\$1000 Solar Water Heating System Efficiency Test for December 4, 2008

 This page uses the data collected on December 4, 2008 to do a rough estimate of the PEX Water Heating Collector efficiency.   The results are about as predicted from the early small panel tests -- the PEX collector shows about 15% less heat output per sqft of collector than a commercial collector would have under the same conditions, but the PEX collector's BTU output per dollar of cost is 6.4 times better than the commercial collector.

The plot below shows the supply and return water temperatures for the PEX collector for December 4, 2008.
The sun intensity is also shown.
This was a mostly sunny day with occasional light clouds passing over.
Ambient temp was low (about 18F).

Efficiency

Rough efficiency evaluated at around 11:45 AM.

T supply = 106.8F

T return = 113.5 F

Delta T = (113.5 - 106.8) = 6.7F

Flow rate = 1.73 gpm = 14.4 lb/min = 861.5 lb/hr

Sun = 1049 watts/m^2 = 332 BTU/sf-hr

Collector area = 48 sf

T ambient = 62F

Sun Energy in =  (48 sf)(332 BTU/sf-hr) = 15936 BTU/hr

Energy Out = (861.5 lb/hr)(6.7F) (1 BTU/lb -F) = 5772 BTU/hr

Efficiency = (Energy Out) / (Energy In) = (5772 BTU/hr) / (15936 BTU/hr) = 36.2%

Using the efficiency calculator, a Heliodyne Gobi flat plate collector with black painted absorber would give 42.6% efficiency under the same conditions.  So, the PEX collector has an output of 85% of the Heliodyne flat plate collector.  Amazingly, this is agrees almost exactly with the earlier small panel tests -- isn't science wonderful :)

If both of these efficiencies seem on the low side it is because the ambient temperature is quite low for a mid day winter temperature.  Even in January, the average midday high in Bozeman is nearly 30F -- this day was 18F.  So, think of these as close to worst case for a sunny day.

BTU per Dollar

If you do the more important calculation of heat output per dollar of collector cost, it comes out this way:

PEX Collector cost at \$4 per sf = (48 sf)(\$4) = \$192

Commercial collector at \$30 per sf = (48 sf)(\$30) = \$1440

BTU per dollar for PEX = (5772 BTU) / (\$192) =  30.1 BTU/\$

BTU per dollar for Commercial = (42.6%)(15936 BTU) / (\$1440) = 4.7 BTU/\$

So, the PEX collector is 6.4 times more cost effective.

The same numbers for the Copper/Aluminum Collector would be:

Copper/alum collector cost at \$6/sf = \$288

Output should be about 96% of the commercial collector, so (15936)(0.426)(0.96) =  6517 BTU

BTU per dollar for copper/alum = (6517 BTU) / (\$288) = 22.6 BTU/\$

So, about 4.8 times better than the commercial flat plate.

This is the screen copy for the efficiency calculation for the Heliodyne Flat Plate Collector

Gary December 5, 2008