Assuming furthermore a heat storage efficiency of 0.93 and reasonable parameters for the determination15 of the diameters of the receiver aperture and of the assumed parabolic dish concentrator, the energetic and geometric characteristics of the receiver were calculated. They are summarized in Table 1. The dimensions of the HPSE are given in the next section. Heat-Pipe/Storage Element Experimental simulation at a geometrically and energetically 1:1 scale has been planned for one of the 61 HPSEs of the reference receiver. For this purpose and as a basis for the calculations, a HPSE satisfying the geometrical and energetic conditions of Fig. 1 and Table 1, respectively, was conceived in detail. Fig. 2 shows its most important features and dimensions, in particular the capillaries at the inner walls of the PCM containers. The geometry of the capillaries meets three requirements: • their combined volume is equal to the volume change upon solidification of the liquid PCM volume in the storage container at 848°C • the ratio of the capillary width to the distance between neighboring capillaries and the ratio of the capillary height to the height of the completely liquified PCM is 0.5 and 0.286, respectively, as required for maximum PCM-to-heat-pipe heat transfer (see Appendix). • The mean absolute width of each capillary (1.3mm) lies in the practical optimum16 between machinability (no too small) and required capillary forces (not too large). Heat transfer from the heat pipe to the working fluid of the Stirling engine (He at 150 bar) is accomplished by metallic tubes extending into the heat pipes. These tubes are the Stirling heater unit and must: (a) have minimum internal volume because large working gas volume outside the Stirling compression and expansion spaces reduces the efficiency of the Stirling engine;13 and (b) be designed to provide a maximum heat transfer coefficient to the fluid flow. This heat transfer coefficient governs the heat flow from the heat pipe to the Stirling heater. For the present calculations, the dimensions of the heat transfer tubes (see Fig. 2) were extrapolated from those of a V 160F Stirling manufactured by Stirling Power Systems Corporation. Terrestrial tests with that engine using a different kind of heat pipe receiver have been reported.12 It uses He for the working fluid like the engine assumed here. The heater tubes of that engine appear to have been optimized between above Requirements (a) and (b) because our experiments showed that the Stirling effeciency was very satisfactory and the heat transfer coefficient in question was 2830 W/(m2K) which is near the upper limit of what can be achieved with He flows at very high pressure.17
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