Calculation Conundrum
After returning to the office from a facility inspection with a list of potential energy savings projects in hand, the following question arises in the mind of an engineer: “How should energy savings be calculated?”
Multiple calculation options exist for any potential energy savings project. For simple projects, such as a light fixture retrofit, the choice may be easy with a simple one-line spreadsheet calculation. Other times, the model of choice may not be so clear.
For example, consider a measure calling for a reduction in ventilation loads, which depend heavily on outdoor air conditions. Such a calculation could involve a spreadsheet with binned weather data, or hourly temperatures for the year. Both of these options may be sufficient; however, another tempting option remains – the whole-building energy simulation.
What is it?
A whole-building simulation models the energy consumption of all equipment in a building. Estimates are calculated with pre-defined algorithms that incorporate user inputs (or defaults) into the model. With a few quick mouse clicks, the energy impact of any potential change to a building system can be estimated. Some tools have wizards, for quick model development and energy comparisons. And to top it off, a fancy building rendering is created.
So what’s the catch?
Temptations can arise to use such a model on any change to a building system. This can easily lead to poor results if the baseline model is not calibrated to the utility bills correctly. Since care and expertise is required to understand what model inputs drive major changes in building usage, whole-building modeling of a relatively small project is typically too tedious and time consuming to be worth it.
Other times, a building system is nowhere near working order, as is often found in retro-commissioning studies. Attempting to calibrate a model of such systems is a fool’s errand.
When does it make sense?
For comparison of design options on a building yet to be constructed, such a simulation makes sense, as utility bill calibration is not necessary, or even possible, since there are no utility bills yet. For large retrofit projects, such as the conversion of a building’s constant volume air handlers to variable volume, a whole-building model should be considered. The time invested in creating a model is likely worth the windfall. Similarly, the interactive effects of many potential changes to an air handling system can be assessed quickly with an accurately calibrated model.
Conclusion
How and when to go about performing a whole-building simulation requires the expertise of an experienced energy engineer who can find the delicate balance between effective results in a reasonable amount of time.
