Augmenting cavity receiver performance: Spillage skirts and secondary reflectors

In this study, Monte Carlo ray tracing (MCRT) is coupled with heat transfer models to analyse the efficiencies and energy losses of a cylindrical cavity receiver in a dish system. The progressive Monte Carlo evaluation (PMCE) method is used to determine good geometric designs with high receiver efficiencies. The spillage skirt, which consists of several coiled pipes outside the cavity inlet, is added and optimised to increase the receiver efficiency. Three different types of secondary reflectors, including conical, trumpet and compound parabolic concentrator (CPC) reflectors are also investigated to decrease the spillage loss. The results show that the spillage skirt helps in greatly decreasing the aperture size, while simultaneously reducing the spillage loss. The conical and trumpet reflectors can increase the efficiency by reflecting rays outside the cavity inlet into the receiver. CPC secondary reflectors experience more internal reflections which cause high secondary absorption loss and consequently cannot achieve the equivalent efficiency compared to other designs. With a combination of spillage skirt and conical or trumpet reflectors, the receivers obtain the highest efficiency, with a 1.0% increment in receiver efficiency and 12.9% relative reduction of losses compared to the best cylindrical receiver. Limitations of the current study are that the effects of skirt and reflectors on convective losses are not considered and that the primary concentrator design is fixed.