[Bldg-sim] Energy Models for Existing Buildings

[Jim Dirkes]

Merci, Mssr. Beaulieu.

I agree about using an 8760 hour simulation.  The big issue is how to define
the level of importance for various inputs and how that affects you speed of
preparing the model. I think certain information can be assumed with
reasonable confidence, some must be measured and some can be figured out by
trial and error.

As you mention below, many owners / operators have no clue about the details
of their systems' operation, so you must attempt some trial and error to get
a good fit, or spend a lot of time in the building.  I like spending time in
buildings, but my clients are not too excited about paying me to do it, so
I'm looking for reasonable shortcuts.

As an aside, I think benchmarking is helpful if your building happens to fit
one of the standard categories.  It is NOT helpful to understand why the
building varies from the category mean.

I am now walking the path of a three step accuracy level for an energy
audit.  

1.       The first step is a "drive by" audit using basic building
dimensions, age and HVAC & lighting system information along with utility
bills.  (No utility bills = no confidence about predictions.)

2.       Step 2 is gathering more detailed information from occupants about
schedules and gathering data about key energy equipment ("gathering data"
means measuring motor power and key operating temperatures, and observing
outdoor air damper positions)

3.       Step 3 gather yet more detailed information when Step 2 results in
more than 10% difference between annual and predicted energy plus worse than
75% R-squared correlation month-by-month.

 

Step 1 can be considered a measure of opportunity for the owner and is
supposed to be very fast and inexpensive.  Step 2 confirms and quantifies
the opportunity, and includes a moderate amount of additional effort.  Step
3 is even more effort and more costly.

I think many owner / managers are hesitant to invest much money when they
are unconvinced that there is any sizable opportunity / savings.  Once it
looks promising, they are more likely to fund additional work.  More likely,
but not highly likely, so that step still needs to be a modest cost
increase.

The just-delivered ASHRAE Journal has an interesting article on
"Streamlining Energy Simulation To Identify Building Retrofits", which is
very timely for me J. The authors did a fairly detailed audit of a 700,000
sq.ft. Federal office building in ~300 hours.  Check it out!  I think we
could have done better, but they seem to have done a very good job  ;)

 

 

 

The Building Performance Team
James V. Dirkes II, P.E., BEMP , LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of JP Beaulieu
Sent: Monday, November 14, 2011 1:21 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Energy Models for Existing Buildings

 

Hi Jim,
Here are my thoughts on the subject. That'll help me sort things out as
well.

I don't do an audit without a full year hour by hour simulation. Why?
Because we offer E Audits as a means to optimise a client's building and to
give all the details to the client in order for him to make a good decision.
BTW, we do small and large commercial and institutionnal bldg simulations
and have been using EnergyPlus for some time now.

I must say that the solutions we are proposing call for something flexible
like EnergyPlus. I was using other softwares before but I must admit that
there where times when there was little advantage of using simulation
because we had to do the savings calculations on the side using much
guessing or assumptions. Now I can make any system I like. Using bldg
exhaust air or outdoor air or a mix of the two to feed the garage
ventilation system would be easy and take out a lot of uncertainties. 
I think that other softwares are fine if you just want to compare two
systemes performances in a real life bldg in a specific climate and with
specific uses. When I think of e-sim as a set of equipement (HVAC) for a
specific bldg (walls and windows, etc.) used in a specific way by the
occupants (schedules, plug loads, etc.) I realise that there is much more
interpretation and uncertainties in the latter anyway. So why bother with
hour by hour e-sim if you are not looking at what's happening every hour. 
In optimisation however, I need to look at what's happening every hour.
While most other softwares are running 8760 hours most do not give you a
good visualisation of the results on that scale (i.e. hour by hour). I want
to be able to graph a pumps runtime with a room's exess heat, concurrent
cooling and heating loads, exhaust air of one room compared to the intake of
another... This is why I use hour by hour e-sim. 

You mention that you are looking for the big picture. I agree but...I tend
to be carefull at not adding (and often multiplying) uncertainties.  you
know what I'm talking about. I think I change my strategy depending on the
type of building use or construction. I won't be looking too much at the
pumps if there are only a few. I might consider more the wall construction
of an older bldg, But bottom line, why bother with a pump's specifications
if you're not even sure of its runtime. What do you think?

Also, matching the bldg's energy bills helps me to validate some schedules
and HVAC operations mainly. I don't really want to be close to the bills
each time. 30% can be acceptable if you have some explanations. I look at
the patterns of real vs simulation data : monthly energy use distribution by
source and by end use, peak power demand monthly. I would like to use
benchmarking more but I didn't find much valuable information for canadian
bldgs yet. 

Anyone has good BENCHMARKING data for canada? by end use mainly. 
Energy Portfolio Manager (EPM) will now collaborate with Canada gov. I hope
that is good. I used it once and had a major issue with source energy since
the bldg was 100% electric. Electricity is 90+% hydro here in Montreal.

"How carefully must the energy model match define ECMs?"
I'd say how important are the results in making the right decision? also,
what does it take to have more precise information? It's a tradeoff between
the two.



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