the temperature gradient between container wall and the next void is less than 0.136 K, provided that there is no PCM movement. Hence a very efficient heat conducting matrix is necessary to avoid heavy containment vessels in this case or the solid PCM layer has to be always so thin that it bursts before the containment does. In the other case—heating and cooling surfaces are different—voids developed at or close to the non-cooled walls could substantially reduce heat transfer and cause hot spots. In both cases it will be necessary to install devices to support heat transfer, e.g. a heat- conducting matrix, thermal ribs, fins or fingers, etc., which will increase the TES unit's mass. In our experiments the LiF did not solidify like a foam with a uniform average density. Void order and structure showed similarities to fractal systems, which can be seen in the scanning electron micrographs (SEMs) shown in Fig. 3. Hence the PCM/void distribution depends on the geometry and proportions of the PCM containers rather than on its absolute dimensions. This means that dividing the PCM vessel into small compartments will reduce the needed heat flux rate through a given surface but will not necessarily solve the problems caused by the volume change. Gravity Independent Convection Marangoni Convection. A change of surface tension caused by temperature or concentration gradients along a free surface is responsible for Marangoni convection. Both
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