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





Direct is defined as the insolation on a plane that is perpendicular to the incoming ray, so it's measured by an instrument that continuously moves during the day so as to stay pointed directly at the sun. That instrument is collimated so that it "sees" only the sun's disk and a small circumsolar region, so doesn't measure diffuse. On the other hand, global is measured by a stationary instrument perpendicular to the earth's surface. It catches the fraction of the direct beam that is the cosine of the angle between the incoming solar beam and the horizontal, plus the diffuse. Though the two are often numerically similar during the middle of the day, the direct beam is much higher early and late in the day.

It helps to see graphs of these to get the relationships in mind. The NREL monitoring station has a great web site that is very instructional on a sunny day.

http://www.nrel.gov/midc/srrl_bms/

Check out the graphical display!

Ed


--- On Wed, 6/17/09, chainsaw367 <chainsaw367@xxxxxxxxxxx> wrote:

From: chainsaw367 <chainsaw367@xxxxxxxxxxx>
Subject: [EnergyPlus_Support] Re: creating 'real' weather files - Wind/solar data
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Date: Wednesday, June 17, 2009, 8:14 AM

Chris,

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...?

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...

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.

Thank you again for your advice,

Paul

--- In EnergyPlus_Support@ yahoogroups. com, Chris <chris@...> wrote:
>
> Arslan,
>
> > Please take a look at page three of the attached document and under
> > the section "Solar and Shading Calculations" you'll find that E+
> > calculates all the radiation values needed from your location and
> > geometry input.
>
> I beg to differ. E+ calculates the "plane-of-array" irradiance using the
> Perez model. But its inputs are the direct and diffuse horizontal
> irradiances, which need to be known beforehand.
>
>
> Paul,
>
> < I have found some wind/solar data for local weather stations, but there are problems, like the solar data was only recorded until the start of 2007, and the diffuse horizontal radiation has been recorded at zero for the whole year, and i cannot seem to find out if this is a real measurement, or that it has not been measured.
>
> Diffuse must be strictly positive any day, so these are missing values...
> There was a recent thread on how to obtain "real" radiation/weather
> data, which you should check.
>
> 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.SolarCon sultingServices. com
>




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