Mind the Gap: Getting Smart with the Building Energy Model
Energy Model Commissioning
A building-energy performance gap is a disparity between the energy consumption predicted in the design stage of a building and the energy use in actual operation. The energy performance gap is considered as one of the most significant issues associated with the assessment of energy consumption in the built environment (1).
Architects and engineers strive to provide clients with high performance buildings; however, in reality, many projects do not achieve their targeted efficiency level. Once a building is complete, its performance is rarely measured to validate the design intent. Systematic quality processes are not being put in place to allow building owners and operators to effectively monitor or maintain their buildings at the intended high efficiency level.
Building Energy Modeling (BEM) is the practice of using computer-based simulation software to perform a detailed analysis of a building's energy use and energy-using systems. Energy modeling is based on a series of assumptions about building characteristics that are based on physical characteristics and anticipated use patterns. However, studies show that energy consumption calculations can have errors of up to 90% with the typical software packages (2,3).
BEM is great for new construction or retrofit building projects; it can reduce operational costs, reduce project first costs, and improve occupant satisfaction. BEM can be used to compare project efficiency options, comply with codes or green building certification requirements, predict savings, and inform actual performance.
However, currently energy models are built to comply with code or with owner energy saving requirements, and these models are rarely correct and do not represent actual results. There are two main challenges BEM software has yet to overcome:
Create a model that accurately represents the physical building and its internal systems.
Provide accurate representation of usage and conditions to which the building will be subjected.
Challenge number one can be mitigated by creating more user-friendly programs with a better system of checks and balances to identify errors in the physical model. Challenge number two is highly complex, often due to the lack of data acquisition to represent the past and the absence of exacting foresight into the uses and conditions of the building.
Using Commissioning to Bridge the Gap
Commissioning is the process of quality assurance. An incorrect energy model is not quality. While it is up to the designers and engineers to create a model that accurately represents the physical building and its internal systems, commissioning firms can provide a check of the assumptions and inputs to verify the model is accurate to the intent of the owner.
More importantly, during functional testing and turnover, commissioning agents have direct knowledge of final system and equipment installations and building operations. We can provide feedback to the energy modelers on how to accurately represent usage and conditions, or even provide an updated model using in-house resources. The process is as follows:
Retain the CX agent early to review goals (OPR/BOD)
CxA will review the model parameters as part of the design
CxA will provide feedback with energy efficiencies and capacities
After turnover, the CxA or the engineer will update the energy model with as-built conditions including actual schedules and occupancies. This is similar to energy model calibrations.
Why Commission the Energy Model?
There are three main reasons why there is value in commissioning the energy model. First, it gives an owner a more realistic expectation of energy savings. During the initial design, many significant financial decisions are made based on energy model results. Owners that prefer to pay more upfront CapEx for less ongoing OpEx will opt for more expensive equipment and systems that are energy efficient. An incorrect energy model leading to faulty decisions may result in overspending on equipment.
Second, commissioning the energy model provides a more accurate energy use and utility prediction. The ASHRAE Guideline 14-2014 Measurement of Energy, Demand, and Water Savings procedures include the determination of these savings from individual facilities and speaks to many of the official mathematical measurements, but it does not clarify accuracy. Some utility companies using the energy model for energy rebates require the annual energy use of the model be within 5% of the annual building energy use with no individual month varying beyond 10%.
Finally, a high-quality, accurate, calibrated and commissioned energy model turned over to the owner at the end of the project provides ongoing operational value. A typical energy model is thrown away after design. The hours used to build the model are wasted when the model’s perceived usefulness fades away. A commissioned energy model can be used for Fault Detection Diagnostics (FDD), future retrofits, and performing what-if scenarios for ongoing commissioning and energy reduction.
Building energy models and simulations provide a virtual laboratory for refining energy saving design. Energy models are utilized in a vast majority of new construction projects in California under Title 24 and buildings pursuing LEED. GMC Cx has completed several models for our clients, who are now reaping the benefits of a quality model. Contact us to find out more.
Gucyeter, Basak. CALIBRATION of a BUILDING ENERGY PERFORMANCE SIMULATION MODEL via MONITORING DATA. 26 Sept. 2018.
Van Dronkelaar, Chris. A Review of the Energy Performance Gap and Its Underlying Causes in Non-Domestic Buildings. 13 Jan. 2016, www.frontiersin.org/articles/10.3389/fmech.2015.00017/full.
Shi, Xing, et al. “Magnitude, Causes, and Solutions of the Performance Gap of Buildings: A Review.” Sustainability, vol. 11, no. 3, 12 Feb. 2019, p. 937, 10.3390/su11030937. Accessed 13 July 2021.