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HOMER Pro 3.13

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Constraints

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The Constraints page under the Project tab allows you to modify system constraints, which are conditions the systems must satisfy. HOMER discards systems that do not satisfy the specified constraints, so they do not appear in the optimization results or sensitivity results.

Variable

Description

Maximum annual capacity shortage

The maximum allowable value of the capacity shortage fraction, which is the total capacity shortage divided by the total annual electric load, in %

Minimum renewable fraction

The minimum allowable value of the annual renewable fraction, in %

Operating Reserve

Note: Under most circumstances you do not need to change the values of these advanced inputs. Their default values are appropriate for most systems.

Operating reserve is surplus operating capacity that ensures reliable electricity supply even if the load suddenly increases, or renewable power output suddenly decreases. HOMER defines the required amount of operating reserve using four inputs, two related to the variability of the electric load and two related to the variability of the renewable power. These four inputs are described in the required operating reserve section of the help. The total required operating reserve is the sum of the four values resulting from these four inputs. In its simulation, HOMER operates the power system so as to keep the operating reserve equal to or greater than the required operating reserve. It records any shortfall as a capacity shortage.

Variable

Description

As a percentage of load:
Load in current time step (%):

HOMER adds this percentage of the primary load in the current time step (AC and DC separately) to the required operating reserve in each time step. A value of 10% means that the system must keep enough spare capacity operating to serve a sudden 10% increase in the load.

As a percentage of load:
Annual peak load (%):

HOMER adds this percentage of the peak primary load (AC and DC separately) to the required operating reserve in each time step. It, therefore, defines a constant amount of operating reserve. For example, if the peak AC primary load is 40 kW and you want to ensure at least 8 kW of operating reserve on the AC bus at all times (maybe to cover an 8 kW motor starting load), set this input to 20%.

As a percentage renewable output:
Solar power output (%):

HOMER adds this percentage of the PV array power output to the required operating reserve in each time step. A value of 25% means that the system must keep enough spare capacity operating to serve the load even if the PV array output suddenly decreases 25%. In most cases, the output of the PV array is less variable than the output of a wind turbine, so this input is usually set at a lower value than for the Wind power output.

As a percentage renewable output:
Wind power output (%):

HOMER adds this percentage of the wind turbine power output to the required operating reserve in each time step. A value of 60% means that the system must keep enough spare capacity operating to serve the load even if the wind turbine output suddenly decreases 60%. The more variable you expect the output of the wind turbine to be, the higher you set this input.

Note: To the right of each numerical input is a sensitivity button (shots_sensitivity-button) that allows you to do a sensitivity analysis on that variable. For more information, see Why Would I Do a Sensitivity Analysis?

See also

Operating Reserve

Required Operating Reserve