This invention accommodates the volume expansion and contraction of water ice as it freezes and thaws, thus enabling the use of water as a phase change material (PCM) for thermal energy storage. Due to the relatively large volume expansion of water upon freezing, and the relatively large bulk modulus of elasticity of ice, it is imperative to accommodate the volume expansion in order to prevent rupture of the containment vessel. In addition to accommodating the volume expansion associated with the phase change from liquid water to solid ice, this invention is usable at temperatures as low as –150 °C, thus enabling the ice to be super-cooled for additional sensible thermal storage capacity. Finally, this invention operates independent of gravity, enabling its use in space applications.

Most PCMs contract upon freezing and expand upon melting. When filling a PCM container with PCM material, typically the PCM material is melted and heated to its maximum operating temperature, then poured as a liquid into the container, which is then sealed. As the PCM material cools and refreezes, it wants to contract, resulting in a negative pressure inside the container relative to the ambient pressure outside the container. Due to the nature of typical PCM containers, and the fact that in a oneatmosphere environment there is an upper limit to the magnitude of the pressure differential across such PCM containers with typical PCM materials, it is usually rather straightforward to design a rigid PCM container.

However, with the use of water as the PCM, the container is filled with liquid water, which will expand upon freezing, imparting a positive pressure inside the container relative to the ambient pressure outside the container. Because of the relatively large volume expansion and bulk modulus of elasticity, extremely large forces could be imparted to a rigid container, likely resulting in rupture of that container. Therefore, the volume expansion must be accommodated.

Flexible foam is placed inside a container as an internal liner to be used for holding the water PCM. Inside this foam is flexible Tedlar bag material. The foam inside the container compresses as the water freezes and expands. Additionally, the foam provides some degree of insulation for the PCM. Conversely, when the ice melts and contracts, the foam re-expands, pushing back on the Tedlar bag material. The function of the bag material is to allow for the expansion and contraction of the water as it freezes and thaws. The greatest challenge with the material selection is the extremely low temperature (approximately –150 °C) to which the material must be usable, resulting in a derived requirement of an extremely low glass transition temperature. Additionally, the material must not be permeable to gas, and flexible but not necessarily elastic. This material still accommodates the volume changes of the water by simply folding (crinkling) and unfolding. The chosen material is polyvinyl fluoride (PVF). The material is flexible down to –100 °F (≈ –73 °C), and usable from –385 °F (≈ –232 °C) to 225 °F (≈107 °C) with intermittent spikes to 400 °F (≈204 °C).

This work was done by Tom Leimkuehler, Grant Anderson, and Tom Morin of Paragon Space Development Corporation; and Brian Walter of XC Associates for Johnson Space Center and Ames Research Center. MSC-25363-1/4-1/715-1

NASA Tech Briefs Magazine

This article first appeared in the November, 2015 issue of NASA Tech Briefs Magazine.

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