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Irishvoyageur

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Reply with quote  #11 
I am aware of the light attenuation by twin wall polycarbonate. In addition, vertically mounted panels are less efficient as well. It is a simple matter to add an additional panel inside my porch window to provide additional power should that be necessary. The glass has little impact on light attenuation. I like the external look of the collector and it works well, even with the internally mounted solar panel.  The 60 watt panel inside the SAH was placed opposite the intake to provide some cooling to the panel and the heat is exhausted with the airflow. I will likely need another solar panel once I add an air filter.  I tried adding a second solar panel today (30 watts). At noon under full sun at an outside temperature of about 60F, the following data were obtained:

Intake Temp   70 F
Outlet Temp  123 F
Delta T           53 F
Airflow        ~156 cfs
solar PV output 53 watts (2.57 amps @ 20.8V)

I will do some testing later this season when the sun angle is lower and the outside air temperatures are colder and with the addition of an air filter before determine the size for the second panel.

dbc

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Reply with quote  #12 
Greg - No, the collector doesn't need to be as thin as I have shown; I was mainly trying to work out something for a 3 5/8 inch stud-track frame.

That said, it seems to me that the ideal depth for a ZP frame would just leave room for the screens, the screen gap, and whatever additional depth is needed to maintain adequate conductive isolation between the absorber assembly and the glazing / back panel.  Extra depth beyond that would just increase the internal volume and surface area (and material cost) to no useful purpose.  I certainly don't have any proof this is true, it's just a hunch.

The cheapest and easiest screen frames in my limited experience are 1x2 wood.  Wood is not especially conductive (thermally), so - if the frames are positioned right against the back panel and glazing, which puts the screen 3/4 inch away from the surfaces, and if the screen gap is 1 3/8 inch, the ideal internal depth (according to my 'formula') is 3/4 + 1 3/8 + 3/4 = 2 7/8 inches.  Add a 1 inch polyiso back panel, with no structural reinforcement, and you're already 1/4 inch too big to fit into a 3 5/8 inch track frame.  That's what started me on moving the back sheet outside the frame.

Back to the DC fans - I agree that bigger fans, such as 120mm (4.72 in.), would be better.  Tilted sounds like a good idea too; it may help direct the air at the optimum angle.

Assume, worst case, the fans were mounted perpendicular to the screens:  You would need around 5 inches minimum internal depth.  You'd have to tilt 120mm fans pretty far to fit them into a 2 5/8 inch space.  3 5/8 inch might be doable, although the fans themselves are at least 1 inch wide, which eats up some depth when tilted.

Alternatively, you could get there with 3 5/8 track stacked with 1 5/8 track, if you mounted the back sheet externally, internal depth would be 5 1/4 inches.  If I was going to the extra trouble of adding a second track, I would go with 3 5/8 inch + 2 1/2 inch track and move the back sheet inside the frame, giving an internal depth of 5 1/8 inches with a simpler to build assembly.  Tilting would then be optional.

One more idea would be to add a 'bump-out' to the intake plenum, leaving the rest of the collector just a single track-width.  Here's a rough sketch (I showed the add-on section as 3 5/8 inch material):

5) Frame with deep intake plenum, 120mm fans, 101119.jpg 

None of this is exactly to scale, and you would need to set the fan angle for best transfer into the screen gap.  You'd want the sheet metal baffles to have the best angle between the fan and screens.  All that would be pretty easy to adjust during construction.

Don't really know which design would work best or be easiest to build.  Sorry to ramble on.

Next up, Irish Voyager - I'll second the above comments about your collector; it looks really sharp.  Well integrated into the setting too.  Interesting idea too about the enclosed porch acting as 'thermal accumulator', especially since you keep the full benefit of the porch windows in addition to the collector.

I'm a little puzzled about your airflow readings with the 60W PV panel vs. 12V battery.  Your airflow with the PV panel was only 58% of what you saw with the battery.  This would indicate the voltage was low out of the PV panel, yet you stated elsewhere your panel was running at 20.8V / 2.57A.  Is this a 36-cell panel (most 12V panels)?  20.8V is typically pretty far down the current/voltage curve from maximum power point, even at STC (standard test condition) temperature of 25C.  At operating temperatures above 25C (almost certainly the case inside your collector), it becomes increasingly difficult to achieve 20.8V even at open-circuit.  Most PV panels have a negative temperature coefficient, which means the output voltage (and power) drops as temperature increases.  My 12V panels are probably typical; their Voc temperature coefficient is -0.08V / degree C.  Don't know if you have this data for your panels, but you can probably find it if you have the model number.  I attached the data sheet for my panels, just to help explain what I'm talking about.  It sounds like you are doing what you can to cool the panel with intake air flow, which is good.

I'm curious what the current was when you ran the fan off the 12V battery.  It may be higher than what your panel can provide.  It could be helpful to add the second panel.  If that doesn't bring fan speed up enough, you could try moving the first panel outside the collector.  It would run cooler if nothing else.  Trade-off is that you'd lose the nicely integrated packaging you have now with the panel inside.  It sounds like you are getting useful heat out of the collector in any case.

 
Attached Files
pdf KD135GX-LP.pdf (739.54 KB, 0 views)

gbwillson

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Reply with quote  #13 
Just a thought...

Instead of moving the polyiso outside the back of the stud tracks, why not add an extension to the front of the collector box to hold the glazing? A rabbit cut into a piece of clear 1x1" hardwood might even make nice mounting for glazing. 

A larger intake would give you as much space as needed to accommodate the internal fans. It makes the box assembly slightly more difficult. The extra space might also allow for an easy access panel to the intake manifold. It would be good if you could easily add or subtract fans until you have the proper airflow since it's unknown how many fans of a given capacity you may need. But thankfully, 12v fans are pretty cheap, so it pays to have an extra or two on hand.

Greg in MN
Irishvoyageur

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Posts: 31
Reply with quote  #14 
DBC - Here is a quick summary of recent tests at mid-day in full sun. Lower solar panel output over rated capacity is due to vertical orientation, attenuation by glazing or glass and warm temperatures. My 12v fan is rated at 80watts but only approaches this power draw at start-up using a 12 volt battery.  Air flow through the collector is primarily related to the amount of available current (see table below). You will note that I have added a second solar panel (30w) that was mounted in a porch window (no screen). The addition of the second panel provided favorable results (lower output temps, adequate air flow and decent BTU).  I plan to repeat these tests at colder outside temperatures and after the addition of a an air filter before finalizing the size of my second solar panel. The 60 watt panel is a Newpowa monocrytalline. The 30 watt panel is a Richsolar monocrystalline. I am also including a photo of the completed project. It looks great (IMO).

Zero pass solar air heater 2.5ftx18 ft (Effective area ~39 sq. ft.)  
          
PowerTempTempDelta TCFMAmpsVoltsWattsBTUComment
SourceInOut       
PV 30w641326862118.1184600External solar panel
PV 60w66130641081.622.0357500Internal solar panel
PV 90w70120501562.620.8548400combined panels
Battery72114421823.212.5408300 

Attached Images
jpeg P1410168.jpg (427.31 KB, 20 views)

dbc

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Posts: 303
Reply with quote  #15 
Irish Voyageur - It looks like you have your operating point pretty well dialed in with 90W of PV.  As your chart shows, you can increase air flow by changing the power source to the 12V battery, which lowers the delta T, and you end up with nearly the same BTU/hr.  Good performance either way.

You really have my curiosity going.  I wondered how a 12V fan would react to a higher voltage (like from a PV panel).  I guess your fan is doing OK with 20.8V?  That's encouraging.

About the fan - you mentioned it was an automotive unit.  Is it a radiator fan?  Heater fan?  Also, where is it located in your system?  I gather it is connected to the input, but is it inside the collector behind the PV panel?  It's hard to tell from the photo how deep your collector is.

You mentioned earlier you are using a 3-level screen assembly.  Can you tell us any more about that?  I'm especially curious how you mounted the center fan.  Thanks, Don C.

Greg - You won't have to work too hard to talk me out of the external back panel idea.  It's always seemed like sort of a band aid to me.  Seems like the corners would still be vulnerable, even with ripped-track covers.  I'm still looking for ideas.

I, too, have thought about adding wood strips to the front face instead.  Don't know about the rabbet joint to form a 'pocket' for the glazing.  I've seen that done somewhere (maybe Gary Reysa did it), but I've never done it myself.  Hardwood would be great too, but I'd have to find a source of used wood; new oak and other hardwoods are really expensive around here.  I remember Bruce's hardwood screen frames came out really great.

One bit of good news - I just inherited second hand a used Ryobi BTS 20 portable table saw (actually a friend inherited it first, but he lives in an apartment, so he generously gave it to me).  It's missing the blade guard assembly and associated mounting hardware, but everything else works.  My search for the missing parts turned up a lot of 'no longer available' messages; I think this saw was last manufactured over 10 years ago.  Just have to be focused and careful using it (like any power tool).  Anyway it might prove useful for rip cuts, better than 'freehand' which I have been doing up to now.
Irishvoyageur

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Posts: 31
Reply with quote  #16 
I will provide construction details soon. Need to get my pictures in order. I use a 12 volt Maxon slimline automotive fan mounted at the intake on my porch. Two aluminum screens are mounted on the backside of the panel. The third screen is mounted 1.5 inches above the two screens forming the airflow channel. The intake and outlets are 6.5 inches in diameter. The frame is treated wood. It is 5.5 inches deep. I used two inches of isocyanurate foam on the backside of the collector, one inch iso was used on the sides. The glazing is 8mm twinwall poycarbonate.
gbwillson

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Reply with quote  #17 
Don-

I don't know anyone who uses a table saw that keeps the blade guard in place. It's like any power tool iand like you said, they bite you aren't careful. I wonder if you could use composite wood trim? Not the vinyl bendy stuff, but the one made from resigns and wood fibers. MiraTEC is a brand sold at Menards. A 1x2x16' board goes for $6. It doesn't split or crack either. I bet you could use a router on it too since there are no knots. Make a frame like the bottom of a cabinet drawer, where a groove or channel would hold the glazing. 

Whether you extend the front or back of a stud track frame is all a matter of preference. But I do think more depth is needed than a standard 3-5/8" track. Being your collector is solidly mounted and shielded somewhat from the elements, I think even with the back polyiso only protected on the corners it would hold up for several years. You could also cover the back polyiso with a protective skin, such as FRP. Seasonal moves really stress any build, unless it's very heavy and solid. But I may not always have help available when needed. My next build, my NEXT build, my NEEEXT build will undoubtedly have all this worked out.

A very light, but incredible strong frame, with the structural stiffness to hold up with repeated moves. My frame consists of 1/16th" thick tube walls and I have had all the pieces cut and ready to assemble since last fall. I'm just able not to complete the build right now. 

Greg in MN


gbwillson

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Reply with quote  #18 
Irish-

I'm looking forward to seeing the build. With you trying to heat what looks like  3-season porch, your heater will have it's work cut out for it. It might be like trying to heat a tent, as virtually every exterior surface will be cold, regardless of the sun coming in through the windows and the heater. But I had a 3-season porch years ago, and even without a heater, the air temp rose to about 40˚F while the sun was shining. Which in the middle of winter, sitting in the sunshine, felt pretty damn good!

Greg in MN
Irishvoyageur

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Posts: 31
Reply with quote  #19 
The porch has interior storm windows. I put thermal curtains on the east and west sides during the winter. I hang a double layer of shade cloth over the south windows to act as a solar collector. On sunny days, the porch heats up nicely and acts like a very large solar air heater. The air temperatures get warm enough to pass the heat into the main house for several hours in the late morning to afternoon. There is enough heat to keep my furnace from kicking in. So far my purpose and use, heating the porch works great and I wonder why you don't see the postive side of this activity. Further, this project give this retired scientist a project monitor and study. :)
gbwillson

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Reply with quote  #20 
Irish-

I'm far from thinking your build is anything less than gold. Your explanation of heat collected on the porch before the addition of the heater far exceeds the heat I enjoyed on my old porch. Come to think of it, it had single pane aluminum windows and faced east. So while I had a few morning hours of sunshine to enjoy, it was also at the coldest time of the day. 

With the early output numbers you posted, plus a better sun angle, and the addition of show on the ground in front of your heater, it wouldn't surprise me if your heater output improved by another 25% for over 10,000 BTU output! That puts your heater output at, or above the output of two electric space heaters! And since you are using PV to power your fans, the cost to operate is zero! Consider how fast your electric meter would be spinning if you had two electric heaters running for several hours every day, not to mention the cost!!

Again, I look forward to seeing some details of you build. Make sure to start a new topic.

Greg in MN
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