[Bldg-sim] Load Calcs with Energy Simulation Software

[Michael Haughey]

Hi Dan,

 

I would clarify two things.  First is that the load calculations are done,
or should be done, for the design condition, not the worst possible
condition plus safety factor.  For cooling that normally means the ASHRAE
97-1/2% condition; for heating, the 2-1/2% condition.  Some programs list
adjustments as safety factors, but there is really a higher purpose.  One is
a morning warm-up factor that allows night setback energy savings to be
realistically achievable.  Similarly, a cool-down factor can be applied to
the cooling load and comes into play often on Monday morning when the system
has been off over the weekend and the building mass has gotten quite warm.
If you can't cool it down, then you can't save as much energy by letting it
get or stay warm for as long prior to occupancy.  The warm-up and cool-down
factors are largely a function of the space mass (the greater the mass, the
larger the factor needed).  There is also the possibility of either adding
capacity for future building or process load additions or designing for the
ability to add the capacity in the future.  It is best to do that in a
modular way the does not penalize efficiency until the future load is
on-line.

 

Second is that load programs do have schedules for people, lighting,
appliances, and power equipment.  It is proper to set those realistically
for the highest anticipated conditions, but not higher.  If set higher, or
at 100%, then the result will have compounding additional capacity (safety
factors multiplied upon each other resulting in grossly over-sized systems).
Heating is traditionally treated differently in that internal loads are
normally not left on for the design condition.  It is possible to turn off
all lights and internal gains during a heating requirement.  Cooling is a
little different in that some loads are simply never all on at the same
time.  People move throughout the building and they also come and go, so
diversity factors are also appropriate.

 

I would expect, or at least want, a program that does both load calculation
and energy modeling to be able to account for morning warm-up and cool-down
(including after a power outage) by sizing the systems to have sufficient
reserve capacity to recover in a reasonable period of time (and have that
time adjustable).  It would need to consider building mass, and look at
setback/setup differentials vs. recovery time and reserve capacity.

 

Michael Haughey,P.E., LEED AP

Silvertip Integrated Engineering Consultants

303-650-1080

mhaughey at earthlink.net

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Dan
Katzenberger
Sent: Friday, January 25, 2008 11:09 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Load Calcs with Energy Simulation Software

 

Jonathan,

 

There are several differences between load calculation and energy simulation
programs, some of which others have mentioned.  Some other differences
include:

 

Summer load calculations are done at the worst summer design conditions
(outdoor air temperature, outdoor humidity, solar gain, etc.), and all of
the internal gains are assumed to be on 100% (e.g., people, lights,
equipment, etc.).  Energy programs use schedules and may not have 100% of
the internal gains on during the peak summer conditions, thereby suggesting
smaller equipment sizes.

 

Winter load calculations are done at the worst winter design conditions
(outdoor air temperature, nighttime, etc.), and the internal gains are all
assumed to be 0% (e.g., no people, no lights, no equipment, etc.).  Energy
programs use schedules, and may not have 100% of the people, lights, and
equipment off during peak winter conditions.

 

Some load programs allow the user to add a safety factor to the heating and
cooling loads (not just the equipment size).  So the heating/cooling load in
your zone is increased, therefore your air supply cfm is increased, your AHU
is increased in size, your plant size is increased, etc.).  I don't know how
to do this directly in any of the energy analysis programs that I use.

 

Because of these and other differences, I have found that energy modeling
programs will give smaller equipment sizes than do load programs.  Engineers
typically error on the side of safety, and therefore they prefer to size
equipment for the worst possible scenarios as described above.  When I was
designing building mechanical systems, I would never count on 25% of the
lights being on at night when it's -20 degrees F outside so that I could
reduce my boiler size.  My loads program would give me a larger boiler,
whereas my energy modeling program would give me a boiler sized as if the
lights will be on.  Also, the local TMY2 weather file that I use for energy
modeling doesn't even have -20 degrees F as an outdoor air temperature, and
this is the temperature that many engineers use to design their heating
systems.  M loads programs allow the outdoor design conditions to be input
directly.

 

Load programs require much less input than energy programs, and generally
don't require any special knowledge, art, or workarounds.  Energy modeling
programs require much more input, require very specialized knowledge and
experience, and always require workarounds and creative inputs (art) to get
the correct results.

 

All that being said, Trane's Trace 700 program does an acceptable job of
being both a loads program and an energy analysis program, although it does
have strengths and weaknesses in both areas.  In Trace, when running loads,
you can disable the energy related inputs.  After you are satisfied that you
have the correct loads, then you can proceed to working on the energy
parameters and inputs.  You can easily switch between running loads only and
loads + energy at any time.

 

Dan

 

Daniel A. Katzenberger, P.E., CEM, LEED-AP 

Principal & Certified Energy Star Green Building Engineer

 

Engineering, Energy, and the Environment, LLC

"The Green Group"

420 North 5th Street, Suite 305, Minneapolis, MN 55401-1348

(612) 327-4210

 

  _____  

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Jonathan Curtin
Sent: Friday, January 25, 2008 3:25 PM 

 
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Load Calcs with Energy Simulation Software

Hello All,

Would anyone out there like to share experiences or precautions in using
simulation models (particularly eQuest, DOE2.1e, EnergyPlus) for load
calculations? Any advantages/disadvantages, tricks, or warnings in using
these simulation models as opposed to dedicated load calculation software
like Elite's CHVAC? Any studies comparing the two?

 

Thanks,

 

Jonathan Curtin EIT, LEED AP

 

W  Y  L  I  E 
C O N S U L T I  N G
E  N G  I  N  E  E R S

6161 Savoy, Suite 700   Houston, Texas 77036
713.781.2526     713.781.2536 fax

www.wylieassociates.com <http://www.wylieassociates.com/> 

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