Modeling cyclic phase change and energy storage in solar heat receivers

Numerical results pertaining to cyclic melting and freezing of an encapsulated phase-change material (PCM), integrated into a solar heat receiver, have been reported. The cyclic nature of the present problem is relevant to latent heat thermal energy storage systems used to power solar Brayton engines in space. Specifically, a physical and numerical model of the solar heat receiver component of NASA Lewis Research Center's ground test demonstration (GTD) system was developed and results compared with available experimental data. Multiconjugate effects such as the convective now of a low Prandtl number fluid, conduction in the PCM, containment tube, and working fluid conduit were accounted for in the model, An ideal-body thermal radiation model was also included to quantify reradiative effects inside the receiver along with losses through the aperture and receiver shell. A high-temperature eutectic mixture of LiF-CaF2 was used as the PCM and a mixture of He/Xe was used as the working fluid. A modified version of the computer code HOTTube was used to generate results for comparisons with GTD experimental data in both subcooled and two-phase regimes. While qualitative trends were in close agreement for the balanced-orbit modes, excellent quantitative agreement was observed for steady-state modes.