[bldg-sim] VAV vs. Constant volume cooling energy

[Mike Roberts]

Anna,

In your experiment, did you use 5 PSZ systems (one for each zone)?  If not,
one zone will be the thermostat zone and control the supply air temperature.
You will probably have temperature control problems in the other zones.

If you are not already looking at it, take a look at the PS-E report in the
.sim output file. This report breaks down the total building energy into
several components including lighting, heating, cooling, fans, etc.  I like
to pull that report into an Excel spreadsheet and create a stacked bar graph
to show the monthly energy consumption.  eQUEST can do the same thing with
the Single Run Report, Monthly Energy Consumption.  Seeing how much energy
you are using for cooling, fans, lights, etc. can be very helpful.

Of course many of the components of the simulation model will affect energy
use in other components.  For example, the default static pressure for a VAV
system is higher than for a PSZ system.  That higher static translates to
fan heat and thus more cooling.

I am not real sure what you are looking for, but, in general, I would expect
economizers to save energy for either system.  I would expect a VAV system
to use less fan energy, even with the higher static, because it does not
have to move as much air in total.

The control sequence for a VAV system will be as follows:
1.  The supply air temperature comes first.  It will be fixed, reset by OA
temperature, scheduled by you, or set to meet the requirements of the
warmest zone.
2.  The economizer will mix outside air and return air if OA temperature is
below a maximum (around 65 degrees).  It will try to bring the mixed air
temperature to the supply air temperature.
3.  If the supply air temperature is not as cool as required by step 1., the
cooling coils will bring the temperature down to that temperature.
4.  The VAV box at the zone will throttle the air between 100% flow and a
minimum flow (defaulted or specified by you) to maintain proper space
temperature.
5.  If the minimum flow will overcool the space, reheat kicks in to bring up
the supply air temperature to maintain the proper space temperature.

That is the simple VAV.  You can make it more complicated with fan powered
VAV boxes, or a reverse acting thermostat.  Some of these options allow you
to increase air flow above minimum when heating.  That can be helpful in
some situations, such as low minimum flows or climates cold enough to
require heating.

As I said earlier, I am not real sure what you are looking for.  I hope I am
not heading off in the wrong direction.  Let me know if I can be any more
help.

Mike

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Zhou, Anna
Sent: Monday, April 12, 2004 7:55 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV vs. Constant volume cooling energy


Mike,

I did experiment by using a 5-zone 1-story building, assigning each zone to
a system in both In the PSZ system case and the PVAVS system case and
resetting the supply air temperature to a higher upper bound to avoid
reheat. it does bring down the cooling energy consumption of the PVAVS, but
the total energy consumption is still higher (each month) than the PSZ.
This is when both PSZ and PVAVS are using OA-temp economizer.

Then, by disable the economizer, the cooling energy consumptions from PVAVS
and PSZ are brought closer to each other. This shows as if the control of
PVAVS in DOE2 tries to reduce air flow first instead of using more outside
air when the outside is cool.But still the PVAVS cooling energy is always
slightly higher than PSZ (under California climate zone 1).I just cann't see
a case that PVAVS uses less energy than PSZ.

Also, doe this say when there are economizers,it's better to use constant
volume rather that variable volume? What's the typical control sequence of a
VAV system among: volume decreasing, temperature reset and economizer? Can
we specify in DOE2 the sequence we want?

Thanks for your reply.

Anna


-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Mike
Roberts
Sent: Monday, April 12, 2004 4:16 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV vs. Constant volume cooling energy


Anna,

A PSZ (Packaged Single Zone) system is constant volume, but only has to cool
when the space calls for cooling.  A VAVS system cools the air all the time
(or may use an economizer part of the time), then reheats the air to the
temperature the space requires.  You can improve on this by resetting the
cold deck temperature, but will probably not get your cooling below a single
zone system.  For comparison, try a RHFS system.  That is a constant volume
system, but cools all the air, then reheats it to satisfy the zone.  It will
generally use more cooling than the VAVS system.

Both DOE-2.1 and DOE-2.2 have some fairly extensive documentation on
controls, related to how the programs control real world systems.

Mike

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Zhou, Anna
Sent: Monday, April 12, 2004 6:34 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV vs. Constant volumn cooling energy


The simulation software was DOE2.2.

-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of Zhou, Anna
Sent: Monday, April 12, 2004 3:19 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] VAV vs. Constant volumn cooling energy


Hi,There

When doing simulation, I noticed that the cooling energy consumption from a
variable air volumn system (VAV,PVAVS) is always larger than that of a
constant volume system (PSZ). Did anybody have this problem, too? Can
anybody tell me what's the possible cause of this?

Thanks a lot.

Anna

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