National Space Development Agency of Japan [NASDA] 1. Introduction The National Space Development Agency (NASDA) of the Science and Technology Agency regards space thermal power generation technology as vital technology to be developed through intensive research for space vehicles which will require massive power supplies in the future. Studies began as early as 1985. Space thermal power generation technology was proposed as a vital mission when the Japanese Experiment Module (JEM) Project was first implemented, and studies began on the technology with the cooperation of other research institutions. 2. R&D of Solar Power System NASDA’s studies may be divided into the following three main themes: 1) Trade off study on thermal power generation system modes. 2) Study of basic technologies. 3) Study of scenarios for technological development (1) Trade Off Study on Thermal Power Generation System Modes The thermal power generation system mode can be classified by the power generation cycle. The target thermodynamic cycles for study are the Brayton, Rankine and Stirling cycles. Based on the estimated technological level as of 1996, studies are based on: Power generation capacity 30 kW, Launching means H-II rocket. Table 1 shows an example of system design. The Brayton and Stirling cycle engines are characterized by a higher working temperature than Rankine cycle engines, while the Stirling cycle engine has a higher working pressure, higher efficiency but shorter life expectancy. Table 2 shows the results of the study of these three cycles. Nine items were selected for evaluation. As shown in the table, the practicality, service life expectancy and specific weight have more importance than other items, since the ultimate purpose is to establish a bus technology for future space vehicles. The studies conducted suggest the Brayton cycle is the optimum candidate. (2) Study of Basic Technologies Studies on basic technologies preferably focus on service life expectancy and lightness. Table 3 shows a breakdown of the weight ratios of systems based on design studies. This table shows that the weight of the dynamic power conversion unit (DPCU) is a relatively small part. The larger portion of system weight consists of the heat re- ceiving/siorage unit and radiator. Studies to make these equipment lighter will raise the system working temperature and aid studies on new technology of heat receiving/ storage units and radiators. Regarding the service life expectancy, low-cycle fatigue should pose no problem, assuming continuous operation, and studies on the chemical denaturing of DPCU gas bearing materials and heat receiving/storage materials will be effective. Basic technologies have been assessed as above, and studies are focussing on the following themes: Heat receiving/storage unit: Studies on high-temperature characteristics and chemical denaturing of materials. Radiator: Studies on new radiation systems. DPCU: Studies on gaseous bearings. (3) Study of Scenarios for Technological Development Some experimental SPS systems now operate on the earth, but no experimental model has been made for conducting
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