2nd Test of Reference Solar Air Heating Collector 10/12/04

This is a several hour test of the reference collector in full sun, winter conditions.

The test was done on a good sun day with an outside temperature around 21F.  

The collector inlet and outlet temperatures were logged along with the ambient temperature and the sun intensity for about 5 hours.  The collector flow rate was also checked at a couple points during the test.

The heat output of the collector is calculated from the above, and a rough estimate of efficiency is also made.

In a nutshell, I think the collector design is performing well.

NOTE: This was the first collector built, and none of the tests below show comparisons to the other collector designs -- see the links on the other collectors for the comparative performance.

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



Pictures below show the setup.  A few small changes were made from yesterdays setup to make it easier to make the measurements and keep things stable.

Collector location.

Inlet and outlet ducting

Revised outlet ducting.
The big yellow bag connects to the 4 inch PVC exit duct
from the blower for flowrate test.

Inlet duct routing.

Sun conditions.  Mostly very good with occasional
thin clouds in front of sun.

The Apogee Pyranometer mounting -- in same plane
as the collector.





This is the plot of temperatures and sun intensity starting around noon and going through about 4 pm.



For the time marked by the vertical line on the plot above:

Collector outlet temp = 123.5 F


Collector inlet temp = 48.7 F


Temperature rise = 74.8 F


Flow rate = 80 cfm   (about 2.5 cfm per sqft of collector area)


Sun intensity on the collector surface was 919 watt/sm


Ambient temperature was 25 F


Pressure drop over the collector was 0.415 inches of water


Air density is calculated as 0.0611 lbs/cf based on 5000 ft altitude and 78F average col temp (see note below)


The flow rate was estimated based on a 42 second fill time for the big yellow bag (56 cf), which 80 cfm. 

A check on this was that the Kestrel wind meter showed an exit velocity from the 4 inch exit duct of 940 fpm, which gives (940 fpm)(0.087 sf) = 81.8 cf.

I still need to do a more careful calculation of the bag volume, so this may change a little.


Collector inlet and outlet temperatures were measured with thermistors in the inlet and outlet ducts.  The thermistors were placed in the center of the ducts.  In the case of the inlet duct, there is about 4 ft of duct between the thermistor location and the actual collector inlet.  In the case of the outlet duct, there is about a 10 ft length of duct between the thermistor location and the actual collector outlet.

The thermistors are from Onset Computer, and are (as I recall) said to be within 1/4 F over their normal range.


Ambient temperature was measured in a shaded spot about 2 inches behind the collector and about 4 ft off the ground.


sun intensity was measured with an Apogee pyranometer that was mounted in the same plane as the collector glazing.  While the sun conditions looked quite good, base on past experience I would have expected readings over 1000 watts/sm  given the clear looking conditions.  Just shows its hard to judge the sun intensity visually.



Heat Output

The heat output for the point marked on the plot above at about 12:53 pm is:


Heat Out = (80 cfm)(0.061 lb/ft^3)(123.5 F - 48.7 F)(0.24 BTU/lb-F) = 87.6 BTU/min = 5256 BTU/hr


The 75F temperature rise with 2.5 cfm per sqft airflow through the collector under winter conditions with a single glazed collector seems quite in line with what a good air collector should be doing to me.




This is a very rough estimate of efficiency for the point marked on the plot above.


Efficiency = (Heat Energy out) / (Solar energy in)


Heat Energy Out is the 5256 BTU/hr calculated above


Solar energy in is 919 watts/sq meter = 291 BTU/hr-sf.


Efficiency = (5256 BTU/hr) / (291 BTU/sf)(32 sf) =  56.4%

This seems fine to me -- better than I would have expected for this type of calculator under winter conditions.



Other Bits

Measured the stagnation temperature before the test started at 190 F.  This was with a little bit of air thermosyphoning through the collector -- you could see a little vapor coming out of what is normally the inlet duct (lower duct).


The dip in sun intensity at about 1:15 (where the double triangles are) is me cleaning off the outside of the glazing with windex -- it still had some shop dust on it.  Did not seem to make much difference.  Inside of glazing was cleaned before installing.



Background on Heat Output Calculation

The heat output depends on the weight flow of through the collector and the temperature rise from inlet to outlet.


Heat Out = (Flow Rate) (Air Density) (Temperature Rise) (Specific Heat of Air)

Flow Rate is the volumetric flow rate in cubic ft/minute (cfm)

Air Density is the density of the air in  pounds per cubic foot

Temperature Rise is the outlet temperature - inlet temperature in degrees F

Specific Heat of Air is the energy in BTu's required to raise the temperature of 1 lb of air 1 degree F  (0.24 BTU/lb-F for air)


I measured the volume flow, and since density of the air (the weight per cubic foot) depends on the air temperature and pressure (altitude), I use this calculator to estimate the density of the air based on our altitude (5000 ft above sea level) and the average temperature of the collector air.

Air Density Calculator: http://www.denysschen.com/catalogue/density.asp



For a collector temperature of (110F + 47F)/2 = 78F, and an altitude of 5000 ft, the density of air  is 0.0611 lb/ft^3   -- this compares to  0.073 lb/ft^3 for 78F at sea level.

I've not taken into account our actual barometric pressure -- will have to look into that.



This is an added set of data for 12/6/10


This was a day that started with pretty good sun, but then developed into an overcast.

I left the collector running through the whole cloudy day just to see how it would do at much lower sun levels.



In the early morning, with the sun up at 700 watt/m^2, the collector was heating from about 42 F up to 108 F for a 66 F rise with 73 cfm flow rate, and with an ambient temperature of about 20F. 


The plot shows what happens as the clouds come in and sun level drop to 200 watts/m^2 and even less. 

The collector hangs in there producing some heat even at sun levels as low as  200 watts/m^2.  

Part of the reason the collector was able to keep producing heat even at sun levels of 200 watts/sm is that the collector inlet temperature was relatively low -- ie the shop was not that warm -- but still, its pretty interesting that its able to produce some heat at these low sun levels.


This is an Added Set for 12/7/2010

Good sun until about noon, then highly variable sun for the rest of the day.



No wind

About 32 F ambient

Good sun early with clouds coming in and out in the afternoon.





Yellow bag fill time of 49 seconds, implying about 67 cfm.

Kestrel wind meter reads near 820 fpm at the exit of the 4 inch duct, implying 71 cfm.

Static pressure drops over the collector of about 0.41 inches of water.


The 75F temperature rise with 51 F inlet temperature, 913 watt/m^2 sun, and 70 cfm seems good to me.


Note that at 4:30pm, the collector started to cool the room instead of heat it.




Gary December 5, 2010, December 6, 2010