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Are Performance-Based Models the Future of Commercial Building Energy Codes?

Commercial building energy codes are largely prescriptive, combining mandatory requirements for lighting controls with limits on lighting loads by application. The typical lighting load metric is lighting power density (LPD) measured in watts per square foot.

This approach is intended to ensure that a building is built (or renovated) to a certain standard of efficiency, but does not require that the building operate within a target limit for ongoing energy use for the simple reason that it does not account for the operating time of the building. Additionally, as energy codes become more restrictive and continue the prescriptive LPD approach, critics charge that they limit design flexibility.

As a result, code authorities are considering approaches to energy codes that are performance based instead of mainly prescriptive. In a performance-based code, the building would be designed so that it would operate within a target limit for energy consumption—using annual kWh/sq.ft. instead of W/sq.ft. as the primary metric. The limit, in turn, might be developed from whole building monitoring, historical data that is considered the most accurate, and/or building modeling.

“The performance basis can seem like a much more straightforward and potentially more effective way to show that a building is energy efficient,” says Eric Richman, senior research engineer for the Pacific Northwest National Laboratory. “Limiting energy use is, after all, the true goal of energy codes.”

He points out that a performance-based code is more directly linked to actual or expected energy use, potentially allows maximum component tradeoff flexibility between building systems, is considered a way to achieve higher energy savings, and can more easily accommodate alternative energy features such as renewable energy.

One approach of a performance method is to compare modeled energy use for a proposed building buildingsagainst a predetermined energy target. In this approach, few items would actually be prescriptively required with the main goal being to comply with the energy consumption limit. This would provide maximum tradeoff flexibility between building systems, but establishing the right (and fair) target would be difficult considering the large variation in building types and uses. Another approach is to monitor the building’s actual energy performance after occupancy over a period of time and compare it against the target. This would provide the most accurate measure of building energy use, but would require building departments to monitor buildings after construction, and begs the question of what happens if a given building fails to achieve its target.

These are questions that must be debated before effective solutions can be considered for policy, but Richman says there is a great deal of interest among the lighting community in a kWh approach as a way to slow down perceived excessive ratcheting down of LPD limits. If target energy savings can be realized through controls, for example, this would give breathing room for lighting power allowances. Energy advocates, meanwhile, are also interested in performance-based codes as a way to harvest missing energy savings.

Lighting sections of energy codes already consider energy consumption, however, by virtue of the fact that each generation of codes contains more extensive mandatory requirements for advanced lighting control strategies such as automatic shutoff and daylight harvesting. Because of this, significant additional energy savings accounting for time of use may not be realized by implementing a performance-based code.

“This is the very reason that it is not clear that a performance-based method will automatically save more energy than the newest energy code requirements,” Richman says. “Building modeling as a compliance method does offer additional flexibility and is currently allowed in all major energy codes. However, it is not clear that setting building energy targets of future building operation as the comparison for compliance is accurate or practical and/or that it will garner additional energy savings.”

As a result, policy makers may find it difficult to turn the ideal of a performance-based code into a practical reality that will be accepted by designers and building departments. But they have time: Richman says do not expect such a code anytime soon—or at least one that functions smoothly or fairly.

“There is work being done on methods to implement performance-based approaches but since changes for the two major national code standards—90.1 and IECC—are already ‘in the can’ for 2010 and 2012, I don’t expect to see anything for a couple of years, at least.”

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