(3) constant solar irradiation and duration of irradiation and eclipse periods of the orbit. Timely variations of these quantities and their effects are included, e.g., in a more rigorous thesis.21 (4) constant mean temperature of the working fluid (He) of the Stirling engine in the heat transfer tube within the heat pipe, both in time and along the tube. Uniform conditions along the tube is a common acceptable simplification. Constancy with time is a twofold simplification, i. e. neglecting both the longterm variation with the heat resistance in the storage unit over the charging and discharging periods, and the high frequency fluctuations caused by the characteristically rapid change of directions of the He flow in the Stirling engine (1500 rpm in our V 160F type12). The former were not considered in accordance with the V 160F engine in which an internal control keeps the temperature constant. The latter need not be considered in order to prove proper heat pipe operation.1 A limited investigation of the effects of the high frequency fluctuations has already been described.12 The heat transfer coefficient to the He flow was assumed to be constant at constant He temperature, and equal to the value derived from prior tests (see preceeding section). (5) the liquid-solid interface in the PCM is a function of the cylinder radius only. This is a reasonable assumption because the heat pipe equalizes the heat transfer over the axial and azimuthal directions. (6) radiation heat transfer through the LiF in the container (LiF is transparent near 848°C) was neglected because the temperature differences involved are small except in certain transitional heating phases which were not considered. The material properties and other parameters used in the calculations are summarized in Table 2. The wall and wick thicknesses in the Na heat pipes chosen were taken on the basis of experience at the University of Stuttgart.22 The other dimensions of the heat pipes were chosen such that all calculated heat flows through the heat pipe walls were well below the upper limits known23,24 for this heat pipe fluid. In determining the heat flows and temperatures as functions of the state of charge, the irradiation through the receiver aperture and the mean temperature of the He flow in the heat transfer tubes of the Stirling heater, THe, were first adjusted to yield the desired heat flows 832.5 W and 1366.1 W (see Table 1) to the storage unit and the Stirling heater, respectively, in storage charging at a state of charge of 0.5. The incident radiation and THe were then held constant during both the insolation and the eclipse periods. Further details may be found elsewhere.19 Results The main results of the calculations are shown in Figs. 3 through 7 which compare Cases (1) through (3) as defined in the introduction.
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