During solar radiation the heat pipes transport heat both to the working gas of the Stirling engine and to the storage units surrounding them. During eclipse, they transport heat from the storage units to the Stirling working gas only. In both intervals, the heat enters and leaves the storage unit at the inner wall of the storage containers. Ideally, the heat flow to the Stirling engine is equal for both the insolation and eclipse periods. LiF with a latent heat of 1044 kJ/kg and a melting point of 848°C was chosen for the phase change heat storage medium (PCM), and toroidal capsules made out of graphite for the PCM containers. These were designed with internal capillaries sized to handle the volume change upon melting of the PCM, AV/VB = 0.196 for LiF. This idea makes use of the fact that LiF does not wet graphite such that the melted LiF only enters the notches against the capillary forces if pressure builds up in it through volume increase on melting. The principle, its advantages, and confirming experiments have been described earlier.7,8 A small specimen has been tested successfully under microgravitiy conditions in a 6 min. TEXUS flight.3 For the usual metal PCM containers, thermal ratcheting and failure of the containers may not occur if the PCM volumes are small, because the strength of solid LiF is much smaller near 848°C than that of the container.1 PCM units of larger dimensions, however, may require an effective storage principle such as the one described above.
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