Robust Design of Emirates Mars Mission’s Electrical Power Subsystem

The Electrical Power Subsystem (EPS) of the Emirates Mars Mission (EMM) is designed to autonomously perform its functions of controlling the solar arrays output current, managing the battery charging, and distributing power with minimal ground intervention. By removing ‘software in the loop’ and simplifying operations, the EPS charge control system is robust and resistant to operator error. The EPS consists of the Power Control Unit (PCU), solar arrays, and battery. The subsystem is responsible for generating power, distributing power to the Spacecraft, and charging the battery through cards within the PCU. The EMM battery is an 8s52p Lithium-Ion battery with a 78Ah capacity; its simple design reduces complexity by having no electronics to balance the cells because cells are screened and matched at the vendor. The solar arrays are designed to output 611W at end of life, 33V, -20C, and 1.666AU. The EMM bus uses a Direct Energy Transfer (DET) system, where the bus voltage is the battery voltage, and there are two battery target voltages of 32.8V and 33.6V. The voltage set-point is maintained by the solar array card which operates autonomously to control segment switches from the solar arrays through a triple voted algorithm, hence controlling the amount of current charging the battery and balancing the bus. Peak Power Tracking (PPT) is known for its higher efficiency and based on the analysis conducted, a PPT system results in a 10% higher power output for the EMM Spacecraft. However, a DET system was selected over PPT due to its simpler design and higher reliability.