Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
You are correct -- there is no specific input exposed in the Green Roof 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Centre for Medium-Range Weather Forecasts (ECMWF) document from 2002, but the link in the source code seems to be out of date.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.
| | 2 | No.2 Revision |
You are correct -- there is no specific input exposed in the Green Roof object 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Centre for Medium-Range Weather Forecasts (ECMWF) document from 2002, but the link in the source code seems to be out of date.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.
