Considerations for building energy modeling


Figure 3: This shows the output of an energy modeling exercise for a project at the proposal phase. The graphic's intent is to compare three different levels of energy-efficient design to existing benchmarked projects in terms of annual energy utilization intensity (EUI). The circles represent the EUI of three proposed levels of energy performance for the proposed project and compares them to four similar projects executed by the interviewing firm. The smaller the circle, the smaller the energy footprint of the building.Step 1: Benchmarking

Regardless of whether energy information was analyzed in the pre-award phase, most projects now incorporate an “absolute” goal-setting activity during the initial phase, called energy benchmarking. Energy benchmarking compares the program or building shell design to existing building metrics. The most readily available form and tool for benchmarking is the Energy Star Target Finder calculator; it is web-based, free, and easy to use.

Target Finder uses the 2003 Commercial Buildings Energy Consumption Database to score buildings between 1 and 100. An average building comparison to existing building stock would have a score of 50. A project with an energy performance goal of 25% better than ASHRAE 90.1-2007 typically would have an energy score between 80 and 85. The score is determined by evaluating the energy utilization intensity (EUI) in kBtu/sq ft. The location, building type/usage, and gross square footage are needed to obtain a score. The tool has an input for projected/actual energy use, so later modeling results can be input and the “simulated score” developed. The final output includes a printable summary page and a statement of design intent, documenting the projected energy performance.

The benchmarking process provides an alternative starting point to U.S. Green Building Council LEED energy goals based on ASHRAE 90.1-2007. Not all projects pursue LEED certification, and the LEED energy targets are based on a theoretical baseline, annual energy cost. EUI for a LEED target is not final until the baseline model is complete. Energy Star data supports teams that have an early, more tangible alternative to an absolute target to be working toward.

Energy Star uses a publicly available database for benchmarking buildings. An alternative to benchmarking against a composite index of existing buildings is to benchmark against similar projects recently designed or currently designed by entity doing the analysis. Most large architectural and engineering firms have signed the American Institute of Architect's 2030 Challenge, which requires reporting energy consumption for all projects to the AIA for compilation and comparison. The reporting tool for this process requires that partner firms track the energy consumption, cost, type, and square footage. The information can then be built into a usable database for tracking project energy performance. A project benchmarking exercise can thus compare similar projects using similar, current, efficient design technologies and engender friendly competition within a firm or group of firms. Typically, benchmarking summaries list the project name, location, square footage, and EUI along with those of similar peer projects. By using existing projects, the benchmarking phase engages the design team to ask questions about why one project is projected to perform better or worse than another, thereby influencing design at an early stage instead of just validating compliance.

Step 2: Analyzing design decisions

Design and energy modeling teams must adjust and provide more information and through efforts in the early project stages. The effort associated with providing additional early analysis should not detract from a final deliverable that holds up to the expectation of building owner paying the utility bills for 40 years. Energy modeling must be tailored to the overall process and each particular project stage. Spending significant time on the appearance or absolute model accuracy at the initial stages is wasteful. Attempting to build on an initial, expedited interview-level model and polish it through a potentially multi-year design process may also be ineffective. It is often easier to build updated models as the design evolves, or use the original shell to inform the design team about general efficiency strategies. If floor-to-floor heights drastically change or if the footprint expands, it likely will not impact the desired orientation from an energy perspective.

During development of initial design decisions, it's important to understand whether a decision or strategy saves energy and the magnitude of those savings. Modeling in early phases should focus on relative comparisons such as orientation or comparing glazing on alternate facades. Absolute savings aren't as important now as providing positive or negative feedback for decision making. Energy deliverables are typically provided at major project milestones, including 100% design development and construction documents issuance, and include energy usage estimates, annual energy cost information, project assumptions, and systems data. The assumptions and systems data should be reviewed by team members for accuracy, and changes tracked throughout the project. Many government and environmentally minded private entities currently require energy deliverables at various milestones. Proactive and forward-thinking design teams already institute these actions and tools internally.

Table 1: Example of internal benchmarking exercise The table shows internal reporting on energy model accuracy and utility reporting for a completed project. The report allows the project to be quickly reviewed and a determination to be made as to whether further model investigation or site investigation may be necessary based on the predicted energy utilization intensity (pEUI). The new project being compared to other current projects is shaded.Reacting to change

After the issuance of construction documents and development of a final energy performance summary, the interactive energy modeling process becomes reactive. A set of documents has been provided, a performance goal established, and measurement tools and strategies provided. Procurement and construction begins and aspects of the project may change.

For a very energy-conscious client, questions may be asked on a monthly, weekly, or daily basis. If a fan goes from variable to a constant speed, what is the energy impact? If the insulation level of the roof increases by 2 in., what are the cost savings? The energy modeler and the design team are called on to provide quick responses. Often delays in the construction process can cost more than the option being considered. The emphasis should not be on updating a modeling report for every change or proposed change.

View the model itself as the living document and keep adjustments short and correspondence brief. If, hypothetically, every design report or study costs $1,000 a page, no one can afford a weekly updated energy report. However, if an e-mail or brief memo about the projected savings and impact is acceptable, then the process can work quite well. The reactive period typically can include finalizing of utility design incentives or submittal for third-party certification such as completion of energy documentation for the LEED process. The design review process for LEED still dictates somewhat of a retroactive process due to the nature of the energy modeling requirements.

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