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Initial Experiences with OpenStudio and EnergyPlus

February 11, 2014

As I mentioned in a previous post, I’ve begun to integrate EnergyPlus modeling into my workflow, using NREL’s OpenStudio to interface to it.  I’ll talk more about my current projects in future posts, but while I’m in the thick of it, I thought it might be worthwhile to give a few initial impressions of OpenStudio v1.1.0 coupled with EnergyPlus v8.0.0.

The Department of Energy (DOE) developed EnergyPlus in the 1990s with the goal of combining the best of the preceding DOE-2 and Blast efforts. Energy simulation engines are big complex pieces of software requiring larger research and development funding than was available through commercial channels. This excellent article shows how the oil embargoes of the 1970s triggered government interest in building energy modeling. And yet as we all know from the 2030 Challenge, building operations still consume way more than their share of national energy expenditures.

EnergyPlus is a simulation engine with a crude 1990s era text-based file format interface.  As it says on DOE’s about page:

“EnergyPlus is a stand-alone simulation program without a ‘user friendly’ graphical interface. EnergyPlus reads input and writes output as text files. A number of graphical interfaces are available.”

The text interface is actually great news, because if you want to examine precisely what is going into and out of the simulation program, with nothing hidden, you can look into those text files and see all the nuts, bolts, bells and whistles. The graphical user interfaces (GUIs) like OpenStudio generate those text files, hopefully with a more-user friendly interface. We see this in daylight modeling tools as well, where Lawrence Berkeley National Laboratory’s Radiance simulation interface has spawned a number of GUI tools to better facilitate creating the complex text-based input files. It is actually funny that the human readable text-files are so complex that they are, well, difficult for humans to read. But when it gets down to the simulation itself, you can always check the details of what is going into the simulator by looking at the text based file generated by the GUI.

Okay then, the goal of a GUI for EnergyPlus is to provide a more convenient way for users to create models. Unlike the energy simulators themselves, they cost a lot less and are much easier to develop. In fact, there are several commercially available GUIs already developed for EnergyPlus. The links to them are right there on the web page. So why OpenStudio? Ordinarily the government does not stifle commercial development by competing directly with it,  so it’s a little unclear to me why OpenStudio exists.

Currently, I have no answer to that question. It’s possible that because the government labs such as NREL are also users, it seemed wasteful for them to have to buy commercial interfaces to their own underlying freely available simulation engines. Or perhaps as expert users they have found the commercially available options insufficient for their needs. Or maybe the cost of the commercial tools is still creating a barrier to wider adoption of energy modeling as a part of the design process. Whatever the reason, over the last few years NREL developed OpenStudio as a freely available GUI to both the EnergyPlus and Radiance simulation engines, utilizing SketchUp as a modeling interface.

But don’t let the SketchUp part fool you fool you into concluding that OpenStudio is either a) not very sophisticated or b) something that you can plug your existing SketchUp design modeling straight into.

The basic hotel example provided by OpenStudio.

The basic hotel example provided by OpenStudio.

As for (a) sophistication, SketchUp is being used to construct the Building Envelope Model (BEM) required for energy simulation. Most of the annotations available to define materials and HVAC systems and whatnot are all still there.  The construction characteristics of each surface must be defined and added to the model, along with temperature set points for the thermal zones and usage characteristics, etc.  There is plenty of room for parameters, materials, set-points, etc.

As for (b) simply plugging in your SketchUp massing model with no further thought, the OpenStudio interface groups room boundaries together and assigns the surfaces to pre-defined layers which denote specific conditions – ground contact versus outside air – for example. Given the way that elements in different groups interact in Sketchup, I found it extremely difficult to create the zone groups from an existing model and much easier to start from scratch alongside it, while using the existing elements as quick references for scale and dimensions.

SketchUp, however, excellent choice for BEM creation because it inherently limits the user to the planarized surfaces required for the energy modeler. EnergyPlus doesn’t understand NURBS, so regardless of how free form your actual building is, you are going to have to planarize it.

Once you have your building thermal zones massed out in SketchUp, the next step is to refine the annotation details using the OpenStudio interface. Here’s where I start to run into difficulties. While on the surface the GUI appears to satisify the goal of a nice, clean user interface, a little below the surface it begins to lose its integrity. First, OpenStudio requires that you start with a building template and there are 16 of them provided for various ASHRAE standard building types. Unfortunately they all come preloaded with a number of predefined construction types and materials that may not suit your needs and become confusing when you are trying to sort out what’s being used and what’s not. I found no barebones template reflective of the simple energy modeling examples provided by EnergyPlus itself. Remember those text files that I mentioned could be examined to verify all the details of your model? OpenStudio refused to import the provided EnergyPlus example files and, worse, when I tried to strip a template of the extraneous materials, I quickly arrived at the point where OpenStudio could no longer run the file, without giving a clear explanation of why, except that it was missing elements that did not appear to be required by the energy modeler itself.

This takes us to the next problem, error handling. An OpenStudio error when constructing a model takes the form of refusing to do something without telling you why. You can’t drag and drop a material into the wrong place, which is good. But it also won’t give you a clue as to why, which is bad for coming up to speed. Meanwhile, if you do create a model that EnergyPlus doesn’t like it is only possible to debug by digging out the appropriate text-based log file and deciphering a cryptic message from the simulation engine, a message that you are less familiar with because you are using the GUI to distance yourself from that very model.

MaterialsShot

The GUI becomes confusing when elements appear in multiple locations without a clear distinction of why they differ.

The GUI itself could use some additional work as well. Again, at the top level it is fairly clean and reasonably well organized. Drag and drop works okay, but is often not needed when a simple double-click-select would do. Once inside some of the tabs, however, things become a lot less clear. On the construction tab for example, there are three different columns where materials appear, one labeled “library”, one “model” and one for…? I guess it’s for materials assigned to actual objects in the model. Honestly I’m still sorting this one out. If what’s in the model file is more than what is assigned to actual objects (which it appears to be because of those templates with all sorts of extraneous materials included), why are they included in the output files instead of being left in the library? What is the difference between the library and model columns if the materials in the model columns are not those assigned to actual objects?

Conclusions? OpenStudio v1.1.0 provides a powerful mechanism for creating input models for EnergyPlus, but there is still a great deal of getting up to speed required and it does not eliminate the need to understand the complexities of the underlying EnergyPlus models. I was especially frustrated by the lack of a basic, empty model with no extraneous, unused parts and I hope that will either be remedied or that someone will point me to it if one exists. All that said, if you put the time in to become expert with it, OpenStudio provides a reasonable avenue to access the EnergyPlus simulator. I hope we will see many future refinements but, in the meantime, it remains a powerful engine that I am happy to be putting to work now.

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One Comment leave one →
  1. Felix permalink
    October 13, 2014 11:53 pm

    Hi, I’m just looking in to using OpenStudio for my house design. I’m not an architect but I am a software and electronics engineer. I read your post with interest. Designing good GUI is hard and it is often just left to the software engineer who is so familiar with the underlying application that they do not see the problems with the GUI that are obvious to new users. I hope they read this! Have you made any progress? Time for an update post?

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