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Re: [EnergyPlus_Support] Phase Change Materials
Jim Dirks, thanks for the message.Â
Jim Farnham,Â
The modeling of Phase Change Materials is indeed possible in Energy Plus. However, the term M-Value as presented by Kosny has its limitations in terms of modeling. The M value is Î?h or enthalpy change of the material. The term M value is also used by some manufactures to specify the total capacity of material by density*thickness*h area to devise an analytical approach for the total capacity of the material to absorb and  release heat in terms of Btu capacity. Using this simple analytical approach you can use M-Value to calculate the magnitude of potential of the PCM, assuming that it makes a full transition. M value is an idea, but not a physical input. The current inputs for PCM modeling are as follows: Thickness, Density, Specific Heat, Conductivity, temperature coefficient for thermal conductivity, variable thermal conductivity and temperature dependant enthalpy.Â
 Energy Plus uses a heat capacity method with that bounds the Stephan problem into fixed grid and converges at a solution using either a fully implicit or Semi-implicit time step scheme to account for latent heat evolution. This approach is based on first principal physics and performs a curve fit up to 16 linear piecewise temperature enthalpy pairs, then uses discretization to divide the surface into nodal temperatures and then calculates the total enthraply at a given temperature for the PCM layers.  This approach is not perfect as it does not take into account sub cooling or hysteresis, nor does it directly correlate convection to PCM heat flux, nor does it provide many outputs to allow for bug checking.  I am the first to admit that at times can produce results that do not seem to make logical engineering since ie negative or null energy savings.  Irregardless of the issues, the Energy Plus has been validated using Analytical,Comparative and Experimental methods, is based on first principal physics and is in my opinion the best whole building PCM modeling tool currently available.
I am working on a detailed design guide and some example files that I can send you in the future if you are interested, but for now-->
Guidance on designing with PCM's:Â
Buildings and locations that have diurnal temperature shift are best for passive PCM applications, (ie no control, or natural ventilation) whereas 24 hour facilities and/or ASHRAE climate Zones 1A,2A,3A, are better for active PCM applications (ie use of the cooling system to discharge the heat from the PCM at night)Â
There are three ways to approach modeling PCM's in Energy Plus:
- Put PCM in a construction and apply it to all surfaces. This method is not recommended because it tends to produce results that show very high costs, very low energy savings/load reduction/comfort enhancement and long paybacks.Â
- Evaluate individual surfaces to find the surfaces that reach the highest daytime temperatures and also have the largest diurnal temperature swings. (D-View or Results Viewer work well for this) The rule of thumb for this approach is to use surfaces that have solar loads on one side and internal loads on the other.Â
-  Parametrically study all possibilities (My proffered approach and most of the time the best solution)  Create a construction for each orientation that should meet the conditions mentioned in #2, and associate the constructions to the surfaces.  Run a parametric study where you alternate the PCM Q value (peak melt temp) and M value for each construction. Then choose the model that meets your design criteria and review each surface to assure that full transition is being met, if necessary reduce PCM from surfaces that are not transitioning.  I suggest jE+ for this approach because it creates summery tables that allow the user to review all results at once.Â
Attached is a dataset of objects for bioPCM that you are welcome to use in your research. Also I have an experimental version of E+ that I may be able to share if you are interested in testing sub cooling or hysteresis, and additional outputs if you are interested let me know and I will check with the developer.Â
On Tue, Mar 12, 2013 at 11:03 AM, Jim Dirkes
<jim@xxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
Â
Jeremiah Crosset is someone who spends a lot of time with E+ and phase change materials.
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Hi,
I'm looking for any information regarding members who have used or are interested in using phase change materials as an energy efficiency measure in the energy modeling/design process- using Energy Plus. I understand that Energy Plus can calculate use of pcm(M value). I'm leading a team with Fraunhofer/Mit Center for sustainability for the R&D of using pcm with fenestration systems.
Thanks in advance.
Jim
--
Jeremiah D. Crossett  | Senior Analyst  |  Phase Change Energy Solutions
120 E. Pritchard St.  | Asheboro, NC 27203  | Mobile 503-688-8951
 Â
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