Drawing inspiration from the veinous ears of jackrabbits and elephants, Drexel University researchers have come up with a new approach to passive heating and cooling that could one day make buildings more energy efficient. Their concept, recently published in the Journal of Building Engineering , embeds a vascular network within cement-based building materials that, when filled with paraffin-based material, can help passively regulate the surface temperature of walls, floors and ceilings.
The approach is an effort to address the substantial contribution of building energy demand — nearly 40% of all energy use — to the production of greenhouse gas. About half of a building's energy use is spent maintaining a comfortable temperature. And while new insulation products and techniques have helped to shore up walls, windows and ceilings, these surfaces remain the biggest challenge when it comes to holding or losing heat — contributing to about 63% of energy loss in buildings.
"Architecturally, it looks nice to have a lot of window area on a building, but this also results in diminished insulation properties," said Rhythm Osan, an undergraduate student in the College of Engineering who was a co-author of the research. "In an ideal world, a building wouldn't lose any heat, but from a realistic constructability standpoint, issues like thermal bridging, air leakage from ducts, material performance and joint detailing will always pose some heat loss."
Turning this frustrating reality on its head, the Drexel team devised a way for these surfaces to contribute to maintaining a desired indoor temperature, rather than being an impediment to it.
"Look at the way our circulatory system is used to regulate temperature. When it's hot out, blood runs to the surface – we might get a little red in the face and begin to sweat through our glands and this cools us down through a phase-change process — sweat evaporation," said Amir Farnam, PhD , an associate professor in Drexel's College of Engineering who was a leader of the research. "This is a very effective, natural process that we wanted to replicate it in building materials."
Farnam's Advance Infrastructure Materials (AIM) Lab is a leader in research focused on nature-inspired methods for making infrastructure materials more durable. They have previously developed concrete that uses phase-change material — similar to the paraffin used to make candles — to melt snow and ice from its surface; self-healing concrete that employs special bacteria that produce calcium carbonate; and 3D printed polymers that strengthen concrete structures.
To create the thermally responsive building materials, the group drew inspiration from several of these endeavors — using a printed polymer matrix to create the grid of channels in the concrete surface before filling them with a paraffin-based material to enable their responsive temperature regulation.
Phase-change materials, like paraffin, are uniquely suited for this application because they absorb and release thermal energy as they shift between liquid and solid states. So as temperatures drop, and the material transitions from liquid to solid, it releases heat energy; conversely, when ambient temperatures rise the material is able to absorb heat energy, producing a cool surface.
"We have previously used paraffin-based material as the phase-change ingredient for self-warming concrete, so we knew that it was a reliable, natural substance that could affect the surface temperature of concrete building materials," said Robin Deb, PhD, a research scientist in the AIM Lab and a co-author of the research. "For this application we selected a phase-change material with a melting temperature around 18 degrees Celsius, a relatively low melting point, to test its effectiveness in cold climates. But this system would allow for tailoring the phase-change material to be responsive in warmer climates as well."
Using a dissolvable, or "sacrificial," polymer template, the team created a series of cement samples with varying vascular channel patterns, including a single channel, multiple channels, parallel channels perpendicular to the edges of the surface, diagonal parallel channels and a diamond-shaped grid of channels; and ranging in thickness from 3 to 8 millimeters.
They tested each sample to determine its mechanical behavior, as well as their ability to slow surface warming and cooling, in relation to ambient environmental conditions, when the channels were filled with phase-change material.
The most effective combination of strength and thermal regulation proved to be the sample with diamond-shaped grid channel architecture. This sample was able to maintain its structural integrity during tests to stretch and compress it, while also slowing the heating and cooling of its surface — to 1-1.25 degrees Celsius per hour — with respect to its environment.
"We found, perhaps not surprisingly, that more vasculature surface area equates to better thermal performance. This observation is similar to physiology of elephant and jackrabbit ears, which contain extensive areas of vasculature to help regulate their body temperature," Deb said. "We believe that our vascular materials could play a similar role in a building by helping to offset temperature shifts and reduce energy demand from HVAC to maintain thermal comfort."
To further bolster the strength of the materials — despite being partially hollowed out by the channels — the team showed adding a fine aggregate material to the cement could improve its durability without affecting the vasculature's ability to circulate the phase-change material.
"While this study was intended to show a proof of concept, these results are promising and something we can build on," Farnam said. "This shows both the effectiveness of this method for regulating surface temperature in cementitious materials, as well as a simple and cost-effective method for producing them. With additional testing and scaling we believe this has the potential to make a significant contribution to the many ongoing efforts to improve the energy efficiency of buildings."
The team's future research will entail testing different phase-change materials and channel configurations in larger cementitious material samples over a longer period of time and a wider range of environmental temperatures, among other variables.
