Sefaira workarounds with EnergyPlus sizing

Overview

Sefaira sizing results do not always match the results that come out of the EnergyPlus reports. 

This article explains where we have diverged from E+ sizing and why. It covers the following specific work-arounds:

  • DOAS (Dedicated Outside Air System) Primary Air loads
  • DOAS system heat recovery
  • Zone equipment sizing with DOAS systems
  • DX cooling coils in VAV systems
  • Natural Ventilation Sizing
  • Enthalpy design days

These issues are covered in more detail below.

Note we have only implemented these corrections for our sizing calculations. We still allow E+ to autosize for annual energy simulations and these changes are not taken into effect on those simulations.

DOAS Heat Recovery

When a heat recovery unit is added to a Dedicated Outside Air System (also known as Primary Air System) in EnergyPlus, the impact of the heat recovery is not taken into account when sizing any heating or cooling coils within the unit.

We do not understand why this happens but have asked EnergyPlus to fix it here if you want to vote for it:

https://energyplus.uservoice.com/forums/258860-energyplus/suggestions/32870554-heat-recovery-effectiveness-should-impact-sizing

Sefaira accounts for this manually by looking at the condition of the air after the heat recovery coil at each time-step and sizing the heating and cooling coils in the air handling unit based on that condition rather than on the pure outside air condition.

DOAS Cooling and Heating Coil Sizing

We discovered that in E+ DOAS heating and cooling coils are not sized equally. Originally Sefaira was using the coil size reported from EnergyPlus, but we found that, particularly for some DX coils, the coil would be reported as undersized for the load it was seeing. We also found that comparing two HVAC systems side by side resulted in very different cooling coil sizes for otherwise identical models. 

As a result, we are using the same workaround described above to address this issue as well. We manually determine the load seen by the coils by looking at the entering and leaving enthalpy conditions and calculating the sizing from that difference. We do this for each time step during all sizing days then take the maximum values to determine the sizes.

DOAS Primary Air Load impact on zone equipment sizing

Many DOAS systems supply conditioned primary air for both ventilation and dehumidification / cooling. Good examples of this are chilled beam systems where primary air is supplied at 10-13C (approx 50-55F) at all times.

For these systems, it is important that the heating and cooling units in each zone are sized to take account of the primary air impact on the space. If this didn't happen, then heating and cooling units in the zone would not be appropriately sized. (in the example above, heating could be under-sized and cooling would be over-sized).

EnergyPlus does support a way of doing this but we did not believe it was intuitive or acceptable. In EnergyPlus you can apply a handle that will incorporate the primary air loads into the space load calculations and report it there. We didn't feel that it was appropriate for space load calculations to change based on changes to primary air design and that space load calculations are expected to be agnostic of ventilation and HVAC components.

As a result, when we size the HVAC zone equipment we adjust the size for each time-step based on the primary supply air condition and then report the sizes once that has been taken into account.

We've put a request into EnergyPlus to fix this and it's here if you want to vote for it:

https://energyplus.uservoice.com/forums/258860-energyplus/suggestions/20298838-account-for-doas-air-when-sizing-zone-units

Zone equipment not sized properly

We discovered in EnergyPlus that in some situations it would have dramatically different zone equipment sizing for a model with identical space loads.  See the results below for a VRF fan coil vs a chilled water fan coil unit (models are identical with identical space loads).

different_zone_equipment_sizes.png

We have decided to instead size based on the "Zone Air Heat Balance System Air Transfer Rate" for all fan coil systems. This is an E+ output which measures essentially the amount of heat added or removed at each time step.

This means our results for these pieces of equipment will not match standard EnergyPlus reports.

You can vote to fix this here: https://energyplus.uservoice.com/forums/258860-energyplus/suggestions/20300374-size-zone-equipment-properly-different-coil-types

DX Cooling Coils in Central Air Handling Units are not sized properly

We found a similar phenomenon in some models with VAV rooftop units using DX coils. As with the above, we report the peak load delivered by the coil from hourly EnergyPlus outputs rather than the coil size reported by EnergyPlus.

VAV box position during "off" hours

When a building is "off" we still support HVAC operation in order to achieve a setback condition. The HVAC system in these cases can operate without ventilation. 

We found that for VAV systems we were not conditioning zones after hours. The reason for this was that if the minimum turndown is set to zero during these hours, the box will not open at all, even if cooling is called for.

To fix this, we had to set the minimum VAV box position to 0.001 (ie 0.1% open). This has virtually no impact on sizing and energy but is still a variation that we needed to add to make the system work. It also means that VAV central AHU fans operate to some degree at all hours.

Sizing for Zone units if switched "off" as part of a natural ventilation system

Users can switch zone equipment off if they have a natural ventilation system applied to their model. We implement this in EnergyPlus by setting the schedule for this equipment to be off at all times. However the auto-sizing for EnergyPlus still sizes the equipment as if natural ventilation was not on and as if these units were in operation.

We therefore override this and set capacity for whichever components in the zone are set to off to zero. This may result in our outputs being very different to the EnergyPlus report. It does not mean the model is not running as intended.

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