Elsevier

Solar Energy

Volume 177, 1 January 2019, Pages 163-177
Solar Energy

Thermal performance of vortex-based solar particle receivers for sensible heating

https://doi.org/10.1016/j.solener.2018.10.086Get rights and content

Highlights

  • First order numerical heat transfer model of vortex-based solar particle receiver.

  • Sensitivity of thermal performance to key operating parameters are assessed.

  • Effect: gas/particle mass flow rate, flow direction, particle size, receiver length.

  • Receiver can be configured to operate as either an air-heater or a particle-heater.

  • Counter-flow direction relative to incident radiation tends to improve efficiency.

Abstract

We report a first-order assessment of a novel vortex-based solar particle receiver and the sensitivity of its thermal performance to a number of key operational parameters. This assessment is made with a one-dimensional numerical model developed here to adapt the zonal method to calculate heat and mass transport within the enclosure of the solar vortex receiver (SVR) and to incorporate radiative and convective heat transfer between the particle phase, the air phase and the receiver wall together with re-radiative and conductive loss from the receiver. This simplified one-dimensional model allows for the systematic assessment of first order trends of mass and energy balance within the SVR and is used here to advance understanding of the dominant mechanisms controlling its thermal performance. Sensitivity studies of the thermal performance of the SVR reveal that the receiver can be configured to operate as either an air-heater or a particle-heater, depending primarily on the particle mass loading. For the present SVR configuration, the critical value of mass loading, ṁp/ṁair ≈ 1 was found to define the boundary, above which the device acts as a particle heater, and below which it acts as an air heater. Furthermore, an assessment of the two-phase flow direction found that a counter-flow (relative to the incident concentrated solar radiation) tends to result in a higher efficiency than a co-flow direction. The first order trends of the sensitivity of thermal performance of the SVR to the particle and air mass flow rates, particle size and receiver length were also assessed, finding that the ratio of receiver thermal input to heat capacity of the two-phase flow has a controlling influence on the thermal efficiency of the SVR, particularly with the front entry configuration. Overall receiver thermal efficiencies of up to 88% were predicted for the SVR operating with high mass flow rates of both particles and air, but it is expected that the thermal efficiency of the device for all operating conditions assessed here would increase with an increase in receiver scale from the laboratory-scale device considered here.

Introduction

New technologies are required to achieve operating temperatures up to and above 1000 °C with concentrated solar thermal (CST) energy. This is because solar receivers in current commercially available systems are limited to ∼600 °C due to radiative flux limitations arising from the use of indirect irradiation and the temperature limitations of the heat transfer medium (Ho, 2016). Temperatures in the range 700–1000 °C are being sought to enable the use of new advanced power cycles for concentrating solar power (CSP), such as Brayton cycles with recuperation and combined bottoming cycles as well as supercritical-CO2 cycles (Besarati and Goswami, 2017, Stein and Buck, 2017). Operating temperatures of a similar range and even higher are also required for the application of solar energy into high temperature thermochemical processes via process heat, such as alumina and lime calcination (Davis et al., 2017, Flamant et al., 1980, Steinfeld et al., 1992), the gasification of carbonaceous feedstocks (Lichty et al., 2010, Z'Graggen et al., 2006), metal reduction (Kräupl and Steinfeld, 2001, Lapp et al., 2012) and the production of solar hydrogen (Kodama and Gokon, 2007, Perkins and Weimer, 2009, Sattler et al., 2017, Steinfeld, 2005). Solid particles are a heat transfer medium which, due to their high surface area per unit mass and capacity for direct irradiation, offer the potential to achieve receiver exit temperatures of over 1000 °C together with a low-cost storage medium (Ho, 2016, Ho and Iverson, 2014, Tan and Chen, 2010, Wu et al., 2014). However, little is known of their thermal performance during transient operation over long periods, which requires models of sufficient accuracy to estimate energetic performance with first-order accuracy and numerical simplicity to allow a model to be solved for half-hour time-steps for multiple years (Saw et al., 2017). The overall objective of the present investigation is therefore to develop a one-dimensional model of a particle receiver with first-order accuracy of energetic performance for potential application in system models within high temperature power and thermochemical process systems.

One class of solar particle receiver technologies that has received significant attention employs direct irradiation to heat particles that are transported by a carrier gas within a vortex flow in a cylindrical cavity, termed the solar vortex receiver (SVR). Vortex-based solar particle receiver-reactors have been used to experimentally demonstrate several solar thermochemical processes (Davis et al., 2017, Hirsch and Steinfeld, 2004b, Steinfeld et al., 1992, Z'Graggen et al., 2006). They have the advantage of highly efficient heat transfer to the particle phase due to direct irradiation, as demonstrated by a high chemical conversion with residence times on the order of seconds (Davis et al., 2017, Z'Graggen et al., 2006). A vortex-based solar particle receiver is one device that enables the heating of particles to temperatures of more than 1000 °C with CST, but can be configured in many alternative ways. A simple one-dimensional model is useful to allow a wide range of configurations to be assessed rapidly within a system during transient operation, which is necessary because transients have a first order impact on performance in solar thermal systems (Kueh et al., 2015). The optimal configuration of a solar thermal particle receiver cannot be determined in isolation from other components but must be calculated together with that of the pneumatic conveying system, thermal storage system and hybridisation system (Nathan et al., 2017a, Nathan et al., 2017b), because their thermal performances also vary with turn-down. Hence, determining the transient performance of the SVR within a system of complex components requires a model that is sufficiently simple to avoid excessive computational expense and sufficiently accurate to account for the dominant physical processes within them. However, no first order model of the SVR is presently available. The aim of the present investigation is therefore to meet this need for a first-order model of the SVR.

A large number of solar thermochemical investigations have been undertaken previously using a SVR in which the concentrated solar radiation enters through a transparent quartz window aligned normal to the axis of a cylindrical cavity, while a gas transporting particles is introduced tangentially at the window-end of the cavity and both the particles and gases emerge axially from the rear of the cavity (Davis et al., 2017, Hirsch and Steinfeld, 2004b, Müller et al., 2017, Z'Graggen et al., 2007, Z'Graggen et al., 2006). The residence time of a 5 kW reactor is on the order of seconds and has achieved temperatures over 1000 °C. However, these experimental demonstrations were each conducted with a single geometric configuration, so that no information is available about how the performance may vary with changes to the relative dimensions. In addition, an alternative configuration of the SVR has recently been investigated with a view to mitigating a key challenge to the original configuration associated with particle deposition on the window. Chinnici et al. (2015) proposed that by introducing the vortex gas-particle flow at the opposite end of the receiver cavity to the aperture and altering the geometry of the cavity (the cone angle and aperture-to-cavity diameter ratio) the propensity for particles to be transported through the aperture, and toward the window, is significantly reduced. The efficacy of the aerodynamic mechanism that facilitates the reduction in transport through the aperture has been demonstrated through validated combination of experiments and computational fluid dynamic (CFD) modelling (Chinnici et al., 2015, Chinnici et al., 2016). However, no previous study of the influence on the thermal performance of the two-phase gas-particle flow direction (front entry or back entry) within the SVR is available in the literature. Since it is expensive and time-consuming to fabricate and experimentally compare the performance of multiple receiver geometries and configurations, simplified models, which enable timely comparison of the thermal performance of many different cases, are required.

Several models of the heat transfer within entrained flow solar particle receiver-reactors have been developed, spanning simple one-dimensional models to more complex three-dimensional CFD models. These are complementary because CFD models are generally too computationally expensive to be used to assess a large number of receiver configurations and operating cases. In contrast, a one-dimensional model of the particle receiver operation can provide powerful insights into the dominant mechanisms influencing the thermal performance, before narrowing down to a limited number of configurations that justify the use of more detailed and computationally-expensive CFD models. Previous examples of first order models include the assessment by van Eyk et al. (2016) of an entrained flow solar reactor for the gasification of coal. They adapted a long furnace model (Kasule et al., 2012) to incorporate high-flux solar radiation in a reactor with a one-dimensional axial flow of gas and coal particles. Such lower dimension models (Kasule et al., 2012, van Eyk et al., 2016) have proven useful for sensitivity studies of a large number of configurations but are not well suited to assess the influence of geometry on heat transfer in a relatively short reactor. In contrast, the zonal model has been used to assess relatively short devices in which radiative heat transfer is important. The method has been used to predict gas and refractory wall temperature profiles in a furnace (Hottel and Cohen, 1958), a rotary kiln (Jenkins and Moles, 1981) and, in a modified form, to investigate radiation exchange associated with the solid particle phase in the furnace of a coal boiler (Cañadas et al., 1990). In each of these cases, radiation is treated as non-directional, as is typical of combustion. Solar concentrators, however, provide highly directional concentrated solar radiation to the receiver-reactor. Therefore, the zonal method of analysis needs to be modified to incorporate high-flux directional solar radiation as the heat input to the enclosure, so that its influence on the two-phase gas-particle flow within the receiver and the receiver’s enclosure walls can be assessed. The directionality of concentrated solar radiation can alternatively be modelled with the use of advanced Monte Carlo and finite volume modelling techniques, as has been developed for the steam gasification reaction of carbonaceous particles in the SVR (Z'Graggen and Steinfeld, 2008a, Z'Graggen and Steinfeld, 2008b, Z’Graggen and Steinfeld, 2009). These models have been validated at 5 kW laboratory-scale and have been used to conduct extensive sensitivity analyses of molar extent of reaction and thermal efficiency to key input parameters, such as solar thermal input, feedstock feed rates and particle size. Despite significant modelling efforts characterising the solar steam-gasification of carbonaceous feedstocks in the SVR, there is a need for less computationally-intensive models of the sensible heating of a range of particle sizes in such a vortex-based solar particle receiver, which can be used to assess the influence of receiver geometry on the distribution of temperature and modes of heat transfer, while incorporating the directionality of the concentrated solar radiation input.

To meet these needs, the overall objective of the present paper is to assess the first-order trends of mass and energy balance within the SVR with a one-dimensional heat transfer model of the device. Specifically, this paper aims to assess the trends in thermal performance of the SVR as a function of the particle/gas flow rates, particle loading and size, receiver geometry and flow direction. The SVR can be configured to heat either the particle phase or the gas phase as the primary energy carrier and useful heat can also be recovered from the other phase through a secondary heat exchanger. Therefore the overall thermal efficiency incorporating enthalpy changes of both the particle and air phases between the inlet and the outlet of the receiver are assessed here, together with the partitioning of solar energy absorption between the particle and air phases, and the air and particle temperature rise through the receiver.

Section snippets

Model description and assumptions

The one-dimensional heat transfer model presented here adapts the zonal method of Hottel and Cohen, 1958, Hottel and Sarofim, 1967 for application to the solar vortex receiver (SVR). This method requires that a simple one-dimensional flow pattern and particle distribution within the receiver cavity be known or assumed. The enclosure is then discretised into a number of volume zones (containing gas and particle phases) and surface zones that are small enough to be considered isothermal. A

Model validation and reference case

The accuracy of the model was validated by comparison with experimental measurements for 16 separate cases of the solar sensible heating of air in the solar vortex reactor (SVR). Table 1 lists the geometric and operational conditions of the experimental measurements, which were used as the input parameters to the present heat transfer model. The overall conduction heat loss coefficient for the SVR used in the present validation cases, hw-∞, was previously reported to be variable along the

Reference case configuration performance

Fig. 6 presents the calculated dependence of the thermal performance of the SVR on the mass flow rate of air for a series of values of particle mass loading, as obtained from the present numerical model. Here, the reference case geometry is assessed (Dap = 0.050 m, Dap/Dc = 0.516, L/Dc = 2.55, α = 45°) with front entry flow direction and a constant solar thermal input q̇s= 2000 kW/m2 (Q̇s = 3.927 kW), particle size dp = 40 µm, and inlet temperature Tin = 300 K. Fig. 6a presents the dependence

Conclusions

The first order numerical model developed here to assess the fundamental aspects of heat transfer within a vortex-based solar particle receiver has been found to yield good qualitative and reasonable quantitative agreement with the available experimental data of operation of the device at laboratory-scale under solar simulated conditions. The first order assessment of the sensitivity of thermal performance of the solar vortex receiver to key geometric and operational parameters revealed the

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.

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