Phase change material used in latent thermal energy storage systems must, of course, be encapsulated. The stiff containers, often made of high-density polyethylene, come in a variety of familiar shapes. The two most common are cylinders and spheres. Fins and dimples can be added to improve melting and solidification rates.
Researchers at Shanghai Jiao Tong University in China say they have identified a better shape. They found it in the circulatory systems of mammals.
Red blood cells, or RBCs, are filled with hemoglobin, not PCMs, and their purpose is to deliver oxygen, not thermal energy. Still, the researchers wondered, is the distinctive biconcave shape conducive to more effective heat transfer?
In a paper recently accepted for publication in Applied Thermal Engineering, “Thermal performance analysis of a novel PCM capsule in red blood cell shape,” Xiwen Cheng and Xiaoqiang Zhai described their investigation, in which they compared the RBCs with four other shapes by experimental and simulation methods.
All five tested shapes – RBCs, spheres, cylinders, drums and ring-shaped capsules – were sized to contain 25.5 grams of PCM (capric acid, lauric acid, palmitic acid). The sphere’s interior diameter was 40 mm. The shell material was nylon 12 with a thickness of 1.5 mm.
The researchers found that the RBC-shaped capsule performed significantly better than the other shapes. The average charging rate of the RBC capsule, for example, was twice as fast as that of the sphere.
Dr. William R. Sutterlin, Entropy Solutions’ chief science officer, who was not involved in this research, said he was intrigued by the results. "Until good alternatives are established to increase the thermal conductivity of the PCMs themselves,” he said, “novel designs such as these, which increase surface area, are welcome developments."