December 2, 2011

Multi-Use Facility Energy Modeling: Lessons Learned

James V. Dirkes II, PE, LEED AP, BEMP

Multi-use facilities are everywhere. I had never modeled one of them, but I
had modeled all of the component occupancy types, including a swimming pool;
how hard could it be? Harder than I thought; read on! (Although I use
EnergyPlus and many of you do not, I think that some of what follows is
generic enough to apply to everyone.)

Lesson #1: Heat Pumps are not very appropriate for certain occupancies

ASHRAE 90.1?s Baseline system selection does not care too much about the
specifics of an application. In this case, an all-electric building in
Florida called for heat pumps (System #2 for guest rooms and System #4 for
most of the other occupancies.) The project site in Florida meant that it
is in Climate Zone 2 and did not require an economizer control.

When you combine a heat pump with no economizer and a zone that is dominated
by internal loads, such as a ballroom or meeting room, two things happen:

? The DX coil tries to operate during cold weather (There are lots
of internal loads that require cooling.)

? Millions of warnings arise (200 million in this case), telling you
that your DX coil is producing sub-freezing outlet temperatures. This is
guaranteed to win the favor and confidence of a LEED reviewer.

Without thinking this through completely, the first attempt at solving this
was to turn off the darn DX coils when the outdoor temperature was below 55F
using the EnergyPlus EMS controls. That got rid of most of the warnings

but the unmet load hours went up significantly. After looking at output
variables, it became obvious: The DX coil is OFF and there?s no outdoor air
cooling assist from the economizer. I solved 200 million warnings, but
created another problem which would not win any favor or confidence from a
reviewer.

The next attempt added an economizer for zones which were dominated by
internal loads. Life was better, but not perfect. I was chafing at the
fact that electricity was not being used by the DX coils to provide cooling
during cold weather, because it would reduce ?savings? compared to the
Proposed system, but could not figure out a better alternative.

Lesson #2: EnergyPlus does not like to autosize systems with large outdoor
air loads

While autosizing is neither a new idea nor a difficult one, a wrinkle arises
for energy models that must comply with ASHRAE Standard 90.1?s Appendix G.
Because ASHRAE 90.1 requires outdoor air amounts to be identical for the
?Baseline? and ?Proposed? energy models, I specified the minimum amount of
outdoor air (to match the Proposed systems) instead of autosizing it.

This resulted in several systems with a high outdoor air fraction (20-50% of
total flow). With more outdoor air, the coil entering air is hotter and
more humid and requires a correspondingly large coil capacity compared to
the air flow rate. EnergyPlus imposed its built-in flow / capacity limits
and it reduced the coil size for several zones in order to meet the limits.
Those down-sized DX coils failed to adequately cool the zones they served

and unmet loads resulted.

I tried an assortment of tactics to convince EnergyPlus to size the coils
properly and failed rather miserably. Increasing the sizing factor in
Sizing:Zone made some difference, but I was using 175% for some zones in
order to get noticeable improvement in unmet loads. That meant much larger
fans and a corresponding larger energy use for the Baseline, which seemed
like cheating because it would be a false comparison to the Proposed system.

Experimentally, I told the Sizing:System object to consider those systems as
?100% outdoor air? and saw some improvement in the coil capacity.

Lesson #3: Multiple Occupancy Start Times Breed Large Unmet Load Hours

I considered all of the occupancy schedules, HVAC system types, temperature
and humidity criteria, compass orientation and glazing when defining zones.
This facility is approximately 100,000 sq., ft. and one could have justified
50 zones; I used sixteen. The key aspect of those 16 zones was that their
fan schedules, considering different days of the week, included nine
different starting hours. In many buildings, it?s common and expected that
you will see an unmet load hour during the first hour of operation. When
the entire building starts at the same time, even if several zones have
unmet loads, they all occur at the same time and result in 1 hour of unmet
load. Nine different starting times, on the other hand, opens the
possibility of an unmet load in each of those hours, for a total of 9 unmet
load hours. This multiplication of unmet loads is, I suspect, not something
considered by the 90.1 authors.

One solution would be to make all of the schedules have uniform start times,
but I had other reasons for wanting to retain many of them (See the next
Lesson). My thanks go out to several Bldg-Sim members, who suggested
relaxing the OutputControl:ReportingTolerances from 0.2C to something
larger. The suggestion was based in the understanding that few HVAC systems
are expected to maintain ? 0.2C. More commonly, ? 0.5C is perfectly
acceptable and that?s what I used for both heating and cooling. Combined
with some sizing factor increase, unmet loads became manageable!

Lesson #4: Hotels are not occupied 24/7

Hotels are 24/7/365 operations in many respects. Their HVAC systems,
especially if they are PTAC or PTHP, are not operating in the occupied mode
24/7/365 if the hotel management has any clues about energy conservation.
They?re ON when the room is occupied and OFF or in a setback mode when
unoccupied. On top of this, more sophisticated owners use a guest room
management system which controls HVAC based on room occupancy sensors and
the rental status. My building has such a system and I wanted to represent
this in the energy comparison.

ASHRAE?s 90.1 User Manual and EnergyPlus?s Schedule dataset for a Hotel show
the fans as running 24 / 7 /365. Occupancy, lighting and plug loads are
represented by typical sets of diversity fractions. This approach
inherently:

? Maintains the entire guest room area at a constant occupied
temperature (for the entire year).

? Runs all of the fans constantly.

? Might not account for slow seasons, during which a large fraction
of rooms may be un-rented.

? Is very easy to model.

? Misrepresents the actual energy consumption by what seemed an
unacceptable amount.

? Does not allow the model to accurately represent additional
unoccupied periods made available by a guest room management system.

In order to capture something closer to the actual operation, you could
define a lot of zones and create schedules and diversity which reflect the
occupancy patterns. This was not an attractive option for reasons of the
additional effort to create all of the zones and the additional computation
time that I anticipated would also result.

With input from the Bldg-Sim and EnergyPlus listserve communities, I elected
to continue using the reduced number of zones (16) and approach schedules in
a way that created several discrete occupied periods throughout the day,
similar to that shown in the table below.

ASHRAE?s 90.1 User Manual and its recommended schedules are equivalent to
about 210 fully occupied days per year. The schedule above has the same
number of equivalent days, but has discrete, multi-hour periods where ?no
one is home?. EnergyPlus treats these as unoccupied, begins cycling the fan
instead of running it constantly, and changes the temperature setpoint
accordingly. Cooling, heating and fan energy are all reduced with greater
realism and life is good.

To model the guest room management system?s ability to do a better job of
controlling rooms, this schedule can be easily modified to add more
unoccupied hours. Until I have a better idea, I plan to add 10% unoccupied
hours as a representation of this improved control, similar to the fraction
allowed for lighting occupancy sensors.

Lesson #5: Outdoor swimming pools are different than indoor pools

Modeling indoor pools is a challenge, but at least there is reliable
information about how to calculate the major energy factor, evaporation. An
outdoor pool?s evaporation rate is subject to constantly changing outdoor
temperature and humidity, as well as variable wind speed. None of these is
true for an indoor pool and it does not appear that anyone has documented
their process for modeling energy consumption of an outdoor pool. Little
did I know
.

Starting with a spreadsheet which calculates evaporation rates for indoor
pools using the Shah method, the TMY weather data, and adding spreadsheet
psychrometric functions from NREL, I calculated evaporation for every hour
of the year.

Then I created a Schedule:File which shows the evaporation as a fraction of
maximum. That Schedule is applied to the flow rate for a WaterHeater with a
fixed temperature rise and results in energy for pool heating. At least it
results in something defensible; more work is needed.

This approach does not account for the effect of wind, but pools are
generally somewhat sheltered and I don?t yet have a method for adding wind?s
impact. The pool also loses heat to the ground. I have not addressed that
part yet.

Lesson #6: My fellow energy modelers contribute substantial wisdom to this
growing industry

and I am very grateful to them for sharing their wisdom and insights. 1+1
= 10 in this case! This is not a finished work, but if I waited until it
was, you?d probably never see it. So, I am posting it in hopes that some of
you benefit from my experience. You might also see a better way to approach
these problems than I did. Admittedly, it?s an intertwined mess of factors
and I may have lost sight of important aspects as I scrambled to figure out
what was happening and how to fix it. Comments always welcome!

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