Energy Modeling
by Jim Dirkes, PE, BEMP, BCxP
What you aim for… matters.
Even if you aim at nothing, you still get what you aim for! Better to aim carefully.
Energy use in a facility is “modeled” in a computational environment (i.e., software). It is a prediction of facility energy use and, just as importantly, energy cost. In our post on commissioning (HERE), we note that “back in the day,” the design and construction world spent more time planning, thinking, designing, and building – but that’s not the norm anymore! This is precisely why energy modeling has become a valuable tool for projects that want to achieve top-tier energy performance while also staying on schedule and budget.
“The difference between good architecture and great architecture is the time you spend on it.” -David Chipperfield
It’s not just “more time,” though; it’s more quality, impactful time. It’s common for design firms to use energy models to demonstrate code or LEED compliance, but that type of energy model is an after-the-fact effort; everything has already been designed. The model’s purpose is only to confirm which boxes can be checked. It may be necessary, but it does not make a difference. The rubber hits the road when you “put your money where your model is.” In other words, you base design decisions on modeling results.
From a Crystal Ball to Concrete Costs
Energy-based decisions without an energy model are more like a “crystal ball,” in which the designer gazes for an idea of whether one approach or another is better. A simple example of such crystal ball gazing is deciding whether a rooftop HVAC unit with an EER (Energy Efficiency Ratio) of 15 is better than one with an EER of 10. The crystal ball says immediately, “Sure! It’s 50% better.” A good energy model turns crystal ball gazing into a science-based evaluation. It moves the decision substantially closer to reality by including detailed consideration of climate, operating schedule, fan efficiency, peak cooling efficiency, and part-load efficiency, plus several other factors. After these considerations are accounted for and modeled, the “50% better” guess becomes concrete and much closer to only 10-15% better. This, in turn, leads to an evaluation of the purchase cost difference against the annual energy savings, i.e., an ROI. An energy model answers the question “How MUCH better?” so the design team and owner can make an informed decision.
Truth be told, comparing two rooftop units is a pretty simple analysis compared to the (literally) hundreds of interactions that are part of a whole-building energy evaluation. For starters, a whole-building energy model evaluates wall and roof insulation, windows, window area, and glazing types, lighting, complex HVAC systems and controls, occupancy schedules … and the list goes on. You cannot possibly evaluate all of those interactions without the capabilities of a robust energy model.
Who Has Time?
This sounds like a lot of work. Who has time for it? Most design firms don’t have the time in their schedule, nor do they budget for “extra” tasks. This particular song has many verses, but all of them are sung without the experience of creating energy models that inform design.
An energy model created very early in the design process can show that a few inches of additional insulation will save only pennies – thus allowing you to save the low impact extra insulation cost.
A model that shows modest savings for a very costly (albeit sexy) feature on a rooftop unit saves the owner from spending precious capital on that pricey feature.
A model which shows much-better-than-expected energy savings for a control strategy has an ROI that benefits the owner “forever”.
Gut checks cannot justify energy-affecting decisions. An energy model can. Beyond that, energy models often show counter-intuitive results – things you might never suspect apart from creating the model. After you dig into those results, it becomes clear that there are interactions that you never anticipated or that were much more powerful than you imagined. Who knew? Nobody would know without the energy model.
It’s worth mentioning that an early-design energy model does not need to have every piece of the detailed information required for a certification or code-compliance model. Early design models look at relative benefits; 5% more or 10% less. In order to do this effectively, an early design modeler must have enough experience to distinguish between the details that matter more, or less, or not at all. Since many details are not known precisely in the early part of the design, sorting and effectively presenting them is essential; the energy “eco-system” has hundreds of levers, and they’re not all the same size. When done well, these early models guide the design process by informing it with realistic information. You may still choose to have 100% of the wall covered with glass, but that decision will be made with a greater understanding of costs and benefits, making it well-informed instead of a guess.
In short, used early in the design process, it’s easy to argue that an energy model saves more than its cost on any project of consequence. (See HOK study)
Consistent Benefits of Energy Modeling
When starting to model energy use about 40 years ago, I thought it would be “interesting.” My early models were simplified “bin method” calculations using Microsoft Basic and a program called ASEAM, which I modified extensively for the University of Michigan buildings I evaluated. Even those primitive models (by today’s standards) revealed that my intuition was completely off, and helped me make better decisions about improving a design.
Today’s tools are light-years more sophisticated and capable. Our team uses EnergyPlus, the Department of Energy’s flagship calculation engine, combined with DesignBuilder’s graphical user interface (GUI). Still, there are numerous other software tools for which results are validated using ASHRAE Standard 140. None of the tools can model everything, but all of them, when used well, provide objective guidance for building designers.
Our team has seen modeling results that inform against a particular approach almost as often as models which validate an approach. In all of them, the modeler’s “gut check” gets honed and their understanding of the type and magnitude of energy interactions becomes better. And, as is the case with much of life, the 10th model goes much faster than the 1st – and the 100th is even better and faster than that; it’s not as time-consuming as it appears when first starting out.
In Short…
Even for a modest project, an early energy model will enable owners, designers, and construction teams to spend the project’s budget more effectively and efficiently.
Invite your design team to make a modeler part of the team and learn together how to design high-performance buildings more effectively.
