Energy Modeling for High-Performance Buildings is Only as Good as the Inputs

By David Reid and John Wilkins  
OTHER PARTS OF THIS ARTICLEPt. 1: High Performance Buildings: Avoiding ProblemsPt. 2: This PagePt. 3: High Performance Buildings: HVAC DesignsPt. 4: USBGC Releases Rankings of States With Most LEED-Certified Buildings

Many facility managers and owners are skeptical of energy modeling because they know of too many buildings that do not perform like the model and fail to return the predicted energy savings.

But owners and project teams alike must remember the "GIGO" principle: "garbage in, garbage out." Energy modeling is only as good as the inputs, and, unfortunately, the quality of these inputs varies. An essential component of energy programming is an honest discussion early in the schematic design phase among the owner, user-group representatives, architect, engineer, general contractor, and construction manager. The goal is to arrive at a consensus about realistic design goals for the tightness of the building envelope and owner operating expectations.

The information derived from these conversations improves the quality of the inputs to the energy model. These inputs include:

  • window to wall ratio
  • tightness of air infiltration
  • thermal wall and roof insulation
  • performance criteria of glass
  • type of mechanical system
  • artificial lighting watts per square foot
  • plug loads wattage per square foot
  • occupancy (capacity, operating hours)

When this discussion is lacking, engineers may include a contingency factor above the projected load — essentially, "over-designing" the mechanical system, which drives up capital and operating costs. They may not trust the construction contractor and subcontractors to build the envelope tightly enough to prevent air, water and thermal infiltration, or they may not trust that the building will be operated as designed.

However, keep in mind that precise building occupancy and hours of operation are difficult to calculate in the programming phase. Design solutions can help mitigate these uncertainties. In a theater, for example, carbon dioxide occupancy sensors ensure that the ventilation system is delivering the appropriate volume of fresh air based on the number of persons in the room.

It is also difficult to predict with certainty whether occupants will use the building as intended — for example, will they open windows to enjoy a summer breeze even though the air conditioner is on? Involving occupant representatives in these early discussions improves the odds that they will understand and buy in to their role in the building's energy performance.


"How many years to payback?"

Owners usually want to know what the payback is in terms of the number of years to achieve a 100 percent return on investment. Instead, they ought to think in terms of energy use in kBTUs per square foot per year, also known as Energy Use Intensity (EUI). And there are additional standards related to various building types and climatic regions.

As the project team evaluates and compares various building systems, they should also consider the projected costs of fuel and electricity over the course of the payback period. Operating costs should then be compared with capital costs.

For example, for a project at the University of Kansas, Edwards, in which engineers ran a comparison of geothermal to mechanical heating/cooling systems, geothermal saved more energy than other mechanical systems, but the difference was not significant enough to justify the higher initial investment.

Payback decisions also depend on the type of building. For example, if a building is operational 24 hours a day, the project team should assess a mechanical system that helps to shed peak load demand, such as an ice storage system. This solution would carry higher up-front cost, but the owner may save significantly on utility costs through peak load-shifting discounts.

— David Reid and John Wilkins

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  posted on 3/7/2012   Article Use Policy

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