0883-6272/86 $3.00 + .00 Copyright ' 1986 SUNSAT Energy Council A NEW APPROACH TO OPTIMUM SIZING AND IN-ORBIT UTILIZATION OF SPACECRAFT PHOTOVOLTAIC POWER SYSTEMS M. S. Imamura and B. H. Khoshaim Midwest Research Institute/SOLERAS P.O. Box 5927, Riyadh, Saudi Arabia ABSTRACT - This paper presents a systems approach to optimization of the size and orbital life of photovoltaic systems via minimizing the nighttime energy demand while maximizing the daytime energy consumption. The Day-Night Management of Load (DANMOE) strategy calls for sizing the system to a pre-selected day/night average load power ratio and operating the spacecraft in orbit within the day and night capacity capability, rather than the conventional single orbital average power capability. Examples for the Space Station and the telecommunication satellites show that the reduction in their specific masses can be substantial using any of the photovoltaic system technologies. The DANMOE scheme may also be used effectively to extend the life of batteries on currently orbiting satellites, and hence prolong their lifetime. The paper also discusses other benefits at the spacecraft level and the method of implementing the DANMOE approach. INTRODUCTION The photovoltaic power system wi 11 . continue to be the principal source of electric power for a vast majority of future spacecraft missions. Two key issues that have confronted all spacecraft in the past are the high specific mass of the photovoltaic system and the battery life limitation. The battery life compounds the mass problem because lowering the depth of discharge increases the battery life but also increases its specific mass. Historically, the mass of the power system for the geosynchronous orbit satellites (e.g., telecommunications) has been 15 to 25 percent of the total spacecraft mass. About a third of this mass (5 to 8 percent) is in the solar array and another third in the batteries. Some of the recent 3-axis stabilized spacecraft have resorted to planar foldout sun-oriented arrays which resulted in a lower specific mass. Even so, there is a severe size restriction on the solar arrays for the GEO satellites becuase of the mass-volume limitations of the present launch vehicles (STS/IUS or PAM, Titan 3D/IUS, and Ariane). In a 1983 forecast of commerical telecommunication satellites for the free world [1], Comsat General Corporation predicted that over 280 satellites will be launched between the year 1985 and 2000. Thus, the reduction of the photovoltaic power system will continue to be a key issue for the telecommunication satellites. The orbit duration on low earth orbit vehicles is in the order of 1.5 hours and is much shorter than the GEO (24 hours). Thus, the energy and cycle-life demands for the LEO are more severe on the energy storage system. For this reason, the specific masses of the array and batteries and the battery life problems are higher for the LEO spacecraft. The best example of the future LEO Spacecraft is the Space Station, recently initiated by the United States with other countries participating. The current prediction is that the power level, meaning the orbital average bus power capability, will increase from 75 kW on the first fully operational configuration in the early 1990 to 150 kW in late 1990 [2] . Fig. 1 illustrates the possible orbital configurations for the above power levels using standardized flexible solar array wing designs. Based on the "heaviest" photovoltaic system (rigid Si array/NiCd battery) with a 290 kg/kW specific mass estimated by NASA [2], the initial 75-kW station would require a power system mass of 21,750 kg (47,850 lbs). Several Shuttle flights are needed to assemble this initial stage and many more for the final 150-kW configuration. The reduction of its specific mass, therefore, is one of the key issues and drivers in the Space Station Program. SIZE AND LIFE OPTIMIZATION VIA LOAD MANAGEMENT Two ways to reduce the specific mass and life of any photovoltaic system can be classified as hardware and non-hardware approach. In the hardware (or design) approach, the designer selects and utilizes the best mix of mass-efficient reliable components and minimizes the energy loss factors such as component efficiencies. The degree to which the system specific mass and battery life can be optimized here is dependent largely on the available technology or hardware. The non-hardware method is essentially a "systems" approach because it resorts to operational strategies to achieve the desired objective. There is one "system" approach that has never been implemented in the past for the specific purpose of reducing the overall mass of the photovol-
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