Temperature-Based MPPT Circuit for a Tumbling PocketQube: Design and Experimental Performance Analysis

Abstract

PocketQubes are a young technology that brings with it a vast amount of research opportunities. These pico-satellites consist of standardised units (P) of 5x5x5 cm. Currently, the Delfi team at the Delft University of Technology is developing the next 3P PocketQube mission. One of the goals of this mission is to continue to advance this satellite technology by learning from the challenges encountered in the previous PocketQube.

The focus of this research is on the power generation system of the satellite. In the previous Delfi mission, a Maximum Power Point Tracking (MPPT) technique called Perturb and Observe was used to extract as much power as possible from the small solar cells. However, it did not generate as much power as expected due to the tumbling of the satellite. Therefore, various other MPPT methods were researched and a technique was found that will likely be more suitable for a tumbling PocketQube: temperature-based MPPT. Based on the temperature of the solar cell, this method calculates the voltage at which the cell will generate the most amount of power (V_MPP) using a linear equation. The objective of this research is to design, manufacture and test a prototype circuit that implements this method so that its performance can be investigated. Based on this, the viability of using this method on a PocketQube can be evaluated.

The prototype consists of three separate Printed Circuit Boards (PCBs). One houses the solar cells and temperature sensors and is mounted on a test stand in front of a powerful lamp that mimics the sun. The second PCB is a demo circuit board for a Direct Current to Direct Current converter (DC-DC converter), which is used to convert the power from the solar cells to the correct voltage for the battery. In addition, it can limit the voltage of the solar cells so that they operate at their Maximum Power Point (MPP). The last PCB is the link between the other two and also connects to the load. Furthermore, it houses the control circuit that drives the solar voltage limit set by the DC-DC converter based on the temperature measurement that comes from the PCB with the solar cells.

This prototype was tested with a resistive load while the aforementioned lamp was illuminating the solar cells. By varying the load resistance, the demanded power could be varied. In this way, the performance of the circuit could be measured while operating under, at, and over the MPP. The solar cells could also be cooled down using Peltier modules on the test stand and were heated by the radiation of the lamp, allowing for different temperature environments. The irradiance was also rapidly varied by dropping an opaque plastic sheet between the lamp and the solar cells. Measurements were made with an oscilloscope or using digital multimeters present on the PCB with the control circuit.

In general, the circuit functions as intended and promises to be a viable option for the power generation system of a tumbling PocketQube. It adapts rapidly to changes in irradiance and demanded power from the load, and it keeps up with changes in temperature. It also achieves an overall efficiency of more than 80%, occasionally reaching more than 85%. One major flaw remains: the temperature gradient of the limit set by the DC-DC converter does not exactly match the temperature gradient of the V_MPP of the solar cells. However, this can likely be solved by placing all circuitry on the same PCB. In addition, small tweaks to the design, such as decreasing the volume, and more tests should be performed before this circuit can fly on the next PocketQube.