Considerations for building energy modeling


LEED certification

Most industry energy modeling has been a direct result of the LEED certification process and its requirements. The current LEED 2009 or v3 energy requirements involve compliance with ASHRAE 90.1-2007 or California Title 24 as equivalent. Since LEED drives modeling, the majority of processes employed by design firms and energy modelers are centered on the LEED process, and a LEED scorecard is often issued for team review. The scorecard has projected targets for water efficiency, materials, energy efficiency, and so on. The energy modeler or modeling team is given a target goal, based on a percent better than ASHRAE 90.1, and then the design progresses and the energy modeler react to changes from the initial design. Systems selected based on the building‘s program dictate the baseline requirements, and the comparison begins.

Given the rules associated with ASHRAE 90.1 and specifically the Appendix G guidelines for energy modeling, it is often difficult to project multiple performance levels when systems and components are fluctuating dramatically. If the design uses natural gas instead of electric heat, that changes the baseline, even if no other design components have changed. The LEED certification process has required tools for early design collaboration; from an energy perspective, specifically the utilization of a basis of design (BOD) document, which records relevant goals and options. However, the BOD's use, accuracy, and effectiveness are entirely at the design team, owner, and commissioning authority’s discretion.

The LEED energy modeling process is impacted by the LEED review process. For an energy model, the documentation level required for certification is substantial. Virtually all initial submissions receive LEED review comments that must be addressed. Changes may be required on both the proposed and baseline model affecting the points earned. Comments vary from the mundane, such as verifying the solar heat gain coefficient of a window type because it seems low, to very detailed, such as reviewing the district thermal energy guidelines for a building tied to a campus plant—justifying the “system loss” factor calculated for a university steam plant. The LEED review comments may ultimately impact the performance of the models and reduce or increase the performance in comparison to baseline.

Table 2: The table shows how the current proposed design compares to the IECC 2012 baseline building in terms of energy use intensity, measured in kBtu/sq ft/year, and is subdivided by energy-consuming process.Current energy code compliance

Virtually all of today’s required energy code compliance documentation uses a prescriptive Dept. of Energy tool called COMcheck. COMcheck allows the design team to input envelope, lighting, and HVAC data into a free tool that generates a design report. The design team signs the report and submits it with permit drawings to demonstrate energy code compliance. The software is regularly updated from each state’s current energy code.

Recent adoptions of the 2012 International Energy Conservation Code and the equivalent ASHRAE 90.1-2010 in some jurisdictions have significantly increased energy-efficiency requirements. In some instances, design teams have opted to use energy modeling to demonstrate code compliance in place of COMcheck to demonstrate whole building compliance and allow the design to exceed prescriptive glazing ratios or other envelope requirements that could not be met without significant hardship. As energy codes continue to update on a three-year cycle and energy-efficiency requirements become stricter, it will be more difficult for designers to be creative and still demonstrate code compliance without energy modeling. Adopting a process now to inform early design will pay dividends as modeling becomes a requirement in the future.

Energy modeling process and verification of performance

At the conclusion of the reactive energy modeling process, roughly a year after construction is complete and the building has been occupied, direct client interaction should be renewed through a review of building performance. Reviewing and verifying the performance of a recently occupied or new building is commonly known as measurement and verification (M&V). M&V requirements have been published by the Efficiency Valuation Organization under the International Performance Measurement and Verification Protocol (IPMVP). The IPMVP provides multiple options for verification of building performance to be administered by an M&V provider. Option D. Calibrated Simulation, outlines a methodology for comparing modeled building energy usage to actual utility consumption to verify performance.

Clients are becoming more aware of their energy consumption and are regularly analyzing building performance to decrease operating costs. More clients are requesting feedback on anomalous utility bills or energy consumption that exceeded expectations. Proactively reviewing the utility usage as part of an ongoing process, prior to a client reaching out, creates an opportunity for design teams to identify potential issues, examine potential saving opportunities, and better inform the next energy model/project. Measuring and verifying building performance is essential to the process. Clients and building owners will always be skeptical of simulated savings until backed up by actual performance data. Not only will the data improve the modeling going forward, but it also demonstrates that design entities are invested in the successful operation of the building.

A successful, interactive energy modeling process provides useful feedback to team members and introduces energy as a motivating factor early on—benchmarking to establish a goal, simple analysis of design impact decisions, practical reporting on progress, reactive feedback to change, and verification of actual results. Using a process with those steps will better inform clients, design teams, and entities reviewing the energy performance of projects.

Patrick Dempsey is a senior associate, mechanical engineering with CannonDesign

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