In connecting the Stirling engine to the heat pipes, for maximum heat transfer area it is important to expose the complete heat transfer tubes to the heat pipe working fluid. This excludes designs with round or flat heat transfer tubes brazed to, or channels integrated with, part of the walls of the heat pipes. The weights contributing to the total weight of the HPSE (Fig. 2) are 1.37, 2.06, and 2.94 kg for the heat pipe (without heat transfer tubes), the graphite container, and the LiF, respectively. The storage medium thus comprises ca. 60% of the storage unit. Since the wall thicknesses of the storage capsule were chosen on the safe side for stability and gas tightness, it is expected that this percentage may be increased. Calculation Procedure and Assumptions The theoretical predictions for the most important heat flows and temperatures in the HPSE as functions of the state of charge of the storage unit were made using previously derived formulas18,19 and the following simplifying assumptions. (1) quasi-stationarity, i. e. sensible heat storage in all walls through which heat passes is neglected. This is permissible20 if the phase change coefficient is much larger than 1 which is true in the case considered. The phase change coefficient is the ratio of characteristic latent and sensible heats.20 (2) constant material properties over complete charge and discharge periods although different values were chosen for the charge and discharge periods to take account of the slightly different temperature levels.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==