Reference > Marc Rosenbaum's Writings

Understanding the Energy Modeling Process:
Simulation Literacy 101
from The Pittsburgh Papers (2003)

WHY MODEL?

Previous Page: INTRODUCTION

LEED requires energy modeling if any of the 10 points possible under Energy & Atmosphere Credit 1, for optimizing energy performance, are to be attained. However, energy modeling predates LEED, and there are more fundamental reasons to model. I view the energy model as a continuous process that gets more detailed and refined (and, hopefully, accurate) as the design process progresses. In the majority of cases I have witnessed, the model is made operational so late in the design process that opportunities to use it to guide design decisions have been lost, and it is merely an after-the-fact, record-keeping exercise. It can be a powerful tool in an integrated design process. The following looks at how an energy model might be used throughout the design phases.

Conceptual Design

During the conceptual design of the project, energy modeling can provide valuable input. In this process, a skilled modeler might quickly assemble a simplified model of the building, perhaps with a single zone per major occupancy type (for example, classrooms, labs, or offices), that can be used to test the effects of site location, building massing, and building orientation. Imagine comparing two design concepts, one a single-story, west-facing building on a flat, open portion of the site, and the other a two-story design partially built into a hillside facing south. Results might look like Figure 1 (page 110).

This kind of feedback is rarely available early in the design process, which is ironic since this is often where the biggest opportunities are! Many modelers are reluctant to build a model when so much is still unknown. Yet good professional experience combined with the default values built into some modeling software (such as eQuest and Energy-10) can quickly yield a model that may not be terribly accurate, but is perfectly adequate for comparing alternate scenarios—relative differences are more important than deadly accuracy.

Schematic Design

During schematic design, energy modeling allows those involved in the design process to optimize their focus on the most promising energy-saving strategies. Seeing how the energy consumption of a building breaks down by fuel type, task, and building component allows the design team to focus on the major drivers of energy use. As an example, imagine that the schematic design model shows the output in Figure 2.

Heating overall is a small portion of the energy bill, so the focus can shift to cooling and lighting. It would be wise to look at strategies that reduce cooling loads. We know that if cooling loads drop, then the energy needed for fans and pumps will also drop, because less energy is moved around the building. We also know that lighting energy has to be removed by the cooling system, so reducing lighting energy will also reduce cooling energy. A logical next step is to look at the breakdown of the cooling load for the building, which may look like Figure 3 (page 111).

Since the two largest components of the cooling load are solar gain through the windows and skylights and heat gain from lighting, strategies to examine may include: a different building orientation, daylighting, more efficient lighting, more appropriate lighting levels, better lighting controls, glazing with a lower solar heat gain coefficient, and shading for glazing.

Design Development

During design development, energy modeling permits parametric studies to be done. Elimination parametrics is a diagnostic technique that allows a better understanding of the energy use of each building component. A series of simulations are done in which one component of energy use is set to zero at a time. When the results are viewed, a clearer picture of how the building uses energy emerges.

Perhaps our early modeling led us to orient our building so that most of the glazing faces south. For some reason (maybe it’s in a historic district), we haven’t been able to include exterior shading on the building. Figure 4 is an example of elimination parametrics for the building:

The larger the difference between the length of the bar for the base case and the length of any subsequent bar, the more that component affects the overall energy use of the building. We can see from this chart that the biggest impact is from changing the solar heat gain coefficient (SHGC) of the window glass, which is the fraction of the energy incident on the glass that gets inside. This result tells us that if we set the SHGC to zero, the building’s energy use would drop by 29%. This building’s energy use is dominated by its cooling load, which is dominated by the solar heat gain through the window glass. The designer should look at the quantity and type of glass, since appropriate shading has been ruled out for other reasons. The second-largest impact comes from setting the lighting energy to zero. This tells us to look at opportunities to reduce lighting wattage, to use better lighting controls, and to see if we can increase effective daylighting without adding more cooling load due to solar gain.

We can see that increasing the insulating value of the walls, windows, and roof from the base case all have modest effects on energy use. This lets us know that additional investment in the insulation is not a priority compared to other strategies.

This result leads us to study whether we have optimized the type and amount of glass in the building, so we should do another type of parametric study. Here we keep everything about the building constant, but vary the area of glass. The base-case building has 700 square feet of south-facing glass. If we choose to look at a standard low-emissivity, argon-filled glazing, the results might look like Figure 5 (page 112).

We can see that there is an optimum area of south glass at around 600 square feet. This may be because cooling load is reduced when the glass is reduced from 700 to 600 square feet, yet 600 square feet is still sufficient to daylight well. If we really like the additional glass for other reasons, we should look at a spectrally selective glazing that lowers the SHGC substantially, increases the R-value slightly, and lowers the visible light transmission slightly. (See Figure 6.)

The more efficient glazing has a different optimum area, and yields a lower energy use at every area studied. The cooling load is notably reduced, and daylighting is still effective. However, the added glazing comes at a cost, both in cost per square foot of glass and in the additional window area. This study enables us to weigh the increased annual savings against the additional up-front cost. Using energy modeling in this fashion often highlights how each additional increment of energy efficiency usually yields a smaller increment of savings. Sometimes savings in other capital systems offset the increased investment (for example, envelope upgrades in a cold-climate building might permit the elimination of perimeter heating).

In a large building with repetitive elements, one way to simplify the modeling process when doing parametric analysis is to model only a representative fraction of the building. On a recent project, a building with approximately 160,000 square feet of office space was modeled using a representative slice of the building that was 1/24 of the actual area (see diagram, left).

Construction Document

During the construction document phase, energy modeling allows comparison of the proposed design to the minimally code-complaint base-case building. This happens to some extent during the modeling for LEED, but the LEED system is hamstrung by using the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) 90.1 Energy Cost Budget Method, so we can’t take credit for all the savings that occur over what might have been the true base case. ASHRAE 90.1’s myriad rules about what we must do for the base case and proposed case underestimate the real savings that result from thoughtful design (see the paper by Dan Nall presented at the 2002 U.S. Green Building Council conference). Outside of the LEED process, we can use modeling to look at the energy-use reduction that results from all of the strategies implemented, including massing, orientation, HVAC-system upgrades, novel control strategies, fan systems optimization, and glazing area optimization.

Next Page: MODELING INPUTS AND ASSUMPTIONS

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