Vented Aluminum Soffit Solar Collector Performance

This is a first brief performance test for the solar air heating collector with the flow through absorber made from aluminum soffit material.

There are also results on:

Collector pressure drop...

Glazing temperatures...

Smoke testing for flow visualization...

The heat output of the aluminum soffit material absorber was about about 7 to 13% greater than the heat output of the backpass collector.  This makes its level of performance about the same as the screen collector.

Back to the Solar Air Heating Collector test program home..




Heat Output


performace plot solar collectors

The plot shows the temperatures for the soffit and backpass (reference) collectors.  Since the flow rates were not equal for the two collectors on this test, to compare the two collectors at any given time, its necessary to multiply the product of the flow rate and temperature rise for that time, and compare the products.


For this test, the Big Bag was used to determine flow rates of the two collectors -- this makes it difficult to get exactly the same flow rate on each collector as each Big Bag test takes about 5 minutes to setup and perform.


The table below gives a few sample times and heat output comparisons.  In general, the soffit collector is producing 7 to 13% more heat than the backpass collector.



Aluminum Soffit Absorber vs Reference Backpass Collector Solar Air Heating Collector Test
January 10, 2011
specific heat 0.24 BTU/lb-F
Col Area 32 sqft
Air Density 0.061 lb/cf
Bag Vol 53.8 cf
Reference Collector  Soffit Collector Summary
w/sm F F F F F fpm sec cfm cfm/sf BTU/hr F F F fpm sec cfm cfm/sf BTU/hr
Time Sun Tamb Tgar Tin Tout dT Vout  Fill Time Flow Rate Flow/sf Ref Heat Out Efic Tin Tout dT Vout  Fill Time Flow Rate Flow/sf Sof Heat Out Efic Hsof/Href
10:41 380.4 3 49 51.5 62.7 11.2 823 40.8 79.1 2.5 778 0.20 52.5 62.9 10.4 833 35.4 91.2 2.8 833 0.22 1.070
10:59 410 3 49 52.9 65.2 12.3 823 40.8 79.1 2.5 855 0.21 53.6 65.7 12.1 833 35.4 91.2 2.8 969 0.23 1.134
11:44:33 972 5 55.5 100.5 45 823 40.8 79.1 2.5 3127 0.32 55.9 101.4 45.5 833 35.4 91.2 2.8 3644 0.37 1.165
11:51:44 1009 6 50 55.9 105.8 49.9 805 42.6 75.8 2.4 3321 0.32 56.8 105.2 48.4 840 36.4 88.7 2.8 3770 0.37 1.135
12:06:23 999 6 52 57.4 105.5 48.1 805 42.6 75.8 2.4 3202 0.32 57.7 103.6 45.9 840 36.4 88.7 2.8 3576 0.35 1.117
12:42:03 982 5 53 59.1 106.6 47.5 805 42.9 75.2 2.4 3140 0.32 59.4 101.6 42.2 1021 29.96 107.7 3.4 3994 0.40 1.272
1:08:23 888 5 53 60.3 104.9 44.6 803 42.9 75.2 2.4 2948 0.33 60.6 99.1 38.5 1021 29.96 107.7 3.4 3644 0.40 1.236
2:04:43 403 7 54 60 76.6 16.6 810 42.9 75.2 2.4 1097 0.27 59.5 72.6 13.1 1021 29.96 107.7 3.4 1240 0.30 1.130


For the last 3 data rows, the soffit collector was being operated at a much greater flow rate than the backpass collector, which made the soffit collector even more efficient -- about 25% more efficient than the backpass.   These last 3 rows don't represent a fair comparison between the two collectors in that any collector will improve in efficiency as the flow rate is increased -- a good reason to keep the flow rates as high as other factors allow.



Pressure Drop

Pressure drop for the two collectors were:

Reference collector 0.467 inches of water


Soffit collector 0.110 inches of water (very low)

These pressure drop measurements include about 20 ft of 6 inch diameter flex duct as well as the drop of the collector itself.


These are both with the 690 fpm duct exit velocity.


The low pressure drop for the soffit collector is a significant plus in that a smaller, lower power fan can be used.



Glazing Temperature

The temperatures of the glazing for each collector were measured in three places.  All three locations were about 2 inches to the right of the vertical centerline.  The locations were 1 ft above the bottom, 1 ft below the top, and half way up.  The temperatures were measured with an IR temperature gun.  A piece of tape was placed on the glazing at each of the temperature reading spots to provide a good surface for the IR gun to read.


  Reference Collector Soffit Collector
Near the top 75 F 97 F
At center 86F 77 F
Near the bottom 77 F 65 F

I did several sets of temperature measurements --the one above is typical.

Ambient temperature was about 20F, and there was a very light breeze.

Flow rate was around 80 cfm.


The average glazing temperature for the two collectors is quite close, but the distribution is quite different.


The mental picture of cool room air at about 60F rising up the glazing side of the absorber, and then heating up as it goes through the vented absorber may not be taking place near the top of  the collector, as the glazing temp gets all the way up to 97F -- not sure what is causing this?


I want to make it clear that this is not a terribly precise test.  The readings on the IR guns tend to change as you take them, and even slight changes in slight breeze move the readings around.  If you take a round of readings and then go back to where you started, you feel pretty good if its still within 5F of where it was.  So, don't over interpret these.  But, I do think the temperature pattern on the soffit collector may be trying to tell us something?



Smoke Testing

I did some smoke testing both to find leaks and to see if the anything could be learned about the flow pattern from the smoke.


There are 3 short videos:

Smoke test setup -- shows the smoke "pills" and duct setup...    (3.7 mb)


First smoke test of soffit collector ...   (8.5 mb)


2nd smoke test of soffit collector ...   (8.1 mb)


On the leak front, I did not detect any leaks.


On using the smoke to visualize flow the results are less certain.


The mental picture I had for the soffit collector was that the cool air from the room gets spread across the width of the collector by the deflector baffle, and then works its way up the glazing side of the absorber until it finds its way through one of the vent holes in the soffit.  So, I expected to see a steady flow of smoke up the glazing side of the absorber, hopefully spreading out uniformly across the absorber.


When I first looked at the video (and during the actual test), my impression was that there was initially some smoke starting from the baffle plate area, and making its way up the face of the absorber -- this initial smoke makes its way perhaps half way the absorber, and then fades.  As the test progresses, the smoke in the lower part of the collector appears to fade way.   I was tempted to think that there was little flow on the front side of the absorber.


But, on looking at the two videos more carefully, there is thin smoke in front of the absorber for the full test.   Its easiest to see if you flip back and forth between a frame near the start of the video before the smoke starts, and a frame well into the video.    What smoke there is in front of the absorber appears to be spread evenly over the full area of the absorber as you would hope.


In the end, I'm not sure exactly what the smoke is telling us about the flow, and would be interested in hearing any ideas on a) what these videos say about the flow pattern, and b) how to do a better smoke test where the results are easier to interpret.  I am thinking that more and thicker smoke might be a start -- anyone know how to make/buy this?



January 4, 2011