once the porch warms up (>70F), the heat is circulated into the house through a window and door. The addition of the long SAH collector will provide even more BTUs to the porch. The porch is essential a very large solar collector with a lot of thermal mass (brick chimney and stone flooring). I built a 16 inch solar powered fan that I plan to mount in the window of my house that will push heated air into the house. Air is circulated through a partially opened door on the opposite side of the porch. The window fan will be run with a separate 160 watt panel that will be mounted outside. 

stmbtwle

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Reply with quote  #9 
The porch idea works very well. For years I've used a small $10 box fan to blow cool air from the house into the enclosed porch, displacing warm air into the house. I keep telling myself I'm going to put a differential switch on it.
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Willie, Tampa Bay
SolarInterested

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Reply with quote  #10 
Quote:
Originally Posted by gbwillson
... Remember though, that twin wall glazing cuts out roughly 18-20% of the sun light reaching the PV panel. An externally mounted PV panel would have provided more power to the fan. ..
PV panels are also more efficient at lower temperatures.

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Both temperature rise and airflow are integral to comparing hot air collectors
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