Heat-Storage Modules Containing LiNO3•3H2O and Graphite Foam
- Wednesday, 30 July 2008
Heat capacity per unit volume has been increased.
A heat-storage module based on a commercial open-cell graphite foam (PocoFoam or equivalent) imbued with lithium nitrate trihydrate (LiNO3•3H2O) has been developed as a prototype of other such modules for use as short-term heat sources or heat sinks in the temperature range of approximately 28 to 30 °C. In this module, the LiNO3•3H2O serves as a phase-change heat-storage material and the graphite foam as thermally conductive filler for transferring heat to or from the phase-change material. In comparison with typical prior heat-storage modules in which paraffins are the phase-change materials and aluminum fins are the thermally conductive fillers, this module has more than twice the heat-storage capacity per unit volume.
The use of LiNO3•3H2O as a phase-change heat-storage material is not new in itself, but heretofore, it has been used with aluminum fins. Open-cell graphite foam has been used as the thermally conductive filler material in conjunction with paraffin phase-change materials in some prior heat-storage modules but, heretofore, it has not been used with LiNO3•3H2O because graphite foam is hydrophobic and, therefore not readily wet by LiNO3•3H2O. The novelty of the present development lies in the choice of materials to make it possible to use graphite foam as the filler with LiNO3•3H2O in order to exploit the greater (relative to aluminum) specific thermal conductivity of graphite to reduce the mass of filler needed to obtain a given level of thermal performance.
The prototype heat-storage module consists of an LiNO3•3H2O-imbued open-cell graphite foam core of 76-percent porosity in an aluminum housing that has a ribbed top that provides a rigid mounting surface for electronics. During fabrication, grooves to receive the ribs were cut into the open-cell graphite foam core (see figure). To overcome the hydrophobicity of the graphite foam to enable the core to absorb the LiNO3•3H2O, an organosilicon surfactant was added to the molten LiNO3•3H2O in the proportion of 0.3 mass percent.
Also added to the LiNO3•3H2O was 1 mass percent of zinc nitrate, which serves as a freezing catalyst to reduce, to an interval of 2 C°, what would otherwise be the susceptibility of LiNO3•3H2O to freezing supercooling by as much as 35 C°. With this catalyst, the LiNO3•3H2O freezes at 28 °C when cooled from a higher temperature and melts at 30 °C when warmed from a lower temperature.