To accomplish item (1) above, an analytical tool for rapid and accurate determination of power demand and capabilities is quite essential even for a low-power spacecraft. The data base for this computer program would include mission sequence and crew activity timeline, load priority and classification of all user loads in terms of: (1) load criticality by various mission phases, (2) load type like essential, non-essential, and emergency, (3) load operating requirement as day only, night only, or both day and night, (4) duty cycle, and (5) power requirements at several operating bus voltages. It is also important to initiate a system-level document covering the above information and maintain it through the life of the program. As the design matures, all specification type power values must be replaced by actual performance data. The required analysis, planning, and validation of the load profile compatibility with the power system capability must always be accomplished a priori by the ground personnel. But future high-power spacecraft with adequate computaional capability like' the Space Station would eventually want the above analytical tool onboard the spacecraft for an automated implementation of the DANMOE requirements. By load shifting and/or eclipse load reduction, the battery mass can be effectively decreased roughly in proportion to the nighttime electrical energy displaced. This will reduce the solar array size, but the average bus power for the entire orbit will not decrease as a result of both solar array and battery size reductions, as will be discussed later. Table 1 summarizes the potential benefits of the DANMOE strategy at the power system and spacecraft levels. PHOTOVOLTAIC SYSTEM SIZING The DANMOE strategy and its principal benefits can be best illustrated by the use of the sizing equations. The following sections discuss the general relationships between array power, day and night power, and orbit average bus power, and between the total specific mass and the above terms that appear in the energy balance equation. The Space Station and telecommunication satellites are cited as examples for the LEO and GEO applications, respectively. However, the DANMOE technique is useful in general to any application that uses photovoltaic/battery combination. Energy Balance Equation The energy balance equation for a given power subsystem design is the principal basis for sizing of solar arrays and batteries. Energy balance simply means the energy developed by the solar array is equal to the sum of the load energies required during the orbital daytime and nighttime. To develop the sizing relationships, the direct energy transfer configuration depicted in Fig. 2 was selected. It is representative of a high power and highly efficient photovoltaic power system. Because of redundancy, component commonality, size availability, and a host of other considerations, a multi-kW photovoltaic power system for the Space Station class or direct broadcast communication satellites is expected to be comprised of multiple power channels or modules that can be easily paralleled electrically at the main output bus. Further, each of these modules is expected to range between a few kW to tens of kW, and to be designed to achieve high efficiency and capable of operating independently of each other, except at the main bus. Also, the Space Station may utilize several isolated power sources, each providing a separate set of captive
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