I am a strong advocate of the work that the SRCC (solar rating and certification corporation) is doing in testing and certifying solar collectors but….  There are a number of factors that can have a significant impact on the certified performance that aren’t published in their certifications and can lead to significant differences in performance as well as longevity.  Below are a few things that have a significant impact on the performance of the solar collector that currently aren’t reported:

1.  Glass

1. Hardiness – as part of the certification process the solar collector glass has to undergo repeated thermal shocks to insure that it doesn’t fail.  It is commonly understood that in order to pass this test the solar collector glass must be tempered.  The tempering process is expensive for glass manufacturers and leads to significantly higher glass prices for the collector manufacturer.
2. Transparency – when the solar collector s are tested for certification they are tested as a whole and the transparency of the glass is not measured separately.  There are two steps that can be taken to increase the thermal transmission of the glass.  The first is going with low iron glass.  This is the most common grade of glass that is used in solar collectors.  The second step that can be taken is going with an anti-reflection coating on both sides of the glass.  This coating allows low angle light to pass through the glass rather than be reflected by it.  Unscrupulous manufacturers could substitute tempered, low-iron, anti-reflection glass for their certified collector (they are only required to produce 5 samples) and then use standard float glass to supply there everyday production.  This higher performing glass would give them a significantly higher performance number from the testing but then they could cut the cost of their glass by 75% by going with standard float glass for their production.

2. Absorber

1. Thickness – as a rule the thicker the absorber sheet the greater it’s ability to carry heat (less thermal resistance).  Unfortunately, a thicker sheet costs more to produce.  Currently, the published data does not specify at what absorber thickness the collector was tested so again an unscrupulous manufacturer could substitute a thinner material once they have received their solar collector certification with higher numbers.
2. Riser quantity – the shorter distance between the fluid channels (tubes) the less heat is lost.  The number and spacing of the tubes is not published.  It is possible (and has been reported to have happened) where a manufacturer increases the spacing between there tubes to save money on the solar collector and the contractor/homeowner are none the wiser since they are sold the solar collector based on the SRCC solar collector rating.
3. tube thickness – in order to pass the rigorous 160 psi standard required for open loop collectors manufacturers can reduce the wall thickness of their copper tube once the certification has been achieved.  This will save the manufacturer money but diminish the expected life of the collectors

Random variation

1. Every process has natural variation including the production of a solar collector.  Currently, the SRCC (or there certified testing labs) tests only 1 collector (from a batch of 5) to determine what the published data for that collector and manufacturer will be.  Even their testing procedures have variation in them (this has been much discussed in the industry).  With only taking a single sample you could infer that Michael Jordan either never missed a shot or never made one (both equally wrong).  Without several data points it is easy to see how seemingly identically constructed collectors have widely difference performance numbers.

So all I have shown is that unscrupulous companies can (and will) take advantage of the system but what can we do about it?

I would recommend that we convert the current system of 1 scheduled inspection every 10 years to one where the certified manufactures pay an annual fee for the testing that will be done that year.  The samples to be tested would then be pulled randomly from the manufacturers on the list based on surprise audits.  This would serve the purpose of leveling out the costs associated with the testing process (fees would be annual based on the number of models needing testing and therefore your chance of being selected in a given year), eliminating the complaints of back-logs in solar collector testing because you would have your certification by paying your fee until your solar collector is inspected,  and freeing the solar collector performance numbers from the influence of gaming that I discussed above.