This paper presents predictions for a HPSE of the above type using a recently developed LiF/graphite latent heat storage unit with capillary notches7,8 to accomodate the volume change upon melting of the storage medium. A large number of such HPSE were assumed to be designed into a reference receiver integrated with a Stirling engine in an SDPS. Characteristics of free-piston Stirling engines for space applications are not yet available, despite considerable study and development.9,10,11 Therefore, data from terrestrial experiments that use heat pipes to transfer heat to a crank-shaft Stirling engine12 were used as a boundary condition in the calculations. The calculations comprise the development with time of the heat flows "solar-to- storage" and "solar-to-Stirling" during the insolation period, and "storage-to-Stirling" during the eclipse period, associated with a relevant spacecraft orbit, and of the most important inherent temperatures, the receiver temperature and the vapor temperature in the heat pipe. The predictions compare three cases of HPSE, • Case (1), a basic HPSE which appears feasible with state-of-the-art heat transfer engineering; • Case (2), same as Case (1) but with an additional intermediate common heat pipe between the bulk of the HPSEs and the Stirling heater, as frequently suggested to accomplish equidistribution of the heat load among the heat pipes and storage units; and • Case (3), same as Case (1) but with the heat resistance at the end of the heat pipe reduced by a factor 2, through a corresponding increase of the heat transfer coefficient to the gas flow in the Stirling heater, as thought possible by refined engineering. A comparison of Cases (3) and (1) is important because the heat transfer coefficient to the fluid in the Stirling heater tube inside the heat pipe is the major restriction for the heat flow to the Stirling engine. Since the respective heat transfer area cannot be made artificially large (because excessive dead volume must be avoided13 in a Stirling cycle) the heat resistance can only be reduced by increasing the heat transfer coefficient to the fluid. The heat transfer coefficient may indeed be increased effectively through surface enhancement, i.e. the creation of additional turbulence near the walls of the flow channels.14 The next two sections give details on the design parameters of the reference receiver and the integrated HPSEs, all chosen in view of future applications. The subsequent section describes the simplifying assumptions and the calculation procedure. The results are presented, discussed, and summarized in the final sections.
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