Thermal Modeling of the Power Train of Solar Powered High Altitude Aircraft

Solar-electric high-altitude aircraft are capable of flying continuously using only electrical energy generated from the sun´s radiation. To enable this continuous operation, the aircraft must be optimized for minimal power requirements, i.e., low surface loads and highly efficient systems. The operating conditions in the stratosphere present another main challenge, characterized by very low temperatures and low air density. To achieve low surface loads, the system mass, particularly the batteries, must be distributed along the wingspan within the structure. To use the battery capacity within the operating conditions of the stratosphere efficiently, the batteries must operate within a narrow thermal range. During the day, the suns radiation poses a potential risk of overheating, while at night, low ambient temperatures without sunlight create the risk of overcooling. At night, it is necessary to heat the batteries to prevent them from reaching critical temperatures. The aforementioned operational challenges require a deep understanding and analysis of the thermal process of such aircraft. Thus, this paper describes the thermal modeling of the propulsion system of a solar-electric high-altitude aircraft, with a particular focus on the battery system and solar panels. The proposed system model is based on a simplified thermal model allowing to simulate weeks of operation within minutes without compromising accuracy compared to high-fidelity solutions. The goal is to use these models in a mission simulation environment to determine optimal operational strategies for the batteries. The models are verified within an intensive simulation campaign.