types of gradients should be small in a TES so it is assumed that Marangoni convection is not a significant problem. Phase Change Convection. The current towards the solidification front caused by shrinkage of the solidifying medium is called phase change convection. Due to the high volume change of the relevant storage media, this convection will have a significant influence on the solidification process in microgravity. The mass transport towards the cooled surfaces generated by the phase change convection influences the PCM/void distribution as mentioned before. Furthermore, this mass transport carries heat towards the solidification front. Thus, phase change convection will enhance heat transfer, especially at the beginning of the discharge cycle. The importance of phase change convection can only be evaluated in microgravity experiments. Wetting and Spreading Properties Good wetting and spreading tendencies of the PCM towards the containment material (CTM) are advantageous concerning the heat transfer from the PCM to the CTM and vice versa. In the ground experiments LiF showed high wetting and spreading tendencies but also the ability to ooze through microscopic cracks in the container walls. High wetting and spreading tendencies will cause higher corrosion rates and accelerate container leakage. Corrosion effects were detected by x-ray structure analysis in LiF probes, especially in those contaminated with hydrogen and/or oxygen. Therefore, microscopic cracks in the container, caused, e.g. by thermal tension or meteorite impacts are really of concern. Based on the experiences with LiF we assume that relevant storage media salts will wet metal containers completely in microgravity. Separation of the components of a eutectic due to different wetting properties is not expected. Crystal Growth Due to the high volume contraction and the low thermal conductivity of relevant PCMs, radiation will play an important role with regard to heat transfer into, inside and out of the storage medium. In advanced REC/TES systems radiation could be the main heat transport mechanism [4]. The transmissivity of the PCM for thermal and solar radiation is influenced by the void distribution, crystallization and contamination. The voids will scatter sunlight and infrared and therefore increase the effective absorption rate. Type and order of crystallization depend on the cooling rate and are therefore difficult to predict for the TES, but it is very likely that polycrystalline solidification will occur. Monocrystalline LiF is transparent to infrared and sunlight and is widely used in optics. Crystal planes of LiF can be seen in the SEMs shown in Fig. 4 and also in Fig. 5 where some dendrites are depicted. The influence of gravity is evident in Fig. 5. Lack of Sedimentation In weightlessness particles, e.g. from corrosion, will remain distributed in the melt. They will minimize undercooling by functioning as nucleating agents. Furthermore, particles suspended in the PCM will decrease melting and solidification temperature, change the PCM transmissivity and consequently influence the performance of the
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