Hello modeling community,
I've always thought that when trying to forecast an EUI for a building with a whole building energy model, the better approach would be to use 90.1 appx G PRM over Section 11 ECB. While I realize that neither one is intended for energy use predictions per se, I am curious what folks think about using Section 11. Thoughts? Pros/Cons?
Thanks,
Shanta Tucker, PE, LEED AP BD+C, BEMP
In our work we are also leaning towards using cost intensities based on a
model rather than energy intensities to "LEED-proof" our early design
comparisons.
Arpan Bakshi, LEED AP BD+C
Shanta/Arpan,
Forecasting an EUI with energy models is a risky proposition to begin with, though many of us are asked to do so on a regular basis. Hopefully all modelers realize that modeling is best used for making relative comparisons rather than predicting absolutes. That said, a model can give you a much better idea of an EUI for a particular building/program than just pulling a value from the air. Hopefully folks are looking to benchmark data to help confirm the reasonableness of their findings.
To your point about which to use, ECB or Appendix G, I would say neither. If you're trying to determine EUI it should be based on the most representative model you can develop, not necessarily restricted by anything dictated by 90.1. To Arpan's point about tracking cost intensities, that makes sense if LEED is part of the project or the owner is concerned with cost. As engineers/analysts we should be thinking about energy and cost, and in some cases CO2e as well. The final decision making metric very much depends on the goals of your client.
The challenge on many projects is a tension between energy and energy cost savings. We have countless projects that save significant amounts of energy but score terribly in energy cost savings (using 90.1 or not as the reason to look at cost), especially with low gas prices these days. This highlights the importance of the client's goal...if it's about saving energy, the necessary design strategies to do so may or may not also save a proportional amount of energy cost. Thus the conundrum, energy cost as a metric is not indicative of the energy savings potential of a design path, but it is likely indicative of the likelihood for strategies to provide an economic return on investment.
Shanta, we've seen that it's good to use 90.1 App G for code and LEED modeling, and keep track of those Base and Proposed EUI's, but also consider the EUI of a "typical baseline" design that the owner/market would build and a "proposed" model that is as reflective of our actual design as possible. It's a lot to track, especially when you add in the cost numbers for each, but it allows us to paint a more complete picture of the options and their outcomes.
Not sure if that resonates with others but would be interesting to hear what others are doing if something different.
Paul Erickson LEED(r) AP BD+C
I would agree that, if the goal is to generate a realistic predicted EUI, there isn't much difference (or value) in choosing ECB or App G since you're not really after a baseline comparison. Focusing on determining the most likely occupancies, schedules and how the building systems will be operated is probably the best approach when trying to predict actual energy performance. Since I think we all agree on how difficult this can sometimes be, educating the client about factors which can negatively impact performance (and hence qualifying your predictions) is a good course of action.
Maybe this goes without saying, but tenant loads and actual sequences of operation play as big, if not a bigger, factor in actual EUI, than the spec'd performance of systems (this is been the biggest lesson learned from our work with benchmarking existing buildings and retro-commissioning). In general, we have found that tenant loads are 1.5 - 2 times higher than ASRHAE defaults and a lot more if there are server rooms (even small ones). You can also usually chalk up a 10-20% increase in EUI from the "ideal" simulation results (using typical schedules and optimal operation of systems) due to changes during construction, lack of completely detailed sequences of operation, incomplete commissioning, tenant demands on schedules and set-points, and general operational "drift" that building undergo over time.
If you are in fact looking at comparisons, I personally think normalized, absolute metrics like cost and energy intensities provide more value to owners, tenants and operators compared to % savings. Benchmarking predicted performance against something like the EnergyStar Portfolio Manager is an interesting and valuable exercise.
I'm looking forward to hearing what others think about this and hearing what experiences are like out there.
Happy predicting,
Luka Matutinovic, P.Eng., LEED(r) AP BD+C
For our projects, we model a "typical construction baseline". This
model represents the Owner's concept constructed to the average new
construction characteristics based on our experience, input from the
design team and input from the cost consultants/contractors. The
Proposed design model is constructed to match the teams "concept
vision". Schedules are set based on the intended programming and
adjusted for typical diversity for such a facility.
We compare these two models as well as give an indication as how the
Proposed design might compare to the "standard" the project might be
trying to achieve - not necessarily LEED. Costs and energy use
intensity is compared.
The issue with absolute EIUs is an accurate base to compare to. As
Luka mentioned, it would be helpful to normalize the Proposed design
EIUs to Portfolio Manager or another source baseline EIUs.
>> Christopher Jones, P.Eng.
Yes, except we call it the "conventional" baseline for cases where the PRM baseline isn't really typical. That's case-by-case, many of our projects do have more or less reasonable baselines...so don't feel like that's a mandatory thing to do.
Basically the same approach and similar experience.
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP