What is the accuracy of building modelling?
A building simulation will not exactly match the reality. The challenge is to recognize where and why models can differ and how can you use it for your advantage.
Occupancy is the big issue in the building simulation!
When you are modelling a new building, one needs to make assumption about occupancy: how many people, what activities, lighting operation, temperature set points, etc. Simply to put how will they use the building?
James V. Dirkes states: “The new building owner does not give a rip what the energy modeller assumed for schedules of operation, sequences of operation, lighting operation, temperature set points or a host of other variables. They’re going to use it however they please; it’s their building, after all.”
It is all about assumptions, which you will make, especially for newly built buildings. You have a model with many variables inputs and for each one of these you will make a guess, an educated guess, but nevertheless it will be a prediction.
“An owner-occupied building is often differently managed than an investor-developed building, but, most owners don’t really know more than basic details of how they will use the building. That leaves the design engineers to make “best professional judgement” assumptions that get passed to the modeller.”
Why? What is the actual performancein the building simulation?
For existing buildings, the reason behind modelling is to find where the problem for large energy usage might be located. The energy calculations help to lower the energy bills, evaluate potential HVAC renovations or building management objectives for the building.
If you want to model your building accurately, you need to know it perfectly and you need to know what is really happening in the building. You need to inspect the buildings several times in several different seasons and during daytime and night time also. And you need a very good knowledge of the original design, construction, maintenance, etc. The building needs to have building management system (BMS) to use the date for the model. In the model further on you need to make sensitivity analysis to verify and locate potential problems with your model. And you need to understand the software limitations and how reality works.
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- Article by BY JAMES V. DIRKES II and BRAD WEAVER: “Modeled Performance Isn’t Actual Performance” published in ASHRAE Journal May 2016 www.ashrae.org