Characterising the Performance of Vortex-Based Solar Thermal Particle Receiver-Reactors

Abstract

This thesis investigates vortex-based solar thermal particle receivers to provide new understanding of the mechanisms controlling their performance. This new understanding is needed to optimise a receiver on a case-by-case basis for the different requirements of a range of alternative potential applications. There is growing interest in the use of solid particles as the heat transfer medium in concentrating solar thermal systems because particles are e&cient absorbers of direct irradiation and have strong potential as a low-cost thermal storage medium. The vortex-based class of solar particle receiver is under development as a device for the solar heating of both inert particles, via sensible heat, and of reacting particles, via chemical (and sensible) heat. Such a receiver typically consists of a cylindrical cavity receiver, through which particles are conveyed by a vortex 'ow of gas so that the particles are directly irradiated by concentrated solar radiation entering the cavity through a circular aperture. This thesis supports the further development of the vortex-based solar particle receiver by characterising three different aspects of the performance of such a receiver. Firstly, the influence of key input and geometric parameters on the receiver thermal performance is presented. Next, fundamental insights into dimensionless parameters controlling particle residence time distributions within such receivers are provided. Finally, the performance of a vortex-based solar particle receiver in a common industrial thermochemical process is characterised. Key results and findings of this thesis have been published in two peer-reviewed research articles in the journals, Solar Energy and Green Chemistry, while a further two articles have been submitted to the journals Chemical Engineering Science and Solar Energy. A one-dimensional numerical model is developed to systematically characterise the heat and mass transport processes within vortex-based solar particle receivers and provide key insights into the factors affecting the thermal performance. The model adapts the zonal method to calculate heat and mass transport within the enclosure of the receiver, incorporating radiative and convective heat transfer between the particle phase, the gas phase and the receiver wall, together with re-radiative and conductive losses. Sensitivity studies of the thermal performance reveal that a vortex-based solar particle receiver can be configured to operate as either an air-heater or a particle-heater, depending primarily on the particle mass loading. Furthermore, assessment of the two-phase 'ow direction finds that a counter-flow (relative to the incident concentrated solar radiation) tends to result in a higher thermal efficiency than a co-flow direction. The first order trends of the sensitivity of the receiver’s thermal performance to the particle and air mass flow rates, particle size and receiver length are also reported, predicting overall receiver thermal efficiencies of up to 88%. It is, however, expected that the thermal efficiency of the device for all operating conditions assessed will improve with an increase in scale from the laboratory to the industrial scale. The model developed in this thesis is thereby used to advance understanding of the dominant mechanisms controlling the thermal performance of vortex-based solar particle receivers. The first direct measurement of particle residence time distributions (RTDs) within a vortex-based solar particle receiver is presented to provide new understanding of the physical mechanisms controlling the two-phase flow behaviour within the device. The tracer pulse method is extended to directly measure the residence time of the particle phase for a systematic and independent variation of particle size, gas volumetric flow rate, inlet velocity and receiver tilt angle (orientation relative to gravity). Two regimes of particle behaviour are identified, characterised by both the Stokes and the Froude numbers of the two-phase flow. In the low Froude number regime, an increase in the Stokes number increases particle residence time, so that large particles have a longer residence time than the smaller particles, as is desirable for the efficient processing of polydisperse particles. In the high Froude number regime, the Stokes number has a weaker influence on the residence time than it does in the low Froude number regime. The higher Froude numbers result in a decrease in the particle residence time. The comparison of experimentally-measured RTDs with those of ideal reactors finds that the two regimes of particle behaviour can be described by a single analytical compartment model consisting of a small plug flow reactor followed by a series of two interconnected continuously-stirred tank reactors (CSTRs). For the low Froude number regime, there is a significant degree of back-mixing between the two CSTRS, while, for the high Froude number regime, there is negligible. Finally, assessment of alternative receiver tilt angles finds that particle recirculation is enhanced and particle residence times are longer when the two-phase flow proceeds in the direction opposite to gravity. This is because the vertical orientation (corresponding to upward-facing receiver tilt angles) aligns the direction of gravity with the reverse flow direction generated by the central recirculation flow zone to augment the recirculation of particles towards the base of the cavity. However, the assessment also finds that the influence of the receiver tilt angle on the particle residence time is relatively weak for smaller particles. This implies that it is preferable to operate tower-mounted systems (i.e. with downward facing receiver tilt angles) with small particles. This study provides both new experimental data and understanding of the factors influencing the particle RTDs within vortex-based solar particle receivers. Finally, a first-of-a-kind demonstration that alumina can be calcined with concentrated solar radiation in a small-scale vortex-based solar particle receiver is presented. In this novel application of the device, simulated concentrated solar radiation at radiative fluxes up to 2.2MW m⁻² is used to directly irradiate and calcine aluminium hydroxide particles at nominal receiver temperatures over the range 887-1277C. This yields chemical conversions of up to 95.8% with nominal residence times of approximately 3 s and solar energy conversion efficiencies of up to 20.4%. Potential improvements in the alumina product quality (particle pore diameter and specific surface area) are also identified, relative to the quality of alumina produced with the conventional industrial Bayer process. This suggests that concentrated solar thermal processing can be used to improve the quality of alumina over existing fossil fuel based processes through a combination of a high heating rate and avoided contamination by combustion products. Furthermore, the solar-driven process has the potential to avoid the discharge of combustion-derived 165kg-CO₂ per tonne-alumina of emissions for the calcination stage of the conventional Bayer process, at least during those periods when the solar resource is available.