Certain SWH systems apparently demonstrate over 100% efficiency!
This of course is clearly not possible and can only come about as a result of an error in measurement or calculation however, there are certain products on the Eskom subsidy scheme which are listed with subsidy values and therefore Q factor measurements, which appear to have been mis-calculated.
It should be noted that this is not a Eskom fault but does possibly indicate an oversight with the issuing of test reports.
In any solar water heating system (as with all other systems) there are theoretical maximums.
SABS test procedures typically issue Q factor measurements relative to 16MJ/sqm/day of solar input.
The maximum Q factor for any product with a collector size of 1 m2 is therefore 16MJ and this would imply 100% efficiency. Energy received being equal to energy delivered.
In the real world, of course, this is not possible since there will always be losses in any energy conversion.
As a case in point one product is calculated to have a 154% efficiency. With a 1.1 sqm collector area and its calculated Q factor of 27.221MJ.
The theoretical maximum of such a product is 16MJ x 1.1sqm = 17.60MJ
It is essential then that, before purchasing a system, you check the measured Q factor for reasonability.
Using the Sky Power product performance calculator available here. http://www.skypower.co.za/content/calculators/Product%20Performance%20Evaluator%201.1.xls
Enter the subsidy value as published by Eskom. The calculator will return the Q factor of the product in kWh. Multiply the kWh figure by 3.6 to convert to it MJ.
Ask the supplier to tell you the absorber area of their product. Divide the Q factor by the absorber area to get energy delivery /sqm
Then divide energy delvery /sqm by the 16MJ X100 (the figure used as the test datum).
The result will be the efficiency of the product as a percentage.
Using Sky Power product as an example:
Q Factor 33.227 MJ and Absorber area 2.4 sqm.
(Q Factor / Absorber area ) = energy/sqm
(energy/sqm /16) x 100 = % collector effeciency
33.227/2.4 = 13.84
13.84 /16= 0.865
0.865 x 100 = 86.5%
Anything over 90%, should be questioned as being unlikely and anything over 100% as impossible.
If in doubt, please call us at Sky Power and one of our technical staff will help you.
Sky Power is a South African supplier of advanced Solar Thermal and Solar Electric systems. We offer a money back performance guarantee on all of our installations.
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