'Active tube area' vs 'aperture area'

[Bargeman]

One for Navitron I think,

The website states that the active tube area for an SFB20 collector is 1.41 m2 (http://www.navitron.org.uk/page.php?26), whereas the EN 12975 certification report for the same collector states the aperture area to be 1.747 m2.

Could someone at Navitron please clarify what is meant by 'active tube area' and how it differs from the more widely recognised 'aperture area' ? This information is used under SAP to calculate collector performance, so I need to be clear on which measure to use.

Likewise the 'panel area' is stated as 2.25 m2 whereas the EN 12975 certification report gives  a 'gross area' of 2.403m2 and an 'absorber area' of 2.655 m2.

regards

[guydewdney]

at a guess here, its the difference between the area of tube outer, vs tube inner, vs total panel size?

[Ivan]

For some reason, SPF, the testing house,  decided that they had to count the entire area of each tube - including the back. SPF are very pro-flat plate, anti-vacuum tube (as we've since found out from some quotes they provided for Irish publications). They don't seem to apply the same policy to flat plate panels.

[Bargeman]

Thanks Ivan,

Hmm, interesting. By counting the entire tube surface area in the aperture area SPF have actually increased the 'useful' heat collecting area of the panel. In the context of the SAP analysis this will produce a larger estimated heat output than would be the case if only 'active tube area' is used. From this it might be concluded that in this instance SPF are actually tending to favour vacuum tubes.

regards

[ericw]

Bargeman,
Do SPF not measure the output of the panel and then calculate a specific set of parameters based up an area. It doesn't actually matter how the area was defined - the value of the parameters will reflect its value.

When determining the panel output, which is what presumably is done within the SAP calculation, don't you have to use one of their areas and the parameters that were calculated for it. They should all give the same answer.

[Bargeman]

Hello Eric,

Three parameter values are used within SAP; aperture area, zero loss collector efficiency and linear heat loss coefficient. These are all taken from the EN12975 report produced by SPF and for the SFB20 are 1.747 m2, 0.561 and 1.61 respectively. My point is that it is in some ways quite fortunate that SPF have chosen to define aperture area as per Ivan's note rather than use the 'active tube area' definition that Navitron themselves have on their website, i.e. 1.41 m2. If SPF had chosen to define aperture area in the same way that Navitron define 'active tube area', i.e. taking only the upper facing surface area of the tube, then SAP would produce a lower estimated output which would cause the collectors to have a lower contribution in SAP based energy models of building performance. Thus as far as SAP calculations are concerned, the SPF methodology seems to favour vacuum tube collectors versus Navitron's own methodology.

regards

[ericw]

Hello Bargeman,

Looking at the SPF results for the three areas defined.
The product of area and neta0 is the same (to within .4%) which means that the SAP  "Solar energy available" [H7 box] would contain same value which ever area (and matching neta0) regardless of which area you chose.
The collector performance ratio is also the same for all three areas.

As the panel area doesn't seem to appear anywhere else in the calculations doesn't this mean that providing you use the matching parameters for that area definition you will get the same overall answer.

If you change the definition of aperture area then the parameter values will also need to change to keep the calculated panel output the same as its measured value. 

[Bargeman]

Hello Eric,

I guess it's just that the Navitron definitions of active tube area and panel area don't seem to correspond to any of the 3 collector areas used by SPF in the EN12975 report.

regards

[Ivan]

The efficiency constants will be related to the area tested. So if SPF decided to arbitrarily count the area twice (this is what they have done by counting the backs of the tubes), the efficiency calculated will effectively be halved to make things add up correctly. Unfortunately, the effect is to make the panels look worse in terms of efficiency - not surprising when you count twice the area that is irradiated (flat plates would appear pretty bad too, if you counted the backs of them too!).