Working with Large Models

DesignBuilder has been used to successfully model and simulate very large models including buildings with over 2000 zones and huge floor areas. This topic provides advice on how best to go about working with large models.

Modelling Strategy

While it is possible to create very large, complex building models using DesignBuilder, you should pause to consider what you're trying to achieve by your modelling exercise before diving in and including every detail of the building. Otherwise you may find you have created a beautiful model which is impractical because of long simulation times. You should consider the following points.

EnergyPlus simulations are slowed down by:

 

  1. Larger numbers of windows
  2. Zones with many windows and other surfaces (large number of surfaces / zone)
  3. Radiant heating systems
  4. Calculated natural ventilation using the Airflow Network
  5. Calculating solar reflections
  6. Reading hourly or sub-hourly results for extended simulation periods
  7. Not enough computer memory (RAM)
  8. Large volumes of output data
  9. Large values of timesteps per hour

You can speed up simulations in a number of ways:

 

  1. In early stage modelling it is sometimes helpful to reduce the number of windows in the model by increasing the Window spacing. Shading issues aside, modelling a small number of large Windows has the same effect as a large number of small Windows provided you get the frame area right.
  2. Reduce the number of surfaces by avoiding unnecessary "wiggles" and indents in block perimeters and partitions - keep it as simple as possible. The larger the model the more important it is to follow this advice. Where an indent in the perimeter gives shading then you could instead model this using a single flat surface without indent and use local shading devices with sidefins to model local window shading.
  3. Avoid very large zones with many surfaces and windows. Contrary to intuition, from the simulation time point of view, it is best to split such zones into more zones with fewer surfaces each.
  4. Use the Lump similar windows on surface option to speed up simulations with many similar windows per surface. Note that switching this option on has no effect for zones using Daylight lighting control or if the Full interior and exterior solar distribution option is selected.
  5. Wherever possible lump similar adjacent zones together. Only create zones to model areas of the building with specific environmental conditions, and HVAC systems or internal gains schedules.  A common mistake made by the beginners is to model each room as a separate zone.  This is often not necessary for building energy simulations because many rooms will have reasonably similar operating conditions.  In this case you can use one of the DesignBuilder Merging features to reduce the number of zones.
  6. If there are multiple identical zones then you can use the Zone multiplier to reduce the number of zones processed.
  7. Use Simple HVAC descriptions.
  8. Use the Scheduled Natural ventilation option.
  9. If you need to use Calculated natural ventilation in your simulations then you should aim to minimise the number of cracks and openings involved in the airflow calculations.  One way to do this is to switch off infiltration calculations. Normally DesignBuilder includes a single crack in each surface in the simulation to simulate infiltration. If you have other openings such as vents, windows, doors etc in a particular block/zone you can switch off infiltration. This approach can be used in roof blocks or calculations such as cooling design in well sealed buildings where infiltration is relatively insignificant.
  10. Switching off calculation of solar reflections in the Model options dialog
  11. If you are interested in analysing a single zone within a much larger model, you could consider carrying out simulations on a single representative or worst case zone, excluding consideration of the rest of the building. You can exclude unnecessary zones by unchecking the Include zone in thermal calculations option.
  12. Fit as much memory (RAM) to your computer as possible (8 GB or more ideally). Paging memory to disk during simulations of large models can cause EnergyPlus to grind to a halt.
  13. Uncheck the Allow custom outputs option to speed output variable processing.
  14. Only request the output you really need. This will speed up simulation initialisation and reading of output (especially hourly and sub-hourly) at the end. If you are only looking at hourly results then you can get noticeably faster initialisation and simulations by switching off monthly and daily results. For example, simulation times for a test winter week simulation on a real model were: 


    1. - With hourly, daily, monthly outputs: 39 min
    2. - Only hourly outputs: 14 min
    3. - Only hourly outputs and checking 'Internal surface as adiabatic': 12 min

  15. If you don't need surface data (Walls, Windows etc. heat transfer) switch the Surface heat transfer incl solar output option off.
  16. Restricting hourly and sub-hourly results to short simulations for investigation of detailed building operation. Note that if you request large volumes of hourly or sub-hourly data this can result is a program crash when results are being loaded into DesignBuilder. To avoid this you should switch off automatic loading of EnergyPlus results to DesignBuilder and view results instead using the Results Viewer.
  17. It is sometimes possible to use less timesteps per hour to speed up simulations. Some models can give reliable results using 2 timesteps per hour, however when complex Detailed HVAC systems are being modelled, 6, 10 or more timesteps may be required. If you aren't sure, compare the hourly results you obtain when using a range of timesteps per hour values and choose the minimum value that gives results equivalent to the 10 timesteps case.
  18. If all else fails then you can model very large buildings by splitting them into smaller units and summing results outside DesignBuilder. Note that we haven't seen a need for this since the very early days of EnergyPlus development.

Tip and Techniques

Tip 1: You can speed up modelling, and in particular the drawing and deleting of partitions in large models, by switching off Automatic block zoning option. After having manually rezoned the block you can also save time when renaming the new zones by a) switching off the Double-click to navigate site hierarchy Program option and b) switching off the Automatically refresh site navigator lists Program option.

Tip 2: Another way to speed up modelling in large models is to zoom into the area of the building or block where you wish to work. This eliminates consideration of objects that are not visible on the screen and allows the screen to refresh much more quickly.