While the above discussion is concerned with the mean temperature THe, Fig. 8 shows the streamwise development of the He temperature in the heater tubes of the Stirling engine over their length within the heat pipes of the HPSEs, for various assumed inlet temperatures THe The outer wall temperature of the heater tube was taken constant and equal to a characteristic vapor temperature Tj = 848°C (= TPC) in the heat pipe. The He flow in the tube was assumed unidirectional for simplicity ignoring the complex oscillatory motion in a real Stirling engine. The calculated streamwise increase of THe is therefore a time average for the mean He mass flow. A relevant mean He mass flow was extrapolated from previous experiments12. Each curve is marked with the corresponding heat flow Q. The curves are valid for both the insolation (Q = QSt) and the eclipse (Q = Q^) periods. For each THein, THe is approximately the temperature in the middle of the curve, and the exit temperature THeoul is approximately equal to the maximim He temperature THemM in the working cycle of the Stirling engine. It is seen from Fig. 8 that while THe is certainly an indication for THemax, THemax becomes increasingly larger than THe as T; - THein or Q increase. If Q is equal to the desired value 1366.1 W (Table 1), the difference between THcmM and THe is 53 K. Therefore, a sizably larger Ar?th may be expected than estimated in the above discussion in terms of THe. Fig. 8 also shows the interesting result that the total temperature difference THe necessary to to drive the desired Q = 1366.1 W is 122 K. Conclusions Theoretical predictions were made of the relevant heat flows and temperatures in a new heat-pipe/LiF-graphite-latent-heat-storage element for a solar dynamic space power system with a 25 kWe Stirling engine. The graphite storage container had internal capillaries to take care of the LiF volume change upon melting. The capillaries were optimized for maximum heat transfer resulting in a storage-to- container weight ratio of ca. 3:2. The calculations were carried out for three cases, a basic heat-pipe/storage element (HPSE), a HPSE with intermediate heat pipe, and a HPSE with a fictitiously increased heat transfer coefficient to the working fluid of the Stirling engine. For the three cases, respectively, • the deviations form desired solar-to-storage and solar-to-Stirlingheat flows during insolation reach 20, 16, and 44%; • the deviations from the desired storage-to-Stirling heat flow exceed 17, 12, and 37%; • the required mean working temperature in the Stirling heater is ca. 675, 624, and 790°C.
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