Optical and thermal radiative performance of bladed receivers for concentrating solar thermal energy systems.

Abstract

The receiver is a key component in a concentrating solar power (CSP) system that converts the concentrated radiative flux into high-temperature thermal energy, for use in electricity generation. In commercial molten salt CSP systems, conventional receivers are typically external receivers with convex banks of tubes formed into a cylindrical or cuboid arrangement. In these convex designs, most of the reflected solar irradiation and thermal emissions are returned to the environment, and result in significant energy losses. In this work, a novel configuration is investigated, where the tube-banks are reconfigured into a 'bladed' structure. This concept aims to overcome the peak flux limitations inherent to molten salt receivers and to achieve a significant performance gain without adding new dependencies on futuristic materials.

An optical model is first developed, to investigate the optimal configuration of blades in terms of spacing, depth and orientation. The best design has many very long blades, and a gain of 4.7% in optical efficiency is attained compared to a flat receiver, albeit with a large increase in the total surface area. A simple optical-thermal model (i.e. isothermal) is next developed, which allows the optimal size of the receiver to be determined, balancing the optical (spillage and reflection) losses with the thermal (thermal emission and convection) losses. The optimal blade depth is significantly shorter, but the overall size of the bladed receiver cannot be reduced due to the tradeoffs. Thirdly, results from a detailed receiver model are presented, showing that a 2.5% increase in overall receiver efficiency could be expected from a bladed receiver adapted to a hypothetical 'high-performance PS10' heliostat field (Planta Solar 10, Seville, Spain).

The resulting molten salt bladed receiver design was then adapted for testing at the CSIRO solar field (Newcastle, Australia). Part of this adaptation required plans for a meaningful test using water/air in place of the intended molten salt. The modelling presented here established how the required spatial and directional flux distributions corresponding to the PS10 field could be created in reduced-scale experiments at CSIRO.

Finally, the thesis presents a novel method for directly measuring radiative losses from the bladed receiver. Using computer vision methods to analyse 62 separate images of the operational receiver under concentrated flux, a directionally-and-spatiallyresolved measurement of the reflection losses from all parts of the receiver was determined, and found to closely match the predicted losses from Monte-Carlo ray tracing.

Overall, this thesis presents an investigation of the optical and thermal radiative performance of bladed receivers for CSP systems. A modest yet significant improvement in receiver efficiency through the bladed receiver design is demonstrated. The thesis also contributes to a rigorous verification of optical modelling tools in CSP community, presents an experiment strategy to test full-scale receivers in a small-scale testing facility, and demonstrates a novel radiosity mapping technique to directly measure the radiative losses of a complex-shaped receiver under in-situ conditions. Show more