Early Energy Modeling with an Energy Conservation Measures Toolkit

By Richard P. Juntunen, P.E., CDT LEED AP

This article is part of Wood Harbinger’s newsletter series.

As mechanical and electrical engineering consultants, we are tasked with providing a wide variety of services to our clients. System design and construction cost estimates are two areas that are easily identified as a typical role we might fill on an A/E team for an upcoming project. A lesser known service that we see requested more often is our expertise in the role of energy analyst. This has always been a core capability of the HVAC designer, but the increased prominence of sustainable principles in our industry has created an ever-increasing demand for these skills. Specifically, programs such as the U.S. Green Building Council’s LEED rating systems have shifted our industry to a place where most projects involve some form of energy analysis as another metric for evaluating a project’s viability.

LEED Energy Modeling

The term “energy model” is a fairly broad designation and, if given without context, it can be used to describe a great variety of information about a building. Due to the LEED rating system’s popularity, most people will likely associate “energy model” with the whole building energy simulation defined in ASHRAE Standard 90.1, Appendix G. This is a comprehensive analysis used to evaluate the energy efficiency of a particular building design. It quantifies efficiency by comparing the hourly operation of a building to an alternative baseline building, which conforms to all of the minimum performance requirements of ASHRAE Standard 90.1. The annual energy costs of both designs are calculated and a percent savings is reported as the basis of comparison.

There are some important things to recognize about this version of an energy model. First, it is mostly a design validation exercise. The inputs for this level of analysis are very specific to a particular design, and the calculations are performed on an hourly basis for a typical year of operation. Second, due to the volume of data and specificity of design elements required to be input into the model, it is very time consuming to complete this effort. These two factors usually relegate this type of analysis to the end of the design cycle.

Energy Life Cycle Cost Analysis (ELCCA) Energy Modeling

Another type of energy model that comes earlier in the design of a building is the energy life cycle cost analysis (ELCCA). This is another comparative analysis approach that is intended to differentiate various building elements by their expected cost over the entire life of a building. This includes initial purchase, maintenance, replacement, and operating costs. This level of analysis, when done properly, is executed early enough in the design cycle that it can inform the decision to pursue a particular type of system or building element over an alternative approach. Depending on the number of systems evaluated, these studies can also be very time consuming and expensive to perform.

Conceptual Energy Modeling

Moving even earlier in the design cycle of a building, we encounter another type of energy model. Conceptual energy modeling is a type of assessment commonly conducted by design teams following an integrated design approach. This kind of energy model is engaged much earlier in the design schedule in an effort to provide information about the energy impacts of design elements at a time when these elements are most easily adjusted. This can be an elusive balance to achieve; the earlier you are in the design, the less that can be specifically known about the energy consumption impacts of various alternatives to be evaluated. Often, this level of analysis can be most beneficial for evaluating design elements, such as building orientation, window-to-wall ratios, or building massing concepts. In order to perform these comparisons, mechanical systems are typically simplified to their energy use intensity (EUI) expressed in in kbtu/ft2 per year and one system type per building.

“Shoebox” Energy Modeling

Another approach to the early energy modeling is to develop prototypical building models. These are sometimes known as “shoebox energy models” and rely on typical design elements common to a type of building use and climate. One of the most prevalent sources of these types of models was developed by the U.S. Department of Energy and three of its national laboratories. The Commercial Reference Building library is a collection of energy models developed to cover 16 building types in 16 climate zones across the United States. Building size, configuration, and systems were all modeled for a typical occupancy type and operational schedule.

The Commercial Reference Building library provides a typical breakdown of building energy consumption by end use. For example, a hotel would have a much higher percentage of energy allocated to domestic water heating than an office building. This suggests a greater opportunity for savings via improvements to a hotel hot water system than the system for an office building and a better opportunity for energy savings likely exists for the office building project.

“Real-Time” Energy Modeling with an Energy Conservation Measures Toolkit

There are many uses for these various types of energy models, but they can be time consuming and require familiarity with specialized software. In an effort to meet the growing inclusion of energy considerations in early design concepts discussions, Wood Harbinger has developed a tool to bring another approach to the energy modeling effort. The Energy Conservation Measures toolkit, or ECM toolkit, creates a more universal capability that can be employed in real-time in client meetings to glean energy information early in the design process. This combines a wide array of source information, including the National Renewable Energy Laboratory (NREL) Commercial Reference Building Library, the Energy Trust of Oregon Energy Breakdown Worksheet, U.S. Environmental Protection Agency (EPA) Clean Energy State Lead by Example Guide, Naval Facilities Engineering Command (NAVFAC) Energy Conservation Measure Selection Toolkit, and the collection of Wood Harbinger simulations for both specific projects as well as more general shoebox-type models.

An illustration of our ECM toolkit spreadsheet, with outputs including potential operating costs and energy savings for a variety of different system option inputs.

An illustration of our ECM toolkit spreadsheet, with outputs including potential operating costs and energy savings for a variety of different system option inputs.

Where a typical energy model would require specialized experience and detailed input, a simple spreadsheet format can be quickly manipulated by a wide variety of users with consistent results. It can be mobile on a laptop or tablet, enabling real-time usage in meetings. With a few inputs like the types of end-use allocations available from the Commercial Reference Building library, combined with the location of a project and information about local energy costs, we can evaluate the relative impact of various energy conservation measures. Expected energy savings, improvement measure costs, and simple payback can all be evaluated in very approximate terms.

Energy Modeling Early and Often

The goal of this effort is to provide a tool to facilitate early design team discussions about various building elements. Design elements and conservation measures can be quickly evaluated for dismissal or further evaluation. This tool should be used to focus the discussion on areas where the greatest impact can be achieved. Whether the goal is reducing energy costs, lowering total energy consumption, or achieving a particular energy utilization index, the ECM Toolkit is ultimately a way to focus on where the greatest impact can be achieved for set of project requirements.

“Early and often” is the mantra of any experienced energy modeling practitioner, but ultimately it is up to the design team to decide what type of analysis is appropriate and what point in the design process to invest in the often time-consuming endeavor. The ECM Toolkit will not displace any of the existing tools available for use, but is instead another option available to improve the decision and design process for better energy results.

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