Tanaka et.al. [1] selected lithium fluoride (LiF) and its eutectics as candidates for the latent thermal storage materials through systematic screening based on heat of fusion of molten salts. The authors studied straight type storage units filled only LiF in inconel canisters in order to investigate heat transfer to the storage units [2,3]- They experimented in several cycles of charge and discharge. Lithium fluoride was confirmed to be a promising material for the latent thermal energy storage on space solar dynamic system. Lithium fluoride increases its volume about 30% at melting, and canisters of the thermal storage unit may be destroyed by thermal stress. When the material freeze, gaps may be formed between the envelope and the material and voids may be generated in the material, and heat input from the sun may be hardly conducted to the material. Several ideas were proposed for solving this problem. Kerslake [4] investigated a thermal storage unit consisting of many separated modules. Michael et.al. [5] developed a light weight heat receiver unit combined with heat pipes and thermal storage containers. Krause [6] investigated a combined receiver unit consisting of a new type of heat pipes and latent thermal storage elements. These units tend to become heavy or still have possibilities of void formation in the thermal storage materials. To overcome these problems, Abe et. al. [7,8] proposed a concept of composites of fluoride salts and porous matrix made of silicon carbide (SiC) or carbon (C). The fluoride salt is held in the matrix of the composite, and it can change its volume with no restriction at melting, because it sinks into the matrix. No voids are expected in the composites during freezing of the salts, so that no additional thermal resistance is created in the storage material. Moreover the matrices have higher thermal conductivity than the salts. The effective thermal conductivity of the composites are expected to be sufficiently high. Tanaka et. al. [9] pointed out the importance of thermal conductivity of the latent thermal storage materials to obtain good thermal characteristics and to reduce the weight of the receivers. The composites proposed by Abe et.. al. were pointed out to be ideal from a view point of heat conduction. These composites were successfully developed and proved to be chemically and mechanically stable by TG(Thermogravimetry) - DTA(Differential Thermal Analysis) [10], Kanari et. al. [11] made a computer simulation on heat transfer in the receiver for various configurations of heat transfer tubes and for various conditions of solar insolation. They showed merits and demerits of a straight tube and a U-tube. The U-tube type is expected to have higher utilization factor of the thermal storage material than a straight type. The U-tube receiver has the inlet of working fluid nearby the outlet. Heat from the working fluid may be transferred between the inlet and the outlet without being carried by the working fluid. In a bayonet type receiver, working fluid flows in a double walled pipe from the inner pipe to the outer pipe. Some part of heat in the working fluid
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