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2018 Edition, 2018

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Building System Efficiency Modeling — Improving the Accuracy of Building Energy Modeling: A Study of the Relative Strengths and Weaknesses of Building Energy Modeling Tools and Recommendations for Improvements

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Description / Abstract:

Executive Summary

Building energy modeling (BEM) tools are used to document compliance with energy codes, in green building certification, for incentive programs, to help optimize new designs, and to inform project retrofits. Organizations like ASHRAE and AIA advocate for the wider use of energy modeling, viewing it as essential for achieving low-energy and net-zero energy buildings. However, discrepancies between modeled and actual building performance have been widely reported, reducing credibility and bringing into question the feasibility of relying on energy models for decision-making

This paper explores reasons for disagreements between the actual and modeled energy use for several building systems, including lighting, motors, and controls. It highlights substantial uncertainty in the impactful modeling inputs for these systems related to occupant behavior, building operation, and actual versus ideal performance. It also reviews the selected capabilities of the simulation tools, including eQUEST, Trane TRACE, IESVE, EnergyPlus, and OpenStudio based on the input from tool vendors, developers, and users.

• eQUEST offers a user-friendly interface, mature simulation capabilities, and transparent reporting. It enjoys a large and loyal user base ranging from beginner to advanced modelers and is used on the majority of commercial modeling projects, based on most sources cited in this paper.

• TRACE 700 has a good balance between usability and simulation features. It is favored by design engineers who can use it for both energy analysis and equipment selection. It is transitioning to using EnergyPlus as the calculation engine, which will enhance its simulation capabilities.

• IESVE is a comprehensive design tool that fully integrates an advanced daylighting simulation with energy simulation, and automated code compliance following several protocols, including ASHRAE 90.1 Section 11 and Appendix G, IECC, AIA 2030, and Title 24 in California.

• EnergyPlus is widely used on research projects, offering flexibility for modeling nonstandard systems and controls. However, it has a steep learning curve and is best suited for advanced users.

• OpenStudio offers the promise of combining the simulation capabilities of the EnergyPlus calculation engine with usable custom interfaces. The tool works best for intermediate users who can access features beyond those supported by the native graphical user interface or through custom (proprietary) interfaces.

A BEM tool capabilities matrix is included in Appendix A. In addition, the relevant existing and emerging industry Standards and guidelines are discussed, such as ASHRAE Standards 90.1, 140, 205, 209, and Guideline 14. The key reasons of misalignment identified in this paper include the uncertainty of the simulation inputs, differences between the modeled versus actual system operation, modeler errors, and the limitations of BEM tools

In conclusion, recommendations for improving the credibility of energy modeling are provided, including the following:

• Enhance the BEM tool features important for commercial models, including the following:

- User-friendly graphical user interface

- Transparent reporting to help with model troubleshooting

- Explicit support of common systems, designs, and operation, including common operational faults

- Automated quality control to flag possible modeling mistakes

- Rapid integration of new systems and components

- Integrating energy analysis with other tasks commonly performed as part of commercial energy modeling, such as modeling-based code compliance, daylighting analysis, and evaluation of design alternatives and energy conservation measures

- Integrated capabilities to compare modeled energy use to measured consumption (e.g., utility bills)

• Updating simulation rules of the relevant Standards, including the following:

- Standard 90.1: Develop and prescribe detailed operating conditions for use in compliance modeling

- Standard 140: Prescribe the acceptance ranges for software tools being tested - Standard 205: Engage with equipment manufacturers to develop methodologies for better capturing impactful aspects of building systems and components in the energy models to help differentiate and encourage the use of efficient technologies

- Standard 209: Develop methodology for establishing modeling uncertainty so that the simulation results of design support models are reported as the ranges of likely outcomes as opposed to a fixed value

• Improve consistency in energy modeling–related policies:

- Align the BEM tool policies of incentive programs, jurisdictions (for energy code compliance), and other adopters of energy modeling (e.g., LEED) with the relevant industry Standards to foster competition between BEM tools and allow users to pick the tools that work best for them while meeting the industry-standard requirements

- Develop an infrastructure for peer-reviewed, unbiased comprehensive comparison of the simulation tools based on their support of systems and components found in real buildings and interface features important on commercial modeling projects

- Establish modeler certification requirements to minimize human error, and require postoccupancy model calibration and measurement and verification (M&V)
ANSI/NEMA MW 1000, 2014