Building energy models are used by owners during the design phase to evaluate what design elements will affect the energy consumption of a new building.
Here, Steve Kemp of MMM Group discusses what energy models tell building owners and how they can be beneficial.
What are building energy models and what are they used for?
Energy models are mathematical constructs used to determine the energy performance of buildings. The math involved can be based on physical principals, such as heat transfer through the envelope based on the laws of thermodynamics and heat transfer, or statistical correlations, such as part-load performance of boilers.
While energy models can be developed as simple spreadsheet calculations, in practice, they are based on sophisticated calculation engines that have been thoroughly vetted by researchers. One of the most common calculation engines is DOE 2.2. The user of the model must balance the sophistication of the tool being used, the level of detail for the inputs they use, and the resulting level of effort against the desired level of accuracy of the results.
When should a building owner consider using an energy model?
A building owner should consider using an energy model to evaluate design options when considering any new building project. Energy models allow for “what if” scenarios to be investigated with respect to building operational energy consumption and associated cost.
Models should be considered as soon as possible in the design process. More commonly, energy models are used to determine compliance with an energy code or green building standard — such as Ontario’s SB-10, Toronto’s Green Building Standard, or LEED (Leadership in Energy and Environmental Design) — as the design nears completion.
Why is it beneficial for building owners to complete an energy model?
It is beneficial for building owners to complete an initial energy model early in the design process to provide assurances that the building design is progressing in most cost-effective way to achieving its design goals.
It’s beneficial whether the goal is achieving minimum energy code compliance, or for more aspirational goals, such as a high level of LEED energy points, a net-zero design or 2030 Challenge target. Final design energy models verify that these targets have been met, but do not allow for alternative design strategies to be investigated.
What will an energy model show a building owner?
An energy model can show the expected energy performance of a building given its design parameters, including enclosure performance, the amount of glass in the building and the type of HVAC system in place.
Most commonly, the energy performance is compared to a modeled “reference” building that reflects a similar design to the owner’s proposed design, but instead simply meeting the prescriptive requirements of a defined building energy code, such as ASHRAE 90.1.
Alternatively, an energy model can be used to show the actual operating energy consumption of the building. This, however, requires significantly more effort to determine as closely to reality as possible the actual operating parameters of the building with respect to receptacle loads and operating schedules.
The users of the building heavily influence these factors. And given human behaviour, users are not easily predicted. This information is best determined from real world sub-metering of similar buildings — information that is not widely available. Finally, predicting the actual energy use of a building requires that the building is constructed, commissioned and operated as per the inputs used in the energy model.
How is it used to improve energy efficiency in a building?
When using energy models to investigate “what if” design scenarios, the operational energy consumption and cost information, combined with the capital cost of the scenario being investigated, allows building owners to optimize their return on investment with respect to operational energy costs or achieving a desired energy target. This is powerful information to have, as it allows for better decision making.
For example, energy models have helped projects evaluate the use of variable refrigerant flow heat pumps. This heating/cooling technology changes the primary heating energy source from natural gas to electricity, albeit using electricity much more efficiently than a natural gas boiler could.
Steve Kemp, P.Eng, is the manager of sustainability at MMM Group. He is a LEED faculty with two decades of experience in green building design and energy modelling.