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netttech

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Reply with quote  #1 
First let me state I have lost all trust in an accurate digital thermometer (pix below). I use these in all my panels, but WOW...what a difference. I had 3 digital thermometers outside for 30 minute prior to the test & all 3 had difference temps. I ended up using an old mercury thermometer to record the outside air temp for this test.

Using pieces of my 4'x6' foam panel I constructed a test panel of collector material. This test was 2 layers of screen, against Cinefoil. If I get another sunny day, I plan to remove the screen & replace it with black luminum flashing for another compare test. The panel is 6'Tx2'Wx4", 2 sections, passive, with heat-shrink glazing.

I kept the cinefoil flat so the amount of collector would be the same as the screen. The cinefoil & screen is suspended 2" from the back & glazing to allow free-flow of air on both front-back. The input vents are the same size (3"x10"). The exit vents is the entire width & thickness of the panel sections. That provided a free-flow of air out. The output thermometers were semi enclosed (top 5" of panel) to keep the cold outside air from affecting the readings.

In short the cinefoil out-performed the screen both in heat-up speed & overall maximum air temp. Use the recorded mercury temperature as the cold air input temp.

Jeff
Central IL
Solar air & water
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gbwillson

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Reply with quote  #2 
Jeff-

I know what you mean about the thermometers. I have tried several different models and brands, including wireless. First thing I do is calibrate a new thermometer. Since I usually buy them in pairs, I set them next to a thermometer I know to be accurate(Kestrel 2000). Since I buy the same model they are likely from the same production batch the readings are usually very close to each other, and the Kestrel. But if they do not match, I can usually find a different model or brand that matches. If THAT doesn't work I either take the unit back or note how much the unit is reading off of the reference Kestrel. I often see a large discrepancy between the base unit and the probe or remote sensor. I would imagine the difference here is from the different type of thermometer normally used between the base and the remote/probe. 

I like side by side tests boxes such as yours. It allows for cheap testing of different ideas and designs. I don't see the airspeed listed on your test. You might consider trying different air speeds since we know some designs work better at different speeds. This is especially true when you are using the Cinefoil as a backpass-type collector. The 2-screen has better flow than a backpass, but your test box is small enough that this difference may not be apparent. Tom showed us that Cinefoil works very well when rolled into tubes. I especially liked his tube rolling method. I'd like to bring out my test box, but sadly, it's being used as a table. Another possible test to try after comparing Cinefoil to aluminum flashing is heavy duty aluminum foil that has been given a light coating of black paint. According to the manufacturer, Cinefoil is coated with a paint specifically designed to absorb visible light. I'd be interested to see how that compares to lightly painted foil you buy from the grocery store.


Greg in MN
KevinH

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Reply with quote  #3 
Basic home digital thermometers can have wide accuracy specs.  Some don't even list a spec.  An old one I have is:
+/- 1.8F from 32 to 104
+/- 3.6F from -40 to 122
+/- 5.4F from -58 to 158

So, the hotter or colder it gets outside its normal range, the less accurate it gets.  An indoor model I have is +/-2, +/-4, and +/-6 for those ranges.

In a 2 screen collector a good part of the heating comes from the black backing.  Using Cinefoil to simulate that or painting the back of the box would have given closer results.

Kevin H
MN

SolarInterested

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Reply with quote  #4 
Jeff was your screen collector configured so that airflow passed through the screens?
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Both temperature rise and airflow are integral to comparing hot air collectors
gbwillson

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Reply with quote  #5 
I've been becoming partial to wireless thermometers since I have such a long duct run to string a wire. Since my collectors are on the ground, changing batteries seasonally is not a problem. But they still have the same spec/accuracy issues.

Greg in MN
netttech

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Reply with quote  #6 
I have no way to measure the air flow. Inside my house I use the garbage bag method, but that wouldn't work outside in the wind.

Since the inlets were the same size, with free flowing air...naturally I would think the hotter panel would flow more air.

SI - No. Both the cinefoil & screen are laid flat, suspended with air flow on both front & back (pix below). That being said, the screen did tend to sag & I placed some little supports in the middle to try holding it up straight. That means, the screen isn't as flat as the cinefoil. I should also mention the collector material doesn't overlap the inlet vents. The screen/foil starts at the openings.

I have always used the digital, wireless thermometers because they are so convenient. I can live with the variations, as long as I know they are flawed, somewhat. Since I was conducting this test outside on a cold windy day, I decide to use the old mercury thermometer, since I knew the digital was incorrect.

I have cinefoil tubes in my 2'x6' under 2 layers of screen. However it looks like it will perform very well without the trouble of making tubes. I truly love the KISS (Keep It Simple Stupid) method. [smile]

Jeff
Central IL
Solar air & water
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stmbtwle

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Reply with quote  #7 
The tubes would probably still be better as they isolate the airflow from the glazing, but is the difference worth the labor?

I have a question. Suppose you were to block off the flow in front of the cinefoil so air could only pass behind it (backpass). Do you think it would be better or worse?

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Willie, Tampa Bay
SolarInterested

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Reply with quote  #8 
Quote:
Originally Posted by netttech
... SI - No. Both the cinefoil & screen are laid flat, suspended with air flow on both front & back (pix below). ...

That would seem to negate the best characteristic of screen - the great surface area available to transfer heat from the absorber to the airflow as it passes through the screen.

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

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Reply with quote  #9 
I agree with SI, that you may be handicapping the screen side. If the screen side air were to pass through the screens it would likely yield a different result. But it will also reduce the flow of air. If you do change around the screen configuration, see how a passive ZP configuration works as well. 

Greg in MN
KevinH

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Reply with quote  #10 
Not sure what the original intent of this test was, but without a black box the screen side isn't going to get close to the cinefoil side no matter how the screen is configured.  Screen over black box or screen over cinefoil backing would likely beat the cinefoil frontpass/backpass side.

Kevin H
MN
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