August 30, 2017 by Judy Peters
Though not a new concept, the intentional practice of calculating in detail how a building will use energy is a critical, yet often overlooked, step in building development.
At its core, energy modeling is the practice of using software to determine how a building will use energy. This is done in a detailed and calculated manner, by identifying how both direct inputs (e.g., lighting power density and miscellaneous electrical loads) and indirect inputs (e.g. how the HVAC system responds to building demands for thermal comfort heating and cooling) affect total building energy consumption.
The optimal design from an energy and economic standpoint is based on a synergy of operating hours, internal loads, local weather patterns, utility costs and the cost of the system itself. A detailed algorithm is required to calculate the best building and HVAC component selections.
Buildings share some commonalities in energy consumption patterns. For example, most commercial buildings in heating-dominated climates such as Canada have 30-50 per cent of their energy consumption for heating; another 30 per cent for lighting and plug loads; and the remaining attributed to fan, pump and cooling energy. While most commercial buildings in Canada share this common framework, they also differ widely in respect to key parameters, such as how much ventilation air is used, operating schedules and size of internal loads. a certain HVAC system might be advantageous for one building type and climate, another building might find a whole different system more advantageous.
And that is where the energy model comes in: helping the contractor or owner make critical, calculation-based determinations for which system will be best in a specific building application. An energy model can facilitate this process through the use of a parametric study, where a single input is changed and the model calculates the interdependencies and impact on the overall outcome.
In the past, owners made decisions based on simple payback – how many years before their investment in high efficiency equipment would be returned. Today owners are more sophisticated and want to understand the total cost of ownership for a piece of equipment or even an entire system. Energy models contribute to this analysis by calculating annual energy costs. Other factors that are included in a life-cycle cost analysis include interest rates, depreciation rates, annual maintenance costs, energy escalation rates and replacement parts. Cost and sustainability are the primary driving factors for this decision-making process.
The term, “green building” has become a fully-fledged priority pillar in building construction. Energy modeling is one means of determining a building’s level of energy efficiency for certification in the Leadership in Energy and Environmental Design (LEED) environmental building certification program. Through the use of a highly detailed energy model, a building is measured against a base model as outlined in ASHRAE 90.1 Appendix G to demonstrate that it achieves an energy cost savings of 10, 20, or even 50 per cent better than the base model – depending on the certification level a building is aiming to achieve. This certification is critical for many office and retail buildings, as corporate customers are demanding energy-responsible facilities to support corporate social responsibility goals. These certifications can drive higher lease rates and stronger lease agreements for building owners working with energy-savvy tenants.
Many governing bodies from both a regional and federal level now require some demonstration of energy efficiency. In many regions ASHRAE 90.1 serves as the energy code. In addition, many utility companies and municipalities offer rebates to encourage energy efficiency thereby minimizing the need to expand the size of the grid. Rebates may be issued prescriptively but may also entail an energy model to demonstrate compliance.
Just who does energy modeling? Typically, it is highly skilled professionals with a background in mechanical engineering. Energy modelers need a strong working knowledge of the building’s thermodynamics to understand heat transfer processes between a building and its environment. An energy modeler must be detail-oriented and knowledgeable about HVAC equipment. Once the model is created, they need to analyze the outcome to determine if the model is accurate in its depiction of building operations.
That is not to say that energy modeling is a one-size-fits-all situation. There is a range of software available, from the highly technical EnergyPlus (developed by the U.S. Department of Energy) to proprietary tools that are more of the industry standard, and even tools designed for the sales process.
The ideal entry point for energy modeling is early in the design process when critical decisions about equipment and building design are being made. Preliminary energy modeling results deliver a high degree of accuracy and are invaluable for making informed decisions. Later as more details are available the mechanical engineer will construct a more detailed energy model.
Going into a project, there may be preconceptions regarding the ideal piece of equipment for a particular building. An example may be selecting a water-cooled versus air-cooled chiller. It is not obvious which one is going to be most cost-effective and appropriate prior to creating the model.
With varying degrees of model sophistication comes variation in how well the energy model represents and explains each project. Awareness of common pitfalls in the energy modeling process can help mitigate their occurrence. These include:
• Improper modeling of HVAC equipment: it can be difficult to secure product-specific equipment efficiency information. This data is often not readily available in the marketplace, partly because manufacturers have a desire to protect proprietary information from competitors. If an energy modeler is in a hurry, he or she may input an efficiency number for a piece of equipment that deviates from reality, significantly skewing the results in the end.
• Inputing the incorrect electricity rate: another pitfall can be to take a shortcut in the electricity rate input by choosing to use an industry or national average instead of the local rates. If the actual rate is more expensive than what is put into the model, the life cycle cost analysis and simple payback calculations will not be accurate.
• Using models to “predict” future energy consumption: Even the best energy model cannot predict future energy consumption because critical parameters are unknowable such as what the weather will be like in a future year or exact building operations including tenant habits and equipment maintenance. Instead, a model provides a relative comparison between the owner’s building and a base building and demonstrates the difference in degree of energy efficiency involved.
• Bringing in an energy model too late: oftentimes energy models are brought into the process too late, after major decisions have been made – such as how the building is oriented, how many windows it has, even what type of HVAC system it is going to have. The result is lost opportunity to achieve exceptional energy efficiency.
The data-based insights that an energy model provides underscores why every project can benefit from energy modeling. It takes the guess work – and disagreement – out of the process, and drives recommendations rooted in hard numbers.
Technology will reduce the tedium of the energy modeling process. Automation in the form of Building Information Modeling (BIM) will continue to help design professionals with plan development and execution. Increased automation will drive the measurement and verification process for building energy performance. Currently, the expense of measurement and verification of energy modeling is a significant barrier to spotlighting when a building is underperforming. As technology continues to advance, measurement and verification of building energy efficiency will increase in prevalence.
As we move forward with smarter, more efficient methods for commercial construction, energy modeling should play a significant role in your next building project – no matter how large or small.
Judy Peters, PE LEED-AP BEMP, is an energy modeling engineer at Daikin Applied.