Fig. 3 shows the variation of the heat flows solar-to-storage, Q^, and solar-to- Stirling, QSt, as the storage unit is charged during the insolation period. In accordance with the adjustment made, for all three cases, these heat flows agree with the desired values of Table 1 at a state of charge x = 0.5 of the storage unit. QSt increases uniformly from values smaller to values larger than the desired value, and Open, vice versa. This behaviour is due to the increasing thickness of solid PCM which governs directly, and QSt indirectly, through the resulting temperature changes. The reduced heat resistance to the He flow of the Stirling engine (Case (3)) makes the increasing heat resistance in the storage medium so predominant that it increases the deviation from the desired value by more than a factor of 2 as compared to Case (1). The intermediate heat pipe (Case (2)) causes only a small increase by a factor of about 1.2. Since and QSt agree with the desired values at x = 0.5, it is not surprising that their sum is nearly constant over x and nearly the same in all three cases, and that the total heat stored over the complete charging period (time integral over 0^) is close to the desired total heat because the deviations at x > 0.5 partially cancel those at x < 0.5. The absolute derivation of both Qpon and Qst from their desired values unfortunately reach rather large percentages of the latter.
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