Solar radiation obstruction by cloud cover

[Tom Foster]

On a similar topic to https://aecb.net/forum/index.php?topic=2621.0 , how should actual weather data (i.e. cloud cover, duration/density) be applied to modify clear-sky solar radiation intensity data? I'm about to see Tas Thermal Modeller do it, black-box style, but wd like to know what the old fashioned manual method is.

CIBSE Guide A 7th ed Tables 2.30 or 2.33 provide “clear day beam and diffuse irradiances (/ W·m–2) on vertical and horizontal surfaces on specified days”. For the chosen orientation (SE) of vertical surface (glazing) I can see both the direct-beam component and the diffuse radiation component in W/m2 at times of day and dates of the year – assuming clear sky.

Elsewhere in CIBSE I'm sure there are data tables giving actual cloud cover, duration/density for the same dates. The cloud cover will decrease the solar W/m2 actually received; it will decrease the direct-beam component greatly, the diffuse component less so (If at all?)
Can anyone point me to such a table, in CIBSE or elsewhere?
How would such a table be converted into %age reduction of a) clear-sky direct-beam, and b) clear-sky diffuse W/m2?

[Tom Foster]

Wot, no comment?
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6172&page=1#Item_26

[Anonymous]

Fostertom wrote:

For the chosen orientation (SE) of vertical surface (glazing) I can see both the direct-beam component and the diffuse radiation component in W/m2 at times of day and dates of the year – assuming clear sky.

Since no one has picked up the thread I might have something usefull here:

http://www.velux.com/Sustainable_living/Model_home_2020/Sunlighthouse/energy_balance/default.aspx

One can not rely on manufacturer's wokshop-test-lab data alone when it comes to reality.

Get your window supplier to hand out real data derived from real structures under real conditions. And these you can compare with the calculated data (PHPP) to find the usual default.

[Tom Foster]

That's very interesting – I was amazed to find that even on an 'average' deepest Dec 24hr, when insolation is weakest and external temps approaching the Jan low, SE and SW windows show near-neutral loss/gain, even slight gain. That's with only one hour of direct Dec morning sun at best, mid day to afternoon cut off by the horizon.

My original question, while struggling with Tas, has been superseded by the wonderfulness of new-version http://re.jrc.ec.europa.eu/pvgis/index.htm – thanks so much for bringing that.

We are discovering some very interesting things by number-crunching Plymouth raw weather data to get a handle on the frequency and amplitude of the hugely varying insolation curves of 'typical' winter days, which are always depressingly averaged-out in e.g. PVGIS. The highs are eminently harvestable even in mid winter – as long as you don't immediately suffer loss into the lows that intervene.

[Author]

Posts