Shedding light on energy codes
As energy codes become more stringent, it becomes more of a challenge for architects and designers to meet the requirements of the current system of prescriptive energy codes. Because of these challenges, advocates of performance-based energy code systems point to them as possible solutions to the problem of meeting the various 30%, 50%, and near-zero-energy building goals supported by propose...
By Eric Richman, Pacific Northwest National Laboratory, Richland, Wash.
As energy codes become more stringent, it becomes more of a challenge for architects and designers to meet the requirements of the current system of prescriptive energy codes. Because of these challenges, advocates of performance-based energy code systems point to them as possible solutions to the problem of meeting the various 30%, 50%, and near-zero-energy building goals supported by proposed legislation and green programs.
The goal of energy codes and standards is, of course, the long-term reduction of actual energy use over time through efficient technology, effective design, and appropriate control. For lighting loads, the ultimate metric of this goal is the kilowatt-hour (kWh) in the form of kWh per square foot per year (kWh/sq ft/year), and it seems reasonable to base lighting energy codes and compliance on this same metric.
In contrast, current prescriptive requirements for lighting systems are based on controls and lighting power densities (LPD) measured in watts (W) in the form of watts per square foot (W/sq ft). These prescriptive requirements restrict wasteful energy use, but compliance is not directly tied to actual future energy use of the building.
Performance-based codes tend to have few (or no) prescriptive requirements. Instead, they typically are based on the total energy use of the proposed building compared to a baseline energy use or target. For example, a building would comply with a lighting-only performance code if:
Actual energy use (kWh/sq ft/year) & energy use target (kWh/sq ft/year).
This seems simple enough and explains why a performance-based code method is considered a direct evaluation of a building's energy effectiveness. A performance-based method also tends to provide maximum trade-off capability. In the calculation above, a prescriptive LPD limit could be exceeded for design reasons and compensated for with more advanced lighting controls or renewable energy sources that would reduce on-grid electricity use to help meet an expected equivalent building energy use.
This performance basis can seem like a much more straightforward and potentially more effective way to show that a building is energy efficient. A performance-based code:
Is considered a direct evaluation of actual or expected energy use
Potentially allows maximum component trade-off flexibility
Can more easily accommodate alternate energy features such as renewable energy
Is thought of by many as an effective method for achieving higher energy savings.
At the same time, current prescriptive-based energy codes are typically viewed by some as:
Not directly linked to actual energy use (although prescriptive requirements certainly save energy)
Composed of building component requirements that can restrict design flexibility
Lacking the ability to accommodate alternative energy efficiency features or sources.
Breaking down the code methods
On the surface, it seems that a performance-based energy code is a better choice all around. But let's explore these two energy code methods in a bit more detail.
A performance-based code compares actual building energy use to an energy target. But where do you get the actual building energy use for compliance? One option is to estimate future energy use based on modeling the building as it is proposed to be built and operated. This is not actual energy use but just an estimate that may—or may not—accurately reflect future energy use.
The second, and most direct option, is simply measuring the energy use with the building energy meter after the building is occupied and functioning. This is very direct, but not currently practical because building jurisdictions typically have no authority or system for comparing energy use of a building after occupancy.
And where do you get the appropriate target energy value? Would it be based on historic data? Could it be developed with energy simulation? Historic data for individual energy-efficient buildings can be found in limited quantities, but compiling a complete set for all building types (and subtypes and operational varieties) would be a monumental task. Energy simulation could fill in some holes, but it is not practical to presume that all buildings and varieties could be provided their appropriate and accurate energy use target.
For example as just one of several primary restaurant types, a fast-food restaurant could itself include many varieties that require unique energy targets due to many energy-related variables including:
Cooking equipment: standard grill operation versus chicken deep frying versus pizza ovens, etc.
Operating hours: breakfast/lunch/dinner versus lunch/dinner only versus late night versus 24/7, etc.
Function intensity: substantial seating versus carryout only versus with/without playground, etc.
In comparison, the prescriptive compliance methods in energy codes strive to provide requirements that:
Are proven energy savers with practical application
Include appropriate requirement details to ensure success in saving energy
Are standardized for consistent compliance and validation.
In addition to lighting power limits, the current and upcoming revisions of nationally available lighting energy codes already include mandatory requirements for most of the practical controls that could be effectively used in buildings. The lighting requirements are based on these two energy effects:
Installed lighting power density (W/sq ft) = allowed lighting power density (W/sq ft)
Specified controls applicable to specific space types and lighting functions must be installed.
This combination of prescriptive power density limits plus controls is driving to restrict the very same energy use proposed as the ultimate metric for a performance-based code. It also is important to note that the nationally applied energy codes are not strictly prescriptive—they already provide an alternative whole building energy simulation compliance path. This method typically simulates (estimates) the energy use of a proposed building design and compares this to the simulated energy use of the same building constructed to meet prescriptive requirements for a relative energy performance comparison. The proposed building design can then include the builder's own preferred mix of energy efficiency components, controls, and/or alternative energy features and sources. Simulation tools can be limited in how they model unusual building features. However, this is the same simulation method that would likely be needed to develop the specific energy targets for a true performance-based method.
Are performance-based codes a bad idea? Certainly not. But while prescriptive compliance code paths are not perfect, it is not at all clear that a performance-based approach will automatically drive buildings to lower energy use unless the performance targets are set intentionally low. Developing these intentionally low targets for all building types and varieties that also are proven to be achievable (i.e., not just an arbitrary low value) will not be a simple exercise.
An additional concern with the performance-based method is the possibility that many or all prescriptive requirements and mandatory controls can be traded away as part of the method's flexibility, which can lead to poor design. For example, with a true performance-based approach for lighting, a building could be designed with excessive LPD and/or inefficient lighting technology but with advanced control strategies or renewable energy sources. If the advanced controls or renewable sources do not perform, the building is stuck with an inefficient lighting system—for the rest of that equipment's life.
Many code developers and enforcement jurisdictions favor code methods that provide at least some minimum prescriptive limits and mandatory basic controls. However, performance-based and similar trade-off methods should continue to be explored and improved. As enticing and simple as a completely performance-based code may sound, it is not yet clear that effective development and application are ready for prime time.
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