Delfi-n3Xt Electrical Power Subsystem
The Electrical Power Subsystem (EPS) consists of solar panels, a battery system, a central power conditioning and management system called Global EPS (G-EPS) and local power control. The G-EPS is developed by SystematIC BV and the solar panels are developped in cooperation with Dutch Space.
The EPS is designed on the required power of the subsystems in the nominal mode in which the satellite will be operating for most of the time (>95%). If the ITRX or STX transmitter is used, the PTRX transmitter will be turned off, yielding an almost power neutral mode transition. Only the ignition of a CGG of the T3µPS microthruster consumes considerable amount of power (~10 W), but this will only last for about 10 seconds which has an insignificant impact on the energy storage of the batteries. All other satellite modes consume less than the nominal mode.
During eclipse, a low power mode is chosen in which the primary radio is not transmitting to save power.
Solar Panels and Configuration
The solar cells which will be used for Delfi-Next are TEC1D triple junction cells (GaInP2/GaAs/Ge) from TECSTAR. These cells have an efficiency of about 23% at a temperature of 28°C and are supplied by Dutch Space as in-kind support. A (deployable) solar panel with dimensions compatible with the long side of the satellite contains 7 of these cells. Under normal incidence of the sun and a temperature of 28°C, a single panel can deliver 5453 mW of power. For the power budget, simulations of real view factors (including Albedo) and temperatures are taken as input.
Though Delfi-n3Xt is equipped with a three-axis active attitude control system, the experimental status of this system and its identified risks has led to a requirement that no other subsystem may be critically dependent on the proper functioning of this system. This means that for defining the solar panel configuration, a (slowly) tumbling satellite about an arbitrary rotation axis must be assumed. To avoid heavy cycling and major dependency on the battery system, a near omni-directional configuration is chosen. This was also the case for Delfi-C³ which had no battery at all and used a tetrahedron configuration. The difference between the minimum and average power can be used to charge the batteries for eclipse operation, limiting battery cycling to the orbital frequency.
A configuration is chosen with double-sided solar panels which are attached to the top of the satellite body and deployed under an optimized angle. Many more possible configurations have been analysed, but most of them yield less power with similar or increased complexity.
Global EPS Design
Maximum Power Point Trackers (MPPTs) will be used to obtain the electrical energy from the solar panels. These MPPTs provide a safe means to get the most out of the solar arrays in all conditions and performance of the solar cells.
Lithium-ion batteries will be used for energy storage for eclipse operation and temporary events such as the ignition of a CGG of the micro-propulsion payload. A battery management device will charge a battery, if not fully charged already, when there is sufficient available power and discharge the battery when there is insufficient solar power available.
A regulated single supply voltage of 12 Volt is used for all subsystems. The main reason to go for a single supply voltage is to keep the system bus interfaces to a minimum in order to limit wiring harness and complexity and to standardize the system bus interface. This main power line is protected by the standard system bus interface.