SKN_N0257R0
Institut für Solartechnik
Hochschule Rapperswil
Oberseestrasse 10, CH 8640 Rapperswil
Tel +41 55 222 48 21, Fax +41 55 222 48 44
www.solarenergy.ch
SKN Proposal: Re-introducing aperture area in collector testing
The exclusive use of gross area was decided with good intensions. However, there are some
problems that might have been overlooked or were not foreseeable when taking this decision.
This proposal is asking to step-back from this decision and to re-introduce aperture area again.
Although the ISO9806 is based completely on gross area, most of the implementation work
remains to be done: Almost all other documents, standards, regulations, scheme rules, subsidy
schemes etc. are still based on aperture area.
One of the key regulations for the near future of Solar thermal are the Energy Labelling
regulations. They are based on aperture area and it has become very clear that it won’t be easy
to change these regulations. For the time being we have decided in the SKN (Brussel 2014) to
extend the Collector datasheet with Aperture areas. As a long term strategy this is nonsense,
meaning that either we have to convince the commission to using gross area OR to give-in and
always publish aperture areas on top of what is available from the ISO9806. In general I
personally would prefer fighting, but in this case there are some strategic considerations that
might make it wiser to step back:
Let me explain: For the time being the energy labelling is based on real test result. Hence for a
certain collector the test result of this collector are to be used for the calculations. For a certain
storage tank the test result of this storage tank have to be used. The family concept as it was
introduced by the Solar Keymark Network is not part of the considerations of the commission.
This is because the family concept is not yet part of the European standards. Strictly speaking
this means that for the time being all collectors of a family might have to be tested (the same
applies for storages). At the moment I fear nobody knows. But it is very clear, that from a formal
point of view the Solar Keymark families have no legal relation with the Energy labelling. For this
reason we should strive to strengthen the family concept in general, with the hope of having it
accepted for Energy Labelling as well.
Coming back to the Gross/Aperture area issue:
Using only gross area for performance calculations leads to some incorrect energy ratings within
collector families (e.g. SK Datasheet, Energy Label, EN12976, EN12977, etc.). The reason is
that the aperture area is relevant for the absorption of light, NOT the gross area. As the ratio
page 1 of 3 pages
SKN_N0257R0
Institut für Solartechnik
Hochschule Rapperswil
Oberseestrasse 10, CH 8640 Rapperswil
Tel +41 55 222 48 21, Fax +41 55 222 48 44
www.solarenergy.ch
between gross area and aperture area is not a constant within a collector family we voluntarily
introduce an error. The pivotal point for performance calculation within a family must be the
aperture area, as it was until now. Using gross area performance data of one member of a
family as pivot for computing the performance data of other members of the family will often
result in wrong energy output figures. The error is depending on the sizes of the collectors, and
the difference between Aperture and Gross areas.
Example:
Standard Flat plate collector family with 2 members “1” and “2”. The frame has a width of 40mm.
Eta0Aperture =0.8
Aperture Areas: AAp1: 920 mm x 1520 mm = 1.398m2 and AAp2: 1120 mm X 2120 mm = 2.374m2
Gross Areas: AG1: 1000 mm x 1600 mm = 1.600m2 and AG2: 1200 mm X 2200 mm = 2.640m2
Collector 1 had the lower performance figures and is therefore taking the lead in the
performance calculations.
Starting with Aperture Area figures:
Eta0Ap1=0.8
Energy output at ΔTm =0 is 0.8*1000W/m2*1.398m2=1118.4W
Eta0Ap2=0.8
Energy output at ΔTm =0 is 0.8*1000W/m2*2.150m2=1899.2W
Eta0G1=0.8*1.398/1.600=0.699
Eta0G2=0.8*2.374/2.640=0.719
So far we considered this as correct. It makes sense because the frame of the collector does not
contribute to the output. For this reason the bigger collector has a better Eta0. Correct
NEW:
Starting with Gross Area figures:
Eta0G1=0.699
Energy output at ΔTm=0 is 0.699*1000W/m2*1.600m2=1118.4W
Eta0G2=0.699
Energy output at ΔTm=0 is 0.699*1000W/m2*2.400m2=1845.4W
Eta0Ap1=0.699*1.600/1.398=0.800
Eta0Ap2=0.699*2.640/2.374=0.777
In this case, the use of gross area results in 2.9% underestimation of the output of collector “2”.
page 2 of 3 pages
SKN_N0257R0
Institut für Solartechnik
Hochschule Rapperswil
Oberseestrasse 10, CH 8640 Rapperswil
Tel +41 55 222 48 21, Fax +41 55 222 48 44
www.solarenergy.ch
Depending on the construction of the collector the error can be smaller or higher of course. In
many cases it will be beyond measuring uncertainty. Just think about shingling collectors with
some overlap. I agree that in many cases the difference it is not really important and for nonfamily-collectors there is no need for discussion. However, there is very fundamental
misconception when using the gross area parameters as pivot and we know that. All in all this
makes the family concept not very credible anymore.
Keeping furthermore in mind what was said at the beginning, i.e. that the transition phase to the
gross area regime has just started, I therefore propose the following:
Aiming at the provision of a solid basis for the implementation of the Energy Labelling,
the Solar Keymark Network decides to use again the aperture area for all collector
performance figures. The standards EN12975-1 and ISO9806 will be adapted accordingly.
Thank you for considering the arguments.
Dr. Andreas Bohren
Head of SPF Testing
Institut für Solartechnik SPF
CH-8640 Rapperswil,
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