Experimentations and Simulations in a 4m Height Heated Ubfb Solar Receiver

Concentrated Solar Power (CSP) plant efficiency is limited by the heat transfer fluid used and especially its operating range of temperature. Indeed, increasing the maximum operating temperature would allow to increase the efficiency of the power conversion cycle deployed.

A new alternative to conventional heat transfer fluids (HTF) is solid particles suspension. It features a good thermal capacity and allows for low-cost heat storage without the usual disadvantages of currently used HTF (operating temperature range, safety issues, storage, corrosion and maintenance costs [1]).

In order to reach heat transfer coefficient from the solar heated surfaces to the HTF in the range of existing industrial receivers, working with conventional fluids, an innovative receiver conception is to use an Upflow Bubbling Fluidized Bed (UBFB) [1][2].The Next-CSP (http://next-csp.eu) European project aims at improving the reliability and performance of CSP plants with this new technology, by demonstrating the concept feasibility with a 4MW_th working pilot, and upscale it to the industrial scale by designing a 150MW_el plant.

The critical design specification for the UBFB receiver is to maximize the heat transfer between the solar heated surfaces and the HTF, which is directly linked to the gas-solid hydrodynamics. In addition, the UBFB technology implies the use of Geldart group A particles [3].

Heat transfer coefficient was measured up to 400-1100 W/m²K during on-sun experiments with small 1m height receiver tubes [4,5] while industrial scale receiver tubes could reach up to 4m height.

However, hydrodynamics in the UBFB strongly depends on the tube height [6]. Indeed, during the fluidization of Geldart group A particles within a tall small internal diameter tube, it has been established that the fluidized bed can be separated into three zones. In the bottom, the bubbles are freely bubbling and starts to coalesce. Then bubbles trend to go large enough to form wall slugs. Finally, wall slugs enlarge to become axi-symetrical slugs[7], which are known to strongly reduce the particle mixing and thus the heat transfer coefficient. In addition, operating temperature strongly impact on the starting height of slugging: operating at higher temperatures trend to shift slugging area higher, and reduce the wall slugging zone.

In order to optimize the design of an industrial UBFB solar receiver, slugging mechanism and especially the impact of temperature should be investigated and understood.

Experimental measurements of fluidization of Geldart group A powder (cristobalite of d_sv= 55µm) in a 3.5m height, 5cm internal diameter, UBFB receiver tube were conducted. The UBFB was maintained by external heating elements at a steady state temperature of 20°C, 475°C and 600°C. Then three-dimensional numerical simulations were carried out using the unstructured parallelized multiphase flow code, NEPTUNE_CFD, based on a Eulerian n-fluid modelling approach [8], including the solving of the coupling between hydrodynamics and heat transfer, imposed by a thermal flux at the wall. The convective/diffusive heat transfer between the gas and particles, and the particle-particle radiative transfer was accounted for [9]. Significant cells size had to be used to ensure mesh independent solution [10].

Numerical simulations were compared to experimental measurements of bed voidage, slugs size and frequency. Emphasis focused on the impact of high temperature on these parameters and numerical simulations allow to study for the trajectory and particles repartition within the receiver. Heat transfer between the tube walls and the particles will be investigated especially through wall boundaries conditions.

Acknowledgements

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 727762, Next-CSP project and supported by the Beijing Advanced Innovation Center for Soft Matter Science and Engineering of the Beijing University of Chemical Technology. This work was granted access to the HPC resources of CALMIP under the allocation P1132 and CINES under the allocation gct6938 made by GENCI.

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