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Scott Davis

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The Zero Pass Hot Air Collector is a newer brainstorm that a couple folks here on the Simply Solar forum are working on, so we've started a new topic for this design.

The idea behind this collector is that any air stream will tend to keep within a predictable range, so the heat absorbers (usually screens) can be placed on the upper and lower edges of the airflow.  That way, the heat is scrubbed off the absorber as the air passes between, but there should be minimal flow resistance.  In addition, in theory, it should keep most of the heat away from the glazing, enhancing performance.

Some side by side tests, comparing the zero pass design to the tried and true, double layer screen reference standard, should be coming soon. 

I haven't tried this design myself, but this video demonstrates the concepts:


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gbwillson

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Hi Gang-

The sun FINALLY came out so testing has begun in earnest. As some may not know I will be testing a basic 2-screen collector versus a relatively new design, the ZeroPass(ZP). I started this testing project about two years ago, but due to the year it took me to re-side the house and the health issues as a result of the abuse I placed on my body, I was unable to complete the collectors until now. It's the first time in years I've looked forward to cold weather so I can test these units side-by-side.

Pic of the two test collectors with the ZeroPass on the right
IMG_0313_2.JPG 

Greg's ZeroPass Solar Collector.jpg 

As a collector, the 2-screen has become the de-facto solar air collector. Testing has shown that the screen collector to be one of the easiest to build, and best performers. Being a newer design, the ZP has not been previously been tested against other collectors, specific the 2-screen. 
Last week I was able to fire up the 2-screen last week and the performance, and the numbers were  impressive. So the ZP will have its work cut out for it. 

http://www.builditsolar.com/Experimental/AirColTesting/Index.htm


Both heaters have 4'x8' heat collector areas. The ZP has an additional 1' manifold section on each end. I don't think the ZP would need this large a manifold, but I wanted the space just in case. Others have noticed that balancing air flow can be easier with a larger manifold. It's also a lot easier to close off a portion, if needed. My original ZP manifold design had the ducting entering from the end of the manifold and tapering down to nothing at the far end of the manifold. This follows a basic principle of HVAC ducting. Where each branch is roughly half the air flow of the previous branch. I also had dampers that allowed me to control the flow of air and balance the flow evenly from side to side. Interestingly, I needed the damper controls at BOTH ends as the air flow was completely different weather you were pushing or pulling the air. Ultimately, I ended up scrapping the tapered manifold. Not because it didn't work, but I learned something about manifolds when I built my 2-screen. With a 2-screen, air flowing evenly from side to side is not so important as the heating takes place when the air passes through the screen. So unless you have rather a hot spot or a cold spot within your collector, it may not affect your efficiency much. On a ZP, due to the way the air is heated, it is very important for the collector to flow and heat evenly from side to side. When I added the diffuser above the intake  on the 2-screen, I noticed the flow of air coming out from under the diffuser was not even. My tests showed that near the edges, the flow was 2-3 times greater than flow coming straight out from the intake hole. Normally, air flow hitting the diffusor would spread in roughly 360 degrees. But since my intake was at the very back edge of the box, the flow was restricted to roughly 180 degrees, with the extra 180 degrees of flow mostly being directed to the edges. I overcame this flow issue by adding a few restrictors that limited the extra flow being sent to the edges. Once The manifold area will allow the air to enter the heating area BETWEEN the layers of screen. So, unlike the 2-screen, the flow of air is not designed to pass through screens to pick up heat. 

For both collector boxes I used 1"x6" boards and ½" plywood for the back and 1" polyiso insulation to line the box. The boxes were covered with 6mm twin-walled glazing. The fans supplying the airflow are a matching pair of 350CFM exhaust fans I scrounged from a couple of downdraft cooktops. 6" flex duct is being used for both test units.
 My test ZP was designed to be modular and easily accessible. I can pull off either manifold unscrewing only 4 bolts. I wanted to be able to bring them out of the cold if I have to work on them. The glazing can be removed with a ¼ counterclockwise turn of hold downs. Once the glazing has been removed, the fixed back screen and adjustable screen can easily be removed. The fixed back screen has two layers and is ¾" above the back wall insulation. I thought about 3 layers to start, but it is a lot faster to add a layer of screen than it is to remove dozens of staples. The gap between the fixed, back layer of screen(s) and the adjustable upper layer will start at ¾". The width of the collector opening is 43". So any screen gap much smaller and I will begin to restrict airflow as the single layer adjusting screen is attached to the dampers that can control air flow(32.25Cu. In. default screen gap versus 28.26Cu. In. ducting). The screen gap can adjust from a little over 2" wide to completely closed off. I used threaded bolts trough the adjustable screen frame. The bolt heads are on the back of the collector. The screen gap adjustment is simply screwing the bold head in or out. The airflow and gaps at either end can be adjusted independently of one another, which could be interesting to play around with. I want to try having a wide intake gap tapering down to a very fine gap, or no gap at the exhaust. It might be too restrictive, but I'm wondering if it would pick up heat along the gap and near the exhaust the air is "forced" through the screen. In other words, use the heat collecting properties of both designs. Hmmmmm.

The area where the ducting enters the house is right above the double laundry tub. It's a very awkward. Where the ducts enter outside is even more awkward as the window is under an 18" cantilever and I don't' fit underneath. Working inside the window, I'm either sitting on the edge or partially standing in the laundry tub. Thank goodness it's made of concrete. Before I turned on the fans, Passively, I have to say the 2-screen was much warmer. Although its design is much more conducive for a passive collector. The ZP airflow would simply pass along the underside of the cold glazing until it reached the top. After I turned on the fans and did a rough adjustment to match the CFM, the ZP was clearly the warmer unit. But with a lot of loose, leaky ducting connections, and a touchy speed controls, I'll reserve judgement for now. For now, I'll be pushing the air through the collectors, before I try pulling the air through the test collectors. Let me know if you have any questions or if there is a test you would like to explore. This testing is as much for the group as anything.

Greg is finally testing in MN [biggrin]








joebehr

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Reply with quote  #3 
What did you use for the clear glazing on your panels?
joebehr
joebehr

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Reply with quote  #4 
Never mind...I just read it in your posting...missed it on the 1st read.
gbwillson

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When comparing two collectors like I am, what is the "proof" of better efficiency? Assuming the collector boxes are the same size and the air flow are matched, is the proof the higher output temps or the net increase in temp from entry to exit?
The collectors have not turned on yet, but the input and output temps of the 2-screen is 15 degrees warmer. I assume it's because the deflector covering the intake is absorbing heat and passing it on the intake thermometer probe. This won't happen on the ZeroPass due to the insulated manifolds. Will this skew the results once the air starts flowing? None of the probes is exposed to sunlight.

Greg is curious in MN [confused]
Scott Davis

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Hi Greg,

Yes, you want identical input temperatures and of course identical air flow through the collector.  Then you will measure the temperature rise of the two collectors.  The collector with the higher temperature rise is the better performer (again, assuming both collectors have an equal amount of air passing through them).

The reason you can't start with a 15 degree higher input in one vs the other is that the hotter a collector is running, the less efficient it will be, losing more heat to the ambient surroundings.  It would not be a true test under identical conditions.

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gbwillson

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Quote:
Originally Posted by Scott Davis
Hi Greg,

Yes, you want identical input temperatures and of course identical air flow through the collector.  Then you will measure the temperature rise of the two collectors.  The collector with the higher temperature rise is the better performer (again, assuming both collectors have an equal amount of air passing through them).

The reason you can't start with a 15 degree higher input in one vs the other is that the hotter a collector is running, the less efficient it will be, losing more heat to the ambient surroundings.  It would not be a true test under identical conditions.


Scott-

The difference as I see it is due to the design difference. One has no manifold, and the other an insulated manifold. So my differences in intake temps readings right now is mainly due to the fact the fans are not running. The differences would be much less once the air was flowing, but still cause a slightly increased air intake reading. So in my case, the 2-screen will always read a higher input temp, due to thermometer probes proximity to the heat absorbing deflector? The 2-screen intake temp probe is in the throat of the intake collar. Could I use the intake air from the basement floor as a constant? Both intakes are about 6" off the floor and about a foot apart. Only difference would be 4' additional flex duct to the ZP intake. 

Greg in sunny and really cold MN [wave]
Scott Davis

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In that case your ducting would influence the test, so that's not a good way to go.  You definitely want to measure right at the input of the collector and right at the output of the collector, to remove as many other variables as possible. 

Let's see what the differences are once the fans are running.  If it is only a couple degrees (much less than 15) at the input I think it will be a reasonable comparison when you calculate total temperature rise for each. 

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Scott Davis

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Hi G_H,

We want to learn how well the solar collector design works by comparing one design to another.  Greg already documented that his temperature entering one collector is 15 degrees hotter than the other.  That's as a result of his duct setup and has absolutely nothing to do with absorber design.  So, if Greg used the floor input temperature, the collector receiving that extra advanced preheating has a 15 degree advantage, which pretty well invalidates the test.

For side by side tests to be useful, the collectors must have all variables identical, with the exception of heat absorber design.  It's important.  Not doing so and putting those results out there may cause casual readers to misinterpret and extrapolate those invalid conclusions. 

Side by side testing is harder than in looks.  Gary and I both worked really hard on this.  it's not an easy thing to accurately do, but it is really important and useful information that is well worth the time to do correctly.



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gbwillson

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Scott-

You are right about the difficulty in testing. While I don't want to seal and tape up all of the leaks for each test, I may have to. I balanced the air flow on both collectors. This was hard enough as both fan setups have a slow ramp up in speed. I stepped away for a few minutes to allow the readings to stabilize. About 10 minutes later I checked the readings. I'm not too concerned with output temps or fan speeds(other than being balanced against each other) at this point. At least not until this intake probe temp issue is resolved.
Here is what I have at this point…


The intake reading on the basement floor was 62.8, so this same air is being fed to both collectors

The intake temp probe for the ZP was 62.4 degrees, so a loss of approx. .4 degrees from the basement floor air, which seems about right
The output temp probe for the ZP was 97.3 degrees, so a gain of 34.9 degrees

The intake temp probe for the 2-screen was 72.5 degrees, so the temp increased 9.7 degrees
The output temp probe for the 2-screen was 109.2 degrees, so an increase of 36.7 degrees

So it would appear the 2-screen has a 1.8 degree advantage over the ZP. BUT, it has a 9.7 degree of artificial temp boost


All external probes were checked against my Kestal anemometer/temp probe and were within .4 of each other. 

Does anyone know what to make of the data or how to correct the artificial temp bump?

Greg is confused in mostly sunny and frigid MN [confused]


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