On the thermo-electric modelling of smallsats

Correct thermal modelling is crucial for smallsats. They can experience higher variations of temperatures and its components usually have a lower range of safe operating temperatures, especially if they are COTS (commercial-off-the-shelf). It must also be taken into account that thermal design of smallsats is, in certain aspects, very different when compared to bigger satellites. One of the main differences is the modelling of solar panels. Large satellites have control systems that guarantee that solar panels always operate at the maximum efficiency voltage. Thanks to this, one hypothesis that can be used in thermal model is that the efficiency of the panels only depends on the temperature. On the other hand, the electrical system of small satellites is usually very simple, often based on Direct Energy Transfer (DET). In these satellites, the operating voltage is not optimal, but mainly depends on the battery voltage. For this reason, previous hypothesis is no longer valid, and it is necessary to consider that efficiency depends on both the temperature and operating voltage. This change is not minor, and not considering it can lead to differences between modelled and real temperatures that greatly exceed safety margins. For correct modelling of DET satellites, one solution is to integrate the solar panel thermal and electrical models, in order to calculate the real efficiency at which it operates. However, this is not simple, since the electrical problem of a solar panel has many unknowns and implicit equations that complicate its resolution. In this work, the authors propose a simple methodology for thermoelectric modelling using only the solar panel parameters provided by the manufacturer. This methodology can be easily integrated into complete thermal models of smallsats using thermal analysis software. Simulation results for a smallsat mission, including differences in temperatures between thermo-electric and traditional modelling are also presented.