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Re: [EnergyPlus_Support] Green Roof
I'm not sure that using either model makes a big difference. As I understand it, an EMPD model is only going to tell you how your interior surfaces buffer interior air RH conditions. An MTF model is attempting to predict the effects of moisture transfer through an assembly. MTF should be better if the thermal changes in the system also stimulate moisture/energy changes. However, E+ has no reliable moisture quantities and MTF assumes a wet absorbing exterior when it rains. The calculations could fail if the absorption is too great (see the reference about MTF instability). If you somehow adjust material properties to keep the model calculating, you have no real basis for believing that you are accurately simulating the moisture effects anyway. MTF is also simulating moisture flow to the inside air, and some portion of your roof had better be highly resistant to moisture flow, or your roof could not really call it a roof....
You generally may be better off to simply simulate the mass of the system and use some shading to limit the solar exposure of the roof (that is the resultant effect of the vegetation anyway). You may be able to "schedule" shading in a manor that more closely resembles the seasonal variations in roof moisture and plant growth. The problem still remains that you will have to have a better understanding of the green roof thermodynamic performance from other sources because you will not get it directly from E+. E+ can only tell you the impact of your roof shade assumption on the building's energy use.
Ned Lyon
Senior Project Manager
Simpson Gumpertz & Heger Inc.
41 Seyon Street
Building #1, Suite 500
Waltham, MA 02453
Phone: 781-907-9000
Fax: 781-907-9009
>>> fluizp@xxxxxxxxx 5/21/2004 11:45:48 AM >>>
About green roof simulation, we would like to make another two questions:
1) If we use CTF or EMPD model, what aproximation would we achieve?
2) What would be the best model to use?
=====
___________________________________________
Engº Fábyo Luiz Pereira
UFRGS - Laboratório de Vapor e Refrigeração
www.mecanica.ufrgs.br/lafrig
Rua Sarmento Leite, 425 - Sala 203 - Centro - 90050-170
Fone: +55 51 3316-3167 - Fax: +55 51 3316-3355
Porto Alegre - RS - BRASIL
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