Power conversion design guidelines for high-altitude electric aircraft

Our modern society is highly dependent on satellites for services such as GPS, weather forecasts, radio and television broadcasts, as well as, more recently, cell phone coverage and widespread internet connectivity. Many private companies as well as national space programs are currently deploying tens of thousands of satellites in low-earth orbit to develop these services. With more and more satellites in orbit, the likelihood of collisions increases dramatically. In the event of a collision, the velocities at play would cause both objects to be pulverized into tens of thousands of pieces of debris. These then present a high risk for the other satellites. In fact, the race for satellite internet coverage may be setting the grounds for a scenario coined the Kessler syndrome, which is a destructive chain reaction that could potentially cause the loss of many, if not all, satellites as well as render Earth’s orbit unusable for decades. In this context, high-altitude pseudo-satellites (HAPS) could be a viable alternative platform for many current satellite services. Furthermore, the cost of HAPS could also be much lower than that of satellites. However, for their development, many challenges remain. The only way to sustain quasi-perpetual flight is by having a solar-powered, electrically propelled platform. This thesis focuses on the challenges related to the development of a high-altitude electrical propulsion system, notably the power conversion. There are two main challenges: the first is linked to the low-pressure environment, which causes the breakdown voltage of air to be lower and the available mass flow for cooling to be lower. The second is the risk linked to the increased radiation levels, which are the result of cosmic rays, that can disrupt micro- and macro-electronics. The first part of this thesis focuses on aircraft propulsion itself. This step involves an overview of the basic principles of flight, drag, and performance, which then allows us to define how the different flight parameters as well as how the propulsive power needs evolve with altitude. Propeller characteristics, as well as their limits, are also studied to define the specifications for the electric motor and its associated motor drive. The second part of the work focuses on the issues related to the low-pressure environment. The theory of breakdown in gases is studied, and an experimental setup is developed to test the breakdown properties of air with different electrode shapes and configurations in a lowpressure environment. Results are then presented, and recommendations based on an adaptation of standards for clearance creepage are proposed. Furthermore, with the flight parameters defined in the first part, an experimental setup is used to measure the performance of a heat exchanger in low mass flow conditions. The results are then used to calculate the expected heat rejection performance with altitude. The third part of the work focuses on the radiation environment and its effect on electronics. The nature and intensity of the radiation are studied and defined, as is the effect shielding has on them. Following this, the effect of radiation on electronics is studied and quantified. Then, design recommendations are proposed to deal with the effects of radiation. The final part of this thesis puts the recommendations that are proposed in the second and third parts to the test. Experimental hardware is developed based on these recommendations and tested in a climatic chamber capable of recreating the high-altitude conditions. The results demonstrate that recommendations are applicable and that they allow electrical power converters that function in harsh high-altitude conditions to be designed.