storage unit, but since the loop is a closed Brayton cycle, it can also be used to investigate the thermal characteristics of the space solar dynamic power generation sys- tem. Computer simulation is also being done on heat transfer and thermal stress characteristics. 4. Large Radiator A heat pump augmented radiator is designed to radiate waste heat from large space structures to space by elevating the temperature using a heat pump. At the space technology section, its feasibility has been confirmed using CFC-11 as a working fluid. Furthermore, for a demand of a larger amount of heat rejection, a liquid droplet radiator has been investigated. This radiator, shown in Fig, 3, emits hot liquid droplets with a 100-200 pm in diameter to space and, after they are cooled by radiation, collects them to circulate as a heat loop. Compared with an conventional radiator, the radiator is composed of an eminer and a collector in principle to make it very lightweight. In addition, the possibility of fatal damage by collision of high-speed meteorites is small, and it is highly promising for use as a large- capacity space radiator. It is demonstrated by experiments that stream droplets are produced from small orifices in a vacuum by pressurizing a silicon based diffusion pump oil as a working fluid to form liquid jets and perturbating it with a piezo-electric oscillator. Fig. 3 Magnified view of liquid droplets formed uniformly at an excitation frequency of 45 kHz Fig.4 Pipe type receiver with AMTEC and latent thermal storage 5. Solar Cell 6. AMTEC Research on solar cells at the Electrotechnical Laboratory includes crystalline and amorphous solar cells. Previously, the laboratory developed a tin oxide/silicon crystalline solar cell, a thin-film crystalline silicon solar cell and a new material evaluation technology. It has also developed technologies for the diagnosis of the gaseous phase and surface reactions in the process of thin-film growth as well as a reaction control technique, which have allowed the quality of alloybased amorphous thin-films to be improved. Through the reevaluation of the Sunshine Project, the Electrotechnical Laboratory started basic research on the following su- perhigh-efficiency solar cells in this fiscal year I) A single-crystalcompound/single-crys- tal Si tandem solar cell with an efficiency of 38%. 2) A single-crystal Si solar cell with an efficiency of 28%. 3) A thin-film solar cell with an efficiency of 20%. (Amorphous silicon, alloy-based amorphous materials and chalcopyrite such as copper-indium-selenium) These solar cells are for use on the earth, but the results of research may be applicable to solar power satellite (SPSs). The Energy Materials Section is engaged in research on the sodium thermoelectric conversion (AMTEC) system, technology for direct thermoelectric conversion by using sodium ion conducting p” alumina solid electrolyte. Present research under the Moonlight Project has the primary objective of developing a distributed type power source for use on the earth, but a power source usable in outer space is also being studied. A power density of 0.79 kW/m1 has been attained by using Mo electrodes. Experiments conducted under the condition of no heat loss have shown an efficiency of 30% at a current density of 0.4 kW/m5. Fig. 4 is an example of the system design with a unit body with the receiver. This AMTEC system has a system efficiency of about 21%. A vital technical theme is the development of a porous electrode withstanding sodium environments at high temperatures and affinitive with P" alumina Research on materials such as Mo and TiN thin-films is being conducted to improve performance and life of the electrode. System research on series-parallel operation, methods for heat supply, and heat loss reduction is also going on. The heat supply method proposed is supplying heat efficiently by radiation and conduction using foamed metal, and the effectiveness has been corroborated.
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