What does the yellow exclamation point that I see in the result mean?
Occasionally an optimized system will show a yellow "!" warning icon in the sensitivity case and optimization case tables.
The warning occurs for systems that have potential instability due to high renewable energy penetration. This can be caused by two conditions:
- a high renewable penetration with no storage and may require additional analysis to determine appropriate controls for the final design, or
- a high renewable penetration with storage, but the storage has a low system autonomy.
Integrating high renewable penetration systems requires special considerations to assure system stability. The simplest, but not necessarily best, way to do this is by using a large amount of storage.
HOMER calculates the instantaneous renewable penetration by dividing the total renewable electrical power output (in kW) by the total electrical load served (also in kW) in each time step. It displays the maximum value under the Electrical tab. For systems with storage, HOMER calculates the battery autonomy by dividing the battery bank’s nominal capacity by the average load. It displays this under the Battery tab.
The values of the parameters that trigger the warnings can be set by the user. They are found under the menu ribbon System > Project > System Control tab. HOMER provides default values that are a rough guide for when more detailed modeling may be required. That situation requires a different kind of model that consider the system’s transient responses to rapid changes in the load balance. See High Penetrations of Renewable Energy for Island Grids for more information on this situation.