The battery size is dependent upon the nighttime energy demand and the depth of discharge (DOD) as given in the following relationship (based on Fig. 3): Where is the rated watt-hours, and C_ is the allowable depth of discharge (in fraction of rated capacity) selected for the particular design. The rated ampere-hours for the batteries and the number of batteries can then be determined using C and battery interconnection configuration . Specific Mass The total specific mass of the power system is defined herein as the sum of the specific masses of the individual components and those of the thermal control system elements (mostly the radiators) to dissipate the waste heat. Mathematically, it is the ratio of the total system mass to the orbit-average bus power capability (P^) which is given by Equation (4). The total specific mass of the photovoltaic system is as follows: Note that each term inside the brackets in Equation (9) represents the mass of the solar array, battery, power conditioning/distribution, and radiators for thermal dissipation in batteries and regulator. Letting PnT = oP , and replacing Pn and P_. from Equation (5) and Equation (2) in Equation (9), W becomes: Equation (10) shows the specific mass of a photovoltaic system as a function of day/night load power ratio (o), the individual component specific mass; and other terms in the energy balance equation. The last three fractions in the bracket in Equation (10) represent the masses of thermal dissipation radiators required by the batteries and regulators. The specific masses of several candidate photovoltaic systems were plotted as a function of the day/night load ratio using the values of parameters listed in Tables 2 and 3. Figs. 6 and 7 show the results for the Space Station and the telecommunication satellites, respectively. These plots indicate that the most significant reduction in the specific mass is in the range of o between one and four.
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