RETscreen Solar thermal Heaters Area

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Good morning,

I am currently modeling with RETscreen a solar water heater system for the
domestic hot water of a hotel.

The collectors are evacuated collectors.

In the ?Solar water heater? part of the ?Energy model? of RETscreen
spreadsheet we need to give the gross area and the aperture area but my
solar collector technical sheet (in Spanish) gives me 4 different areas:

- Gross area: 3.74m? (collector length : 3065mm; width : 1220mm).

- Aperture area: 6.2 m?

- ?Channeling? area: 4.88m? (in Spanish : ?captacion area?)

- ?Efficient? Area: 3.66m? (in Spanish : ?efectiva area?

Do you know in such a case which values I should enter in the model?

It seems easy to enter the Gross area (3.74 m?) and the Aperture area (6.2
m?) but it does not seem right that the aperture area is bigger than the
gross area.

What do you think?

What is exactly the aperture area?

Thank you for your Help

Best Regards,

Charline Seytier, LEED AP BD+C

Co-Owner, CEO


155 rue de Belleville

75019 Paris, France

charline.seytier at

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Joined: 2015-03-03
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Hi Charline.

I?ve copied the definitions from RETScreen help below. They define the
aperture area to be the area covered by the tubes only while the gross area
also includes the area in between the tubes. I presume the gross area
should include the header as well.

If your manufacturer isn?t included in the RETScreen database, you may be
able to find out whether they were the original equipment manufacturer ?
often times I find companies will buy products from one of the large Chinese
manufacturer?s and put their own logo on the product. RETScreen has several
of the large Chinese manufacturer?s.

Lastly, if you decide to input area and efficiency on your own, ensure that
the efficiency is in relation to gross area. European standards use
aperture area leading to efficiencies in the 80% range while those drop down
to the 40-60% range when using North American Standards with gross area.


C - Differences between gross and aperture area

Collector tests report efficiency relative to gross collector area, aperture
area, or both.

* Gross area is the total area occupied by the collector, including
its frame. It is simply the product of the outside length and width
dimensions of the collector.

* Aperture area is the maximum area of the collector over which useful
direct solar radiation can be collected.

* For most unglazed collectors, gross and aperture areas are identical. For
glazed collectors, aperture area is equal to gross area minus the area
occupied by the frame. For evacuated tube collectors, aperture area is the
area covered by the tubes themselves, whereas gross area includes the area
between tubes.

RETScreen expects collector efficiencies expressed in terms of gross area.
If the efficiency is expressed in terms of aperture area, the following
conversion can be used:

eta_g = eta_a (Aa / Ag)

where eta_g is the efficiency based on gross area, eta_a is the efficiency
based on aperture area, Ag is the gross area and Aa is the aperture area.

Example. An evacuated tube collector has a gross area of 2.140 m2 and an
aperture area of 1.412 m2. The efficiency equation of the collector, based
on aperture area, is:

eta_a = 0.813 - 1.32 (DT/G) - 0.035 (DT2/G)

Then the efficiency equation based on gross area is obtained by
multiplying the coefficients of the equation above by 1.412 / 2.140, hence
the efficiency equation based on gross area is:

eta_g = 0.536 - 0.871 (DT/G) - 0.023 (DT2/G)

Aaron Smith, P.Eng

M&R Engineering

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Joined: 2011-09-30
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