Solar Exposure: Toronto Solar Radiation Levels Accurate?

[Anonymous]

I am working with the Toronto Weather file that comes with the software and I have noticed large discrepencies between the solar incidents given by the software and the one given by NRCan's Retscreen tool per facade. The model set up to produce this is just a cube, with no other factors at play.



Direct Solar only is unchecked, as is ground reflection.



Can anyone explain this or give reasons why it may be so different? If these numbers are incorrect, how can I have confidence in any outputs given by the software?



Image of graphs of output are attached.



THANKS.

[drajmarsh]

Hi Kay,

Without much to go on, my first guess would be that the issue lies with the time period over which the calculations are taking place.

Looking at the two graphs, the values for the roof in July are above 6000Wh/m2 in RetScreen and about 4700Wh/m2 in Ecotect. Given that the maximum instantaneous radiation at the Earth's surface is about 1100-1200W/m2 and the latitude of Toronto, I would guess that the period is about a single day or perhaps an average day.

If it is a single day then, due to cloudiness and local conditions, the hourly weather data can vary significantly so it would be a good idea to compare the actual daily profile for direct and diffuse from both RetScreen and Ecotect (you can use WeatherTool for this). If it is an average over the whole month, then I would expect the values to be closer.

You may also want to check the exact time period used in Ecotect - it may be set 0800 to 1800 which, in July, may be ignoring a couple of hours just after sunrise and just before sunset. Better to use a period 0000 to 2400.

If these don't solve the problems, can you post the two weather data sets you are using and we'll have a closer look.

Kind regards,
Andrew

--
Dr. Andrew Marsh
Principal Software Engineer
AEC-Simulation
Autodesk, Inc.