I will start out by saying I am not an EnergyPlus expert. I am trying to
convert one of the DOE commercial building E+ prototypes from a CAV system
to a VAV system to determine the energy savings. I have modified the
baseline CAV model for one of the zones thus far to a VAV system, but the
VAV model is using more energy than the CAV model. I also have a lot more
unmet hours for both heating and cooling. I cannot figure out what is
causing these issues (I assume they are related). My current guess is that
I continue to use the same cooling coil (Coil:Cooling:DX:SingleSpeed) in
the VAV model, rather than converting the coil to Coil:Cooling:DX:TwoSpeed
as I see in most of the other DOE prototype VAV models.
My questions are:
1. Is it ok to use the Coil:Cooling:DX:SingleSpeed in a VAV system, or
should I convert that to a Coil:Cooling:DX:TwoSpeed?
2. If it I convert the VAV model to the Coil:Cooling:DX:TwoSpeed is this
analogous to modifying the baseline compressor to a variable speed
compressor?
Thanks for any help.
Ryan
Ryan,
In terms of debugging your model, it would be constructive to know where the energy use differs: increased heating, cooling, or fan energy? Also, you may want to take a look at the table file and double-check that the coil and fan sizes, and design OA flow rates are identical between the two models. It's possible that in your conversion you modified something that is causing your system to size differently (presuming you're not hard-sizing everything). That's where I'd start.
Thanks,
Daniel Studer
Thanks for the quick reply Daniel.
I have checked the Report: Equipment Summary tables. Here all is identical
between the two models, except for the Heating Coil for the zone I
modified. For the VAV system, the Heating Coil nominal capacity is 28,369 W
vs 68,845 W for the CAV system. All the other heating, cooling and fan
systems are sized the same between the models.
In the Report: HVAC Sizing Summary everything is identical.
For the zone I switched the Time Setpoint Not Met has increased
significantly for both cooling and heating, and during occupied and
unoccupied periods.
In the Outdoor Air summary: both mechanical ventilation and infiltration
are minimally different in all zones (differences are in the hundredths).
As for where the energy differences are:
Fan Energy: slight decrease in overall fan energy (I have only changed a
small zone)
Heating: increase in heating energy
Cooling: increase in cooling energy
I think this sums it up. From this, maybe the issue is with the heating
rather than the cooling coils. There is a significant difference in the
heating coil nominal capacity.
Ryan
Hi Ryan,
Check VAV minimum airflow set point within the zone which might be a big
reason to increase the reheat load and the cooling load accordingly.
Regards,
Wahid H. Mohamed, LEED AP BD+C