Solar Array basics 3-axis
The solar arrays are designed to provide power based on the load requirements of all units including the payload operating on a 3-axis satellite. To determine the load all operational configurations are assessed based on the units that will be powered on and the power they require. Solar array efficiency is maximized when the Sun’s angle of incidence is at a 90 degree angle to the panel, therefore the maximum power generated will be at equinox and the minimum power produced will be during the solstices. During eclipse periods, power is supplied by the batteries, this will require additional power for battery charging post eclipse. Eclipses occur each day over a period of approximately 45 days centered around the equinox. The Power budget is developed for each configuration and the solstice period efficiency and maximum eclipse recharging requirements are used to determine the power required from the solar arrays during the worst case. A margin is applied to account for potential damage and solar cell degradation to ensure antiquate power is available throughout the mission life.
Solar arrays are comprised of a number of panels and each panel is broken down into a number of strings. Starting with the strings, they are made up of a number of solar cells connected in series to obtain the necessary voltage. For example if you require 36 volts and each cell produces 0.5 volts, then the string would have 72 cells connected in series (in series voltage adds and current remains the same). To protect the string from a damaged cell or during eclipse diodes are installed. By connecting strings in parallel (the current adds and the voltage remains the same) the output current is increased. By use of Ohm’s Law, you can use the voltage and current to determine the panel’s output power generated. Panels are then added in parallel to achieve the total power needed.
Any excess power generated by the solar arrays is controlled by the voltage regulator and power distribution section of the EPS subsystem. Based on design a number of approaches have been taken to address this, ranging from shunting the excess current, to the use of switching transistors that add or remove strings connected to the power bus.
To maximize performance the solar arrays must maintain pointing at the Sun. Solar array drive motors are used to actively track the Sun or can be stepped to keep pace with the Sun. The drive motors are connected to the solar array drive electronics providing an interface for control by the EPS subsystem, ACS subsystem or the Flight Computer based on design.