My Master’s thesis was an analytical and experimental approach to exploring the feasibility of solid-liquid phase change materials (PCMs) for extending the lifetime of CubeSats in low-Earth orbit, conducted in the Electrochemical Science and Engineering (ESE) Group at Imperial. I saw this project through from beginning to end: researching the idea during the summer before my senior year, mapping out a research strategy with a PhD student in the ESE Group, formally proposing the research as my thesis topic, sourcing the hardware, then running the simulations and experiments. Many lessons learned along the way!
The basis of the research was to extend the lifetime of CubeSat battery packs by dampening temporal fluctuations and spatial temperature gradients across the pack. Since the battery is charged whilst in sunlight and discharged in eclipse, a typical Li-ion battery on-board a CubeSat in LEO will therefore be required to complete up to 5,000 charge-discharge cycles per year. This is highly demanding! Particularly considering that the pack undergoes simultaneous thermal cycling during each orbit, which rapidly accelerates the degradation of Li-ion cells. Since a high proportion of CubeSat failures are traced back to the battery pack, there is a clear incentive to design reliable packs robust to the harsh environment in space.
The lifetime of Li-ion batteries is highly sensitive to thermal conditions (particularly cell-to-cell gradients and temperature variations in time). In automotive packs, PCMs have been shown to limit both of these effects. Applied to CubeSats, PCMs have the advantage of being fully passive, simple, reliable, and scalable in modular form. Provided that PCMs can effectively control battery temperatures in the space environment, they have the potential to reduce cell degradation rates, extend the operational lifetime of CubeSats, and maximize mission success.
The aim of the research was to design, build, and test a Li-ion battery pack with integrated PCM thermal management system for the INSPIRESat-1 CubeSat. To do this, I developed a multi-node thermal model of the spacecraft as well as an FEA model at the pack-level. In parallel, I conducted cell-level tests and assembled a prototype pack for model validation.
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Manufacturing prototype pack
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Cell-level fuses perform their job after a short-circuit while routing the BMS!
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Cycle test set-up inside thermal isolation chamber
Cell-level test set-up for heat generation data input to thermal models
Body-fixed reference frame for satellite-level model
Global reference frame for satellite-level model
Predicted cell and pack heat generation during 10 simulated orbits, based on test data and cell internal heat generation model
Temperatures across spacecraft during 10 simulated orbits from satellite-level thermal model
Top-view of simplified FEA thermal model of battery pack
Temperatures across pack during minimum (left) and maximum (right) incident solar flux
Comparison of pack temperatures between the two models
Parametric study showing effect of melting temperature and PCM conductivity on temperature fluctuation amplitudes (top) and pack gradients (bottom)