RE: [EnergyPlus_Support] Re: Modifying a weather file to reflect "real" weather

["James V. Dirkes II, PE" <jvd2pe@xxxxxxx>]

Whoa!  I think I'm over my head already!  I'll take a look at this info and see if I can absorb it enough to understand properly.  Thanks very much.

 

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

 

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From: EnergyPlus_Support@xxxxxxxxxxxxxxx [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Chris
Sent: Monday, June 01, 2009 4:35 PM
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: Re: [EnergyPlus_Support] Re: Modifying a weather file to reflect "real" weather

Assuming you have global horizontal radiation data from a site within,
say, 10 km of your test location, you need to separate the direct and
diffuse components, which can be done with one of many possible methods,
including Maxwell's. Then you have to calculate the sky diffuse and
reflected diffuse components for your walls and other tilted surfaces.
These two steps currently rely on empirical models, which have two known
problems: (i) they introduce considerable random hourly errors and
sometimes significant bias errors, and (ii) you never know where model A
will perform better than model B, precisely because of their empirical
nature.

I recently published on this issue in Solar Energy, using high-class
measured data from Golden (NREL) to test these various combinations.
There were many surprises in the results, but for Michigan, North
Carolina or elsewhere, other combinations of models might have to be
recommended, since the cloud, aerosol and water vapor regimes are different.

In many instances, particularly for prevailing clear-sky conditions, my
tests also showed that it is more accurate to use meteorological data to
calculate the direct and diffuse components directly than to separate
them from measured global radiation.

Chris

>
> In North Carolina, we have a series of EcoNet monitoring stations for
> agricultural research which capture global horizontal radiation. You
> can map this into direct beam and diffuse radiation with the
> correlation developed by Maxwell. See:
>
> http://www.nrel.gov/rredc/pdfs/3087.pdf
>
> I don't know where you can get a ready-made program to do this
> computation. I wrote one myself. It has given nice results in
> comparing measured solar thermal yield with modeled predictions under
> a variety of partly cloudy to full sun conditions using the global
> horizontal value as the input, so I have come to trust it. My program
> is proprietary and not validated by external authority, so I'm not in
> a position to offer it at this point. However, this is one way to
> accomplish what you want if you have a local source of global
> horizontal radiation and ability to turn Maxwell's method into a
> computer program.
>

--
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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