Reference > Marc Rosenbaum's Writings

Understanding the Energy Modeling Process:
Simulation Literacy 101
from The Pittsburgh Papers (2003)

MODELING INPUTS AND ASSUMPTIONS

Previous Page: WHY MODEL?

A model is usually an attempt to simulate the energy operation of a yet-unbuilt project. As such, many unknowns about that building must be estimated for the model. A typical set of modeling inputs might include the following.

Location:

• Climate data and

• Interior conditions and setpoints.

Envelope:

• Area, orientation, solar absorptance, and U-value of all opaque building surfaces;

• Area, orientation, SHGC, visible light transmittance, U-value, and shading of all glazing components;

• Mass of building components; and

• Infiltration rates.

Internal Gains:

• Lighting in watts per square foot;

• Plug loads in watts per square foot; and

• Sensible and latent (moisture) loads from people.

Schedules:

• Lighting schedules;

• Plug-load schedules; and

• Occupancy schedules.

Systems:

• Heating system type, including the source, distribution, and terminal units;

• Cooling system type, including the source, distribution, and terminal units;

• Ventilation system type;

• Fan and pump inputs;

• Economizers and/or heat recovery systems;

• Domestic hot-water system;

• Specialty systems (commercial kitchens, for example); and

• Renewable-energy systems.

Literally hundreds of inputs will need to be entered to build a model. Some user-friendly programs have built-in industry standard defaults that speed up early model creation. The responsibility for the accuracy of these inputs resides with different team members. The clients must supply their best estimate of the occupancy of the building daily and seasonally, in detail. They must supply location data—weather data if there is no nearby standard weather station data, and all pertinent site features, such as shading of the building by topography, vegetation, or adjacent buildings. The clients should supply a list of plug loads likely to be in the building and their anticipated frequency of use. Nameplate data for wattage of most plug-in equipment will be higher than what the equipment uses, so actual measured data is always more accurate (in one case the mechanical, electrical, and plumbing [MEP] engineer estimated over 10 watts per square foot for plug loads for a building, yet the prospective occupants had a measured usage of one watt per square foot in their existing building. Making an error of this magnitude results in a drastically oversized cooling system, adding useless capital cost). The architect must communicate the building envelope inputs, paying special attention to unusual items such as high-performance glazing, shading devices, or unusually lightweight or massive construction. The lighting designer should be asked to estimate lighting wattage for the different occupancy types. The design team should discuss with the MEP engineer appropriate mechanical system types to put into the model.

If the modeler is the MEP engineer, that person must understand very clearly the difference between an annual energy use simulation, which seeks to model the building throughout an entire year with its typical usage, and design load modeling, in which the goal is to size the heating and cooling system equipment to handle the reasonably expected peak heating and cooling loads. Engineers tend to add safety factors in a lot of places, and the peak design loads they calculate, especially for cooling, tend to result in systems that are oversized. (This is not news….) Oversizing has many penalties for the building owner, including higher capital outlay. The architect or owner who wants to protect the budget should look just as carefully at the inputs and assumptions for the design loads as for the annual simulation. Some things worth checking include:

• What are the assumed outdoor and indoor design conditions? If the peak is designed for an outdoor condition that occurs once a year for a few hours, and simultaneously the system is being asked to maintain 72°F inside, an oversized system will result.

• Is this a building carefully designed to be daylit, yet the model has all the lights on at full tilt while the building is experiencing peak solar input?

• Are there more people in the building than reasonably possible because every space is calculated at peak occupancy, even though those people packing the conference rooms can’t be in their offices at the same time?

With annual simulation, the design team attempts to predict actual building operation, not its peak; schedules and other inputs should reflect that objective.

The people for whom the model is being done (generally the owners or the architects) should ask the modeler for a complete list of all the inputs used for the model and should check to see that they represent the proposed building accurately. They should ask for a document with a table for each major occupancy and all of the inputs for each occupancy. They should ask questions pertinent to their building type; ask, for example, how the model sets ventilation air quantities—does it vary according to occupancy or is it constant volume? And, they should review the inputs before modeling is begun so they can ensure that their intent is accurately represented.

Common errors include:

• Plain old slipped-a-decimal-place data-entry errors (100 square feet of glazing instead of 1000, for example);

• Incorrect lighting and plug-load power densities (usually too high);

• Incorrect glazing characteristics; and

• Peak occupancy in all spaces at once, which virtually never happens. (An example of this is a dormitory in which all student rooms and all public spaces are simultaneously set to peak occupancy. This may lead to the model calculating ventilation air at three or four times the actual amount needed by the occupants.)

The watchword of any simulation process is “garbage in, garbage out.” The team needs to take joint responsibility to ensure that the inputs are reasonable. Don’t get hung up on whether the office lighting will be 1.1 or 1.0 watts per square foot at this stage! I usually ask the modeler to first produce the base-case building model, because the inputs are likely to be familiar and because a good baseline is a solid foundation for all the work yet to come. Also, the energy use of a minimally code-compliant building is more likely to be familiar, so the modeler can more easily evaluate whether the model is sufficiently accurate.

Next Page: GETTING, AND VETTING, THE OUTPUT

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