Ben Welter - Tuesday, October 17, 2017
The latest version of EnergyPlus modeling software includes an updated module for measuring the impact of phase change material on energy use in buildings.
Developed by the U.S. Department of Energy, EnergyPlus is an energy simulation program that can be used to calculate heating and cooling load in a building, based on detailed information about the building’s physical make-up and mechanical systems.
With the support of a $100,000 grant from the Department of Energy, Jeremiah Crossett, chief technology officer of NRGsim Inc., worked with Dr. Edwin Lee of the National Renewable Energy Laboratory to incorporate the updated PCM module into EnergyPlus.
The PCM module was originally developed more than five years ago by Ramprasad Chandrasekharan, then a graduate student in mechanical engineering at Oklahoma State University. Crossett assumed responsibility for the module soon after, debugging the FORTRAN code and modifying the logic to include the effects of subcooling and hysteresis. He began using the module in a customized version of EnergyPlus on all PCM modeling projects he worked on. The module, however, was not compatible with public versions of EnergyPlus, which was converted to the C++ programming language in 2014.
Crossett, right, sought the DOE grant to fund the conversion of the PCM module to C++ so that it could be incorporated into EnergyPlus. The module features two major improvements to the original PCM module.
“The new model includes the effects of hysteresis,” Crossett said. “The input object is called Material Property Phase Change Hysteresis and includes two [temperature/enthalpy] curves. Instead of going up and down in the slope of a single curve, the model either stores the energy in the storage range and then releases it when the model hits the heating curve. Or the model hits the heating curve immediately, depending on the properties of the actual phase change material. This model takes into account the histories of building surface temperatures, on into the current. It does a better job of accounting for the phase change material’s performance over time.
“In the previous version, what would happen is any time temperatures would fall and then rise again, and rise and then fall again, any time temperatures were not fully linear, [the module] would show additional energy consumption. Essentially, you had this single curve and as it would get cooler it would drop the heat back into the space. So say your material’s melting at 74, 75 Fahrenheit, and you’re saving on cooling energy, and then it drops down to 73, and now it drops all the heat right back into the space. So now you’ve got more energy you’ve got to deal with in your cooling system. That’s not how it actually works in the real world. …
“That caused a whole lot of studies, done by many, many different people over the years, to be wrong. It essentially gave a false indication that phase change materials don’t work nearly as well as they really do. …
“[The new module also] models subcooling and supercooling. Each PCM has a theoretical melt and freeze point. That’s normally how these products are rated. Some companies call it a Q-value. Other companies just say it as a number, such as a phase change 23 or a phase change 29 in degrees Celsius. But in the real world, how these products actually perform is that there’s some amount of subcooling: the freezing point is not identical to the melting point. You can have supercooling as well, where once your material is frozen, you have to reach a point above the theoretical melting point in order to melt it. This model allows the thermal simulation to have different melting and freezing points. ...
"Now that EnergyPlus can accurately model PCM's performance in real buildings," Crossett said, "it is well-suited for evaluation of PCM technologies for energy codes such as California's Title 24 and green building standards such as ASHRAE 189.1."
Here's a screen grab of a portion of the new EnergyPlus PCM module:
Crossett and Lee completed work on the module in August. Version 8.8.0, released on Sept. 30, is available for free at https://energyplus.net/downloads. They plan to publish a paper on their work later this year.
Over the next six months NRGsim plans to work with PCM manufacturers to add their products to a database for the EnergyPlus program, based on the ASTM C1784-14 protocol, "Standard Test Method for Using a Heat Flow Meter Apparatus for Measuring Thermal Storage Properties of Phase Change Materials and Products."