Solar thermal propulsion (STP) offers an unique combination of thrust and efficiency, providing greater total ΔV capability than chemical propulsion systems without the order of magnitude increase in total mission duration associated with electric propulsion. Despite an over 50 year development history, no STP spacecraft has flown to-date as both perceived and actual complexity have overshadowed the potential performance benefit in relation to conventional technologies. The trend in solar thermal research over the past two decades has been towards simplification and miniaturization to overcome this complexity barrier in an effort finally mount an in-flight test.
A review of micro-propulsion technologies recently conducted by the Air Force Research Laboratory (AFRL) has identified solar thermal propulsion as a promising configuration for microsatellite missions requiring a substantial Δ V and recommended further study. A STP system provides performance which cannot be matched by conventional propulsion technologies in the context of the proposed microsatellite ''inspector" requiring rapid delivery of greater than 1500 m/s ΔV. With this mission profile as the target, the development of an effective STP architecture goes beyond incremental improvements and enables a new class of microsatellite missions.
Here, it is proposed that a bi-modal solar thermal propulsion system on a microsatellite platform can provide a greater than 50% increase in Δ V vs. chemical systems while maintaining delivery times measured in days. The realization of a microsatellite scale bi-modal STP system requires the integration of multiple new technologies, and with the exception of high performance thermal energy storage, the long history of STP development has provided "ready" solutions.
For the target bi-modal STP microsatellite, sensible heat thermal energy storage is insufficient and the development of high temperature latent heat thermal energy storage is an enabling technology for the platform. The use of silicon and boron as high temperature latent heat thermal energy storage materials has been in the background of solar thermal research for decades without a substantial investigation. This is despite a broad agreement in the literature about the performance benefits obtainable from a latent heat mechanisms which provides a high energy storage density and quasi-isothermal heat release at high temperature.
In this work, an experimental approach was taken to uncover the practical concerns associated specifically with applying silicon as an energy storage material. A new solar furnace was built and characterized enabling the creation of molten silicon in the laboratory. These tests have demonstrated the basic feasibility of a molten silicon based thermal energy storage system and have highlighted asymmetric heat transfer as well as silicon expansion damage to be the primary engineering concerns for the technology. For cylindrical geometries, it has been shown that reduced fill factors can prevent damage to graphite walled silicon containers at the expense of decreased energy storage density.
Concurrent with experimental testing, a cooling model was written using the "enthalpy method" to calculate the phase change process and predict test section performance. Despite a simplistic phase change model, and experimentally demonstrated complexities of the freezing process, results coincided with experimental data. It is thus possible to capture essential system behaviors of a latent heat thermal energy storage system even with low fidelity freezing kinetics modeling allowing the use of standard tools to obtain reasonable results.
Finally, a technological road map is provided listing extant technological concerns and potential solutions. Improvements in container design and an increased understanding of convective coupling efficiency will ultimately enable both high temperature latent heat thermal energy storage and a new class of high performance bi-modal solar thermal spacecraft.
|Commitee:||Erwin, Daniel, Ronney, Paul D., Sadhal, Satwindar S.|
|School:||University of Southern California|
|School Location:||United States -- California|
|Source:||DAI-B 78/04(E), Dissertation Abstracts International|
|Subjects:||Aerospace engineering, Energy|
|Keywords:||Microsatellites, Solar thermal propulsion, Thermal energy storage|
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