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 gd parameter for plant specific characteristic. Looking at the source code where this equation is applied in EnergyPlus, it looks like gd is a constant value of 0. This corresponds to most vegetation except for trees, in which case gd 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Centre for Medium-Range Weather Forecasts (ECMWF) document from 2002, but the link in the source code seems to be out of date.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 gd 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 gd parameter for plant specific characteristic. Looking at the source code where this equation is applied in EnergyPlus, it looks like gd is a constant value of 0. This corresponds to most vegetation except for trees, in which case gd 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Centre for Medium-Range Weather Forecasts (ECMWF) document from 2002, but the link in the source code seems to be out of date.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 gd 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.
