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Packed beds of crushed rock particles are typically encountered in the thermal energy storage system at a solar thermal power plant. Rocks have irregular shapes, and tend to pack with their long axes in a horizontal plane. As a results its resistance to airflow is anisotropic. Designing a bed for effective charging and discharging cycles, depends on an accurate prediction of the flow distribution through it. Ellipsoids are mathematically simple shapes that characterise crushed rock particles reasonably successfully. A packed bed was generated using a discrete element model, and the flow through the bed calculated with computational fluid dynamics. The resistance tensor for the bed was extracted from the data. The tensor is almost symmetrical, with the diagonal terms dominant. Incorporating this tensor as a source term in a porous medium presentation of a packed bed, should yield more accurate results than, say the Ergun equation, at very little extra computational overhead.

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