Elsevier

Solar Energy

Volume 190, 15 September 2019, Pages 126-138
Solar Energy

Particle residence time distributions in a vortex-based solar particle receiver-reactor: The influence of receiver tilt angle

https://doi.org/10.1016/j.solener.2019.07.078Get rights and content

Highlights

  • Direct measurements of influence of receiver tilt angle on particle residence time.

  • Vortex-based solar particle receiver investigated with tilt angles spanning 180°.

  • Tilt angle influence is significant for large particles, weak for small particles.

  • Preferable to operate tower-mounted receivers with smaller particles.

  • Residence time behaviour modelled with combination of classical ideal flow reactors.

Abstract

We present the first experimental assessment of the influence of receiver tilt angle on the particle residence time distribution (RTD) of a two-phase solar particle receiver. The tracer pulse response method is used to measure the particle RTD within a laboratory-scale vortex-based solar particle receiver, with the particle phase itself used as the tracer. The experimental parameters of particle size, transporting gas inlet velocity and a range of receiver tilt angles – spanning 180° from vertically upward to downward facing – were systematically varied to determine the influence of key controlling parameters on the particle RTD within the receiver. It was found that the Stokes number of the two-phase flow evaluated at the receiver outlet, Skout, has a controlling influence on the residence time and that the influence of the receiver tilt angle is significant for large particles (Skout > 10) but weak for small particles (Skout ~ 1). This implies that it is preferable to operate tower-mounted systems (i.e. with downward facing receiver tilt angles) with Skout ~ 1. Furthermore, a preliminary scale-up assessment suggests that the influence of tilt angle on the residence time of particles 200 µm and smaller will be insignificant for a nominal 50 MW-scale receiver, which will provide flexibility in the design of industrial-scale devices. Finally, the residence time behaviour for the range of tilt angles assessed can be well described by an analytical compartment model consisting of a small plug flow reactor, followed by two continuously-stirred tank reactors in parallel with a second plug flow reactor.

Introduction

Understanding the influence of the receiver tilt angle on the performance of solar cavity receivers is a necessary component in the development of optimal concentrating solar thermal (CST) systems. This is because the receiver tilt angle relative to gravity has a bearing, not only on the heat and mass transport within the receiver itself, but also on the layout of the heliostat field and the height of the tower on which the receiver is mounted (Behar et al., 2013, Ho, 2017, Ho and Iverson, 2014, Li et al., 2016). While many investigations of the influence of receiver tilt angle on the thermal performance have been performed for single-phase gas cavity receivers, particularly with respect to their convective losses (Clausing, 1983, Lee et al., 2018, Leibfried and Ortjohann, 1995, Wu et al., 2010), this is not true for two-phase devices that employ a gas-particle suspension flow within the cavity. In such solar particle receivers the particle mass transport has potential to be strongly influenced by the direction of gravity. One measure of this particle mass transport performance of a solar particle receiver is the particle residence time distribution (RTD) within the receiver, which describes the probability distribution of residence times a particle spends within the receiver for a given set of operational conditions (Danckwerts, 1995, Fogler, 2006, Levenspiel, 1999). However, no data is presently available as to the influence of the cavity tilt angle of a suspension flow solar particle receiver on its particle RTD. Furthermore, as an alternative to the tower-mounted systems, beam-down receivers are also under development (Kodama et al., 2014, Yogev et al., 1998), for which the receiver will be oriented upwards, together with many variations between the two extremes of beam-up and beam-down. Hence, the objective of the present investigation is to support the further development of suspension flow solar particle cavity receivers by providing first-of-a-kind measurements of the influence of a wide range of receiver tilt angles on the RTD of particles within the cavity.

The vortex-based solar particle receiver-reactor, typically referred to as the Solar Vortex Receiver (SVR), is a particle receiver concept in which the alignment of gravity relative to the direction of particle mass transport is important. In this class of receiver, particles are conveyed by a vortex flow of gas through a zone of direct, concentrated solar radiation within a cylindrical cavity with a circular aperture at one end (Chinnici et al., 2015, Steinfeld et al., 1992, Z'Graggen et al., 2006). SVRs have been demonstrated experimentally at laboratory-scale for a number of different solar thermochemical particle reactions, in which the concentrated solar radiation is typically introduced into the cavity through an aperture sealed by a transparent quartz window (Hirsch and Steinfeld, 2004, Kräupl and Steinfeld, 2001, Z'Graggen et al., 2006). These devices achieve highly efficient heat transfer to the particle phase due to direct irradiation as demonstrated by the high values of chemical conversion reported for residence times on the order of seconds (Davis et al., 2017, Müller et al., 2017, Z'Graggen et al., 2006). In addition, SVRs also have the potential to operate without a window, so that the cavity is open to the aperture (Chinnici et al., 2015, Steinfeld et al., 1992). Whether the SVR operates atmospherically-open or as a closed system with a window, detailed data for both cases is required, including RTD measurements in a closed system. However, all previous thermochemical assessments of SVRs have been performed with the cylindrical cavity oriented horizontally so that gravity acts normal to the central axis. This introduces some potential differences to tower-mounted cavities in which the axis of the receiver is likely to be directed with a downward tilt to align it with the heliostat array. Hence there is a need to investigate the effect of orientation on particle residence time.

A new type of SVR, termed the Solar Expanding Vortex Receiver (SEVR), has recently been developed with a view to capitalising on the high energy conversion efficiency of previous prototypes whilst also mitigating some of their limitations (Chinnici et al., 2015). These limitations of the SVR design concept have been identified as a particle residence time distribution that is approximately independent of the particle size, and a tendency of particles to egress through the aperture and deposit onto the sealing window or be lost to the environment (Chinnici et al., 2015). The SEVR prototype reverses the axial flow direction relative to the aperture to reduce particle flow through the aperture by an order of magnitude, and also features a radially-oriented outlet that helps to recirculate larger particles to the base of the receiver where velocity is greatest to increase their residence time. Chinnici et al. (2015) also identified some potential advantages of a vertical orientation of the SEVR in further augmenting the retention of large particles within the receiver to increase their residence time relative to small particles, notwithstanding other advantages of the tower-mounted receiver orientation. Recent experimental measurements have confirmed the potential of the device to increase the residence time of large particles relative to smaller ones for the vertical orientation relevant to beam-down solar concentrating systems (Davis et al., 2019). This is a potential benefit to processing polydisperse particles, of which larger particles require a longer residence time to undergo a given extent of heating or reaction than smaller particles. However, no experimental measurements of the influence of the orientation on residence time have been reported previously for any type of SVR. Hence, we aim to assess the role of alignment relative to gravity of the SEVR in influencing the particle RTD.

Previous work has found that the SEVR can be configured to operate in two different flow regimes: The Froude-Stokes regime and the cyclonic regime (Chinnici et al., 2015, Davis et al., 2019). In the Froude-Stokes regime, the residence time of larger particles can be increased due to their higher inertia that inhibits them from leaving the receiver, which can also be augmented by gravity for certain orientations (Chinnici et al., 2015, Davis et al., 2019). This applies where a characteristic Froude number of the two-phase gas-particle flow is below a threshold value. In the cyclonic regime the particle behaviour is somewhat analogous to that in a cyclone, so that particles are centrifuged toward the wall of the receiver when the characteristic Froude number is above a certain threshold (Davis et al., 2019). In this regime, the residence time is relatively independent of particle size and is shorter than for the Froude-Stokes regime due to the reduced significance of recirculation (Davis et al., 2019). However, these two regimes have only been reported for the vertical orientation, so that there is a need to expand this understanding for other orientations. Hence an additional aim of the present paper is to expand understanding of the two regimes of operation for the SEVR with a range of tilt angles other than vertical.

In summary, the overall objective of this work is to provide new understanding of the influence of receiver tilt angle on the residence time distributions of particles within a solar particle receiver. Specifically, this paper aims to assess how the alignment of the central axis of the SEVR relative to gravity affects the RTD of particles with large and small inertia, for operation within the low and high Froude number regimes. It has become common to use a compartment modelling approach to model a real device as the sum of ideal flow reactors by comparison with measured RTDs. This analytical approach advances understanding of the device and provides a useful design tool. Hence, we also aim to describe the influence of receiver tilt angle on the residence time behaviour of particles in the SEVR with a compartment model. Finally, we aim to project the particle residence time characteristics of a scaled-up SEVR by providing a preliminary scaling analysis of the particle RTD measurements from a laboratory-scale SEVR. The measurements presented here are for the SEVR operating with isothermal room temperature conditions because of the need to understand the isothermal particle residence time behaviour before assessing the effects of temperature and buoyancy.

Section snippets

Experimental technique

The particle residence time distribution within the Solar Expanding Vortex Receiver (SEVR) was measured for various orientations relative to the direction of gravity with the tracer pulse response method described by Levenspiel, 1999, Fogler, 2006 and illustrated in Fig. 1. The experimental technique follows that described previously by Davis et al. (2019) with the key features reported here. This technique injected a short pulse of particles into the cavity of the SEVR, so that the particle

Measurements of the particle RTD

Fig. 3 presents the measured particle RTDs within the SEVR for seven receiver tilt angles in the range ψ = −90° to +90°, for two outlet Stokes numbers (Skout = 19.4 and 1.2), a single inlet velocity Uin = 30.7 m/s and constant inlet diameter Din = 6 mm, generating a nominal residence time τnom = 3.2 s, where Uin = air/(2 × (π/4) × Din2) and τnom = Vr/air. The distributions have been smoothed for clarity with a moving point average spanning 0.125 s of measured data.

Fig. 3a and b present the

Conclusions

The systematic variation of receiver tilt angle with two inlet velocities, and two particles sizes has revealed that alignment of the central axis of the SEVR relative to gravity has a significant influence on the particle RTD for large particles (Skout > 10), while it is relatively weak for small particles (Skout ~ 1). This implies that it may be preferable to operate a tower-mounted SEVR (ψ > 0°) with Skout ~ 1, so that the receiver’s particle residence time characteristics are independent of

Acknowledgements

We gratefully acknowledge the financial support of the Australian Solar Thermal Research Initiative (ASTRI), a project supported by the Australian Government, through the Australian Renewable Energy Agency (ARENA). Dominic is also grateful for additional assistance in the form of an Australian Government Research Training Program Scholarship. We have no conflict of interest to disclose.

References (25)

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