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Re: [EnergyPlus_Support] Re: creating 'real' weather files - Wind/solar data

Paul,

> Thank you for your advice, i had seen the previous post, but some was not relevant as i am in the UK. However since yesterday i have found more solar data, which reports global and direct radiation, but not diffuse, am i right in thinking that E+ will work with these two variables....but you have reservations about the method used...?
>   

I have no reservation whenever you can obtain locally measured data of 
at least two components (e.g., global and direct, or direct and diffuse) 
and they are quality controlled. But this is rarely the case.
I'm quite sure that E+ can derive diffuse from global and direct 
transparently, but this would be a question for Linda. See also the next 
question below.

> The other major point is that the data is approx 30km from the site, is this too far away? this is the closest with current data however...
>
>   

This is quite good, specially if the terrain is relatively flat with no 
lake or coastal area around.

> I also have reservations about the data i have, from the definitions of the terms from where the data is from, it appears as i read it that global is the radiation from the whole sky, and direct is from the sun only, so if this is true i would expect global data to be larger than direct...as global would include the direct data, however in the data this is not true...sorry if i am making a basic error, but an explanation woudl be very useful.
>   

You understood the definitions correctly. However, there are two ways of 
reporting direct irradiance: either with reference to the horizontal 
plane (in which case you should have global = direct + diffuse, as you 
surmised), or wrt the normal plane to the sun. This is most often 
referred to as 'direct normal irradiance' or DNI. In that case, the 
relationship becomes Global = Direct*cosZ + Diffuse, where Z is the 
sun's zenith angle. Depending on Z, DNI might be larger than Global.

Another reason why Direct could be larger than Global is that one of 
them is not recorded correctly due to instrument failure, etc.

Chris

-- 
Chris A. Gueymard, PhD
Solar Consulting Services
P.O. Box 392
Colebrook, NH 03576, USA
Tel. (603) 237-8550
Fax  (603) 237-5314
http://www.SolarConsultingServices.com



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