of Tj as calculated for the heat flows of Figs. 3 and 4, for the insolation and eclipse periods, respectively. The phase change temperature TPC of the storage medium is also shown. As there must always be a driving temperature difference, Tj > TPC if the storage unit is being charged, and Tj < TPC if it is being discharged. Because of the increasing heat resistance in the storage medium the difference | T, - Trc | increases as charging or discharging proceed. In none of the three cases compared in Fig. 6, does the departure of Tj from TPC exceed 39 K during insolation and 19 K during eclipse. An adjustment of THe as shown in Fig. 5 would make the departure for the eclipse comparable to that for the insolation period. The receiver (internal) temperature Trec is needed for receiver design. Since a lid was assumed to cover the receiver aperture during eclipse, Trcc is practically equal to the vapor temperature as given in Fig. 6 for that period. This is not true for the insolation period. Because the heat flow (Q^ + QS1) as driven by AT = Trec - Tj is practically constant with x and nearly equal for the three cases considered (see Fig. 3), the Trec curves are parallel to the Tj curves, nearly equal for the "basic HPSE" and the "HPSE with intermediateheat pipe" and only slightly different for the "HPSE
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