[BLDG-SIM] Statistical Estimate of Number of Units ON (peak

Steven D. Gates steve-gates at softcom.net
Fri Jun 4 10:14:49 PDT 1999

While all versions of DOE-2 use an hourly calculational timestep, DOE-2.2
contains a refinement that allows the program to better approximate the subhour
demands associated with packaged rooftop equipment.   As in previous versions,
the program calculates DX unit's energy consumption using an hourly timestep.
Then, the program looks at the zonal temperature profile that occurred during
the hour, and allocates the energy consumption into 5-minute bins according to
the variation of zone temperature within the cooling setpoint throttling range.

For example, consider a zone's behavior during the first hour of operation
(morning startup).  If the temperature at the beginning of the hour is above
the top of the zone's temperature setpoint throttling range, and the
temperature at the end of the hour is within the throttling range, then clearly
the unit must have been running continuously at the beginning of the hour, and
is cycling on/off at the end of the hour.  The program therefore allocates more
of the hourly energy into the 5-minute bins at the beginning of the hour, and
less to the bins at the end of the hour.

This allocation assumes the temperature change within the hour is linear, which
is a reasonable assumption for most equipment (it becomes less accurate for
equipment that is significantly oversized).

The electric utility rate calculations can take into account either fixed or
sliding demand windows, of variable length, with the smallest possible window
being 5 minutes, and the longest being 60 minutes.

The above applies to packaged rooftop equipment serving either single or
multiple zones, and for both heating and cooling.  This logic has not been
extended to central plant equipment (boilers, chillers).

I am one of the original authors of the DOE-2 program, and have been affiliated
with J. Hirsch & Associates since 1992.  I also have 10 years of experience in
designing HVAC systems, and in the configuration and programming of DDC
controls.  Jeff and I are responsible for the majority of the new mechanical
system simulation capabilities implemented in DOE-2.2, including the above.

Michael J. Witte wrote:

> On  2 Jun 99 at 18:51, BKoran at aol.com wrote:
> > If I have a large number of units, the number of units likely to be
> > operating simultaneously will approach the product of the total number of
> > units times the fraction of time each unit is on.
> >
> > I suppose I need to use a threshold probability to constrain the problem.
> > So, with a probability greater than 70%, what is the maximum number of
> > units operating simultaneously?  Even better, what is the maximum average
> > number of units operating simultaneously over a 15-minute period?
> >
> > How do DOE-2, Trace, HAP, BLAST, etc. calculate a 15-minute peak demand?
> >
> DOE-2.1E and BLAST do not do anything special to calculate subhourly
> demand windows.  The electric consumption for the hour is averaged
> over the hour.  As you said, this is a good approximation for larger
> facilities with more than a few units, but it could underestimate
> demand charges for facilities with a few units or in applications
> where return from setback would cause all units to kick on at the
> same time and run for say 30 minutes to cool down the facility.
> Does anyone out there know whether Trace or HAP or DOE-2.2 have any
> subhourly demand tracking capabilities?
> Mike Witte
> ========================================================
> Michael J. Witte                        mjwitte at gard.com
> GARD Analytics, Inc.                        www.gard.com
> 1028 Busse Highway                    Phone 847-698-5685
> Park Ridge, Illinois 60068-1802         FAX 847-698-5600
> USA          Energy, Environmental and Economic Research
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