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# Green roof with Ecoroof module in Energy Plus

Hi,

I am working on project to see the impact of green roof performance into improvement of building thermal quality. I want to see the impact of different plants strategy to the result. Previously, I read both Engineering References and Input Output Reference to have a reliable modeling process.

The model requires input for minimum stomatal resistance, leaf thermal, solar and visible absorptivity, and plant height to represent the plant species properties implemented in the actual building.

I read in the Engineering Reference that stomatal resistance value will be changed according to solar irradiance, soil moisture content and vapor pressure deficit in the plant itself during the modeling process. Regarding the vapor pressure deficit, there is one property of gd which represent the plant specific characteristic and will only be non-zero for trees as shown in the image below. .

However, I didn't see any specific input regarding this coefficient and there is no reference on this value either in the plant database. I want to ask if Energy Plus have some reference value for it, to see whether my type of plants will have significant value of gd or not since I think it will be important to understand the result? How it will work in the algorithm itself to determine the gd for the calculation of stomatal resistance? Is it omitted during the process by assuming that gd will always be zero since the green roof will only have small plants like shrubs and grass?

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You are correct -- there is no specific input exposed in the Green Roof material object of EnergyPlus to set the $g_d$ parameter for plant specific characteristic. Looking at the source code where this equation is applied in EnergyPlus, it looks like $g_d$ is a constant value of 0. This corresponds to most vegetation except for trees, in which case $g_d$ should only increase to 0.03. The source code references a European Centre for Medium-Range Weather Forecasts (ECMWF) document from 2002, but the link in the source code seems to be out of date.

If you truly want to test the impact of $g_d$ between simulations, then you will need to alter this value in the source code to be a different constant or expose this as a new input field for the user to change. Then, you would build yourself a unique version of EnergyPlus with those source code changes. Since it has such a small potential range (0 - 0.03), I think that you will find this would have a very small impact on overall energy use results of an annual simulation.

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