Dear friends,
I am modeling the active chilled beams system using an induction unit. I
have created a separate treated outside air system (TOAS) from where the
fresh air would be supplied to
the induction unit. I have hard coded the fresh air requirement for each
zone at the outdoor air CFM field under outside air tab as well as under
design flow cfm field under air flow tab
so that the only the fresh air requirement is supplied to the zone. The
cooling will be done by chilled beams modeled as induction unit.
when I see the simulation report the amount of fan energy consumed is huge
which is nearly about 20% percent of the overall energy consumption.
So then I zeroed out the fan energy i.e kW/CFM for fans =0 and only the TOAS
has the fan energy but the fan energy consumption decreased very little.
I have no other source in the model for which the fan energy could be
accountable for.
Can anyone please tell what can be the reason for this or if I am missing
out on something?
Am I modeling the system right? How is the fan energy for this system to be
modeled?
Vishnuraj Nair
Vishnu,
I am not sure I completely follow what you did but perhaps you are not aware
that the induction unit system includes a central fan for pretreating
outside air delivered to the induction units. So if you used the "outside
air from sys" approach,the central fan of the induction unit would still be
modeled with its default parameters. Also, if you don't set the induction
ratio high enough (e.g. default is 3, max value is 10), you will get return
air mixing with your supply (e.g it will no longer be 100% OA), which will
make your supply air flow higher. This will increase your fan energy use as
well.
I don't think the eQUEST documentation includes a description of the
induction unit system (TPIU or FPIU) but its relevant keywords can found in
the DOE2.1E BDL Summary. You could also insert the following commands in
your input file.
DIAGNOSTICS DEFAULTS .. $ before the SYSTEM command and
DIAGNOSTICS CAUTIONS .. $ after the SYSTEM command
Adding the above will show in the BDL file the value for all keywords not
explicitly listed but with default values being used.
Ellen
The other thing at work here is that the Induction units and the DOAS are
constant volume, so you're going to pay a fan energy penalty for this - so
first, make sure you only have one zone per Induction unit in eQUEST. In
real life, the DOAS can send variables amounts of air (based on load) to
chilled beams, but this can't be modeled with the Dummy zone/DOAS approach.
If you change the system type to a Power Induction Unit (which is a VAV) and
you zero out the fan power, this might work.
There is no induction ratio parameter then, but I have played around with
that parameter in eQUEST and seen almost no impact from varying it.
I am attempting to modeling an active chilled beam system served by a VAV
DOAS right now, so if anyone else has comments on the Power Induction Unit
approach, please let me know!
Kendra Tupper, PE, LEED AP
Kendra,
?
I don't think the PIU approach is really the same -?for one you are not modelling the cooling at the zonal level with chilled water like a chilled beam works.
?
I haven't modeled a chilled beam with a VAV supply before - mostly because in the projects I've used chilled beams (labs)?you restrict the supply to only ventilation air which is a constant set normally to around 6ACH. Have you tried using the Induction unit and assigning a schedule to the fresh air? I think you can vary the amount of outside air to each zone that way.
?
Vishnu - if this is a lab building 20% for fan energy is not completely out of the question. Couple of things to check:
?
1. When you come out of th eQUEST wizard the fan energy tab is normally described in terms of static pressure and not kW/cfm. Make sure you set those to default to activate the kW/cfm. You might have already done this because the kW/cfm option generally doesn't show up unless you do this.
2. to check if the fan energy is coming from your DOAS - try zeroing out the fan energy on the DOAS and see if that reduces your fan contribution. That way at least ?you'll know if your barking up the right tree.
3. If this is a lab, make sure you model your exhaust correctly. While Chilled beam systems don't use fan energy on the supply side, the labs will use fans on the exhust side. These will need to be captured on the zonal exhaust level.
4. Unless your supplying air neutral from the DOAS, you will need to capture the cooling effects of the supply air (Kendra - this is where it gets tricky for a VAV supply).
?
Hope this helps
Vikram Sami
?
Oh, I know its not the same - I just couldn't come up with anything that
solved both problems! Trading one evil for another I suppose.....
This is an active chilled beam system in an office building, so the outside
air supply is varying based on cooling load and CO2 sensors - the mechanical
engineer is used VSD ECM motor series FPBs to modulate the outside air
delivered to the chilled beams at the zone level. so it would be impossible
to manually schedule the outside air according to load (that's why we're
using an energy model!). Modeling constant volume induction units conditions
far more outside air than in the actual design, so I figured it might be
more important to account for this, vs the chilled beam cooling at the zone
level.
Kendra Tupper, PE, LEED AP