Effect of Biot number on the thermocline evolution of a packed bed TES system

Abstract

Thermal Energy Storage systems are becoming of paramount importance in the context of an efficient and sustainable energy use utilization. These systems are particularly attractive for Concentrated Solar Power plants where the energy surplus is stored as heat and dispatched when solar radiation is insufficient to drive the power block. Among the different storage alternatives, sensible heat storage in granular media is widely used but strongly affected by the behaviour of the thermocline, which directly depends on the particle Biot number. The effect of the Biot number on the thermocline performance of a packed bed Thermal Energy Storage system of sensible heat based on granular material is analysed through particle resolved simulations and an analytical model of a bed of particles. The models used for analysing this kind of systems usually neglect intraparticle conduction. This simplifies the solution of the numerical models and it is a reasonable assumption provided a small enough Biot number. Nonetheless, certain combinations of particle properties and operating conditions might promote non-negligible intraparticle conduction (viz. a sufficiently high Biot number) being required to be considered. Particle Resolved Simulations are required to analyse the coupled problem of the fluid percolating through the particle voids and the heat transfer both inside the particles and from its surface to the fluid. The main novelty of the work is the coupled analysis using two different modelling approaches of packed bed Thermal Energy Storage systems and, more particularly, the successful modelling of the Particle Resolved Simulations. The results indicate a great variation of the thermocline thickness when the Biot number changes, penalizing its performance when the thickness grows. This increase is observed not only when the Biot number augments but also when the Biot number approaches 0, showing an optimum behaviour in a range of Biot numbers, roughly, from 0.1 to 1.5. • The effect of Bi on the performance of the thermocline of a TES system is studied. • The TES system is a packed bed of sensible heat based on granular material. • Particle Resolved Simulations and an analytical model have been used in combination. • The results suggest that there is an optimum range for the Bi . • In this range, the thermocline thickness is minimum, optimizing its performance.

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