Energy Modeling, ASHRAE 90.1 and LEED
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The most challenging prerequisite in the LEED for New Construction certification process is the Energy and Atmosphere Prerequisite 2: Minimum Energy Performance.
Any project seeking Certified, Silver, Gold or Platinum certification must demonstrate a 10 percent improvement above a building designed to comply with ASHRAE 90.1-2007 to as a prerequisite. (For major renovations, the requirement is 5 percent improvement in the building's performance.) The most effective means of earning this credit is through an application variously called the "Energy Model," "LEED Model," or simply the "Model," but officially the "Whole Building Energy Simulation Model." Whatever it's called, when done correctly, it can provide an extraordinary amount of information about a building's potential for energy savings, well before the first brick is even laid.
The energy model is a computer software simulation that starts with specified materials and systems for a building, calculates the energy cost for one year, and creates a report of the anticipated energy performance of the building. The model will reveal how energy efficient the building can be, while there is still time to enhance it.
An important concept to understand is "percent improvement." The ASHRAE standard and the LEED model compare the annual energy cost of the proposed building vs. the baseline building that just meets code. Annual energy cost is used because it is readily understood by engineers, architects, building owners, executives, accountants, and occupants. Everyone can understand how one item may cost 10 percent less, but not everyone has a frame of reference for British Thermal Unit-hours (BTUh), kilowatt-hours (kWh), tons of carbon dioxide and the like.
Early Start Essential
Naturally, consideration for the energy model should begin as early as possible, even before the design development phase. Conceptual sketches and diagrams shape future design and greatly influence building skin material, solar orientation, glazing and so forth, which affect energy performance. Even an early model with general assumptions can inform decisions in the design or identify road blocks.
More typically however, the energy model is created during the latter part of the schematic design when the building becomes more clearly defined dimensionally. At this point, one is able to specify wall construction, lighting density, plumbing, heating and cooling equipment and even the number of computers and occupancy schedule. All of this goes into the model, which is then compared with a baseline building set to minimally code-compliant values. Points are then awarded above and beyond the prerequisite based on the difference between the annual energy use of the building as proposed and the baseline building. For example, a building designed to be 30 percent more efficient than code would earn 10 points toward LEED certification.
Creating the model is typically an iterative process. Many simulations must be run to fine-tune and calibrate it. Once calibration is complete the engineer is ready to begin to test concepts and strategies for reducing the energy use of the building. By changing the variables one can directly or indirectly affect energy use. Variables fall into three categories: weather and climate; energy and utility; and building components. Weather and climate refers to temperature as well as solar orientation, solar intensity and humidity Ñ these variables that can't really be changed, however. Energy and utility rates refer to the cost of energy paid per unit by the building owner. This includes any fees or surcharges that may be applied. Standard data sets for the HVAC load are calculated from the 8,760 hours in a design year. Building components are anything that is used to construct the building, systems that serve the building, or occupants that use the building. This is the broadest of the three main categories.
Spaces and Systems
Building components can also be broken down into "spaces" and "systems." Spaces refer to the building as a whole and the rooms that comprise the building. Spaces are made up of walls, windows, roofs, doors, floors and shades. Implicit in this list are factors like orientation, construction, and dimension. Also included are lighting levels and plug loads.
Systems include HVAC and plumbing equipment, elevators, escalators and site lighting. Implicit in this list is the capacity of the units, equipment efficiency and energy use. Detailed information must be known to correctly model HVAC equipment, equipment efficiency at peak and partial load, control strategies, and system parameters like pump head and static pressure, for example.
In both cases the occupancy schedule plays an integral role in the calculation of energy use. Unoccupied weekends and holidays are important for this annual calculation, as are 24-hour facilities, or those closed temporarily, like schools.
The software runs the information entered through a series of algorithms to calculate the heat gain and energy use by lighting and equipment, heat load or gain through perimeter walls, and ventilation requirements for occupants.
Once the simulation is complete, a wealth of data is available to analyze and review. Each software package is different but all have the ability to review data hour-by-hour and category-by-category. The energy model links information and systems together in an extremely helpful way. For instance, using a better insulating glass is likely to reduce the overall demand charge for the entire year. Thus the building may actually require smaller-sized, less costly HVAC equipment. By understanding the nature of a component's use one can propose better strategies for mitigating energy cost.
The energy simulation software is not a flawless crystal ball, however. Most facility executives report it takes a minimum of two to three years before kinks are worked out of building system and a building is operated as it actually will be throughout its life. This means that the model shouldn't be taken as an ironclad illustration of exactly what the building's energy performance will be. Metering, measurement and verification, and efficient operations must be a part of the plans to ensure that energy savings are realized.
Additionally, determining the carbon dioxide output from utilities of a building has proven difficult. The calculation of a building's annual carbon dioxide emission is complicated because the source of electricity can be coal, natural gas, nuclear power, or sustainable sources, while the demand and availability of each energy source ebbs and flows. Also, the anticipated annual energy cost should more accurately identify embedded energy costs as well; after all, a utility must pay for raw materials, distribution, regulations, emissions, facilities, and overhead.
Finally, because when energy is used is almost as important as how much energy is used, strategies such as ice or thermal storage can use standard equipment, but at off-peak times, which saves on the cost of energy. The singular piece of equipment may not be highly efficient but the system it is a part of can be.
If an energy model is needed late in the design, perhaps in the construction documents phase, HVAC load calculations that engineers routinely perform can provide the basis for the simulation. Engineers use software much like an energy model to determine the maximum anticipated load that will occur in a building, using peak values for climate data and internal data. This software is not concerned with generating simulation results that could reflect the conditions at any point in the design year, however.
Both the process to generate the model and the analysis of the results create a better-designed building. The most important aspect of the energy model is the careful planning that it encourages us to do in designing healthy buildings that are welcome additions to the built environment.
Andrew J. Kamnik, is a mechanical engineer with EwingCole, a nationally recognized architecture, engineering, interior design, historic preservation and planning firm of more than 350 professionals. Based in Philadelphia, the firm has offices in Irvine, Calif., and Washington, D.C.
U.S. GREEN BUILDING COUNCIL
Energy Modeling, ASHRAE 90.1 and LEED