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Best Practices for Successful Building Energy Modeling with EnergyPlus

Energy modeling or building simulation is a powerful tool for predicting or analyzing building energy use. We use EnergyPlus and OpenStudio frequently when developing whole building energy models. After years using these tools in a consulting environment and sharing that knowledge on the Building Energy Modeling (BEM) user’s group, Unmet Hours, I know the value understanding key factors can have on obtaining model results you can have confidence in. Here are my top five tips to consider for developing energy models that are well-calibrated and useful.

1.    Know the tool

EnergyPlus is a BEM simulation engine developed with support by the US Department of Energy. Widely considered one of the most feature-rich tools available for simulating building energy performance, its use among energy consulting and design firms is increasing thanks to recent improvements in usability, especially with the graphical user interface OpenStudio.

EnergyPlus is an expansive, energy modeling tool that provides a large amount of sometimes redundant capabilities for representing different building energy systems and configurations. Because it shares calculation methods and terminology with other building energy simulation engines, it can be tempting for new users to apply their experience with other tools and their intuition in building models and are often left puzzled why the subsequent outputs don’t match their expectations.

With simulation models in general, and EnergyPlus in particular, it is critically important for users to fully understand the workings of the tool – not only what input data is required for a simulation to run, but how the program uses that input in the calculations of heat transfer, energy consumption and system control. While it can be daunting, reading and understanding the engineering and input/output references is invaluable for EnergyPlus users to get the most out of their models.

In addition to the formal documentation, the code for EnergyPlus, OpenStudio and other related simulation and modeling tools are open-source and available on the online code hosting site Github. As an open-source project, EnergyPlus and OpenStudio are available for anyone to browse the source code (recently rewritten from the original Fortran to a more modern, object-oriented C++), fork (copy) and customize or contribute changes to the main project. In addition, the Github repository hosts the list of known issues, which can be very helpful as a user to be aware of known bugs, or for reporting problems found while using the programs. In fact, part of the issue reporting includes new feature proposals, for which users can voice their opinions on changes they’d like to see.  While it can be daunting at first, having access to the source code is a great resource for knowledge of these energy modeling tools and their inner workings.

2.    Know available sources of input data

EnergyPlus models require input of building conditions, much of which is specific to a building or analysis objective, such as geometry, internal loads, system configuration and control. Other input data is obtained from external sources, which define intensive properties of building materials, system performance coefficients, and the local weather conditions.

Most modelers are probably familiar with the Typical Meterological Year (TMY) weather files, representing ‘typical’ or ‘average’ conditions observed at a location over several years. In addition to these files, observed historical weather conditions over a particular year are available for cases when the measured performance of an actual building is paired with a simulation model, such as for retrocommissioning or measurement and verification projects. Actual weather files can be purchased from weather.whiteboxtechnologies.com.

Recently, with increased focus on the impact of climate change, interest has grown in accounting for changing climate conditions in building energy analysis. Projects such as Arup’s WeatherShift attempt to address this demand with weather files that account for changing climatic conditions based on various IPCC emissions scenarios.

For other general model input data the standard EnergyPlus installation comes with a ‘DataSets’ folder full of input files of HVAC performance coefficients, material properties, economic data and other data useful for a wide array of simulation cases. While some of the HVAC information is from academic studies conducted years ago, it is still a good place to start, especially when creating models of existing buildings and systems. Material properties from the ASHRAE Handbook of Fundamentals are included in standard EnergyPlus input format, as well as numerous other information useful for many simulation use cases.

3.    Automate input and output processing

For most detailed building energy models, the sheer amount of input required for a fully-featured model can be huge and very time-consuming to obtain, process, and enter into the model input file. A multitude of different interfaces, libraries and tools have been developed to help minimize the input required, to various effect. However, since building energy modeling can be used for a multitude of different purposes, it can be difficult to define a single workflow that supports all potential uses.

The most exciting development in the energy modeling toolkit has been the ability pair modeling software to general-purpose scripting tools, allowing users to customize the workflow and data exchange required to build models and analyze results. At kW, we have leveraged the software development kit (SDK) integral to the OpenStudio platform to create custom tools that allow for the rapid development of EnergyPlus models, in order to reduce the time burden of engineers, as well as provide a repeatable and auditable model input process that helps ensure the models are built correctly.

4.    Review detailed output

A downside to the flexibility of EnergyPlus is its complexity. Particularly on the HVAC modeling side, many system control inputs are scattered among different component input parameters, or dedicated control components. Knowing how a model is reacting to user-defined input, especially in more complex or unique models, requires careful evaluation of model output beyond the typically-reported total energy consumption and demand.

A critical step in model troubleshooting is looking at timeseries output, i.e. the simulation state at successive timesteps, and using engineering judgement to verify that systems are operating in the simulation as you expect they would in the real world. All EnergyPlus modelers should be familiar with checking the reporting data dictionary (.rdd) file for the available simulation output variables and requesting this output at the reporting frequency necessary to capture the appropriate dynamics. Keep in mind that HVAC system performance is calculated at sub-hourly timesteps, so system output reported hourly might be averaging the response to control signals in a way that masks the magnitude of the response.

5.    Experiment

EnergyPlus has modeling capabilities beyond what many BEM tools provide, such as thermal energy storage, renewable energy systems and advance control algorithms. Once you understand the basics of how EnergyPlus works, and how to efficiently generate model inputs and review outputs, modelers faced with a new or unique building or system configuration should feel comfortable reviewing the documentation of the model objects most likely to be used in the model, and the example files for those objects included in the EnergyPlus installation for how the objects are implemented in a model. Simulating the example models with various input values and examining the effect on the detailed model output can give a deeper understanding of how those components work and are impacted by other model elements.

Start energy modeling like a pro!

Building Energy Models like EnergyPlus can be very powerful tools for analyzing the energy impacts of design decisions – but only when they are used correctly. Keeping your eyes on these 5 best practices will help you provide sound advice based upon a realistic representation of your building and yield better energy modeling results. If you have any questions about how to check if your building model is working the way you think it is, let us know, we’re here to help.

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