Electric Power Subsystem Design

Abstract

The Electric Power Subsystem (EPS) of a satellite is a heavy and expensive subsystem. It is often about 25% of the weight and 25% of the cost of a spacecraft. Electric Power is also often underestimated, resulting in insufficient power to support the “mission creep” requirements of the spacecraft. The EPS design procedure is outlined below. Each of the steps will be discussed and illustrated in more detail later. (A) Determine the Required Spacecraft Orbit Average Power List all of the electronic components of the satellite and the voltages and currents that each component requires Determine the power drawn by each component in each of the spacecraft operating modes. Augment these by the appropriate DC/DC conversion efficiency to obtain the OAP drawn from each voltage source in each spacecraft operating mode. Determine the peak OAP required (B) Determine the Battery Capacity Required and Choose the Battery Bus Voltage Based on the power drawn during the eclipse (and the maximum eclipse duration), determine the battery WH requirements Select the battery cells that will be used Applying the battery output vs. input efficiency, determine the battery WH used during the eclipse Select the maximum Depth of Discharge below which the batteries should not be discharged. Apply this, and a large safety factor, to obtain the battery WH to be installed. Choose a battery bus voltage and divide by the cell voltage to determine the number of cells in series (in a string) of cells. Divide the total battery current by the current each parallel string will supply to determine the number of parallel battery strings. (C) Select a Solar Panel Configuration and Compute the OAP it can Supply Select the solar panel configuration (the orientations and areas of each panel relative to the spacecraft axes). Also, determine how each panel will be stowed and released. Compute the instantaneous power generated by each panel as the spacecraft moves around an orbit. The total power vs. time is then computed, as is the OAP. Repeat this for all Beta angles (the angle between the sun line and the orbit plane) to determine what the minimum OAP is. Ensure that the minimum OAP generated is equal to or greater than the spacecraft OAP required. (D) Draw the EPS Block Diagram Given the panel configuration, the various required voltages and the number of battery strings and cells per string, the EPS block diagram can now be drawn. Consider which groups of components should be turned ON/OFF on command, and whether the switch to turn these groups ON/OFF should be ahead or after the respective DC/DC converters that supply the voltage to the group. (E) Miscellaneous EPS Design Steps Often, an EPS computer is included to collect telemetry regarding the state of health of the EPS, the battery capacity status, component temperatures and EPS status. This computer may also be used to turn ON/OFF power to the various electronic components. The Separation Switch that signals release from the launch vehicle and the start of spacecraft operations is also part of the EPS. The functions enabled or disabled to ensure that no electric power is drained from the spacecraft prior to launch are used to determine where in the spacecraft circuit the Separation Switch should be located.