Best of ‘Ask a Modeler’: Simulation across Space and Time
Best of ‘Ask a Modeler’: Simulation across Space and Time ‘Ask a Modeler’ is an advice column for the building simulation community. Each month, committee chair Nathaniel Jones and members of the Emerging Simulation Technology subcommittee pose a question submitted by an IBPSA member to recognized experts to get their unique perspectives. Through this column, we hope to expand communication and create a sense of community among practitioners, researchers, and academics at all points in their building simulation careers. Below, we are reprinting some expert advice from the past few months. We hope that sharing these questions and insights will bring value to your work and possibly make you think about building performance modeling from a new point of view.
Sometimes energy modelers and mechanical engineers use different values for details like lighting power density or receptacle loads. How can we maintain better compatibility between models, or do we need to? — Looking for Consistency Dear Consistency, We have been down this path before, especially if you have an involved mechanical or electrical engineer. It is way into a construction document or permit set development cycle, and everyone is anxious to get the documents out the door and know what the energy model is predicting for design EUI or LEED points. That glorious moment when you receive the email from the engineer, who after having examined the energy end use breakdown notices the plug load consumption is far off from their estimates. “Can we set up a GoTo meeting to discuss?” is what usually follows. To start, is the model that the energy modeller produces going to inform anything in the mechanical design? Always ask yourself what the purpose of the model is – be it code compliance, LEED documentation or informing design decisions, among others. Is the modeller simply producing a code compliance report that the authority having jurisdiction (AHJ) is going to see, check the box, and never speak about it again? If that is the case, the simple truth is that it does not matter if those values for lighting power density or receptacle loads align. That is not to say that the model produced for code compliance does not have to align with the installed lighting or equipment design – we know those two things must align. But we also know nine times out of ten the mechanical engineer is not doing space by space lighting power density calculations to inform their load calculations. They know an office building is around 1 W/sqft and if it is a lab, then perhaps the LPD is around 1.6 W/sqft. And the best part is, they are right in doing so. Build a quick 50,000 square foot office building in your favorite piece of energy modeling software and assign a LPD of 1.0 W/sqft to the building. Then, create an alternate and assign a LPD of 1.6 W/sqft. Check out the results. Did your building peak cooling load or airflow really change to any significant degree? Not substantially; what did change was your annual energy use.
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volume 31 number 2
