Optimisation Constraint

Accessed from Optimisation Analysis Settings dialog

Optimisation Constraints allow you to impose upper or lower limits on a building Key Performance Indicator (KPI) such as Cost or Discomfort. Examples of constraints that might be applied to an optimisation analysis are "Discomfort hours must be less than 200", "Daylight availability must be greater than 50%" or "Construction cost must be less than $6m".

Note: The constraint KPI can be the same as a KPI used for an Optimisation Objective.

When viewing optimisation results with constraints applied you will find that some of the results generated don't respect the constraints. This is normal, however you should also see that many more new points are generated that do meet constraints because the optimiser favours such design variants when creating new generations.

When using constraints you will sometimes find that the solution will appear not to converge to as clean a Pareto front as occurs without constraints applied. This is likely to be due to the way constraints are modelled as modified objectives in DesignBuilder. In a problem with 2 objectives and 1 constraint there are effectively 3 objective functions which will give rise to a 3-D Pareto front. This cannot be adequately visualised on a 2-D scatter graph, but it is likely that the Pareto Front would be cleaner were it being viewed in 3-D with the constraint as the 3rd dimension.

Name

Your name for the constraint (e.g. "ASHRAE 55 discomfort hours < 200")

Constraint KPI

Select the Constraint Key Performance Indicator (KPI) from the list:

• 1-Cost - total construction cost as reported on the Cost and Carbon tab.
• 2-Operational carbon - annual operational carbon emissions due to fuel consumption in the building. Fuel consumption is calculated through an EnergyPlus simulation. Carbon emissions are calculated from fuel consumption using the Carbon emission factors on the Legislative regions dialog.
• 3-Discomfort hours - number of discomfort hours based on ASHRAE 55 method. Area-weighted average values for individual zones are used for the building level assessment. Note that this objective is not the same as the building level aggregation method used by EnergyPlus in the Summary output html data which adds up hours in which any zone in the building is uncomfortable. If you would like to only consider occupied hours in your comfort analysis then make sure this option is set in the main Simulation options dialog.
• 4-Daylight availability - % of the building working plane having natural daylight levels higher than the threshold defined on the Daylight screen (2% daylight factor by default).

Note: The 4-Daylight availability constraint is not available in current versions.

Less than or Greater than

Select whether the criterion refers to an upper or lower limit.

• 1-Less than - the KPI must have a value less than that entered in the next text box.
• 2-Greater than - the KPI must have a value greater than that entered in the next text box.

Value

Enter the upper or lower limit value for the KPI.

Units

Enter text to define the units of the value. This text does not affect the calculations.