Considerations for building energy modeling

Here’s a review of initial energy modeling performed during a project from the interview phase to development of final models used for documenting compliance to energy codes or for LEED certification, through a client’s survey one year after occupancy. Here’s what works, what doesn't, and what value can be achieved from each step in the process


Figure 1: This is an example of the output of an energy modeling exercise for a project at the proposal phase. It shows the projected annual energy use intensity (EUI) and an estimate of annual energy cost (in dollars) for three different options based on an increase in project scope for a renovation and addition project. All images courtesy: CannonDesignWhen a customer arrives at a car lot, typically the first question posed is about the vehicle's gas mileage and, relatively quickly, a rigorously EPA-tested fuel economy rating is provided. The customer can use this information to calculate the car’s annual energy cost using simple arithmetic. In August 2013, the average new car cost was $31,252 according to an article in USA Today with an average length of ownership of 71.4 months per Polk. The annual operating cost of a vehicle can be tabulated with minimal effort.

A 10,000- to 15,000-sq-ft renovation or new construction project may cost $3 million or more, and is expected to last 15 years without a major upgrade. It is unlikely that any contractor or designer, prior to signing a contract, would provide a client with a projected annual operating cost or an energy use estimate on the spot.

How can a purchase that’s 100 times cheaper, with a lifecycle roughly five times smaller, provide such detailed energy consumption information by comparison? The simple answer is that buildings aren't cars, as the design of a building is much more customized. However, that answer is quickly becoming inadequate. As clients, designers, team members, contractors, and municipalities require more information regarding building energy consumption, the feedback on energy consumption related to potential configuration and proposed systems must follow the same fast pace as in other industries. The challenge is establishing a process for developing information useful for decision making in a timely fashion without overanalyzing or getting lost in the details.

Energy code history, modeling origins

Energy modeling is rooted in the development of standardized energy codes. Prior to the development of a model or state energy code, ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings was created. The first recorded ASHRAE energy standard was 90.1-1975. Developed and amended several times in response to the energy crisis of the late 1970s, ASHRAE 90.1-1975 was the standard for state energy code development for over a decade; subsequent versions of 90.1 were issued in 1989 and 1999.

Beginning in 1998, the 2000 International Energy Conservation Code (IECC) was developed from the Model Energy Code, which was first published in 1983 by the Council of American Building Officials (COBA) and Building Officials and Code Administrators (BOCA). The two primary energy codes, ASHRAE 90.1 and the IECC, were so similar that they achieved equivalency. The IECC references ASHRAE, and with a two-year lag in equivalency, IECC 2012 is equivalent to 90.1-2010. Both codes include a compliance path allowing whole building energy modeling if the prescriptive or mandatory requirements can't be met.

The process for demonstrating code compliance by energy modeling requires building a separate model for the building as designed and comparing the model to a code minimum baseline. As energy-efficiency requirements dictated by model codes increase, the need to use modeling to demonstrate code compliance will become more common on all but the most basic building types.

Figure 2: This graphical example shows the output of an energy modeling exercise for a project at the proposal phase. The graphic is intended to show the amount of photovoltaic arrays necessary to offset the energy usage of three different scales of project for a renovation and addition project.The energy modeling process

In its infancy, building energy modeling was a comparative exercise used to validate a design after major decisions were made. Energy-efficient design was based on general best practices, determined by lessons learned, and from minimum energy code standards for insulation, lighting levels, and ventilation. An energy-efficient design charette could examine reducing glazing, improving envelope performance, or optimizing the building’s orientation or the performance of building systems. The charette’s results would be incorporated in design documents.

The finished design might be modeled in a Dept. of Energy 2.0-based hourly simulation program providing some basic information on energy consumption and annual energy cost. This post-design processing allowed energy modelers to limit deliverables and avoid repetitive modeling. The major weakness in this process was that the energy modeling was being used to validate design as opposed to influencing it, which restricted the ability to achieve substantial improvements in building performance.

Today’s energy modeling is driving an interactive design process providing guidance for teams, from before a project is awarded through construction. For larger project pursuits in the proposal stage, design teams are coming to the interview table with a fleshed-out design concept and energy consumption estimates based on preliminary modeling/benchmarking. In some cases, multiple design concepts have been modeled and comparative data is provided—an activity that previously would have occurred at the design development phase has now moved into the marketing phase. The benefit of incorporating energy information at the proposal phase goes beyond positioning to win the project. From the onset, the design team shows awareness of the building’s energy impact and will consider it prior to decision making. At the very least the team is cognizant of the issues and will be more likely to communicate their concerns as the project moves forward.

To facilitate extremely early energy analysis, there are numerous tools for gathering quick and reliable energy performance information. Benchmarking tools such as Energy Star Target Finder or the Labs 21 Energy Benchmarking Tool offer useful data for early analysis. Taking it a step further, a quick energy simulation using the eQUEST schematic or design development wizard can generate a rough estimate for the concept’s relative energy conception. As with any energy modeling software, the output is only as useful as the input. The more time dedicated to accurately identifying the operating schedule and upfront usage, the better the generated information will be. Using similar projects or existing facility utility data can also be a useful tool for an interview when working in the initial stages.

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