Understanding the dynamics of volumetric solar receivers*

Volumetric solar receivers are known to exhibit choked flow regions and hotspots above a critical irradiance value, resulting in catastrophic failure. This phenomenon is commonly and incorrectly attributed to "flow instabilities". A dimensionless lumped model is formulated to investigate the coupled dynamics between the mass flux of fluid, the fluid temperature and the solid matrix temperature in the receiver. The choked flow solution is successfully identified as the upper branch of a cusp bifurcation, where the bifurcation parameter involves the ratio of irradiance to pressure drop through the absorber. The phase portrait of the system is depicted for different parameter values to provide physical intuition of the dynamics of the absorber for different operating conditions. The present analysis elucidates limitations and special considerations for volumetric solar receivers that are essential to advance the maturity of the technology.