A Cassegrain type solar collector is employed in the present system. Three subsystems are provided for emergency use. The first one is a heater in the cooling loop to prevent cooling water from freezing and the second one is the dummy load to regulate the load power, and the last one is a vent valve for the working fluid. Table I indicates the size, weight, output power and durability of the primary components. Thermal Storage System Various types of energy storage systems (thermal energy, electrical energy and mechanical energy storage) have been proposed in order to supply continuous power during eclipse. In the present system, a thermal energy storage system is employed as shown in Fig. 4(a). Thermal energy is stored by the latent heat of a molten salt such as LiF or LiH. The advantage of this system is that the output power of the engine can be half compared to that of other systems shown in Figs. 4(b) and 4(c) and so its weight and size can be kept to a minimum. In both the electrical energy storage system with a battery and the mechanical energy storage system with a flywheel, the engine has to produce, during insolation, the excess electric power necessary during eclipse. In the system using a flywheel, auxiliary subsystems such as a clutch and a speed change- over transmission have to be provided. The characteristics of latent heat storage materials (LiF and LiH) appropriate for the present system are shown in Table II. LiH is easily decomposed and the hydrogen decomposed transpires out of the space through a metal wall of the container. On the other hand, although LiF has the disadvantage of relatively low latent heat, it is a stable and harmless material. However, LiF has very corrosive characteristics against a metal and also a large volume change occurs in the phase transition from solid to liquid. The latter problem is reduced by mixing MgF2* and by applying a bellows mechanism as shown in Fig. 5. It is also necessary to find the best configuration of the thermal storage system in order to optimize heat transfer. Performance of the Free-Piston Stirling Engine Fig. 6 shows the heat flow for the present solar thermodynamic power generation system at rated operation. The efficiency of the Stirling engine generator is around 31% and overall system efficiency is around 25%, assuming 81% for the solar collector. The efficiency of the linear induction generator is 87%.
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