Evaluation and Optimisation of Tubular Receivers with Varied Tube Configuration, Working Fluid and Tube Material
Creators: name: ZHENG, Maggie
Publisher: name: The Australian National University
Year: 2023

description: Central tower concentrating solar power (CSP) systems typically focus solar radiation upon a tubular receiver where radiation is absorbed and then transferred, by conduction and convection, into a heat transfer fluid (HTF). High-temperature receivers are critical for third-generation (Gen3) CSP technology to achiever high system efficiencies, and play the role of converting concentrated sunlight into heat. Despite extensive literature on alternative receiver designs, there have been limited efforts to compare optimised receivers with different designs and working fluids using a consistent analysis technique, which leads to a question that there has not been consensus on what the best fluid is for the central receivers. This thesis aims to examine what the optimal receiver design would look like, from the receiver thermal performance point of view. The theme of this thesis is a unified comparison on different working fluids and different tube materials, using one single rigorous model with unified model assumptions, to explore if an optimal receiver performance is caused by the intrinsic benefits of a particular working fluid or the artifact of a particular approach. First of all, the analysis of tubular receivers for concentrating solar tower systems with a range of working fluids (i.e. molten salt, liquid sodium, air, sCO2 and water/steam), in exergy-optimised flow-path configurations is conducted. The effects of varying the tube diameter, and tube wall thickness are studied. Results show that liquid sodium performs the best. The performance of the receiver is strongly constrained by material limits which in turn limit the allowable flux on the receiver. So next, the study seeks to understand the benefits which arise at the receiver as a result of adjusting the flux profile, comparing a simple Gaussian 'spot' with a linear 'ramp' pattern, while respecting an upper limit on the allowable film temperature of the molten salt working fluid. A novel receiver design concept is capable of improving the efficiency of the receiver by adding a series of horizontal tube banks (aka blades), when compared to a conventional flat receiver. Under a 'virtual experimental' approach, both optimised receivers are designed, built and tested (at CSIRO, Australia). The bladed receiver shows a promising technique performance, in relation to the flux limit remission and the light-trapping enhancement, when compared to the flat receiver. Optimised design models are then reconciled with experimental data, studies show that models are consistent with the earlier experimental results, even though a few discrepancies exist in the temperature- and the pressure-matching due to limited experimental data. The relationships between material costs, receiver efficiency, and system level design, for a range of working fluids are conducted and developed under a unified analysis, so that a globally optimal choice can be made. The approach of the exergy analysis has the extra benefit of allowing fair comparisons between receivers operating at different temperatures. After optimising across three different operating fluids, three different tube materials, temperature ranges, tube dimensions and flow paths, the optimal configuration is found under a very extensive free-parameter search, which should have sodium as the working fluid and Alloy 740H as the tube material, in the context of the simplified thermal stress and cost analyses. Lastly, it is found that the exergy destruction in the heat exchanger is not an issue for an optimised sodium-salt system, when compared to an optimised chloride salt system, from a performance point of view. After conducting all the projects mentioned above, it is found that liquid sodium always performed the best in the receivers due to its better heat-transfer characteristics.