Heat Transfer Predictions for a New Heat-Pipe Latent-Heat-Storage Receiver Element for Solar Dynamic Space Power Systems S. ORTNER & S. KRAUSE1 SUMMARY An array of cylindrical heat pipes with annular latent heat storage units is generally recognized as the most promising heat-transfer/thermal storage alternative for spacecraft solar dynamic power systems (SDPS) receivers. The DLR-ITT has developed a new storage unit containing LiF in graphite containers with internal capillary notches to handle the LiF volume change upon melting. Sucessful terrestrial tests have been reported recently. This paper presents theoretical predictions of the relevant heat flows and temperatures in heat-pipe I storage elements (HPSE) employing such storage units. The HPSE were sized for a receiver combined with a fictitious 25 kWe Stirling engine in an SDPS. The capillary notches were optimized for maximum heat transfer. The results show that the required heat flows are essentially feasible, and what deviations are to be expected and must be handled by the spacecraft energy management during a typical orbit. The knowledge of the receiver and internal heat pipe temperatures is important for design purposes. Introduction The central and least developed component of solar dynamic power systems (SDPS) for space applications is the solar receiver with thermal energy storage. A receiver with cylindrical heat pipes for energy transport and concentric latent heat storage units on the heat pipes has generally been recognized as the most promising concept.1 Contrary to designs with heat extraction from the storage unit by gas flow through tubes, melting and solidification of the PCM are from the same surface which permits smaller thermal resistances and hence smaller driving temperature differences. Furthermore, melting, solidification, and heat flow are all axisymmetric. This avoids incomplete melting which would necessitate a large reserve of storage medium, and reduces the danger of thermal ratcheting and rupture.2 Basic solidification tests of latent heat storage media have been carried out in space.3,4 Terrestrial tests of complete heat-pipe/storage elements (HPSE) with internal (not external) storage units have been described and compared with theoretical predictions.5,6 S. Ortner, Graduate Student, Institut fur Raumfahrtsysteme, Universitat Stuttgart, Pfaffenwaldring 31, 7000 Stuttgart 80, Federal Republic of Germany; S. Krause, Institut fur Technische Thermodynamik, Deutsche Forschungsanstalt fur Luft- und Raumfahrt e.V./ DLR, Pfaffenwaldring 38-40, 7000 Stuttgart 80, Federal Republic of Germany.
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