A team of Naval Postgraduate School (NPS) researchers recently published the results of their successful experimentation using phase change materials (PCMs) to develop a formulation with potential to regulate the temperature of living and storage spaces via thermal energy storage (TES). Professor and Associate Chair of Mechanical and Aerospace Engineering (MAE) Claudia Luhrs, Associate Professor of MAE Emre Gunduz, National Research Council (NRC) postdoctoral research associate Richa Agrawal, and recent NPS graduate Ensign Joshua Hanna conducted the research, which appeared in the May 2019 journal “Molecules” and marks another in a long line of innovations in materials science at NPS.
Luhrs initially conceived of the idea while on a recent sabbatical at the Spanish Naval Academy, where she was asked by faculty about materials that could control temperatures inside buildings. Looking into the topic, she became interested in PCMs due to their ability to function without external energy inputs, present reversible phase transformations over multiple cycles and be added to the walls of existing structures. Additionally, Luhrs recognized that PCMs formulations, if optimized, could be used in high temperature locations where NPS students conduct operations, lowering the energy costs of maintaining temperatures at a comfortable range and helping to maintain electronic equipment at ideal temperatures.
“When we talk about development of engineering materials, we need to target the formulations and processing routes to what the application is,” Luhrs said. “We determine what properties are needed, and then devise what materials and microstructures will help us achieve them.”
After careful consideration, Luhrs proposed the use of epoxy-PCMs systems, described in the NPS group’s article as “substances that absorb/release thermal energy during a phase transformation, which is typically melting/solidification.”
One thing that Luhrs and her colleagues were looking for was a way to take existing PCMs technology to the next level—specifically, to use PCMs in a more easily transportable, self-contained system conducive to military needs. “Many buildings already use phase change materials, but they have them in big containers [necessary to store them when in their melted phase],” Luhrs said. “It’s difficult to transport them, and we wanted something that we could use as a coating and have it either readily applied or transported.”
Seed funding from the NPS Foundation and additional funding by the Office of Naval Research’s (ONR) Energy System Technology Evaluation Program (ESTEP) allowed her to take the next step: working with Agrawal and Hanna to develop a material that could hold large amounts of PCMs while producing homogeneous distributions. The team then tested the PCMs that had the best chance of successfully fulfilling the required specifications.
For their study, Luhrs, Hanna and Agrawal tested two alkane hydrocarbons/paraffins for TES applications, and found that n-nonadecane exhibited thermal activity within the desired temperature ranges (30 to 42°C). An epoxy resin was added as a support matrix material (alleviating paraffin leakage), with Carbopol being utilized in order to minimize any phase separation that might occur during synthesis, thus ensuring that when the resin cured, the PCMs would be spread throughout it.
After testing various conductive agents, the NPS researchers found that adding boron nitride (BN) was the best choice to enhance thermal conductivity of the epoxy-paraffin composite.
“We added thermal enhancers,” Luhrs said, “which will make it melt or solidify a little bit faster.”
The group then completed a proof of concept via a laboratory experiment in which the various PCMs being tested were embedded in a hot sand bath. The PCMs that used BN as conductive agents performed best, taking more time to reach the temperature of the sand and remaining about five degrees centigrade cooler than their surrounding environment.
The material was also printed onto polyester/nylon fabrics using Gunduz’s vibration-assisted 3D printing method in order to demonstrate its compatibility with being integrated onto removable liners. Luhrs sees the liners as both practical and valuable for military operations, describing her vision of them as “a very thin piece of fabric that we could place inside a tent or a portable container—any of the working, living [or] storage spaces that we use in our operations—the liner can be put into place when temperatures are rising, and taken out once it has performed its function [of temperature control].”
Modeling performed by collaborators at the Spanish Naval Academy suggests that this formulation will save up to 22 percent of the energy that would traditionally be used in cooling these environments for specialized storage, human use and habitation. The result is a material that offers not only easily integrated options for operations that take place in extreme environments, but also increased efficiency and monetary savings.
In order to further explore the business aspects of potential products resulting from the study’s findings, Associate Professor of Economics in NPS’ Graduate School of Defense Management Marigee Bacolod is working with the team. Bacolod will develop a business case and economic cost-benefit analysis, which Luhrs says will explore “what the life of the material will be, how much money will be saved [by using it], and what products are the most likely to give us an economic benefit.”
Moving forward, Luhrs also wants to explore the possibilities of storing the energy produced during the PCM’s phase change—perhaps for use in powering small electronic devices. In the immediate future, however, she and her colleagues plan to apply for more funding in order to move ahead with further testing and development.
This will include the important step of conducting testing on the life cycle of the material for a full picture of how it will behave “from cradle to grave.”
While the researchers know that the material can be used for multiple cycles without losing effectivity, Luhrs wants assurances that it will not be susceptible to repeated usage in other ways, and additional testing can help to answer important questions ... Will it embrittle, change color (fade) or degrade with extended exposure to sunlight? How will the material degrade over time? Will it pose any risk to successful usage, such as detaching from the liner or drying out and crumbling off of it?
Luhrs wants to be sure before putting the material to work in situations where service members depend on it. “I wouldn’t like to insert it in any kind of system without knowing what will happen after many uses or having a very clear idea of how to dispose of it, when the time comes,” she said.